The report details the design of a test rig to measure pressure correction factors for medical vacuum filters produced by Walker Filtration. Background research was conducted and an initial test rig prototype was built and tested. The results from this prototype suggested improvements and informed the design of a finalized test rig. The final design incorporates more accurate measurement instruments and additional components to further challenge the airflow, in order to obtain more precise pressure correction factor data for Walker Filtration's filters. This new data has the potential to save the company money through more optimized filter designs.
2. Test rig design for medical vacuum flow
correction factors
Link Engineer: Dr. Simon Bartram
Link Teacher: Mr. Andrew Morton
Team Members: Lauren Baldwin, Amelia Dickinson, Kathryn Harder, Tz-Shing Hwang
Central Newcastle High School
4. Introduction
The aim of the project was to design a test rig for medical sterile vacuum filters,
produced by the company, Walker Filtration. The filters that Walker Filtration produces
are air filters, which are often used to sterilise air used for medical and dental
procedures. This tends to involve removing oil particulates that enter the air due to the
oil used in lubricating machinery.
Although Walker Filtration currently has data (see fig. 8) about their filters from their
own testing, the company suspects that the data, having been calculated in the 1960s,
may be inaccurate. Therefore, it was felt that more accurate experimental data could be
found today, using a test rig with higher resolution and more accurate machinery.
The test rig was designed to be able to measure pressure loss across the filter,
temperature, humidity and air flow through the filter, in order to obtain a pressure
correction factor for each specific filter. There was an additional ambition to be able to
add components to the design to measure the number of particles left in air following
filtration, in order to measure filter efficiency.
Pressure correction factors are specific to particular pressures, and give the fraction of
maximum air flow (which is given for a filter at atmospheric pressure) that will be able
to flow through a filter at a particular, not-atmospheric pressure. So, if a company that
uses an air filter knows the pressure at which the filter will be used, the maximum air
flow given for their particular filter can be multiplied by the pressure correction factor
that corresponds to the pressure at which the filter will be used, to give the maximum
air flow for their filter at their particular pressure.
The ultimate aim of the project was, inevitably, to save money for Walker Filtration.
This would be achieved by making Walker Filtration’s pressure correction factors more
accurate than their competitors’, either enabling them to produce smaller filters at
lower cost with the same efficiency as their current filters, or to prove that they are
currently producing the best type of filters that they can; both possibilities would
improve their company image and could therefore increase profit.
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5. Summary
The report follows the design of a suitable test rig, through combining research with
experience of using a draught version of a test rig, to improve upon initial ideas.
The results from experiments using an early prototype of the test rig have been
obtained to suggest how the completed test rig will differ in results from the data
Walker Filtration has at present.
The data obtained suggests an overall change in the initial design, recommending a final
design consisting of more accurate measuring instruments and additional elements to
challenge the airflow.
The work done on the design and in the report to fulfil the task set will go to make
differences in the company Walker Filtration and have real implications in the wider
industry.
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6. Background Research
Walker Filtration
Walker Filtration formed in 1983 by husband and wife team Brian and Carol Walker
and the company has continued to successfully grow over its history. Serving a global
market Walker Filtration manufactures an extensive range of high efficiency
compressed air filtration and drying equipment. Other suitable uses include the
compressed gas, vacuum and medical markets, and they are locally produced in
Washington, Tyne and Wear.
The model used in the original test rig at the workshop was the A108, an alpha series
model. Alpha Series medical vacuum filters are specially designed to protect plant
installations from liquid, solid and bacterial contamination and conform to the
requirements of UK Health Technical Memorandum 02-01. The medical vacuum filters
Walker Filtration produced are designed and constructed using oleophobic borosilicate
media and an open cell reticulated foam pre filter. This unique material construction
minimises pressure drop delivering reliable filtration with improved energy efficiency.
Elements are drop-fit in installation to prevent vibration and improve stability and
drainage.
Filtration Methods
Air filters do not simply trap larger particles and allow smaller ones to pass through.
Instead, they are often able to trap the larger and smaller particles more effectively than
medium-sized particles. This is a result of inertial impaction, Brownian motion and
electrostatic forces.
