Innovative Heat Exchanger Design
for Wind Turbine Applications
In corporation with Vestas
• Project Supervisor Professor Winoto SH
• Group members Tan Tze Peng Wilson
Clarence Chen Jun Jie
Tan Bao Lun
Teh Ter Chen
Yeo Shu Feng Doreen
Ye Xiu Yin
Scope of Presentation
• Heat Exchanger Model for Experiment
• Proposed Experiment
• Fouling of Heat Exchanger
• Conclusion and Recommendations
• Previous Semester
• Verification of Calculations
• Designing the Heat Exchanger for Experiment
• Procedures and Guidelines being drafted for the
experiment in wind tunnel.
• Comparison between Experimental and Theoretical
• Fouling as a Menace in the Heat Industry.
• Innovative and cost-effective methods.
• We hope to be able to manufacture a heat exchanger which we have
previously designed and test it out via experiments.
• Our initial plan was to source for any local heat exchanger
companies to help fabricate the design. However we faced two major
▫ The first constrain was that no local company fabricates plate-fin heat
▫ Secondly, the budget not enough to custom build a heat exchanger and
purchasing equipments for the wind tunnel testing.
• We have decided to do the theoretical calculation for the scaling
down as well as to brainstorm for innovative anti-fouling ideas.
• Facilitating our juniors who will undertake the project and carry out
the verification and testing of the heat exchanger
2. Heat Exchangers Model for
• Evaluation of Plate-Fin Surfaces
• Selection of Fin Configuration
• Scaling Law of Heat Exchangers
• Sample Calculation
Evaluation of Plate-Fin Surfaces
5 Categories of Plate-Fins
• Offset strip
• Pin Fin
• Straight fins that are uninterrupted (uncut)
• Any complex shape can be formed as desired, depending on how the
fin material is folded.
• Rectangular and Triangular passages are most common
• Used in applications where the allowable pressure drop is low
• Preferred for very low Reynolds numbers applications.
▫ -with interrupted fins, at low Re, the advantage of the high heat
transfer coefficient value of the interrupted fins is diminished
while the cost remains high.
• Uncut surfaces in the flow direction, and have cross-sectional shapes
similar to those of plain surfaces. However, they are wavy in the flow
• The waveform provides effective interruptions to the flow and
induces very complex flows
- higher heat transfer coefficient for wavy fin to that of an
equivalent plain fin.
• However, the heat transfer coefficient for wavy fins is lower than that
for interrupted fins such as offset or louver fins.
• Since there are no cuts in the surface, wavy fins is preferred when
potential fouling or clogging problem might occurred.
Offset Strip Fins
• The fin has a rectangular cross section, and is cut into small strips of
length. Every alternate strip is displaced (offset) by about 50% of the
fin pitch in the transverse direction.
• Heat transfer coefficients for the offset strip fins are 1.5 to 4 times
higher than those of plain fin geometries. However, the
corresponding friction factors are also high.
• The ratio of j/f for an offset strip fin to j/f for a plain fin is about
• Offset strip fins are used in the approximate Re range 500 to 10000.
• Louvers can be made in many different forms and shapes.
• The j factors are higher for louver fins than for the offset strip fin at
the same Reynolds number, but the f factors are even higher than
those for the offset strip fin geometry.
• Since the louver fin is triangular, it is generally not as strong as an
offset strip fin which has a relatively large flat area for brazing.
• Can be manufactures at very high speed continuously from a wire of
• Surfaces compactness achieved by pin fin geometry is much lower
than that of offset strip or louver fin surfaces.
• The overall exchanger performance is also lower than other types of
• Potential application for pin fins is at very lower Reynolds number
• Due to vortex shedding behind the round pins, noise and flow
induced vibration may be a problem.
Selecting of Fin Configuration
Optimize Heat Exchanger Design based on the following factors:
• Pressure Drop
• Heat Transfer
To help us in our selection of the best fin configuration, we plotted
graphs for comparing different fin configurations under the listed
factors. From there we will be able to study the different fin types and
formulate a table as a basis for comparison.
