Design Project HKU

The University of Hong Kong
Department of Mechanical Engineering
MECH3008 Design Project
2013 – 2014
Project Title: Design of a silencer for a VE75 soil suction machine
Group Number: 5
Group Members: Chu Ka Choi Robert 2011708401
De Michelis Kevin 3035088697
Haughton Tim 3035088685
Lo Ka Yin Hendrix 3035009419
Macfarlane James 3035082370
Supervisors: Dr. C.K. Chan
Dr. Y.H. Chen
Moderator: Dr. K. W. Chan
Sponsor: Towngas
Advisor: Mr. Ray W.C. Fung, Senior Engineer
Date of submission: 7th March 2014

Design of a silencer for a VE75 soil suction machine 2
Acknowledgement
Many thanks are given to the project sponsor, Towngas, for giving us the opportunity to
undertake a real life engineering project and setting aside time out of their busy schedule
to have a site visit and meeting.
Further thanks is given to project supervisors Dr. C.K. Chan and Dr. Y.H. Chen for their
guidance throughout this year.
A special thanks is given to Dr. K.W. Chan for his feedback and support throughout the
year and giving up his time to mark this final report.
Contribution from the group was equally spit and acknowledgments of work completed
can be found in the Gantt chart in Appendix XII.

Abstract
The primary aim of the project was to reduce the noise level of a Towngas VE75 soil suction
machine. The sound level needed to be reduced to 75 dB while not inhibiting the performance
of the machine. A detailed design has been developed meeting all the goals of the User
Requirement Specification (URS)
After a site visit to Towngas, results were analysed that directed the prototype development
and build. Lab sessions were carried out, testing various prototypes, to verify their sound
reduction performance. Results showed that a baffle and insulation design was the best
solution for reducing sound levels.
This final silencer design achieved a 22.0 dB reduction along the exhaust and 15.8 dB reduction
at the exhaust outlet, in the most sensitive frequency range for humans. It is estimated that
the VE75, with the silencer design implemented, should have a sound output of 63 dB along
the exhaust and 74.6 dB at exhaust outlet.
Suppliers have been found that will manufacture the final silencer design below the target
cost set by Towngas - $12,225. The first silencer costs $4967.2 and all subsequent silencers
would cost $2772.2.
It is recommended that Towngas acquire the final silencer design and test it on their VE75.
Positive results from the test should lead to full implementation across all three of Towngas’s
VE75 soil suction machines.

Table of Contents
Acknowledgement........................................................................................................................ 2
Abstract.............................................................................................................................................. 3
Table of Contents........................................................................................................................... 4
List of Figures.................................................................................................................................. 6
List of Tables.................................................................................................................................... 8
Nomenclature.................................................................................................................................. 9
Introduction ...................................................................................................................................................10
Section 1: Utilivac VE75 .......................................................................................................................... 11
1.1 Current design ....................................................................................................................................11
1.2 Site visit ..................................................................................................................................................12
1.3 Analysis of VE75 .................................................................................................................................12
1.3.1 Air velocity..............................................................................................................................12
1.3.2 Temperature .........................................................................................................................12
1.3.3 Sound........................................................................................................................................12
1.4. Results of data.....................................................................................................................................14
1.4.1 Sound........................................................................................................................................14
1.4.2 Fluid flow and pressure calculations ..........................................................................15
Section 2: Research and Concepts ..................................................................................................... 17
2.1 Initial ideas............................................................................................................................................17
2.2 Research on sound.............................................................................................................................19
2.2.1 Absorptive silencers.................................................................................................................20
2.2.2 Reactive silencers......................................................................................................................20
2.2.3 Diffusers.........................................................................................................................................21
2.3 Design concepts...................................................................................................................................22
2.3.1 Noise reduction ..........................................................................................................................23
2.3.2 Filtration........................................................................................................................................24
2.4 Justification of no mathematical analysis.................................................................................25
Section 3: Prototype Build ......................................................................................................................26
3.1 Objectives...............................................................................................................................................26
3.2 Justification of size.............................................................................................................................26
3.3 List of experiments ............................................................................................................................26
3.4 List of features ....................................................................................................................................27
3.5 Prototype building procedure.......................................................................................................29
3.5.1 Construction of test rig and experimental datum ........................................................29
3.5.2 Construction of the prototypes............................................................................................29
3.6 Constraints and implications.........................................................................................................31
Section 4: Experimental Procedure.................................................................................................. 32
Section 5: Results and Discussion ..................................................................................................... 34
5.1 First stage of prototyping................................................................................................................34
5.1.1 Results......................................................................................................................................34
5.1.2 Discussion...............................................................................................................................37
5.2 Second stage of prototyping...........................................................................................................39
5.2.1 Results......................................................................................................................................39
5.2.2 Discussion...............................................................................................................................40
Section 6: Development of Final Design......................................................................................... 41
6.1 Soundproofing vs. sound absorption .........................................................................................41
6.2 Insulation...............................................................................................................................................42
6.3 Baffle........................................................................................................................................................44

6.4 Filter.........................................................................................................................................................48
6.5 Airflow simulation .............................................................................................................................49
Section 7:Final Design.............................................................................................................................. 51
7.1 Description and CAD images..........................................................................................................51
7.2 Installation procedure......................................................................................................................53
7.2.1 Assembly of the baffle.......................................................................................................53
7.2.2 Installation of insulation...................................................................................................54
7.2.3 Installation of the baffle....................................................................................................54
7.3 Maintenance schedule......................................................................................................................55
7.4 Bill of materials and cost.................................................................................................................56
Conclusion ...................................................................................................................................................... 58
Recommendation ....................................................................................................................................... 59
References ..................................................................................................................................................... 60
Appendices ........................................................................................................................................................ I
I. User Requirements Specification ................................................................................ I
II. 2D Drawings of Current VE75 and Exhaust......................................................... III
III. Raw Data Obtained from Site Visit.............................................................................V
IV. Flow Rate and Pressure Calculations....................................................................XIII
V. Simulations & Results of Current VE75 ...............................................................XVI
VI. Raw Data Obtained from Prototype Testing....................................................XVII
VII. Measurement Instrument Specifications..............................................................XL
VIII. STC....................................................................................................................XLIII
IX. Compressor Research ..............................................................................................XLIV
X. SolidWorks FloXpress Report for Final Design ............................................. XLIX
XI. 2D Drawings of Final Design ........................................................................................ L
XII. Gantt Chart....................................................................................................................LVIII

List of Figures
Figure 1: VE75 components
Figure 2: Exhaust and housing
Figure 3: Birds-eye-view of datum readings and sound measurement locations
Figure 4: Background sound level of the car park when all machines were off
Figure 5: Sound level with only the air compressor on
Figure 6: Sound spectrum when both the air compressor and the VE75 were running
Figure 7: Sound level of air compressor and exhaust recorded along the exhaust
Figure 8: CAD image of VE75 exhaust
Figure 9: Labeled 2-D technical drawing of exhaust (units in mm)
Figure 10: Mood board
Figure 11: Categories of decibel levels (Dangerous Decibels), (Scribbd)
Figure 12: A typical absorptive silencer with a baffle in the airflow
Figure 13: A trumpet diffuser with inflected walls
Figure 14: Half splitters in a diffuser
Figure 15: Dimpled pipe
Figure 16: Holed pipe
Figure 17: Baffle
Figure 18: Initial filter design
Figure 19: Locations along the exhaust – inlet (i), midway (m), outlet (o) and spacing (s)
Figure 20: Test rig and mounted experimental datum
Figure 21: Assembled baffle
Figure 22: Inserted dimple feature
Figure 23: Insulated exhaust
Figure 24: Indication of sound measurement locations along the exhaust
Figure 25: Measuring sound level at air inlet during prototyping
Figure 26: Comparison of two air inlets
Figure 27: Comparison of Towngas VE75 (TG) with equivalent prototype, (RS)
Figure 28: Average decibel readings along the exhaust for all prototypes
Figure 29: Outlet sound levels for all prototypes
Figure 30: Sound reductions of all prototypes relative to experimental datum (ES)
Figure 31: Comparison of insulation (I) vs. experimental datum (ES) along the exhaust
Figure 32: Arial view of exhaust displaying angles of sound dispersion
Figure 33: Outlet sound levels for the final prototype (I2BoFm)
Figure 34: Sound reduction of final prototype (I2BoFm) relative to (ES)
Figure 35: Comparison of final prototype (I2BoFm) to best prototype from stage 1 (2BoFi)
Figure 36: 2-D schematic of best prototype, I2BoFm
Figure 37: 3D image of insulated exhaust
Figure 38: Installation of Rockwool RockTech SPI (Roxul 2009)
Figure 39: Sound absorption of Basotect G, as a function of the thickness according to ISO
10534-2 (Andy Yang), vs. eggcrate foam
Figure 40: Prototype baffles constructed in the lab
Figure 41: Stainless steel perforated tubes, to act as support structure for Basotect G sound
absorption foam
Figure 42: Cross-sectional view of baffle – stainless steel case and perforated tubes
Figure 43: Baffle core made from Basotect G, with cylindrical internal tubes
Figure 44: Variation in thickness along the cross-section of Basotect G baffle component
Figure 45: Exploded 3D CAD image of filter
Figure 46: Stainless steel baffle case, end plate and handle
Figure 47: Airflow simulation of the Towngas VE75 exhaust
Figure 48: Airflow simulation of the modified exhaust incorporating the final silencer
design

Figure 49: Cross-section of VE75 exhaust with final silencer design
Figure 50: Fully assembled baffle housing
Figure 51: (a) Filter, (b) Basotect G and (c) stainless steel case
Figure 52: End plate of stainless steel housing
Figure 53: Exploded final silencer design on VE75 exhaust
Figure 54: Insertion of Basotect G into baffle case
Figure 55: Attachment of filter to baffle case
Figure 56: Insulated VE75 exhaust
Figure 57: Notch located at the bottom in the interior of the exhaust
Figure 58: Insertion of baffle into outlet of the insulated VE75 exhaust
Figure 59: Securing baffle in VE75 exhaust