Inertial impaction prevents larger particles from passing through filters. Larger
particles will be unable to quickly adjust their pathway (as a smaller particle might)
when they reach a change in streamline direction near a filter fibre. This means that the
particle will hit the filter fibre and be unable to pass further through the filter.
Brownian motion is the random movement of gas particles as a result of their frequent
collisions with other particles making them have a zig-zagged pathway. The diffusion
mechanism of particle capture in filters relies on Brownian motion, as smaller particles
can be caught in filters because gas particles will bump into them, causing them to have
a random, zig-zagging motion, which means they are very likely to collide with and stick
to a filter fibre.
Electrostatic forces of attraction between filter fibres and particles can enable the
particles to be trapped in the filter. This can happen because both the fibre and particle
are charged (oppositely) and so are attracted to each other. A neutral particle can also
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7. be attracted to a charged fibre by induced dipole-dipole attraction, just as a charged
particle can be attracted to a neutral fibre.
Bourdon Pressure Gauge
The Bourdon pressure gaugeuses the principle that a flattened tube tends to straighten
or regain its circular form in cross-section when pressurized. Although this change in
cross-section may be hardly noticeable, and thus involving moderate stresses within the
elastic range of easily workable materials, the strain of the material of the tube is
magnified by forming the tube into a C shape or even a helix, such that the entire tube
tends to straighten out or uncoil, elastically, as it is pressurized
Bourdon tubes measure gauge pressure, relative to ambient atmospheric pressure, as
opposed to absolute pressure; vacuum is sensed as a reverse motion. Some aneroid
barometers use Bourdon tubes closed at both ends (but most use diaphragms or
capsules, see below). When the measured pressure is rapidly pulsing, such as when the
gauge is near a reciprocating pump, an orifice restriction in the connecting pipe is
frequently used to avoid unnecessary wear on the gears and provide an average
reading; when the whole gauge is subject to mechanical vibration, the entire case
including the pointer and indicator card can be filled with an oil or glycerine.
Valve Selection for the New Test Rig
Globe valves can be used to control air flow, and are appropriate for gradually
adjusting how much air can pass through a system, as (unlike other types of valves) they
are seldom damaged when partially opened. They are operated by turning a handle that
moves a plug up or down, thereby changing the size of space through which air in a
system can flow. This means that they can be very useful for air flow control, although
pressure is affected more significantly by adjusting them than for other valves, because
the pathway through the valve is ‘s’ shaped. Globe valves can also be damaged by
closing them too far.
Gate valves are appropriate for controlling whether access through a pipe is open or
closed. They work (like a gate) by lowering the gate to obstruct air flow, or lifting the
gate to allow more air flow. Gate valves are used less often than globe valves to control
air flow, because they can be damaged when partially open, however can be used to
open or close off a particular route.
A pneumatic valve is used to ensure that the air in a pipe or system does not flow
backwards, but stays in the desired direction, thus controlling the flow of the air.
Valves are often either 2-port or 3-port (although higher numbers are also possible). A
2-port valve has 2 openings, and some form of mechanism to adjust air flow within,
where air tends to enter by one opening and leave by another.
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8. A 3-port valve will have 3 openings- either 2 paths for air to enter and one to leave, or
vice versa. So, a 3-port valve can be used to bring two pipes together, or to split air
flowing through one pipe into 2 separate routes. The ports of a 3-port valve can
generally be opened and closed independent of each other.
Laskin Nozzle
A Laskin nozzle produces a high concentration of liquid droplets in a polydisperse
aerosol, which means that the aerosol contains particles of a variety of sizes. The
particles that can be added to the aerosol include motor oils, meaning a Laskin nozzle
can be used to test a Walker filter, since motor oils are commonly what they must
remove. This would improve the design of the test rig, by creating a similar situation to
that in which the filters would be used, rather than the previous design, which involved
very little for the filters to remove.
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9. Method
Following initial research a basic plan for the test rig was begun (see fig. 1) and was
refined through communication with the link engineer to include more specialized
equipment so more precise measurements could be taken.
At the university, the improved test rig design (see fig. 2) was constructed with the
materials available. However, as only certain components were available it meant the
constructed rig was a more basic version. This rig enabled any primary issues in the
design to be identified and the solutions to these were then discussed.
