Heat exchanger report

by

Brendan Carberry

Paul Bergin

Cathal Waldron

Keith Quinn

Mr Cian Bregazzi Nevin, Lecturer

A team report submitted in partial fulfilment

of the requirements for the

Degree of Bachelor of Engineering with Honours

in

Mechanical Engineering

Athlone Institute of Technology

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1 Introduction .............................................................................................. 1

1.1 Aim ................................................................................................................................. 3

1.2 Objectives ...................................................................................................................... 3

1.3 Heat exchanger performance improvement ............................................................ 4

2 Materials and methods ........................................................................... 6

2.1 Materials ........................................................................................................................ 6

2.2 Methodologies .............................................................................................................. 7

2.2.1 Experimental methodology ................................................................................. 7

2.3 Finite element analysis methodology ....................................................................... 8

3 Results and discussion ......................................................................... 10

3.1 Experimental results .................................................................................................. 10

3.1.1 Comparison of cavity layers ............................................................................. 10

3.1.2 Comparison of test liquids ................................................................................ 11

3.2 Finite Element Analysis Results ............................................................................... 12

4 Conclusion .............................................................................................. 14

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List of figures

Figure 1-1 heat exchanger friction rig 3D model ................................................................ 2

Figure 1-2 cross-section view of the heat exchanger .......................................................... 3

Figure 2-1 FEA mesh setup for the chassis and water displacement ........................................ 9

Figure 3-1 time taken for various cavity layers to reach 37°C ........................................ 10

Figure 3-2 time taken and temperature reached for the test liquids ............................. 11

Figure 3-3 temperature contour plot of full system ................................................................ 12

Figure 3-4 temperature contour cut-plot of system ................................................................ 12

Figure 3-5 temperature contour plot of the heat exchanger base .................................. 13

Figure 3-6 temperature contour plot of the heat exchanger base ............................................ 13

List of tables

Table 1-1 test liquid and properties (30°C) .......................................................................... 5

Table 1-2 cavity layer material and properties ................................................................... 5

Table 2-1 Pros and Cons of tested cavity layers and test liquids ..................................... 7

Table 2-2 Test combinations .................................................................................................. 8

Table 2-3 material properties for FEA model ........................................................................... 8

Table 2-4 Boundary conditions for FEA model ........................................................................ 9

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1 Introduction

A heat exchanger is a device that is used to transfer thermal energy (enthalpy)

between two or more fluids, between a solid surface and a fluid, or between a solid

particulates and a fluid, at different temperatures and in thermal contact (Ramesh,

2003).

Heat exchangers can be classified on the basis of the transfer process (direct or

indirect), the number of fluids, the surface compactness (gas-to-fluid or liquid-to-

liquid), the construction features (tubular, plate, extended surface, etc.), flow

arrangements (single-pass or multi-pass), and also the heat transfer mechanisms

(forced or free convection). It can be appreciated a wide variety of heat exchangers

exist; however, the heat exchanger used for this lab report does not discreetly fall

under any area specifically. The heat exchanger used has characteristics similar to that

of the panel coil heat exchanger, in that it uses a fixed plate and pipe system to transfer

heat energy from the passing fluid inside the pipe to a separate fluid of interest.

The heat exchanger which has been used for this lab consists of an aluminium

chassis with a single serpentine path with an inlet for the hot fluid to enter and an

outlet. The aluminium chassis consists of a cavity open to the atmosphere which

contains the fluid which is to be subject to the heat transfer process. Inside this cavity

various layers of materials are used interchangeably; these materials are specific to the

rig application and are by no means utilised to aid the heat transfer process. The rig

has been designed for friction testing of hydrophilic polymers whilst replicating in-

vivo conditions. It is for these reasons the cavity containing the fluid must be heated

in an indirect fashion. Since the heat exchanger is designed for in-vivo condition

simulation the cavity fluid is to be maintained at approximately 37°C; hence the fluid

passing through the serpentine path will be higher. From observations the

temperature of the fluid entering the path will be maintained constant at

approximately 42°C at the inlet.

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Figure 1-1 illustrates the heat exchanger friction rig; the blue hoses contain the

primary fluid passing through the serpentine path, the open cavity illustrates where

the secondary fluid resides.

Figure 1-1 heat exchanger friction rig 3D model

Figure 1.2 illustrates a cross-section view of the heat exchanger; the path shown is

that in which the primary fluid passes. It can be seen number of various materials are

used in the construction of the rig, from the aluminium chassis, the polymer hoses,

the brass fittings, and the stainless steel jubilee clips. These various materials all have

different heat capacities; therefore it can be expected that the heat conduction rates

will vary accordingly. This will be compensated for in the lab procedure, as mentioned

in the methodologies.

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Figure 1-2 cross-section view of the heat exchanger

1.1 Aim

The aim of this project is to analyse and improve the performance of a heat

exchanger.