If pressure drop and heat transfer are the most critical factors, then the
heat transfer per unit of pressure drop (i.e. j-Colburn factor divided by
f-friction factor) will show a good comparison between different types
of extended surface.
Heat Transfer over Pressure Drop Figure
j/f (j-colburn / f- friction)
0.3 strip fin
0.2 wavy fin
0 2000 4000 6000 8000 10000 12000
If size is the overall driving design factor then a good figure of merit is
heat transfer per unit height.
Size Figure of Merit
Heat Transfer per unit height
0.002 pin fin
0 2000 4000 6000 8000 10000 12000
If weight is the overall driving design factor, then a good figure of merit
is heat transfer per unit weight.
In general, pin fins, which are incorporated directly into the casting of
the heat exchanger, are the least expensive.
The other fin type cost about the same with some minor differences in
set up costs.
Pin fins 1
Plain fins 2
Wavy fins 3
Offset Strip fins 4
Louvered fins 5
Comparison of All Parameters
Ranking 1 being the most desirable and ranking 5 being the least desirable
∆P Size Weight Cost Average
Plain fins 1 5 4 2 3
Louvered fins 3 2 1 5 2.75
Offset Strip fins 2 3 3 4 3
Wavy fins 4 4 2 3 3.25
Pin fins 5 1 5 1 3
Plain fins for water-side (taking fouling into consideration)
Using excel to find the best combination for air-side fin type
The following ratios apply to the geometric
Hydrauli Length Surface Flow Volume
c Area, As area, Ac
dh2 = dh1/N L2 = L1 /N As2 = As1/N2 Ac2 = V2 =
where N = scaling factor
• The number of tubes in the heat exchanger is the
• The physical properties are fixed, based on the
• The inlet temperatures are fixed.
• Each flow stream is treated the same.
1. Same mass flow rate, (for each stream)
1. Same Reynolds Number, Re
1. Same flow velocity, G
1. Same pressure drop, ∆P
Based on the book:
Compact Heat Exchangers, Selection, Design and Operation
by J.E. Hesselgreaves
We aim to find the following:
• Q2, heat load removed by the prototype(scaled
• The pressure drop ∆p.
• Flow regime of the original 3x3x0.15 m3 heat
exchanger is laminar
• Hence, when applying the scaling laws, flow regime
of the prototype must be consistent, ie, laminar
• Keep Reynolds Number the same
• Nusselt number for laminar flow,
• If turbulent, NuD = 0.023ReD0.8Pr0.4 (Dittus-Boelter)
For mass flow rate
Re1 = Re2
1 - original
2 - prototype
• For the overall heat transfer coefficients, U
• NTU – Number of Transfer Units ratio
Note: NTU2 = NTU1 means e2 = e1,
What if NTU2 ≠ NTU1?
•e2 ≠ e1
•Obtain NTU2 from
the relevant ratio
• Using the NTU2
value together with
Cmin/Cmax ratio get a
new e2 value
• Heat load, Q2
• Note: only inlet temperatures are involved
• Ratio of pressure drops
Where f = Fanning friction factor
• With ∆P2 known, we can size the pump.
For the Experiment
Parameters that we can control:
• Inlet temperatures, Twater,in Tair,in ≈Tamb
• Wind tunnel air speed, u2
• Fluid properties, e.g. cp
• Mass flow rate of water, determined from the
• Unable to scale down DH
• Actual DH = 3.23 mm (wavy, air side) and DH = 2.87
mm (plain, water side)
• Difficult to manufacture
• After scaling down, flow passage will be too narrow
• Heat loss may not be through convection only
• Possibly through conduction
• Build a scaled down HX
• Maintaining the original DH
• Since the entire HX consists of stacks of plates
with the fins attached,
• Can conduct experiments to calculate how much
heat, qp, 1 plate can remove
• ∴Total heat load = N x qp
3. Proposed Experiment
• Experiment Setup
• Startup Procedure
• Tables and Program
The purpose of this experiment is to collect
experimental data from a scaled down prototype of
the wind turbine and verify the data against
theoretical values derived from the excel spreadsheet
calculations to prove that it is within acceptable
range of heat energy dissipation by the actual heat
1. Manufacture a smaller prototype of heat exchanger
based on the scaling down theory calculation
2. Modify and introduce measurement apparatus in
wind tunnel to accurately collect selected design
parameters and data.