After the rig was assembled, results were recorded (see fig. 6). This was achieved by
controlling the variables of pressure (mBar) and air flow simultaneously using gate
valves, which affected the temperature, humidity, and differential pressure.
The test rig consisted of a vacuum pump sucking air through a series of pipes and
measuring devices across a filter, belonging to Walker Filtration, this allowed for
measurements of the differential pressure across the filter using a barometer that
calculated difference in pressure before and after air had flowed across the filter.
The residential workshop began with recording rough measurements and logging the
data. This enabled comparison to the original results, where it was discovered that the
lowest differential pressure value was 2%, which created a target to obtain this 2%
value, or as close as possible, at all pressures in the second experiment.
This was done by setting the vacuum pump on a particular pressure, and adjusting the
pressure in the pipes by adjusting gate valves on either side of the filter, which changes
the airflow. To measure a particular pressure, the team had to drill/insert a new valve
into the system which allowed the measuring devices to measure air flow in two
different location, one before the filter and the second after the filter. When the
differential pressure was shown to be 2% of the pressure of the overall system results
were recorded for all the variables. This process was repeated several times. Once the
results were compiled, graphs were produced (see results page) which were plotted in
comparison to the walker filtration correction factors.
The new results showed what conditions the filter could withstand. This allowed a new
plan for a new test rig to be designed (fig. 4), testing the filter under varying conditions,
such as with more power or a dirtier vacuum and with different filter sizes, and
including equipment to measure other factors, for example a particle counter, which we
could use in the previous rig.
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10. 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 200 400 600 800 1000
PressureCorrectionFactor
Pressure (mBar)
Relationship between Pressure and Pressure Correction Factor
Walker correction factors
Own correction factors
Results
From the tests that were done at the residential, air flow corection factors were
calculated (see fig. 6) which are plotted on the graph below.
The graph demonstrates a positive correation between air pressure and pressure
correction factors; at higher pressures tested (those closer to atmospheric pressure) a
greater volume of air could flow through, meaning the pressure correction factor is
larger. So, the maximum air flow at higher pressures is a larger percentage of the
maximum air flow at standard conditions of pressure, temperature and humidity.
The results follow a linear pattern, and show very strong correlation, all lying on the
line of best fit on the graph, implying that the results are precise. The pattern also shows
that, up to atmospheric pressure, pressure correction factors and pressure are close to
directly proportional.
In addition, the data shows the test rig to be very efficient as the points are generally
lower than, or, as at 900 mBar, equal to, the points obtained by Walker Filtration.
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11. Discussion
The data appears reliable and accurate as the correction factors calculated from the
results obtained during the residential workshop have a line of best fit that is very close
to all of the plotted points. This shows there is little random error in the methodology.
The results are quite different from the Walker Filtration factors, which suggests that
the test rig designed gave more accurate results than the test rigs previously used. The
pressure correction factors gained at the residential are lower than those of Walker
Filtration, which is likely due to the use of more recent methods of measurement (the
Walker Filtration values were found C. 1960.) This means it is likely that the affects of
pressure loss over the filter have been taken into account more by the residential
pressure correction factors.
After each trial experiment, more elements were added to the pipe system to create
more challenges. For example, different measuring instruments with different degrees
of accuracy and different types of valves to restrict airflow to different degrees.
The initial test rig plan will be redesigned with more accurate measuring instruments
and the removal of bends in the pipe system.
Overall the aim described in the brief title is achieved as a test rig was successfully
planned and built as well as used to obtain sufficiently accurate air flow correction
factors for the air filters.
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12. Conclusion
The final design (see fig. 4), contains the most effective components for testing the
filters, and would be capable of testing many different sizes of filters, the design fulfils
the criteria which was set at the start of the project as it has components for measuring:
Airflow: a digital air flow measuring device is located at the inlet, as it is more
accurate placing it later on in the system, and is more accurate than using an
analogue system which would be easy to obtain human error from.
Humidity: The humidity is measured with two hygrometers, which is a precise
method for measuring humidity. By taking two readings the resulting difference in
humidity can be analysed, can provide more information regarding the filter. Data
loggers are also going to be used so that measurements can be recorded without
human error.