1.2 Objectives

Analyse the heat exchanger with the four cavity layers using water as the

test liquid and identify the most efficient cavity layer

With the most efficient cavity layer identified analyse the heat exchanger

with the four test liquids, this should yield the best pairing of cavity layer

and liquid

Investigate improvement techniques that may yield further improvements

Conduct FEA to use for comparative analysis relative to the experimental

Compile results and document in report

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1.3 Heat exchanger performance improvement

This report is experimental based, which means that certain constraints are in place

such as the circulating laboratory heating water bath used for experimental has a fixed

mass flow. If this was theory based increasing the mass flow may result in less time

taken for the heat exchanger to reach temperature. Another constraint was that the

heat exchanger has fixed area from which to exchange heat, in that the diameter,

length and number of holes cannot be changed, if this was a theory based exercise

then increasing the area of heat exchange would also improve performance. However

with this project just as with any projects in the real world there will be constraints,

this was the rationale for the approach considered in completing this assignment.

There were however two methods chosen which would affect and hopefully improve

the performance of the heat exchanger, they are detailed in the following text which

also puts context to the reasoning behind the importance of the heat exchanges

performance.

The heat exchanger friction rig used in this experiment has particular requirements,

the main one being its ability to reach body temperature [37C] in order for tests to be

carried out on it. After each experiment it has to be emptied of all fluids and dried

thoroughly and then brought back up to body temperature, if a number of tests are to

be carried out a lot of time is lost due to this operation as the operator is idle waiting

for the heat exchanger to reach body temperature. The two aforementioned methods

to improve performance include; the liquid used in the cavity and the material used

as a cavity layer.

This experiment was carried out using three liquids, the names and properties of

which are provided in Table 1-1, and four cavity layers, the name, geometry and

properties of which are provided in Table 1-2.

The intention being to identify the best combination of test liquid and cavity layer

which will allow the heat exchanger heat the liquid to body temperature thus allowing

for the quickest turn around between tests, minimising idle time during testing.

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Table 1-1 test liquid and properties (30°C)

Test

liquid

Density (ρ)

[kg.m-3]

Specific Vol. (v)

[m3.kg-1]

Specific heat capacity (cp)

(kJ.kg-1K-1)

Surface

tension ()

10-2 [N.m-2]

Water 995.7 1 4.179 7.12

Glycerine 1259 794.28*10-6 2.43 63.4

Methanol 782 1.28*10-3 2.51 2.18

Table 1-2 cavity layer material and properties

Material Thermal conductivity (k)

[W.m-1.K-1]

Density (ρ)

[kg.m-3]

Specific heat capacity (cp)

(kJ.kg-1K-1)

Thickness

[mm]

Glass 1.05 2800 670 6

Silicone 1.55 1500 1050-1300 6.5, 13

HIPS 0.18 1050 1400 2

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2 Materials and methods

The materials and methods utilised for both the experimental and FEA elements of

this project are detailed in this section.

2.1 Materials

It has been previously stated that there are two pragmatic methods of improving

the performance of the friction rig heat exchanger:

Improve the cavity layer

Improve the test fluid

The rig itself has been described in the introduction to this report. For this reason this

section will provide an outline of the cavity layers and test liquids. Tests were

conducted using four different cavity layers; 2[mm] HIPS, 6.5[mm] Glass, 6.5[mm]

Silicone, 13[mm] Silicone. Tests were also conducted empty (i.e. no cavity layer). Tests

were conducted using four different test liquids; water, methanol, glycerol and a 50:50

(by mass) water:glycerol mixture. Tests were also conducted empty (i.e. no test liquid).

A list of the pros and cons of each cavity layer and test liquid is provided in Table 2-1.

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Table 2-1 Pros and Cons of tested cavity layers and test liquids

Pros Cons

Cavity layers

Empty No added thermal resistance Surface roughness and modulus do not

mimic physiological conditions

2[mm] HIPS A thin cavity layer suggests a low

thermal resistance

Surface roughness does not mimic

physiological conditions

6.5[mm] Glass High thermal conductivity suggests a

low thermal resistance

Surface roughness and modulus do not

mimic physiological conditions

13[mm] Silicone Mimics physiological conditions in

terms of both surface roughness and

modulus

High thermal resistance

6.5[mm] Silicone Mimics physiological conditions in

terms of both surface roughness and

modulus

High thermal resistance

Test liquids

Empty No thermal mass Does note exploit the hydrophilicity of

the polymer sample

Water Fully exploits the hydrophilicity of

the polymer sample

High specific heat capacity

Methanol Low surface tension Does note exploit the hydrophilicity of

the polymer sample

Glycerol Low specific heat capacity Does note exploit the hydrophilicity of

the polymer sample

Water:Glycerol Exploits the hydrophilicity of the

polymer sample and has a similar

viscosity to blood

Requires an additional preparation step

2.2 Methodologies

The methodologies implemented for both; the experimental and the FEA are

detailed in this section.