List of Tables
Table 1: Fluid velocities and pressure drops along the exhaust
Table 2: Comparison of modification vs. replacement of current exhaust
Table 3: List of experiments and abbreviations
Table 4: Prototype features used to replicate VE75
Table 5: Sound reducing features
Table 6: ES vs. RS
Table 7: Observations from the first stage of prototyping
Table 8: Comparison of expected implications and observations from experimentation
Table 9: PIL decibel reduction for Rockwool RockTech SPI (Rockwool 2013)
Table 10: Rockwool RockTech SPI prices per standard stock size (Alan Leung 2014)
Table 11: NRC values of 38 mm thick eggcrate foam (Fomo Products), according to ASTM
C423
Table 12: Bill of materials and their costs
Table 13: Final design cost and weight

Nomenclature
A Area m2
𝐴 Effective flow area m2
c Speed of sound m/s
d Diameter of Pipe m
𝑓 Friction factor
K Friction coefficient for nozzle or diffuser
l Length of pipe m
∆𝑃 Change in pressure Pa
𝑄̇ Volumetric flow m3/s
𝑢 Mean velocity m/s
𝑣 Frequency Hz
𝜌 Density of fluid kg/m3
𝜆 Wavelength m
Definitions
𝜌 = 1.165 kg/m3 at 30 ℃
Equations
𝑐 = 𝑣𝜆 (1)
Abbreviations
dB Decibel
URS User Requirement Specification
* Denotes a note at the end of the paragraph
STC Sound Transmission Class
NRC Noise Reduction Coefficient
PIL Pressure Insertion Loss

Introduction
Soil suction machines are used for trenchless excavation to lay gas pipes and electrical
cables. They work on the premises of creating a vacuum to suck up soil and grit, leaving
behind a space to lay pipes and cables. This can be used to excavate paths underground
without having to disturb the surface. Doing so allows soil suction machines to be used
in densely populated cities as minimum disruption is incurred. The project sponsors,
Towngas, own three VE75 soil suction machines.
The drawback of the VE75 is that the exhaust air stream exits at a loud decibel level of
90.4 dB. The primary aim of the project is to create a silencer for the exhaust of the VE75;
reducing the sound to 75 dB. This will minimise disturbance to passing pedestrians and
local businesses by decreasing sound pollution.
Through investigation and integration of existing products and new technologies, and
prototype testing this project provides a highly effective, inexpensive and efficient
silencer. The final silencer design has made a considerable improvement to the VE75 by
reducing its sound output significantly.
The silencer takes advantage of soundproofing and sound absorption technologies and
materials, combining them into a unique and unprecedented silencer.
In addition to the project aim, the project meets a series of goals as set out in the User
Requirement Specification (URS), a summary of which can be seen below and the full URS
in Appendix I. The URS was formed from a site visit and meeting with Towngas, and
comparison with the existing product.
The silencer must:
1. Be shorter than 2.2 m in length
2. Be operable by the current VE75 workforce
3. Have a lifespan of 10+ years
4. Cost less than HK$12,275 – 15 % of the entire VE75
5. Have a minimal and quick maintenance using hose and brush
6. Be designed for manufacture on the small scale
By determining the exact project requirements and their evaluation methods early on in
the design process, all future work may be assessed against the brief and URS in a
qualitative and quantitative manner.

Section 1: Utilivac VE75
1.1 Current design
The Utilivac VE75 is a soil suction machine used by Towngas that uses compressed air to
‘suck’
soil
from
a
desired
area.
A single VE75 machine costs $81,830 (Utiliscope Corp.
2013). The VE75 consists of three parts: the housing, the drum and the exhaust. Figure 1
shows the VE75 components. Figure 2 shows a more detailed section of the exhaust and
housing.
Figure 1: VE75 components
Figure 2: Exhaust and housing
The housing consists of three chambers with venturi tubes connecting each section.
Compressed air at 540 kPa enters the housing through the inlet nozzle into the first
chamber. The air then flows through the venturi tubes causing a pressure drop, which in
turn generates a flow of entrained air from the drum to join the main stream. After
undergoing this process a series of times, a vacuum is generated in the drum. The vacuum
draws soil up the suction hose and into the drum. Two of the chambers have valves that
Exhaust
Drum
Housing
Pedal controlled compressed air
Soil suction hose
Inlet
nozzle
Chambers
Pressure valves
SackPipe section

will open or close automatically if there are pressure differences; this is to minimize the
amount of debris that joins the airflow and to ensure a better-generated vacuum. Finally,
the air passes through the exhaust and into the surroundings. At the end of the exhaust
there is a sack. This is to capture any of the very small debris, typically sand, that may
have entered the airflow and prevent it from being blasted out at any passersby.
1.2 Site visit
A demonstration of the VE75 was held at the Towngas headquarters’ car park. The air
velocity, air temperature, sound level and sound frequency were measured during the
demonstration. Datum readings were taken before all measurements were recorded. This
was to account for the noise from the background and compressor. A Bruel & Kiaer 2238
Mediator was used to measure decibel and frequency levels; with a tolerance of ± 0.05 dB.
A Uni-t ut362 USB anemometer was used to measure air velocities, with a tolerance of ±
0.05 ms-1. Full specifications can be found in Appendix VII. Pressure measurements were
not taken due to a lack of available instrumentation.
During the visit it was observed that the internal surface of the exhaust was covered in
cement and sand deposit from previous operations. Conversations with employees
explained this was due to a lack of maintenance. Primarily, to clean the internal surface,
the sack needed to be removed and re-installed – a lengthy operation that could not be
justified. It was apparent that a minimal, quick and easy maintenance was required for
any provided solution. Additionally, the sack had never been washed and thus emitted a
lot of dust when the machine was running.
1.3 Analysis of VE75
1.3.1 Air velocity
The air velocity was recorded from exhaust outlet, in increments of 0.5 m, until a 10 m
distance was reached. The air stream came out in a jet and dispersion was negligible. The
air velocity decreased from 28 ms-1 to 0 ms-1 within 3.5 m without the sack and 4 ms-1 to
0 ms-1 within 1 m with the sack. This analysis shows that, given the sack was always
attached to the VE75 exhaust during previous operations, exhaust air was not a problem
to pedestrians.
1.3.2 Temperature
The temperature of the air flow was measured during the site visit. Temperature showed
very little variation (29 ℃ to 32 ℃) within 2 m downstream of the exhaust exit and
remained constant thereafter. The heat from the exhaust was negligible and is therefore
not an issue that needs to be addressed.
1.3.3 Sound
Sound measurements were taken at various locations around and along the exhaust. The
measurements were broken down into two categories: measurements along the exhaust
– E0 to E4, and measurements around the VE75 – N0 to N10, W1 to W10 and S1 to S10. A
birds-eye-view of these are shown in Figure 3. All raw data measured during the site visit
is shown in Appendix III.

Figure 3: Birds-eye-view of datum readings and sound measurement locations
Three measurements were taken at D0:
 The background environment
 The air compressor on and the VE75 off
 The air compressor on and the VE75 on
The first two measurements were used to determine the sound level and frequency of the
background and the air-compressor. No high decibel reading at any specific frequency
was identified for the background noise, as shown in Figure 4. A sound level of 96.8 dB at
a frequency of 63 Hz was identified for the air compressor as indicated by the appearance
of a peak between Figure 4 (background environment) and Figure 5 (air compressor on).
Consequently, the decibel level at the frequency of the air compressor was ignored for all
future sound readings.
With the VE75 running, frequency and decibel measurements were taken along the
exhaust between E0 and E4. All sound measurements fell in a 1 – 20 kHz range, as shown
in Figure 6. This frequency falls into the most sensitive hearing range of the human ear (2
– 5 kHz) (R Nave 2012). Therefore, the highest decibel level within that frequency range
was recorded for every location in Figure 3. Coincidently, this peak decibel level was
consistently found to be at 2 kHz. Consideration for different height levels were accounted
for; measurements were taken at 1.7 m and 1 m from ground level; the average height of
a Hong Kong adult and child respectively. Measurement showed height was not a factor.
E0
E1
E2
E3
E4
N5
N0
N1
N2
N10
W1W2W10 W5
S1
S2
S5
S10
D0

With these frequencies identified, the background sound level measured 56.3 dB at 2 kHz.
The raw data can be found in Appendix III, Table VI and VII.
Figure 6: Sound spectrum when both the air compressor and the VE75 were running
1.4 Results of data
1.4.1 Sound
Looking at Figure 7, the exhaust decibel level varies marginally from E0 to E3. When the
exhaust air stream exits at point E4, there is an increase in decibel level. This is due to a
sudden expansion at the outlet, shown in Figure 7.
Decibel levels at locations around the VE75 (N0 to N10, W1 to W10 and S1 to S10)
decreased with increased distance from the exhaust.
Figure 7: Sound level of air compressor and exhaust recorded along the exhaust
Figure 4: Background sound level of the
car park when all machines were off
Figure 5: Sound level with only the air
compressor on

The highest sound levels were recorded in the North direction, N0 to N10 (in reference to
Figure 3). This was because the measurements were taken in the exhaust air stream. With
the sack attached, the outlet decibel level was 90.4 dB. Without the sack attached, the
exhaust outlet sound level was 99.6 dB. However, the sound level along the exhaust
remained the same.
1.4.2 Fluid flow and pressure calculations
During the site visit the principle source of the sound was identified to originate from the
exhaust, more precisely; from the turbulent air within. Consequently, analysis of the
airflow in the exhaust was required.
Figure 8: CAD image of VE75 exhaust
In order to carry out simulations and build prototypes, analysis of the fluid flow at various
points along the exhaust was required, specifically flow rate and pressure drops. To do
so, the velocity at the exit of the exhaust, the only data it was possible to obtain during the
site visit, was measured. With the exit velocity it was possible to determine the flow rate
and thus the velocities at other points in the exhaust. However, some other assumptions
were made to find the pressure drop across the exhaust.
 Firstly, it was not possible to measure the exit pressure due to a lack of
instruments available at HKU. So to find a realistic assumption, advice was
sought from Prof. Chun-Ho Liu. He stated that assuming atmospheric would
be an overestimation and thus acceptable for calculations in this project.
 Secondly, the inside of the pipe had never been cleaned, so the mild steel was
very dirty and an absolute roughness had to be estimated. Again, Prof. Chun-
Ho Liu was consulted. His advice was to assume a friction factor 𝑓 = 0.02
 Thirdly, the exit velocity was low enough to assume that the airflow in the
exhaust was not compressible. Hence an incompressible flow analysis is
acceptable throughout this project.
Pressures and mean velocities at various points along the exhaust, with reference to
Figure 9, are shown in Table 1. The calculations of these values are shown in Appendix IV.
After inspecting the data it is apparent that, though there were some overestimations, the
pressure drop was negligible (0.898 kPa) in comparison to the pressure from the air
compressor at the start of the machine (540 kPa). Therefore, it is possible to neglect that
factor and consider only the air velocity when testing the prototypes. Data in Table 1 and
Figure 9 will be used to simulate the airflow in the current design as well as the final
recommended design.