Differential Pressure: A digital barometer has been included, located across the
filter, so it can automatically calculate the pressure difference across the filter.
Temperature: A digital thermometer will be inserted into the pipe after the filter, so
that the temperature within the system can be measured (this is more effective than
having the thermometer outside the test rig, as less other external variables could
differ the result form its true value.
Oil content in air: will be measured in the test rig by two different devices
o An air sample would be collected in a device which had an oil solvent within
it, the oil solvent would remove oil from the system, and this oil would be
able to be detecting by weighing the solvent before and after
o A particle detector, this would digitally calculate the different sized particles
in the air passing the device, using software provided by walker filtration
Moreover, the new test rig consists of a vacuum pump to draw air through the test rig
and filter, which is essential for the design to work.
Pipe adaptors will be inserted into the rig to make it more compatible for various types
of filters with different diameters to be tested. As shown in the background research a
selection of different valves were chosen as these ensure more controlled air flow,
which ultimately give more accurate results.
The results show through the difference the pressure correction factors obtained by the
walker filtration results and the results obtained from the research, that the newly
designed test rig, will produce more current data, which can be used by Walker
Filtration, which allows the company to sell a small filter to its customers, over more
this would reduce costs in production, and give a larger margin in profit.
Therefore the test rig that has been designed meets the engineer’s project scope
effectively and accurately.
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13. Recommendations
Following the results gained at the residential, the test rig could be improved by making
changes to the pipe system and vacuum, the measurement devices and to use
equipment that will challenge the filter.
By altering the pipe system, such as removing any bends from the pipe system and
increasing the pipe length the air flow becomes less turbulent. This means that the
results for air flow will be more accurate. Adding reducers and adaptors into the
pipework means that the system could be used to test various filters, instead of the
single filter that was tested. Additionally, to test the larger filters a larger vacuum pump
would be needed because the current vacuum pump is unsuitable for filter sizes of 3”.
Measurements of the system could also be improved by recording the results using data
loggers, which would digitally record a wider range of values, meaning the values do not
need to be read out and so less time is used. In addition, better measurements could be
taken if: a hygrometer, a digital device which measures humidity, and a particle
detector, which would record how efficient a filter was at removing waste particles,
were added.
Challenging the system is another way the rig could be improved, because it would
allow filters to be tested in replicated real-life situations, as the system may have an
older, dirtier vacuum, or the filter may be wet out (it cannot take any more) and so this
would test the limits of the filters in inadequate situations.
It is recommended that the system should be adjusted in the future to include these
changes.
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14. Bibliography
Vacuum VIP, Filter for Medical Vacuum, 2010. Bea Technologies. [18/12/13]
A Beginner’s Guide to Humidity Measurement, October 2011. Stephanie Bell. [04/12/13]
Vacuum Pump Protection Filter, 07/06/13. f & t. [18/12/13]
Measurement of Oil Mist from mineral oil-based metalworking fluids, June 1997. Health and
Safety Executive. [12/12/2013] http://www.hse.gov.uk/pubns/mdhs/pdfs/mdhs84.pdf
Method for measuring oil contained in Air-conditioning components, 2010. University of
Illinois at Urbana-Champaign group.
http://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=2034&context=iracc [12/12/2013]
Medical Sterile Filters, unknown. Nano Purification Solutions. [17/11/13]
Breathing Air Purifiers, 2010. Parker Domnick-Hunter [18/12/13]
Oil X-Evolution, 2007. Parker Domnick-Hunter [18/12/13]
Medical Vacuum Filters, 2002. Parker Domnick- Hunter. [18/12/13]
Calculation of correction factors for variable area flow metres at deviating working
conditions, unknown. Georg Rollmann. [17/11/13]
http://kt-web.de/pdf/physik/korrekturfaktorenberechnung_gb_2.2.pdf
Humidity Sensors and Signal Conditioning Choices, 13/10/2011. Steve Taranovich.
http://www.digikey.com/us/en/techzone/sensors/resources/articles/humidity-sensors-and-
signal-conditioning-choices.html [04/12/13]
Mechanisms of Filtration for High Efficiency Fibrous Filters, 29/08/12. TSI Incorporated.