3. Design and build a liquid flow system (hot side) to
accurately facilitate liquid-side flow and heat transfer
4. Compare the experimental results against the
theoretical excel spreadsheet calculations.
Determination of surface characteristics:
In the experiment:
• the flow rates on both fluid sides of the exchanger are set at
constant predetermined values.
• Once the steady – state conditions are achieved, fluid
temperatures upstream and downstream of the test section
on both fluid sides are measured.
• The upstream pressure and pressure drop across the core on
the unknown side are also recorded to determine the friction
• The tests are repeated with different flow rates on the
unknown side to cover the desired range of Reynolds number
for the j vs. RE characteristics.
Heat capacity ratio For an unmixed –
unmixed crossflow exchanger, the relationship
For Cr 0,
the heat capacity rate is determined from the
measured mass flow rate on each fluid side and
the specific heats of the fluids at their average
temperatures. On the known side, the fluid
properties are evaluated at the average
temperature Ts. On the unknown side, the fluid
properties are evaluated at the log-mean
The overall heat transfer coefficient Ua based on the total
airside surface area Aa is then evaluated from NTU as
The overall heat transfer coefficient Ua is considered as having
three components in series:
• Air side thermal resistance, including the extended surface
• Wall thermal resistance
• Steam side thermal resistance, including the extended surface
The test cores are generally new, and no fouling
or scale resistance is on either side, so the
corresponding resistance is not included above.
The liquid side heat transfer coefficient must
also be evaluated separately for each core and
should be known a priori. However, the liquid
side resistance is generally a very small
percentage of the total resistance, and a
reasonable estimate will suffice.
The term in equation 1.3 is the extended
surface efficiency of the air side surface and is
related to the fin efficiency of the extended
surface by the following formula:
For many plate fin surfaces, the relation for the
straight fin with constant conduction cross
section may be used to a good approximation. In
Once the surface area and the geometry are known
for the extended surface, h and are computed
iteratively from equations 1.3 to 1.5.
The Stanton number St and the Colburn factor j
are then evaluated from their definitions:
The Reynolds number on the unknown side for
the test point is determined from its definition:
G= ρ V
Determination of the factor is made under steady fluid
flow rates. For a given fluid flow rate on the unknown f
side, the following measurements are made:
1. Core pressure drop
2. Core inlet pressure and temperature
3. Core outlet temperature
4. Fluid mass flow rate
5. Core geometrical properties.
Here Kc and Ke are sudden contraction and expansion
pressure loss coefficients presented in Appendix 1. Tests
are repeated with different flow rates on the unknown
side to cover the desired range of the Reynolds number.
The Reynolds number is determined in the same way as
described in determination of the airside film coefficient
Liquid Flow System
• Stainless steel piping
• Gear pump
• Resistive Heater (Watlow®)
• Mass flow rate meter (D25 MicroMotion®)
• Thermocouple (type T) x 2
Air Sampling System
• Thermocouple (type T) x 2
A type T thermocouple with a
measurement range of -200
to 350 degree Celsius is placed at
the entrance of the wind
tunnel air outlet. And for the air
outlet temperature, a similar
thermocouple is place near the
outlet of the heat exchanger
Pressure Drop Measurement
• Pressure differential Transducer (Validyne DP-103) x 2
To measure the pressure drop
across the heat exchanger, a
total of two differential pressure
sensors, one upstream and one
downstream, are incorporated
into the flow system.
Schematic diagram of Test Setup
MASS FLOW SENSOR
P PRESSURE DIFFERENTIAL SENSOR
Data Acquisition (DAQ)
Acquisition of signals &waveforms &processing the signals to obtain desired information.