Figure 9: Labeled 2-D technical drawing of exhaust (units in mm)
Mean Velocity
(ms-1)
Pressure Drop
(Pa)
Absolute Pressure
(kPa)
A 32.51 - 102.223
A-B 47.94 364.51 -
B 71.57 - 101.858
B-C 71.57 328.88 -
C 71.57 - 101.53
C-D 45.39 198.86 -
D 28.69 - 101.331
D-E 28.69 5.67 -
E 28.69 - 101.325
Table 1: Fluid velocities and pressure drops along the exhaust
A B C D E

Section 2: Research and Concepts
2.1 Initial ideas
Initial considerations for the design are displayed in Figure 10 as
a
“mood
board”.
The

mood board is a visual summary of conceptual brainstorming ideas, existing technology
and systems that influenced the direction and development of research and design.

ConceptualIdeas
ExistingTechnology
Filtration
System
Sound
Absorption
Airflow
Reducing
turbulenceand
vibrationtoreduce
noise
Airflowsimulation
Carexhaustsystems,
windtunnels,fans
Baffles,Helmholtz
tuners,containment
chambers,airflow
dispersion
Modificationor
replacement
Outercasing,
insulation,air
dispersion,diffuser
technology
Absorption,
reflectionor
insulationofsound
Cleanexhaust–
reduced
turbulence
Quickandeasyto
change,low
maintenance–
likecameralenses
Gauzesizefor
variousdebris
materials
Figure10:Moodboard

Prior to further design, research was conducted into sound reduction technologies and
systems identified in Figure 10. With the primary focus being on their working principle
and applicability.
2.2 Research on sound
Sound is measured in decibels on a logarithmic scale – i.e. a 10 decibel reduction is
equivalent reducing the sound to one tenth of its original level. Therefore, any reduction
in decibels cannot be measured in percentages. Decibel levels can be categorized as
shown in Figure 11.
Figure 11: Categories of decibel levels (Karen S. Finch 2014), (Dang-dang Siggaoat-Copiaco 2010)
Towngas originally specified that the noise level of the VE75 was to be reduced to 60 dB.
However, observation of Figure 11 shows this to be equal to a standard human
conversation. This aim is unreasonable because the typical work environment of the VE75
is that of a busy city – 85 dB. Therefore, a reasonable
aim,
considering
Towngas’
original
request, is to reduce the peak sound level down to 75 dB; less than industrial manual
machines that operate at 80 dB. According to National Institute on Deafness and Other
Communication Disorders, people exposed to sound at or above 85 dB can suffer from
noise induced hearing loss (NIH 2013). Considering this factor, the aim of the project is
further justified.
Note: The background decibel level recorded (56.3 dB) during the site visit is not a realistic
representation of actual conditions. It was conducted in an elevated, sheltered car park, significantly
distancing the street sounds. No other activity, or conversation was going on during measurements.
35
45
55
65
75
85
95
105
115
125
Raindrops
Normal
Conversation
Industrial
Manual
Machines
Busycity
traffic
RockConcert
Jack
hammers
DecibelLevel
Decibel levels in certain environements

Sound can be classified in terms of low (30 – 300 kHz), medium (0.3 – 3 MHz) and high (3
– 30 MHz) frequency. Frequencies measured during testing of the existing exhaust fall
into the low frequency band. Thus research was focused on low frequency sound level
reduction. There are many ways of reducing the sound level from an exhaust or air
stream. The types of silencers researched for silencing the exhaust of the VE75 were
absorptive silencers, reactive silencers and diffusers. Each method has different
advantages in reducing sound levels or frequencies.
2.2.1 Absorptive Silencers
Absorptive silencers work on the premise of reducing sound level by absorbing materials
attenuating the sound waves (EngineeringToolBox 2013). The sound energy is dissipated
as heat and vibration of the material as the sound wave passes through fibrous cavities.
(Walker 2013). The absorbing material can be wrapped around the exhaust or placed
inside, in the form of a baffle. Absorption is best suited to > 500 Hz, which is ideal for the
given project (see Section 1.3.3).
A baffle consists of a series of perforated tubes wrapped in sound absorbing material. The
baffle is placed in the airflow causing the air to go through the perforations and into the
material, shown in Figure 12. An increase in material thickness will provide improved
attenuation at low frequencies. The diameter of perforated tubes is determined by the
frequency of the sound source. However, the tube diameters must also consider air flow
restrictions. Decreasing the diameter too much will increase the resistance of air flow.
This is turn will generate noise in the silencer, countering the effect of the silencer in the
first place. Downstream of the silencer the air expands to either the exhaust dimensions
or (worst case) atmosphere. The expansion causes baffle generated noise. The higher the
pressure drop, the greater the generated noise levels and insertion loss values. Thus there
must be an upstream expansion chamber after the baffle to calm the turbulent flow.
The key factors of acoustic performance depend on the following (Arca53 2013):
o Sound absorption coefficient of the duct lining material
o Thickness of the absorption material
o Diameter of perforated tubes
o Length and location of baffle in duct
Figure 12: A typical absorptive silencer with a baffle in the airflow
2.2.2 Reactive silencers
Another way of reducing sound levels is to use a reactive silencer. Reactive silencers are
designed to change or eliminate noise by reflecting sound waves back towards the source
using destructive interference. This is achieved via a Helmholtz tuner (for low
frequencies) or a pinch (for high frequencies). Thus the Helmholtz tuner is relevant for
this project.
A Helmholtz tuner operates on the theory that when sound waves pulse through a
constricted area into a large closed area, the sound energy will be reduced. This is usually

achieved via a closed expansion chamber surrounding a perforated duct. At a specific
frequency the cavity will resonate and the waves in the exhaust pipe are reflected back
towards the source. Calculating the required size and shape of the expansion chamber,
and openings between exhaust and expansion chamber are a complexity beyond the
scope of this project. An advantage of a reactive silencer is that it is non-fibrous, so easy
to clean and will not absorb liquids. This is ideal for this project as it will minimise
maintenance.
2.2.3 Diffusers
A diffusers purpose is to produce an increase in static pressure without further energy
input; by reducing outlet velocity of fluid flowing through, while minimising stagnation
pressure loss. The reduction in dynamic pressure at the diffuser outlet reduces the exit
loss (less kinetic energy discarded). Reducing the exit loss reduces the compressor work
(thus increasing suction capacity) (Sims-Williams 2012). Simultaneously, noise and
vibration are reduced. Research suggests that a trumpet shaped diffuser with inflected
walls is best suited to this project (Macfarlane 2012) (ESDU 1974) as shown in Figure 13.
Figure 13: A trumpet diffuser with inflected walls
However, manufacture of such a complex geometry may be difficult and expensive. Thus
a conical diffuser is more feasible. Optimum length and area ratios are obtainable using
experimental data from ESDU 1990.
Separation of flow through a diffuser also severely affects diffuser performance, due to a
reduction in the effective area expansion ratio. This is particularly appropriate to this
experiment due to turbulent flow and the overall exhaust length restriction of 2.2 m
(stated in the URS). This in turn restricts the length of the diffuser. Thus to achieve a
reasonable expansion area ratio, large wall angles are needed, which tend to lead to
separation of flow.
Splitters are thin plates that divide the diffuser into a number of smaller diffusers, as
shown in Figure 14. Any separation that does occur is confined, rather than affecting the
whole diffuser area. Splitter material thickness is key in prototyping. If too thick, splitters
cause shear drag and reduce AE (effective flow area). Thus either very thin splitters or half
splitters should be added to the exhaust diffusers.
Figure 14: Half splitters in a diffuser
Splitters
Inflected wall shaped trumpet shaped
diffuser
Airflow

2.3 Design concepts
After considering the initial ideas and research, it was apparent there were two
approaches that could be taken; either the current exhaust was modified, or an entirely
new exhaust fitted. Table 2 measures the positives and negatives, against the URS, of
modification vs. replacement of the current VE75 exhaust.
Table 2: Comparison of modification vs. replacement of current exhaust
Modification of Exhaust New Exhaust Design
PossibilitiesofDesign
 Integration of absorptive
silencer technology - internal
baffle
 Integration of outer sound
insulation casing
 Manipulation of metal structure
to create Helmholtz tuner (but
existing geometry will limit
effectiveness of this)
 Integration of absorptive silencer
technology - internal baffle
 Integrated reactive silencer
technology – Helmholtz tuner
 Ability to incorporate more
advanced diffuser with splitters
 New shape – offering vertical or
horizontal design
 Easier access for cleaning of
exhaust
 New exhaust material with better
sound absorption coefficient
DesignLimitations
 Accessibility of current exhaust
to suppliers for modification
 Structural integrity of exhaust
steel must not be compromised
 Additions must not exceed
overall length of 2.2 m
 Weight must not unbalance
VE75 unit
 Removal of existing exhaust
without damaging VE75 unit
 Difficulty of attaching to the VE75
housing
 Suction of VE75 must not be
compromised
 Overall length must not exceed 2.2
m
 Weight must not unbalance VE75
unit
Manufacture
 Added complexity due to retro
fitting exhaust
 Easy manufacture can be
incorporated into design
 Larger volume to manufacture
Comments
 Constraints of adapting to
current exhaust
 Thorough clean of existing
exhaust is needed
 Total freedom of design (both
advantageous and a disadvantage -
where to start)
 One piece design gives better
structural integrity
Cost
 Adaptations may make
components more expensive
due to non-standard sizes
 Transportation of VE75 to
factory for modification and
fitting
 Ability to base size on standard
stock sizes
 Additional cost of new exhaust as
well as silencer components
 Removal of existing exhaust
 Fitting of new exhaust