[10/12/13] http://kt-web.de/pdf/physik/korrekturfaktorenberechnung_gb_2.2.pdf
Medical Sterile Filters, 2003. Walker Filtration. [17/11/13]
What is isokinetic sampling? 25/09/13. John Zactruba
http://www.brighthubengineering.com/power-plants/98903-what-is-isokinetic-sampling/
[17/02/14]
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25. Project: Test Rig Design for Medical Vacuum Flow Correction
Factors
Risk assessmentcompletedby: 4/12/2013
1. Whomight be harmed?
The team members are susceptible to injury
The technicians are susceptible to injury
Lead Engineer: Dr. Simon Bartarm is susceptible to injury
2. Hazards: Please listinthe boxes
providedbelowall of the
hazards associated withthe
project activitiesto be carried
out at the workshop. (A hazard
is anythingwith the potential to
cause harm eg. sharp objects,
heavy loads,slips,trips or falls,
electricity).
3. Please assessthe severityof potential injuryand
likelihoodof accidenthappeningforeach hazard
listed
Severityof potential injury Likelihoodof accident
happening
1 2 3 1 2 3
Minor
injury
Major
injury
Fatality Unlikely Possible Likely
i. Burns from equipment at
high temperatures
X X X
ii. Falls when working X X
iii. Scratches & grazes from
building the rig
X X
4. Considerationsandcontrol measures:Please thinkabout what checks you will put in place
for
each hazard identifiedabove toavoid accidents.eg. the wearing of safetygoggles,
attendingappropriate briefings,keepingworkspace tidy).i. Wearinggloves and hand protection when working with hot equipment
ii. Attending the briefingsso instructions and hazards are clarified
iii. Keepingthe workspace tidy in order to avoid slips,trips and falls
iv. Taking care when using tools and equipmentto avoid injury
v. Wearingsensible clothing and footwear to avoid snagging and injury
We have consideredthe severityof the hazards and the likelihood;however,the considerations
(above) seemappropriate.
Fig. 10: Risk Assessment
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26. EES Minutes: Meeting with Walker Filtration 15/11/13
Project Title: Test rig design for medical vacuum flow correction factors.
Project Scope: Build and implement a test rig that can provide relevant data for
determining flow correction values for systems operating at various pressures.
Equipment that we will need to measure:
1. System pressure
2. Pressure loss across filter
3. Air flow rate
4. If possible- filtration efficiency (preferably bacterial)
Specifically of interest are the flow characteristics of a medical vacuum system, so this
project should be based on air flow at a pressure below 1 bar (absolute)
Project Deliverables:
1. Test rig, (workshop at Newcastle University can be used to investigate
requirements/limitations to be considered for the design)
2. Sourcing parts and assembly of test rig (WFL to carry out this part)
3. Standard operating procedure documentation
4. Test results relevant to medical vacuum application
5. Competitor analysis
Equipment needed for testing
Compressors
Vacuum pump
Particle detector (if possible)
Systems to contain the air and to move the air
Measurers for air flow
Thermometers
Humidity measurers
Purity measurers
Pressure measurers
Bacteria to challenge filters (if possible)
Valves, fittings and pipes
To research
Medical sterile filters
Pressure flow correction values
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27. EES Minutes: Company Visit to Walker Filtration 29/11/13
Prior to the tour of the company, more was learnt about the ethos and values of
the company and the way in which their filters worked via a brief presentation
Questions about the project, including expectations, deadlines, rig size and more
information on the vacuums used, were answered
A rough plan of the work planned during the residential was decided upon- the
test rig would be built, measurements taken, improvements decided and the test
rig improved
A discussion of the hazards was also undertaken and the main hazard was found
to be the heat of the machinery
A tour showed the vacuums that would be used and how the filters were
produced
It was confirmed that the flow correction factors are multiplied by the generic
flow values according to the pressure of the filter system to give their specific
flow value.
Actions for next meeting:
Create a draft flow-diagram for the design of the test rig.
Research into: impaction, inertial, Brownian motion and electrostatic.