Laboratory Virtual Instrumentation Engineering
Graphical approach allows non-programmers to build programs simply
by dragging & dropping virtual representations of familiar lab equipment.
Start Up Procedure
Power up the DAQ unit
Switch on the gear pump to start water circulation
Switching on the heater after full circulation of water
Set the heater to the specific heat load
Start Up Labview Program
Begin Data Aquisition
DAQ with Labview
The NUS industrial
wind tunnel at
Interior of a wind tunnel
The Wind turbine
4. Fouling of Heat Exchangers
• Literature Survey
• General Methods of Fouling
• Heat Exchanger Cleaning
• Detailed Analysis of Anti-Fouling Methods
▫ Air Side
▫ Water Side
• Advantages and Disadvantages
• Fouling is defined as the accumulation of deposits on the
surface of the heat exchangers
• Deposits reduce the amount of heat transfer and thus
reducing the efficiency.
• In most of the cases, accumulation of deposits is from the
Deposition of Foulant
• Region A represents the initiation of adhesion
• Region B represents the steady growth of deposit on the surface.
• Region C shows that a steady state is reach between deposition and
Why is Fouling bad?
Fin Cooler tubes severely corroded Deposits build-up on the inside of a heat Wall thinning led to this catastrophic
exchanger tube failure of an exchanger tube
• Fouling is highly related to conservation of energy
• Fouling decreases the efficiency leading to heavier
consumption of energy.
• Increase in the resistance also cause the pump to work
General Methods of Fouling
• Particulate deposition
• physical interaction between the foulant and the
heat exchanger surface.
• 3 types of interaction:
▫ long range attractive forces: Van Der Waals forces
▫ Bridging effect: mutual diffusion between substances
of the particle and the surface
▫ short range forces: adhesion at molecular level such as
hydrogen bonds and other chemical bonding
• Particle arriving at a surface can be by two
▫ gravitational and: stationary fluids
▫ particle transport within a fluid as it moves across the
surface onto which the particles deposit: Moving fluid
• transported by either or both of the following
▫ Brownian motion and
▫ turbulent diffusion
• Corrosion means the breaking down of essential
properties in a material due to chemical reactions with
• origin of the corrosion is the fluid itself
• or a constitute of it: impurities
• Corrosion process is often accelerated by the presence of
scales or other deposit
• corrosion could also form a protection layer (oxide layer)
Heat Exchanger Cleaning
• Circulation of Sponge Rubber Balls
• Brush and Cage System
• Air and Gas Injection
• Magnetic Devices
• Soot Blower
• Water Washing
• Galvanic Protection
Circulation of Sponge Rubber Balls
• Sponge rubber balls which are slightly larger (by 2mm)
then the diameter of the condenser tubes are being
circulated to remove precipitates.
• Different types of sponge balls are available to suit different
heat exchanger e.g. rubber balls coated with a layer of
carborundum can be used for more heavy duty application.
• The automatic cleaning system is being controlled by a
Programmable Logic Controller.
• The system requires a source of water which has pressure
higher than the cooling water in the condenser inlet. This
water is used for soaking the balls and injecting them into
the Cleaning Systems.
Brush and Cage System
• The operating method of this system is very similar to that of the
Sponge rubber Ball.
• It involves the use of a brush, made of wire or polymer filament
fitting into the size of the condenser tube.
• The brush is made to oscillate from end to end of the condenser tube
to remove the deposit by abrasion.
• The cage is located at the end of the tube to catch the brush after
• The addition of the brush and cage create restriction in the
flow, increasing the pressure drop across the condenser.
• The other drawback of this system is that the reversal of the brush
would cause instability in the flow thus affecting the heat transfer
Air and Gas Injection
• Air and gas injection is usually used in areas where
accessibility is difficult e.g. shell side of shell and tube heat
• The injection of air reduces the formation of deposits on the
heat transfer surfaces.
• The surge of air creates a region of high turbulence near the
• This method is ineffective against deposits that requires
higher amount of force to remove.