Both options have multiple positives and negatives. While replacement of the exhaust
would appear to be the superior approach, it was decided to go with modification of the
current exhaust. The main reason being:
 The implications of removing the current exhaust from the VE75 are unknown*.
It would be very costly and require direct consultation with Utilivac to ensure no
detrimental effects to the VE75 unit. Attachment of a new design would prove
equally challenging.
Also, it was estimated that the cost of modification would be far less, given the fewer
components and smaller volume being manufactured. Therefore, further development of
designs is based on a modification approach to the current exhaust, whilst taking into
consideration the sound reduction methods discussed in Section 2.2.
*For the same reason, it was decided not to include a diffuser. Splitters were also decided against to
maintain simplicity in design and build. Also, construction and attachment of the diffuser was deemed
too difficult for the lab conditions. Finally, if splitters were installed at the outlet of the exhaust then
insertion of any component would not be possible. As such, diffusers are not discussed any further in
this report.
Additionally, it is believed that a clean exhaust would generate more laminar flow (hence
less related noise) than a dirty exhaust. An important assumption was that most of the
noise was coming from the turbulent airflow and vibration. Therefore, attempting to
achieve laminar flow was considered during design. Thus a new component was
introduced; a debris filtration system. Such a system would also prevent any blockage of
holes, gaps and foams, or buildups in any of the components.
Given the chosen approach, the design was broken down into two key areas:
a. Noise reduction
b. Debris filtration
2.3.1 Noise reduction
Dimpled Pipe - By making the inner wall of the pipe dimpled, sound waves would reflect
around the pipe, thus annihilating similar sound waves. The dimpled pipe is shown in
Figure 15.
Figure 15: Dimpled pipe
Holed Pipe - The idea behind this concept was to diffuse the air along the length of the
pipe, thus decreasing the velocity of the airflow in the pipe. Consequently, the airflow
enters the atmosphere at a lower velocity and reduces associated expansion losses at
the exhaust outlet, as shown in Figure 16.
Airflow
Dimples in pipe

Figure 16: Holed pipe
Baffle - The sound reduction theory of a baffle was discussed in Section 2.2.1. For silencing
to be effective in the baffle, the perforated tube diameters must be less than the
wavelength of the sound passing through. The speed of sound is 340 ms-1, and the highest
frequency from the exhaust 4 kHz. Resulting in a wavelength of 8.5 cm using (1). This
consideration was taken during testing along with number of tubes. An example baffle is
shown in Figure 17.
Figure 17: Baffle
To minimise insertion loss, a full length and half-length baffle were tested. Typical
insertion loss expected for a 100 mm diameter pipe of length 600 mm, is 34–45 Hz
(EngineeringToolBox 2013). Possible materials for sound absorption are: mineral wool,
glass pack and glass fiber. Due to their fibrous nature, sound enters and dissipates as heat
and vibration.
Insulation - By wrapping a soundproofing material around the exhaust, the sound waves
emitted will be contained. Thus reducing the overall noise of the VE75. The principle
works on reactive sound reduction.
2.3.2 Filtration
In the current model there is a sack attached to the end of the exhaust to capture any
debris. However, a lot of the debris builds up inside the exhaust; increasing the surface
roughness, and in turn generating more turbulent flow. So it was hypothesized that a filter
located at the entrance of the exhaust would eliminate the current problem of debris build
up inside.
Consequently, a method to easily insert and maintain the filter for an initial design was
drawn up. A small slit in the top of the pipe would be cut, allowing insertion of the filter
without changing the shape of the exhaust too much. To make this hole as small as
possible, a foldable filter was designed. Once in the pipe, the user would turn the hinges
to unfold the filter in the pipe. Two holes in the bottom would allow for the rods of the
filter to secure it in place as shown in Figure 18.
Airflow
Perforated tubes
Sound absorption material
Airflow Holes in pipe
Diffused air

Figure 18: Initial filter design
2.4 Justification of no mathematical analysis
During the research and concepts process of the design, it was found that the
mathematical analysis and proof of sound reduction was far too complicated for the
project. After having these doubts confirmed by Prof. Chun-Ho Liu, it was decided that an
experimental approach would be taken. This approach had the benefit of providing
results that could be analysed and interpreted, which could then be applied to the final
design. Therefore, prototypes were built and tested.
Gap to insert filter
Hinges
Filter
Holes for filter
Folded filter
Pipe
Airflow

Section 3: Prototype Build
3.1 Objectives
The objective of the prototype build was to construct a realistic and accurate test rig of
the VE75 currently used by Towngas; enabling identification of the best solution from the
options in Section 2.3.1. During construction, experimental measuring techniques were
considered to ensure appropriate tests and results could be obtained from the prototypes.
Once constructed, different prototypes could be tested on the rig to see the success at
their sound reducing properties.
3.2 Justification of size
An initial idea for scaling down was proposed and some dimensional analysis was done.
However, after considering how to build the prototype and the complexity of the
calculations associated with scaling decibel levels, it was decided that the prototype
would be a 1:1 scale of the original. This eliminated errors and assumptions related with
scaling the model down to a smaller size. The 1:1 scale was also appropriate for the
materials used in the prototype; a standard stock size PVC pipe was almost identical in
diameter. Also, scaling the model down would have implied an increase in air velocity,
which would have only complicated the experimental procedure given airflow velocity
problems discussed in Section 3.6.
3.3 List of experiments
After careful consideration a selection of experiments was planned to ensure a wide range
of sound reduction techniques were tested. The list of experiments and their
abbreviations are shown in Table 3. Abbreviations consist of an upper case letter for a
feature and proceeded by a lower case letter for position in the exhaust if needed. The
feature abbreviations are listed below:
Baffle B
Dimpled pipe D
Experimental datum E
Filter F
Holed pipe H
Insulation I
Rough R
Sack S
Towngas VE75 TG
The
‘2’
signifies
two
of
the
feature
that
proceed
it. The position of the feature can be either
at inlet (i), midway (m), outlet (o) or spaced (s) as illustrated in Figure 19. The various
positions were to see the effects certain features would have in relation to their sound
reduction qualities.

Figure 19: Locations along the exhaust – inlet (i), midway (m), outlet (o) and spacing (s)
Experiment name Reference
Towngas VE75 TG
Experimental datum E
Experimental datum with filter EFi
Experimental datum with sack ES
Rough R
Rough with sack RS
Dimples D
Dimples with filter DFi
Baffle at inlet Bi
Baffle at inlet with filter BiFi
Baffle at outlet Bo
Baffle at outlet with filter BoFi
Baffle at outlet with filter midway BoFm
2 Baffles with spacing 2Bs
2 Baffles with spacing and filter 2BsFi
2 Baffles at outlet 2Bo
2 Baffles at outlet with filter 2BoFi
Insulation I
Insulation with filter IFi
Holed pipe H
Holed pipe with filter HFi
Table 3: List of experiments and abbreviations
3.4 List of features
All individual features were identified and established from Table 3. The features are split
into two sections. Table 4 shows features constructed in the lab to create a realistic
replica. Table 5 shows features built to reduce sound level. All materials used for the
prototyping stage were primarily chosen for their suitability while considering
availability and cost. The materials were sourced from local hardware stores in Hong
Kong.

Feature Description Towngas VE75 Prototype
Rough
Surface
Sand glued to
paper and inserted
into PVC pipe to
replicate buildup of
sand and cement in
VE75
Sack
Sack connected to
the end of
prototype to
duplicate the
current sack on the
VE75
Table 4: Prototype features used to replicate VE75
Feature Description Image
Baffle
Chicken wire case
filled with shredded
eggcrate foam
Dimpled
pipe
Paper lined with BB
gun pellets
Filter
Air conditioning filter
sheet
Holed pipe
PVC pipe with drilled
holes
Insulation 38 mm eggcrate foam
Table 5: Sound reducing features

3.5 Prototype building procedure
This section discusses how the features in Tables 4 and 5 were built. The building
procedure consists of two parts; building the test rig and the experimental datum,
followed by fabricating all the necessary features that would modify the
experimental datum.
3.5.1 Construction of test rig and experimental datum
The test rig is a support stand to mount the prototype replica of the VE75 exhaust.
Once built, prototypes could be mounted and experimented on. The test rig was
made from a simple plywood base with polystyrene supports. The experimental
datum is an unmodified exhaust that consists of a clean interior PVC pipe and PPE
frustums that act as the nozzle and diffuser. The PVC pipe was cut to the correct
lengths of the VE75, 590 mm using a hack saw. The conical frustums for the
diffuser and entrance nozzle were made from flexible PPE that were cut with a
Stanley knife and fixed into the correct shape with epoxy glue and clamps. The
frustums were connected to the PVC pipe with duct tape. A completed setup of the
test rig and experimental datum can be seen in Figure 20.
Figure 20: Test rig and mounted experimental datum
3.5.2 Construction of the prototypes
Rough and sack -To replicate the Towngas VE75 a simulation of the rough internal surface
was created; by gluing sand to a piece of paper. The paper was then inserted into the
exhaust pipe using duct tape to secure it in place. To reproduce the sack, an air
conditioning filter sheet was fashioned into a sack and attached to the outlet with multiple
elastic bands. Combining both features gave prototype RS, which acted as the Towngas
equivalent.
Baffles - The baffles were constructed by encasing sound absorbing material around
perforated tubes. The tubes were made by wrapping lengths of chicken wire around a
small diameter rod. The diameter of the rod used was 25 mm. This adhered to the
conditions required to attenuate noise below 4 kHz, as discussed in Section 2.3.1.
However,
a
variation
of
the
perforated
tubes’
diameters
was
not
feasible
due
to
building

constraints. Four wire tubes were assembled with a staple method that ensured rigidity,
and then placed in a cylindrical chicken wire case as shown in Figure 21. The gaps
between the tubes were filled with eggcrate foam that absorbed and trapped sound waves.
The housing allowed easy insertion and extraction from the exhaust. Each baffle was 200
mm in length.