EES Minutes: Flow Chart Design and Risk Assessment Form4/12/13
The variables that would be recorded from the rig were decided:
-System Pressure
-Pressure loss across filter
- Air Flow rate
-Particle count
-Temperature
-Humidity
A rough flow chart was then made
Next the devices needed to measure the above were researched, these were:
-Pressure- Barometer
-Air Flow- digital timer, air flow detector
-Purity – particle counter/ detector
-Temperature – digital thermometer
-Humidity – Capacitive sensors or resistive sensors humidity
The risk assessments and the word document the university required were
completed
The universities equipment requirements online form part 2 were filled in
A neat flow chart on Microsoft publisher was made
Finally, the Gantt chart was updated
Page 25
28. EES Minutes: Residential Preparations and Research 10/12/13
Available equipment at the university was inquired about and a list of equipment
that the company would need to bring therefore was complied
Secondly, following an email from our lead engineer asking about valves, it was
discussed what valves we would need and where in the rig they should place
them
Next different types of valves were researched and 3 were selected to be
included in the rig- global needle valve, pneumatic valve and a gate valve
Lastly the flow chart was updated with the new valves in place
EES Minutes: Further Residential Preparations and Research 12/12/13
The university was asked if they had a vacuum pump with an air flow greater
than 25m3/h
The flow chart was updated, so that the vacuum pump was placed at the end of
the system, rather than placed at the start in response to an email from the
Engineer saying that vacuums suck
The ways to measure oil in the air researched - the theory is called flushing, it
includes using solvents and a 3 port valve
The “ Bourdon Pressure Gauge” was researched and a word document was
produced explaining how it works
A diagram for the oil measuring device was created
EES Minutes: Recommendations for the Rig 16/01/14
Recommendations on how the rig could be improved in the future were
discussed
Laskin Nozzles were researched in order to find a way to challenge the filter
The project report was started
Other filter companies were emailed about specific regulators
Other company correction factors were researched
EES Minutes: Further Recommendations for the Rig 23/01/14
Recommendations for the test rig were discussed and clarified
Page 26
29. Replies were accessed from the other companies for specific regulators
A particle detector, to be inserted into the new rig, was researched
The report structure was considered and the information for it was collated
EES Minutes: Working on the Report 30/1/14
The front cover page was designed for the report
The appendices was arranged for the report
The website addresses were compiled into the bibliography
EES Minutes: Report Work 13/02/14
Emails were sent to the engineer, Dr. Simon Bartram to show the final design for the
test rig.
Roles for the report were organised.
The methodology was written.
EES Minutes: Continuation of Report 27/02/14
Aspects of the report that individuals had produced were checked by the whole
team and added to the report.
The results and analysis of them were completed.
EES Minutes: The Report was Continued 06/3/14
The discussion was started
The background for Walker Filtration was created
Improved the results, writing them up and adding a graph
Introduction was started
The improvements were discussed
Page 27
31. A digital humidity measurer, a digital thermometer and two barometer (from left to
right)
Page 28
32. The Bourdon Pressure Gauge
Acknowledgements
There are many people we would like to thank for their help in completing our project.
Firstly we would like to thank Dr. Simon Bartram our assigned engineer from Walker
Filtration, who contributed a substantial amount of time and effort to our project, his
knowledge and experience was highly valued by the team, and we thoroughly enjoyed
working with him.
In addition we would like to express our gratitude towards Walker Filtration, for the
invaluable opportunity and equipment they gave to us in particular Jen MacEwan and
Lianne Walker, as they were particularly enthusiastic and supportive.
Next we would like to thank Bowman Bradley our mentor for his all-round knowledge
and advice on how to tackle the report.
Moreover, we would like to express our appreciation Mr. Morton and Mr Ivison for
allowing us to use the labs and constant encouragement, support and good humour.
Furthermore, we would like to thank Newcastle University for allowing us to use their
laboratories, equipment and providing us with technicians, in particular Mr Iain Strong,
who never failed to meet our many requests.
Finally we would like to thank the Engineering Development Trust for organizing the
Engineering Education Scheme, in particular Charity Watkins for her dedication and
enthusiasm to the event.
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