• Extreme care has also to be taken when the process liquid is
volatile in nature; this is to avoid possibilities of forming
• The main application of magnetic devices is to eliminate
scale formation in pipes.
• It is could be supposed that slight soluble compounds
such as calcium carbonate (exist in water as charged
ion), could be affected by the application of an electric
• One example is the use of hard water to rinse the treated
steel where the hard water could give rise to
precipitation of calcium phosphate and consequent scale
• Till date, this method is still not very commonly use and
there are still doubts and critics on this method.
• The fundamental of the soot blower is to remove soot that is
built up in steam boiler.
• Steam is channeled to the soot blower pipeline and a drain
valve will drain off any water in the steam (Ensures that steam
• The drain valve is shut off, and the soot blower is turned on.
• The steam shoots out from the soot blower tube that is inside
the boiler fireside. Many small holes for the steam to emerge
are drilled along the length of the tube.
• As the tube rotates, the position of the steam jet will also
move with it. After a full rotation, all the areas around the soot
blower tube should be clear of soot. After completing the soot
blowing, the steam supply is shut off again.
• Water washing of heat exchanger is the process of
introducing jets of water into the system. This has
been in practice over many years and is one of the
most commonly applied techniques in cleaning of
• It is important to note that the water used in this
technique is to be as pure as possible as any particle
in the water could deposit itself in the process;
increasing the foulant amount.
• The jets are introduced intermittently and the turn
off of this method could be that the thermal shock
from such action could cause cracking and
subsequent spalling of the foulant.
• Galvanic protection involves the study of electric
current that is involved in the corrosion of metal.
• By understanding how electric current is involve in
corrosion of metal, we look at various methods
applied in the industry to tackle corrosion problem.
• Methods of Galvanic protection include
▫ Cathodic Protection
▫ Sacrificial Anode Technique
▫ Impressed Current Technique
▫ Anodic Protection.
1. fouling deposit provides additional thermal
resistance to heat transfer besides those originally
present due to the inherent design of the heat
2. flow area is decreased and most foulant, having a
rough surface will result in an increased pressure
drop through the exchanger
• increased capital investment
• additional operating costs
• loss of production
• cost of remedial action
thermal resistance Rf across a solid barrier is given by
l is the solid thickness and kf is its thermal conductivity
heat flux q is given by
∆T is the change in temperature
• boundary layers, situated
between the deposits and the
fluids, provide small amount of
thermal resistance to heat flow
due to their almost stagnant
• metal partition wall has good
thermal conductivity, negligible
temperature change at T3 and T4
• foulants have low thermal
conductivities, large temperature
differences required to transmit
heat across these depositions
we can write Rf in terms of the overall heat transfer coefficient for
general formulas for overall heat transfer coefficient, including the
fouling factor, at the air and water side are respectively
• Besides increasing the thermal resistance, foulant layer have two
additional negative effects
• Firstly, once deposit reaches a significantly large thickness, cross-
sectional area for fluid flow reduced
• For the same volume flow, due to a smaller area, fluid velocity will
have to increase and for identical conditions the Reynolds number
• Secondly, increase in roughness of deposit
surface results in higher turbulence level,
and eventually a greater amount of heat
(three fundamental stages that describe process of deposition)