Figure 21: Assembled baffle
Dimples - An initial attempt was to melt the PVC pipe from the outside to create
depressions that would result in dimples on the inner diameter. However, this did not
work as the solder just melted a hole through the pipe, leaving a very small dimple. This
idea was abandoned and instead the dimples were made by gluing plastic BB gun pellets
to a sheet of paper. The sheet of paper was then inserted in a same way to that of the
rough surface feature. An illustration of the inserted dimple feature is shown in Figure 22.
Figure 22: Inserted dimple feature
Holed pipe - The holed pipe was constructed by drilling thirty holes using a handheld
power drill. The holes were equidistant around the circumference and along the pipe. The
holes were drilled in 6 parallel lines along the pipe.
Insulation – The eggcrate foam was wrapped around the exhaust and secured by duct tape,
as shown in Figure 23.
Figure 23: Insulated exhaust

3.6 Constraints and implications
There were five constraints that were encountered due to the budget of the project and
the available equipment in the laboratory, as described below.
1. Jet stream - There were two airflow sources in the lab that could be used; a fan,
and compressed air from a handheld nozzle. Both methods, however, had some
issues.
The fan was too loud and its noise source could not be isolated from the
measurements, thus skewing results. Furthermore the airflow was considerably
lower (12 ms-1) than that of the VE75. Conversely, the handheld nozzle provided
an air velocity of 40 ms-1, similar to that of the VE75. Additionally, the sound
generated from the nozzle was not enough to affect any measurements. However,
the nozzle generated a jet flow instead of an evenly distributed airflow like that of
the VE75.
A third alternative was to buy an air compressor that would simulate the VE75,
but no viable option was found within budget. This is shown in Appendix IX.
Therefore, after much discussion it was decided that the handheld nozzle would
be used despite the jet stream issue.
Implications: It will be quieter along the exhaust due to air stream around jet
acting like insulation but louder at outlet due to greater expansion.
Note: No barrier at inlet was needed as the nozzle generated a suction effect at inlet during
operation. This was caused by the low pressure generated in the jet stream. As a result, the
exhaust inlet remained open to atmosphere while testing the prototypes.
2. Incorrect air velocity - The air velocity used in the replica was not identical to
the velocity measured at the site visit with Towngas. The mean velocity was 32.51
ms-1 at the inlet and 28.69 ms-1 at the outlet of the VE75, as shown in Section 1.4.2
Table 1. The velocity achieved in the lab was 40 ms-1 at the inlet and 13.07 ms-1 at
the outlet. This constraint was a direct consequence of the equipment available in
the lab and could not be changed.
Implications: The prototype may have a lower datum sound level
3. VE75 non-standard dimensions - The pipe used for the prototype was not the
identical diameter to the VE75 design. A 105 mm internal diameter PVC pipe was
chosen from a hardware store – the closest stock size to that of the VE75 exhaust
pipe, 107 mm.
Implications: Minimal
4. Materials - It was unfeasible to make a 1:1 model out of the tempered steel that
used for the VE75. Instead a PVC pipe was used to model the exhaust pipe, while
sheets of PPE were used for the more complex geometries such as the conical
frustums. Furthermore, the connection method of the pipe to the conical frustums
was duct tape instead of welding.
Implications: Lighter material and non-smooth connections between PVC pipe
and PPE frustums may cause noise generation from vibrations.
5. Open at inlet - The air inlet of the prototype exhaust was open to the environment
unlike that of the VE75. An attempt was made to seal the entrance of the
prototype, but complicated alignment of the direction of the jet stream with the
axis of the exhaust pipe.
Implications: New sound source that will need isolating

Section 4: Experimental Procedure
The primary purpose of carrying out experiments on the prototype was to determine
which method was the best at reducing the decibel level – first stage of prototyping. After
testing each individual feature and a succession of combinations, analysis of the results
showed which arrangement of sound reducing methods was best. Thus, a final design was
assembled, incorporating several of the best features found in the lab – second stage of
prototyping. The same measurement instruments were used as those from the site visit.
The experiment was carried out with four aspects to ensure reliable and accurate results.
1. Location of measurements - The measurements were taken at five points along
the exhaust; as illustrated from locations 1 to 5 in Figure 24. These five locations
were deemed most important after analysis of the results of the site visit. Air
velocity was consistently measured at exhaust outlet.
Figure 24: Indication of sound measurement locations along the exhaust
2. Repeats - Sound readings were taken three times at each location; enabling
elimination of any anomalies. All results discussed in Section 5 are an average of
the three measurements for each location. Figure 25 shows a sound measurement
being taken at position 1.
Figure 25: Measuring sound level at air inlet during prototyping
3. Consistency - All experiments were carried out by the same members of the
group doing the same job every time. This ensured consistency in the readings as
two different people may have had different readings for the same result. All
experiments were done in the same section of the lab on the same day. This
eliminated external factors that would affect one set of results from another.

4. Elimination of peak frequency - As found with the air compressor on the site
visit, the jet airflow in the lab generated a background peak frequency. Therefore,
a measurement of the frequency at the air inlet was taken, which was then
excluded from the following measurements along the exhaust. This was done to
eliminate sound levels that would skew results; due to the air inlet being open to
the environment, unlike the real exhaust that is welded to the housing of the
Utilivac VE75, as shown in Figure 26. All other decibel levels were recorded at the
next peak frequency.
Figure 26: Comparison of two air inlets

Section 5: Results and Discussion
This section summarizes all relevant results from prototype experiments. The full set of
raw data can be found in Appendix III. It should be noted that most of the frequencies
measured were lower than those discussed in Section 1.3.3. However, this only implies
that decibel levels between 2 – 5 kHz were less than those measured at the peak frequency.
Thus the decibel levels stated are an overestimation. However, the stated decibel
reductions achieved with each feature are independent, therefore unaffected. All results
plotted in this section are an average of the three sound measurements taken for each
location.
The results labeling is consistent with the referencing system used in Section 3.3. It should
be noted that all decibel reductions discussed throughout this section are in relation to
the prototype designs only. Therefore modifications to the Towngas VE75 cannot be
expected to have identical outcomes. However, they give an accurate representation of
the sound reduction improvements. Consequently, these decibel reductions will be used,
in collaboration with material specifications, to provide an estimated decibel reduction of
the recommended final design.
Analysis of the data collected for outlet airflow velocity showed to be unreliable. This was
because all, except one, of the standard deviations were > 5 % of the associated mean.
Thus
no
further
analysis
was
conducted
with
it.
Each
test’s
average
outlet
airflow
velocity,

along with the associated standard deviation and its percentage, are shown in Appendix
VI, Table CXIII. The proposed explanation is that, due to the nozzle generating a jet flow,
it was difficult to align the flow with the pipe axis. Thus, a small deviation of the
instrumentation from the centre of the jet caused a large variation in the measured value.
5.1 First stage of prototyping
5.1.1 Results
Firstly, a comparison of the VE75 and the equivalent lab replica was made. Figure 27
shows the comparison of the dB levels recorded along the VE75 exhaust (TG) with those
obtained from the closest replica that could be achieved in the lab (RS).
Figure 27: Comparison of Towngas VE75 (TG) with equivalent prototype, Rough with Sack (RS)
70.0
75.0
80.0
85.0
90.0
95.0
Inlet Start Middle End Outlet
Towngas VE75 vs Rough with Sack (dB)
TG RS

The inlet sound level is not discussed nor analysed in this section for reasons mentioned
in point four, Section 4. Readings taken along the prototype exhaust were measured to be
lower than the VE75 exhaust. This was reasoned to be due to the prototype containing a
jet flow of air, rather than a uniform air stream; a cause of compressor constraints. Details
of the compressor constraints can be found in Section 3.6. Material properties may also
have caused differences (plastic rather than tempered steel exhaust). Looking at Figure
27, the sound level at the outlet of the prototype exhaust was within 3 dB of the VE75
exhaust. This was deemed to be an acceptable limit (given the prototyping constraints)
as opposed to the 5 to 12 decibel disparity along the pipe section of the exhaust.
The decibel level recorded at the outlet was consistently the largest, by at least 7 decibels,
reading for all prototypes – as shown by the average decibel readings in Figure 28.
Figure 28: Average decibel readings along the exhaust for all prototypes
It is most important to reduce peak decibel levels and thus the exit decibel levels. The exit
sound level is the loudest and the most comparable to the VE75, therefore the analysis
will focus primarily on the exit decibel level.
All future prototype designs were compared against a datum prototype. As most designs
required the internals of the exhaust to be clean, ES was used as the datum prototype;
representing a cleaned version of the existing Towngas VE75 currently in operation.
Table 6 shows the comparison of the datum (ES) with the equivalent Towngas replica
(RS).
Prototype
Exhaust exit
dB level
ES 89.0
RS 87.2
Table 6: ES vs. RS
Table 6 shows that RS is actually quieter, by 2 dB, than ES. A result that contradicts an
assumption made in Section 2.3. One proposal is that the rough internal surface induced
a higher friction at the internal surface boundaries; thus decreasing airflow velocity along
the exhaust. This would reduce expansion losses and noise at outlet. ES will be used as a
datum for the rest of the experiments.
All prototypes were compared against each other in ascending order to better visualize
which features worked best at reducing the sound level. Figure 29 shows the decibel level
at exhaust outlet for all the individual prototypes tested in the lab.
70.00
75.00
80.00
85.00
90.00
95.00
Start Middle End Outlet
Average sound for all prototypes (dB)

Figure 29: Outlet sound levels for all prototypes
The five quietest results obtained were with various baffle combinations, between 81-84
dB, compared to the 89 dB datum. While some prototypes actually increased the decibel
level at exit relative to ES.
Figure 30 shows the performance of the decibel reductions the prototypes achieved with
reference to ES.
Figure 30: Sound reductions of all prototypes relative to experimental datum (ES)
Results on the left hand side of the graph, in the positive region, show a decibel reduction
from the datum (ES). Figure 30 also shows prototypes that did not aid in reducing the
sound level, thus making the exhaust louder. These are depicted as negative decibel
reductions; the prototypes that yielded these louder exit dB levels were not further
developed in the second stage of prototyping.
5.1.2 Discussion
This set of discussions identifies which prototype designs were developed further for the
second stage of prototyping and which were eliminated.
80.0
82.0
84.0
86.0
88.0
90.0
92.0
94.0
96.0
98.0
100.0
2BoFi
2Bo
Bo
BoFm
2BsFi
RS
ES
BoFi
2Bs
IFi
BiFi
EFi
Bi
I
Hfi
DFi
R
D
H
E
Outlet sound level for all prototypes (dB)
-12.0
-10.0
-8.0
-6.0
-4.0
-2.0
0.0
2.0
4.0
6.0
8.0
10.0
2BoFi
2Bo
Bo
BoFm
2BsFi
RS
ES
BoFi
2Bs
IFi
BiFi
EFi
Bi
I
Hfi
DFi
R
D
H
E
Decibel Reduction from ES (dB)

An important observation made during the experimentation process was that there was
a significant backflow generated when either the filter or the baffle was placed at inlet.
This is undesirable because it may reduce the suction performance of the VE75. When the
filter was placed midway and the baffles at the outlet, this problem was avoided. When
this was done in the lab, the backflow generated was negligible. Fortunately these
locations also reduced the exit decibel level by 6 dB and 10 dB respectively.
Table 7 summarizes other observations made:
Observation Quantification Comments
Baffle at outlet is better
than none
Bo (83.1 dB) vs. E (98.7 dB)
– 15.1 dB reduction
The baffle theory works
Two baffles are better than
one
2Bo (81.2 dB) vs. Bo (83.1
dB)
Two baffles allows more
time for the sound energy
to be absorbed than one
baffle
Two baffles at outlet are
better than with spacing
2BoFi (80.9 dB) vs. 2BsFi
(83.8 dB)
The spacing between the
baffles caused an
additional entry loss
A filter at inlet, reduced
sound relative to no filter
with the exception of BoFi
and Bo
2Bs > 2BsFi 2Bo > 2BoFi
I > IFi D > DFi
H > HFi E > EFi
Bi > BiFi
Filter decreased air
velocity, see Appendix VI
Table CXIII, and thus
expansion losses; reducing
sound.
Holed pipes and dimples
were louder than datum
H and HFi > ES
D and DFi > ES
Dimples: Created more
turbulence than intended
reflection of sound waves
Holed pipe: air did not
diffuse through the holes
as anticipated, instead
more turbulence was
generated in pipe
Two baffles at the outlet
with a filter (2BoFi),
achieved the largest sound
reduction
8 dB reduction from datum
(ES)
This is the best design in
the first stage of
prototyping
Table 7: Observations from the first stage of prototyping
While the insulation around the exhaust did not reduce the exit sound level, it reduced
noise levels along the exhaust by an average of 1.4 dB, as shown in Figure 31.

Figure 31: Comparison of Insulation (I) vs experimental datum (ES) along the exhaust
Though not the primary focus, reducing the sound level along the exhaust will reduce the
sound level in a large angular dispersion– section
denoted
by
angle
α as shown in Figure
32.
Figure 32: Arial view of exhaust displaying angles of sound dispersion
The section denoted by theta shows the area where the sound from exhaust outlet
dominates, while section alpha shows the area where sound emitted from the pipe
dominates.
The sound reduction due to the insulation is not remarkable, however, this is due to a
poor choice in material (sound absorbing rather than sound proofing) and thus any
reduction at all shows promise for a suitable material.
Given the discussion above, the recommended design components to incorporate in the
second stage of prototyping were:
1. Two baffles at outlet
2. Filter at midway
3. Insulation
θ
α
75.0
77.0
79.0
81.0
83.0
85.0
87.0
89.0
91.0
93.0
95.0
Start Middle End Outlet
Sound levels along the exhaust (dB)
ES I

Lastly, a comparison was made between the expected implications discussed in Section
3.6 and observations made during experimentation. These are shown in Table 8.
Constraint Expected Implication Observation
Jet stream
Quieter along exhaust
Louder at outlet
True – shown in Figure
27
Incorrect air
velocity
Lower sound levels
True, but good similarity
to Towngas VE75
Non-standard stock
sizes
Minimal
True – no noticeable
effects
Materials
Additional vibration and
noise generation
False – effects minimal
Open at inlet
New sound source that
will affect readings
True – isolated by
identifying frequency
Table 8: Comparison of expected implications and observations from experimentation
5.2 Second stage of prototyping
5.2.1 Results
Design 2BoFi was identified as the best from the first stage of prototyping. Therefore,
2BoFi, was taken forward to a second stage of prototyping. The design was developed by
considering the recommendations stated at the end of Section 5.1.2. Thus, 2BoFi was
modified by wrapping insulation around the length of the exhaust and moving the filter
to midway, giving I2BoFm (insulation with two baffles at outlet and filter midway along
the pipe).
I2BoFm was compared to 2BoFi to show the improvement achieved by implementing the
recommendations. I2BoFm is also compared to the datum (ES) and Towngas equivalent
(RS), to show the overall decibel reduction achieved. These improvements are shown in
Figures 33, 34 and 35.
Figure 33: Outlet sound levels for the final prototype (I2BoFm)
79
81
83
85
87
89
91
I2BoFm
2BoFi
RS
ES
Outlet sound level for final comparison (dB)

Figure 34: Sound reduction of final prototype (I2BoFm) relative to experimental datum (ES)
Figure 33 shows that the exhaust exit decibel reduction of the final prototype from the
Towngas equivalent is 7.5 dB. Figure 35 shows the dB reductions along the exhaust.
Figure 35: Comparison of final prototype (I2BoFm) to best prototype from stage 1 (2BoFi)
Figure 35 shows that I2BoFm also reduced the sound levels along the exhaust by an
average of 0.83 dB due to the insulation.
5.2.2 Discussion
The recommendations from Section 5.1.2 have proven to be correct. Therefore, I2BoFm
was the solution taken forward to the final stage - material sourcing, finalization of design
and manufacture methods.
0
1
2
3
4
5
6
7
8
9
10
I2BoFm
2BoFi
RS
ES
Decibel reduction from ES (dB)
72.0
74.0
76.0
78.0
80.0
Start Middle End
Sound levels along the exhaust (dB)
2BoFi I2BoFm

Section 6: Development of Final Design
The final design is based on the best prototype, I2BoFm. This incorporates insulation
wrapped around the entire exhaust, two baffles located at outlet, and a filter positioned
midway, with reference to Figure 19, as shown in Figure 36.
Figure 36: 2-D schematic of best prototype, I2BoFm
Consequently, this section discusses the detailed design, material specifications and
manufacturing methods of the three features to be incorporated into the Towngas VE75.
For the design of the three features, emphasis was put on obtaining the objectives stated
in the URS, found in Appendix I:
1. Prioritising sound level reduction
2. Retaining portability of VE75
3. Be operable by the current VE75 workforce
4. Have a lifespan of 10+ years
5. Cost < 15 % of the VE75 price
6. Minimal, quick and easy maintenance
7. Manufacture on the small scale
6.1 Sound absorption vs. sound proofing
This section is necessary for material specification of the insulation and baffle.
Sound reducing materials can be classified into two types; soundproofing and sound
absorbing. Soundproofing materials block the passage of sound waves, preventing them
from passing from one space to another. Sound absorbing materials dissipate sound
energy into the form of heat (as discussed in Section 2.2.1). Thus soundproofing materials
are used for reducing sound between spaces, and sound absorbing materials are used for
reducing sound in a space containing sound generation. For this reason the insulation will
be a soundproofing material and the baffle will be a sound absorbing material.
Every material varies in sound reduction quality. Generally speaking high frequency
noises are much easier to reduce than low frequency noises. Two standard sound metrics,
used in industry, are Sound Transmission Class (STC) and Noise Reduction Coefficient
(NRC). Both are completely independent of one another. A third sound metrics,
specifically for pipes, is Pipe Insertion Loss (PIL). This is a very new metrics, and thus has
limited data (Scott Miller 2014). Essentially, it is an equivalent to STC for pipes. Further
detail of STC tests and ratings can be found in Appendix VIII.

PIL tests consist of recording the decibel level from a bare, noisy pipe, and then the decibel
level after sound insulation is added to the pipe. Subsequently, PIL is a decibel reduction
value. The test is carried out at different frequencies, as stated by ASTM E 1222. According
to Scott Miller (2014), sound insulation thickness should increase with pipe diameter.
NRC is a scalar representation, from 0 – 1, of how much sound energy a material absorbs;
0 being perfect reflection and 1 being perfect absorption. Thus low-density materials are
best suited for a high NRC. The NRC value is an average (to the nearest 0.05) at the one
octave band over 125, 250, 500, 1000, 2000 and 4000 Hz; adhering to test standard ASTM
C423.
Both PIL and NRC values were used to select the best possible materials for the insulation
and baffle respectively. Using material PIL and NRC values also enabled effective
comparison of noise reduction achieved during prototype testing and that expected with
the final design to be recommended to Towngas.
6.2 Insulation
Insulation surrounds the entire length of the exhaust, as shown in Figure 37
Figure 37: 3D image of insulated exhaust
During experimentation the insulation jacket was made from eggcrate foam. This material
is primarily a sound absorption material. Thus it only has NRC values and is not
necessarily applicable to PIL or STC values. Discussions with Industrial Noise Control (INC
2007) confirmed that no PIL or STC data would exist for eggcrate foam. Despite this, the
eggcrate foam provided a 0.83 to 1.4 dB reduction when used as a soundproofing material
around the exhaust during prototype testing.
Prioritising PIL values during research, Rockwool RockTech SPI is recommended for the
insulation. Rockwool RockTech SPI is a non-directional fiber composite made from basalt
rock and slag. The low-density (120 kg/m3), fiber make-up and tight, seamless joints give
it outstanding soundproofing characteristics. It is specifically designed for moisture
intense applications, such as steam and process pipe systems, by impregnation of water
repellant characteristics. Thus making it ideal for external use in the humid Hong Kong
climate. Its water repellant characteristic protects the pipe around which it is wrapped
and prevents fungi and mildew growth. It can operate in temperatures up to 650 ℃ and
is fire-resistant, non-combustible and non-corrosive. Rockwool RockTech SPI offers a
durable insulation casing, with longevity of 10+ years, meeting the URS in Appendix I.