1. The diffusional transport of the foulant across the
boundary layers beside the solid surface within the
2. Adhesion of deposit both to the surface and to
3. The removal of deposit from the surface.
By combining these three components, rate of deposition
growth can be defined as the difference between the rates
of foulant deposition and removal
• A = straight line
relationship, deposit thickness
increases at a constant rate with
time after the initial adhesion
• B = falling rate of deposition
thickening with time
• C = asymptotic curve like
curve B at the beginning but
reaches final plateau steady state
or asymptote (rate of deposition
= rate of removal)
Change in Heat Transfer
Change due to
Change due to Change due to change in
thermal resistance of Re caused by presence
foulant of foulant
Detailed Analysis of Anti-Fouling
Methods : Air Side : Sonic Technology
• Similar to vibration caused by sound energy to dislodge
the deposits off metal surfaces
• Device for producing the sonic wave can a sonic
horn, usually operating at audible frequency of
approximately 220 Hertz with intensity of about 130
• Low frequency resonant sound ranging from 0 to 20
Hertz capable of setting up sound field to remove
soot, loose and brittle dust particles in the air side of heat
Sonic Technology (cont’)
• commercially available sonic horn model #CS-125
designed by IAC (Industrial Accessories Company)
• capable of removing build-up particulates using a
fundamental frequency of 125 Hertz and SPL (Sound
Pressure Level) of 146 decibels, operating at 10 seconds
every 5 to 10 minute interval
• cleaning device can be used to clean agglomerated dust
on collector plates of an electrostatic precipitator, which
is similar in structure to a plate fin heat exchanger
• ionizes the fluid so as to create a plasma around
• may be positive or negative
• Neutralises static charges on the surfaces of the
heat exchanger’s plate fins
• Benefit of keeping the fin surfaces neutral in
Water Side : Electronic Anti-
• Most prominent is formation of scales
• Hard water is heated
• Degradation in the performance
• Decrease the flow rate or increase the pressure
• Solution :Electronic Anti-fouling technology
Electronic Anti-fouling technology
Overview of Electronic Anti-fouling technology
Electronic Anti-fouling technology
• Induced electric field to increase the ions and crystals
• Produces a pulse input current to create time-varying
magnetic fields inside the pipe
Cross section of pipe where induced electric field oscillates
Electronic Anti-fouling technology
• Resulting in collision, precipitation as well as
• Converted into insoluble mineral crystals of
submicron sizes, easily removed
• Level of Super-saturation of the water will
• Scale-causing mineral ions and particles were
eliminated from the water
Electronic Anti-fouling technology
Pressure drop across HE for 4 different cases
Conclusion and Recommendations
Heat Exchanger Model for Experiment
Fouling of Heat Exchanger
▫ Recommendations for Future Work
Conclusions : Heat Exchanger Model
• Manufacturability constrains.
• If we are to scale down the hydraulic diameter, Dh, by a desired scaling factor, it’s
almost impossible to fabricate such small passage.
• We suggest to build a smaller heat exchanger keeping the geometrical data (Dh)
unchanged. From this smaller size prototype,
• Two most important date: Heat transfer coefficient h, which relates to the Stanton
number St and the Colburn factor j, and the flow friction factor f, necessary for
pressure drop analysis.
• After obtaining the experimental h and f, the rating of heat exchanger procedure can
then be applied to calculate the experimental heat load Q and pressure drop ∆P.
• Furthermore, if manufacturability is not an issue, the hydraulic diameter of the
chosen fin type may be varied and better h and f can be obtained.
Conclusions : Proposed Experiment
• By using the experimental results, we are able to
determine Stanton number St, Colburn factor j
and the Reynolds number .
• This will enable us to find out the Q heat load of
the experimental heat exchanger, and based on
this heat load we can compare it with the heat
load derived from the excel spread sheet
Conclusions : Fouling of Heat
• With innovative methods employed on the air side and
water side of the heat exchanger
• We can be confident that fouling can be reduced, or even
• The use of sonic vibrations, coupled with the phenomena
of corona discharged air ions will safely prevent the
buildup of dust particles on the plate-fin surfaces.
• As for the water side, experiments have been conducted
to validate the theory being
• The downtime of the wind turbine can also be reduced
and maintenance can be done online.
Conclusions : Recommendations for
• With the stage set for the experiment to be conducted, our future
juniors undertaking this project can carry on from where we last left
• The theory of the scaling laws can also be further analyzed and
researched in a much deeper scope.
• Feasibility study regarding the innovative method of anti-fouling for
the air and water side of the heat exchanger can also be conducted
• Purchase the equipment and implement it on a real heat exchanger.
• End result will be very beneficial to the industry and will indeed
benefit the future generations to come.
• On this note, there are still many areas of this project that has the
potential for future advancement.