Installation is very simple, and can be carried out by any two individuals. Rockwool
RockTech SPI is supplied in two semi-circular pipe casings, which can be cut to exact
lengths with a serrated knife. Around the frustums of the exhaust, some additional
shaping will be needed to maintain a tight fit. As shown in Figure 38, the Rockwool
RockTech SPI is secured in place with metal wire wraps. Figure 38 shows a section being
installed. It is recommended to wrap a plastic sheet around the insulation, protecting it
from debris – ensuring that material properties are not compromised.
Figure 38: Installation of Rockwool RockTech SPI (Roxul 2009)
Table 9 shows PIL decibel reductions achieved with Rockwool RockTech SPI at four
different frequencies (corresponding to the frequency range recorded during the site
visit) and three different thicknesses. The data is specific to a stainless steel pipe and
conforms to ASTM E 1222. Although the decibel reduction values will not correlate
exactly to the exhaust in this project, they show an accurate representation of what can
be expected.
Table 9: PIL decibel reduction for Rockwool RockTech SPI (Rockwool 2013)
Table 10 shows the most relevant standard stock sizes provided by Rockwool, along with
their corresponding prices.
Table 10: Rockwool RockTech SPI prices per standard stock size (Alan Leung 2014)
Internal Diameter
(mm)
Length
(m)
Thickness
(mm)
Price ($)
115 1 35 98.6
115 1 63 254.2
115 1 100 487.4
170 1 35 134.4
170 1 63 332.7
170 1 100 595.6
PIL (dB)
Frequency
(Hz)
35 mm
thick
63 mm
thick
100 mm
thick
2000 22 23 25
3150 26 26 28
4000 26 26 28
5000 30 30 31

To insulate the entire exhaust two rolls of the Rockwool RockTech SPI will need to be
purchased. One 1 m roll with a 115 mm internal diameter (for exhaust pipe section) and
one 1 m roll with a 170 mm internal diameter (for the exhaust frustums). Though the sizes
are slightly too big, the wire wraps can be used to ensure a tight fit around the exhaust.
Considering the URS, found in Appendix I, the decibel reductions and cost, a thickness of
35 mm was chosen for the insulation. Increased thickness offers minor additional decibel
reductions, but add a considerable volume to the exhaust; the added size would hinder
the portability of the Towngas VE75. It is estimated that 35 mm thick Rockwool RockTech
SPI insulation will give a 22 dB reduction along the VE75 exhaust, with reference to Table
9.
Supplied from Rockwool Building Materials (Hong Kong) Ltd, the two rolls of Rockwool
RockTech SPI (115 mm and 170 mm internal diameters of 1 m length) will cost $233.
There is an additional $500 delivery charge.
6.3 Baffle
The baffle is located inside the exhaust as shown in Figure 36. The two baffles from the
experimentation phase are now discussed as one 400 mm long baffle for the final design.
During experimentation eggcrate foam was used as the sound absorption material for the
baffle. The NRC values of the eggcrate foam, of 38 mm thickness, are displayed in Table
11.
Table 11: NRC values of 38 mm thick eggcrate foam (Fomo Products), according to ASTM C423
In Section 5 the peak decibel levels were in the 250 – 500 Hz range. At these frequencies
the eggcrate foam should have absorbed around 24 - 46 % of sound inside the exhaust.
From experimentation this equated to a sound level reduction of 15.6 dB at exhaust outlet
when compared to a clean and featureless prototype (E) (Section 5.1.2, Table 7). Table 11
also shows that as frequency increases so does the NRC value. Therefore, given that the
human sensitive hearing range is between 2 – 5 kHz, the relevant sound absorption
should be 58 % +.
Prioritising NRC values during research, Basotect G sound absorption foam is
recommended for the baffle. Basotect G, produced by BASF chemical company, consists
of a fine open cell structure, making it flexible, lightweight and excellent at sound
absorption. Its sound absorption qualities have led to it being used in many industrial and
commercial applications, including the Beijing Olympics Aquatics centre, wind tunnels,
studios and engine test benches. Basotect G can operate in temperatures up to 240℃ and
has excellent chemical resistance properties (resistant to all organic solvents). It contains
no hydrocarbons, no flame-retardants, nor any toxic metals, and does not contaminate
water.
It
is
also
free
of
blowing
agents
and
meets
the
‘no-hazardous-labeling’

requirements under German law, certified by Oko-Tex Standard 100. It is also fiber free,
making it ideal for inserting into the exhaust, as no added debris will be created. A silicon
emulsion can be impregnated into Basotect G, without decrementing any of its properties,
to make it 100 % water repellant (hydrophobing). Thanks to its high temperature
resistance and low flammability, Basotect G can be cut easily into 3D shapes by milling or
box cutters. Minimal amounts of expansion and contraction should be allowed for. It is
expected to have a lifespan of 2 years.
Frequency
(Hz)
125 250 500 1000 2000 4000
NRC 0.18 0.24 0.46 0.56 0.58 0.67

The open cell structure allows sound waves to be absorbed and dissipated in the material,
giving NRC values shown in Figure 39. The NRC values are much higher than that of the
eggcrate foam used in prototyping – the values of which are superimposed on Figure 39
with purple dots and line.
Figure 39: Sound absorption of Basotect G, as a function of the thickness according to ISO 10534-2
(Andy Yang), vs. eggcrate foam
The yellow line drawn in Figure 39 identifies the beginning of the most sensitive
frequency range for humans (2 – 5 kHz). Hence, it is apparent that Basotect G has a far
superior performance in the significant frequency range, a 60 % improvement to the NRC.
Thus, it is expected that a decibel reduction of 25 dB* will be achieved with a baffle made
of Basotect G.
* 60 % improvement on 15.6 dB reduction achieved from the eggcrate foam.
Looking at Figure 39, the NRC of Basotect G is heavily dependent on thickness. Both sound
absorbing material thickness and perforated tube diameter affect baffle performance (the
latter being discussed in Section 2.2.1). During experimentation the baffle consisted of
cylindrical perforated tubes, as shown in Figure 40. Cylindrical perforated tubes were
used for two reasons;
1. Existing baffle designs incorporate cylindrical perforated tubes
2. Ease of manufacture during prototyping
Figure 40: Prototype baffles constructed in the lab
38 mm

Manufacturing capabilities during prototyping prevented the option to vary the
perforated tube diameters as discussed in Section 3.5.2. To optimize thickness of Basotect
G and airflow through the baffle, contracting and expanding conical tubes were designed
to maximize sound reduction. Figures 41 and 42 show this configuration.
Figure 41: Stainless steel perforated tubes, to act as support structure for Basotect G sound
absorption foam
Figure 42: Cross-sectional view of baffle – stainless steel case and perforated tubes
The baffle housing consists of a stainless steel case, perforated tubes, end plate, filter
hoops* and handle, as shown in Figures 41 and 42, with Basotect G packed around the
tubes. A cylindrical stainless steel case is used to protect the Basotect G when removing
and installing the baffle from the exhaust. Stainless steel is chosen in particular for its
chemical, corrosion and water-staining resistance. Highly applicable properties given the
environment it will be contained in.
*Refer to Section 6.4
There were two options for manufacture of this configuration.
1. The stainless steel perforated tubes act as a support structure for the foam, which
would be made up of sections of standard sheets packed together.
2. The conical geometry is milled out of the foam directly, negating the need for
stainless steel perforated tubes.
However, it has unfortunately not been possible to source suppliers capable of producing
the stainless steel perforated tubes or Basotect G with conical geometries. Therefore,
cylindrical tubes are recommended as the final baffle design. Figure 43 shows a graphical
representation of the Basotect G component of the baffle. This cylindrical tube geometry
offers a far simpler manufacture that can be prefabricated to design specifications;
negating the stainless steel perforated tubes. Offering a cheaper and lighter design than
the conical tubes baffle.
Case to
protect
Basotect G
foam
Handle for
installation
and removal
of baffle
Conical
perforated
tubes
End plate to
hold
perforated
tubes and
Basotect G in
place

Figure 43: Baffle core made from Basotect G, with cylindrical internal tubes
The ratio of internal tube diameter to Basotect G thickness was investigated with a test
sample of Basotect G for optimisation. After contacting a supplier, Basotect G samples of
100 mm diameter and 100 mm length were obtained and tested. One sample had four 25
mm tubes drilled, and the other 32 mm diameter tubes. These diameter sizes were chosen
because 32 mm was the maximum size possible, given the outer diameter, and 25 mm
was the same used in prototyping. The raw data from which can be found in Appendix VI,
Table CXII. Results showed that the 32 mm tubes achieved a lower sound level at exhaust
outlet (81.4 dB vs. 91.2 dB)*.
*These decibel levels were higher than they should have been due to the sample being short in length
and having an incorrect outer diameter. Each sample was located at the end of the exhaust pipe, and
had a filter placed before. No insulation was used.
Therefore, 32mm gives the best ratio of tube diameter to Basotect G thickness; enhancing
NRC and reducing airflow restriction. As it is the largest possible tube diameter, no
improvements of the ratio can be made. Dimensioned drawings of the Basotect G
component of the baffle can be found in Appendix IX Figure XXIV.
In relation to Figure 39, it is not straightforward to predict the amount of sound
absorption with the Basotect G geometry; as the thickness varies along the cross-section
from 5.5 – 28 mm. Thicker sections should absorb more sound than the thinner sections,
as illustrated in Figure 44. Therefore, the estimated 25 dB reduction can only be
confirmed by implementation of the final design of the VE75 exhaust.
Figure 44: Variation in thickness along the cross-section of Basotect G baffle component
Thick 28 mm
section
Thin 5.5
mm section

The Basotect G component will be supplied by Guangzhou JunYiHui Auto Technology Co.
Ltd. An initial outlay of $1874 is required to create a mould to manufacture the Basotect
G component. The mould is restricted to 200 mm in length. Thus the baffle will require
two pieces. The cost of one 200 mm long Basotect G is $24.4. Shipping costs would be
incurred on top of this. An additional cost would be applied if the hydrophobing was
wanted by Towngas.*
* It should be noted that the hydrophobing would need to be carried out in Korea or Europe as no
supply company offers this service in Asia.
The stainless steel components are discussed further and priced in Section 6.4.
6.4 Filter
As discussed in Section 2.3.2, debris contaminates the airflow through the exhaust. So to
avoid material build up and subsequent blockage in the baffles; it was decided to place
the filter prior to the baffles, as shown in Figure 36. The filter is designed to prevent grit
and dust from entering the baffles by using a wire mesh. Though it was desirable to
prevent particle build up along the whole exhaust, design constraints mean that it could
only be placed directly before the baffle.
Specifically,
the
filter
consists
of
a
wire
mesh
that’s
attached
to
a
steel
hoop
that
will be
screwed to the baffle case, as shown in Figure 45.
Figure 45: Exploded 3D CAD image of filter
The diameter of a sand particle ranges between 62.5 and 2000 microns (Various 2013).
Thus a mesh size of 74 micron rating was chosen; allowing only particles smaller than 74
microns in diameter to pass through. The mesh is made of stainless steel to resist
corrosion and increase its lifespan. A 74 micron rating is classed a series 200 wire mesh
(Cleveland Wire Cloth 2012). Supplied by Utah Biodiesel Supply, a 200 series sheet of
304.8 mm x 304.8 mm costs $116.4 + shipping (Utah Biodiesel Supply 2012). A complete
dimensional drawing of the filter can be found in Appendix XI, Figure XXV. The filter
diameter required for the final design is 106 mm; therefore, four filters can be cut from a
single sheet. Considering the application of the filter, it is recommended that the filter is
rinsed at the end
of
each
day’s
use
– preventing cement drying, solidifying and thus
blocking the airflow. It is estimated that maintenance procedure would take a maximum
of
10
minutes.
Replacement
of
the
wire
mesh
will
be
down
to
operators’
inspection.

Hoop 2 – end
of stainless
steel case
Hoop 1

The stainless steel case, end plate, filter hoops and handle are estimated to cost $2000.
Figure 46 shows all stainless steel components bar hoop 1.
Figure 46: Stainless steel baffle case, end plate and handle
6.5 Airflow simulation
The final design consists of certain features inserted in the exhaust, therefore, an airflow
simulation was run for the original Towngas exhaust, and then for the final design. The
aim of running the simulation was to analyse the airflow and check for backpressure. The
CAD package, SolidWorks FloXpress was used to generate full reports that can be found
in Appendix V and X. An illustration of the simulations can be seen in Figures 47 and 48.
Figure 47: Airflow simulation of the Towngas VE75 exhaust
Figure 48: Airflow simulation of the modified exhaust incorporating the final silencer design
Hoop 2
Stainless steel case
Handle

First of all, it should be noted that the color scale denoting the velocity along the exhaust
differs between Figures 47 and 48.
The only sign of any backpressures in the airflow simulation, is the presence of small eddy
currents upon exit from the baffle, as shown in Figure 48. These are due to the sudden
expansion following the inner tubes of the baffle. The velocity of these eddy currents
appears to be very low, and is thus not deemed to be an issue. Experimentation did not
show a problem with eddy currents either.
The simulation demonstrates that the highest velocities occur in the pipe section and
more specifically in the baffle for the final design. The highest velocities are 94.7 ms-1 and
242.4 ms-1 for the VE75 and the final design respectively. It is believed that, given the
material properties of the Basotect G (discussed in Section 6.3), it could withstand such
high velocities. In the final design, the sound absorbing material was set as foam. Despite
this, SolidWorks Xpress did not manage to simulate any of the flow dispersing into the
foam – as was observed during experimentation. Thus, it is not a wholly accurate
representation of the actual system. Regardless of this, Figures 47 and 48 show that there
is no noticeable change in the exit velocity.
In conclusion, the simulation provides reassurance that there are no problems with the
final design regarding airflow.

Section 7: Final Design
This section describes the final silencer design; its components, the installation
procedure, the required maintenance schedule and the overall cost. The Rockwool
RockTech SPI insulation is expected to reduce the sound level by 22 dB along the exhaust.
Whereas the baffle is expected to reduce the sound level by 25 dB at exhaust outlet*.
These sound reductions suggest, that the VE75 with final silencer design implemented,
should have a sound output of 63 dB along the exhaust and 74.6 dB at exhaust outlet - as
denoted by angle α and angle θ, respectively, in Figure 32. A significant improvement from
the current VE75 design; 85 dB along exhaust pipe and 90.4 dB at exhaust outlet.
*From comparison to an exhaust without a sack at outlet.
7.1 Description and CAD images
The final silencer design consists of an outer insulation and an internal baffle and filter,
as shown in Figure 49.
Figure 49: Cross-section of VE75 exhaust with final silencer design
The fully assembled baffle housing that will be inserted into the VE75 exhaust, is shown
in Figure 50.
Figure 50: Fully assembled baffle housing
Airflow
Baffle
VE75 exhaust
Filter
Insulation
Handle of baffle
housing for insertion
and removal

The baffle housing consists of a stainless steel case, Basotect G sound absorption material
and a filter, as shown in Figure 51.
(a) (b) (c)
Figure 51: (a) Filter, (b) Basotect G and (c) stainless steel case
The stainless steel case has an end plate with a handle, as shown in Figure 52. The holes
of the end plate are aligned with the tubes of the Basotect G. Fully dimensioned drawings
of the stainless steel case can be found in Appendix XI, Figures XXI and XXII.
Figure 52: End plate of stainless steel housing
The Basotect G is inserted into the stainless steel housing from the opposite end to the
handle. Subsequently, the filter is then screwed onto the stainless steel case – completing
the baffle housing.
To complete the silencer, a 63 mm Rockwool RockTech SPI insulation wraps around the
entire exhaust. A final exploded view of the baffle, filter and insulated exhaust is shown in
Figure 53.
Figure 53: Exploded final silencer design on VE75 exhaust

7.2 Installation procedure
7.2.1 Assembly of the baffle
Assembly of the baffle consists of two parts; insertion of the Basotect G and attachment
of the filter.
The Basotect G comprises two 200 mm length sections, of identical geometry. These are
inserted into the open end of the stainless steel baffle case, as shown in Figure 54. Care
must be taken to ensure that the tubes of the two pieces are correctly aligned.
In reality hoop 2 is welded to the end of the stainless steel case, but for the purpose of
clarification, it has been detached in Figure 54. This is because hoop 2 has a smaller
internal diameter than the external diameter of the Basotect G. Therefore, to insert the
Basotect G, it must be squished a little –made possible given its foam structure.
Figure 54: Insertion of Basotect G into baffle case
Following the insertion of the Basotect G, the filter can now be attached. The filter is made
by cutting a 106 mm diameter circle from the supplied stainless steel wire mesh sheet.
Four filters can be cut from each sheet. The wire mesh circle is secured in place by
screwing hoop 1 to the end of the baffle case (hoop 2), as illustrated in Figure 55.
Figure 55: Attachment of filter to baffle case
The fully assembled baffle housing is now ready, shown in Figure 50, to be installed into
the VE75 exhaust.
Hoop 2
Hoop 1
Wire mesh

7.2.2 Installation of insulation
The insulation is cut and wrapped around the entire VE75 exhaust, and secured in place
with wire wraps, as described in Section 6.2 and illustrated in Figure 38.
A picture of the insulated VE75 exhaust is shown in Figure 56.
Figure 56: Insulated VE75 exhaust
7.2.3 Installation of the baffle
The Baffle has been designed for quick installation with minimal effort. To secure the
baffle in the exhaust, a pin and notch system is used. Two notches are located on the top
and bottom in the interior of the exhaust; at the beginning of the diffuser, as illustrated in
Figure 57.
Figure 57: Notch located at the bottom in the interior of the exhaust
The baffle is slid into the exhaust outlet, as shown in Figure 58. Then rotated 90 degrees
clockwise, mating the pins into the two notches; securing the baffle in place. Securing of
the baffle is shown in Figure 59. The removal procedure is the reverse of the installation
procedure, and is required during maintenance as discussed in Section 7.3.
Figure 58: Insertion of baffle into outlet of the insulated VE75 exhaust

Figure 59: Securing baffle in VE75 exhaust
In conclusion, the overall installation procedure is straight forward and can be carried out
by two people. The wrapping of the insulation only needs to be carried out once, while
the
installation
of
the
baffle
and
filter
is
needed
at
the
end
of
each
day’s
use;
and
is

estimated to take 5 to 10 minutes. Thus meeting the URS requirements, stated in
Appendix I.
7.3 Maintenance schedule
The maintenance for the final silencer design is minimal. The stainless steel baffle housing
and Rockwool RockTech SPI insulation have a lifespan of 10+ years, and do not require
any maintenance. Thus the lifespan of the stainless steel housing and insulation meet the
10 year recommended working lifespan, stated by Utiliscope, of the VE75. However, the
filter mesh and the Basotect G will need some maintenance.
The
filter
needs
to
be
rinsed
and
scrubbed
at
the
end
of
each
day’s
use,
as
discussed
in

Section 6.4. Replacement of the filter will be subject to operator inspection – checking for
holes or damage. To inspect the filter, the entire baffle needs to be removed – the
procedure for which is outlined in Section 7.2.1 and 7.2.3. A procedure that is estimated
to take 5 – 10 minutes. The filter can be replaced three times before a new sheet of
stainless steel wire mesh needs to be purchased – at a cost of $206.4.
The Basotect G is estimated to have a 2 year lifespan, and thus will need inspection and
possible replacement every 2 years. Replacement of the Basotect G would cost $148.8 per
baffle.
In conclusion the filter mesh requires daily inspection and the Basotect G bi-annual
replacement. Thus the requirement set out by the project sponsor Towngas for quick,
easy and minimal maintenance, is achieved; meeting the URS found in Appendix I.

7.4 Bill of materials and cost
The overall cost of each component of the final design is broken down in Table 12; along
with each components weight and supplier.
Table 12: Bill of materials and their costs
A summary of the design’s total cost and weight is summarised in Table 13.
Total
Cost of Req.
Qty. ($)
Shipping
Cost ($)
Weight (kg)
Single
Silencer
4272.2 695 4.49
Three
Silencers
9068.6 695 -
Table 13: Final design cost and weight
The total cost of the final silencer design for one VE75 machine is $4967.2 (including
shipping). This is 6.1 % of the total VE75 cost; 40 % of the cost limit set by the project
sponsor, Towngas. All components of the final design have a combined weight of 4.49 kg.
A small additional weight that does not compromise the portability of the exhaust. A
prerequisite set out by Towngas. Thus, the cost and weight of the final design meet the
URS found in Appendix I.
Item
Cost
($)
Req.
Qty.
Qty.
unit
Cost of
Req.
Qty. ($)
Shipping
Cost ($)
Weight
(kg)
Material Supplier
Insulation
- sound
proofing
material
98.6 1 Roll 98.6 255 2.11
Rockwool
RockTech
SPI
Rockwool Building
Materials Ltd.
Insulation
- sound
proofing
material
134.4 1 Roll 134.4 250 2.11
Rockwool
RockTech
SPI
Rockwool Building
Materials Ltd.
Baffle -
sound
absorbing
material
mould
1874 1 Mould 1874 - - Die steel
Guangzhou
JunYiHui Auto
Technology Co. Ltd
Baffle -
sound
absorbing
material
24.4 2 Custom 48.8 100 0.02 Basotect G
Guangzhou
JunYiHui Auto
Technology Co. Ltd
Filter wire
mesh
116.4 1 Sheet 116.4 90 -
Stainless
steel wire
mesh
Utah Biodiesel
Supply
Baffle
housing
2000 1 Custom 2000 - 2.36
Stainless
steel
Estimate by HKU
technician

Design Project HKU

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