2.2.1 Experimental methodology

The Prism heating unit was attached to the heat exchanger and the set point of the

heating unit was set to 42[°C]. The heat exchanger, containing no cavity layer, was left

to heat for 2[hrs] so as to ensure that all components of the device reached steady state.

A cavity layer and test liquid, both of which were at ambient temperature, were added

to the cavity of the heat exchanger and the temperature of the test liquid and the inlet

and outlet temperatures of the heating stream were measured using Type K

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thermocouples and a PA Hilton data logger .. Tests were first conducted to determine

the optimum cavity layer using water as the test fluid. The results of the initial set of

tests suggested that the 6.5[mm] silicone cavity layer was most suitable. Tests were

then conducted using a variety of test liquids and the 6.5[mm] silicon cavity layer in

order to determine the optimum test liquid and cavity layer combination. The full

combination of tests conducted are presented in Table 2-2.

Table 2-2 Test combinations

Test number Cavity layer Test liquid

1 Empty Empty

2 Empty Water

3 2[mm] HIPS Water

4 6.5[mm] Glass Water

5 13[mm] Silicone Water

6 6.5[mm] Silicone Water

7 6.5[mm] Silicone Methanol

8 6.5[mm] Silicone Glycerol

9 6.5[mm] Silicone Water:Glycerol 50:50

2.3 Finite element analysis methodology

The finite element analysis (FEA) was designed in order to evaluate the thermal

diffusivity characteristics of the heat exchanger at steady-state when the cavity is filled

with water. SolidWorks Flow Simulator was used with the material properties and

boundary conditions stated in Table 2-3 and Table 2-4 respectively. The mesh control

was of a fine tetrahedral nature with 83,982 elements in total and 127,628 nodes, as

illustrated in Figure 2-1

Table 2-3 material properties for FEA model

Material Property Value Unit

Water Density (𝜌) 1000 Kg.m-3

Water Thermal conductivity (𝑘) 0.61 W.m-1.K-1

Water Specific heat (cp) 4200 J.kg-1.K-1

Aluminium 6061 Density (𝜌) 2700 Kg.m-3

Aluminium 6061 Thermal expansion coefficient (𝛾) 2.4e-5 K-1

Aluminium 6061 Thermal conductivity (𝑘) 170 W.m-1.K-1

Aluminium 6061 Specific heat (cp) 1300 J.kg-1.K-1

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Table 2-4 Boundary conditions for FEA model

Location Condition Value Unit

Outer chassis walls Convection 10 W.m-2.K-1

Cavity-water interface Convection 20 W.m-2.K-1

Serpentine path Initial temperature 42 °C

Figure 2-1 FEA mesh setup for the chassis and water displacement

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3 Results and discussion

Included in this section are results from both the experimental and FEA methods.

3.1 Experimental results

The experimental results include comparisons between the various cavity layers

and also the various test liquids.

3.1.1 Comparison of cavity layers

The initial experiment was conducted in order to determine the optimum cavity

layer for the heat exchanger friction rig. The results of the experiment are presented

in Error! Reference source not found..

Figure 3-1 time taken for various cavity layers to reach 37°C

As expected the test fluid reaches 37[°C] quickest when no cavity layer is used.

However it is not practical to use no cavity layer as this does not replicate in-vivo

conditions. The two cavity layers which are most similar to in-vivo conditions in terms

of both modulus and surface roughness are the 13[mm] and 6.5[mm] silicone layers.

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20

25

30

35

40

0 5 10 15 20 25 30

Tem

per

atu

re (

T)

[°C

]

Time (t) [mins]

Empty

Glass

6.5[mm] Silicone

13[mm] Silicone

HIPS

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It takes the test fluid 12[min] to reach 37[°C] when using the 6.5[mm] silicone layer

and over 25[min] to reach 37[°C] when using the 13[mm] silicone layer. For this reason

it ca be concluded that 6.5[mm] silicone is the optimum cavity layer.

3.1.2 Comparison of test liquids

It was determined in the initial experiment that 6.5[mm] silicone is the optimum

cavity layer. For this reason the test liquids were evaluated using this cavity layer. The

results of this testing are presented in Error! Reference source not found..

Figure 3-2 time taken and temperature reached for the test liquids

Methanol did not reach 37[°C] over the duration of the test, for this reason it is not

a suitable test liquid. The glycerol and the water glycerol mixture took approximately

the same amount of time to reach 37[°C]. As glycerol will not activate the

hydrophilicity of the polymer sample it is deemed unsuitable. Water reaches 37[°C] in

a shorter period of time than that required by the water glycerol mixture however it

20

25

30

35

40

0 10 20 30 40 50 60 70 80 90

Tem

per

atu

re (

T)

[°C

]

Time (t) [mins]

Water

Glycerol

Methanol

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does overshoot the 37[°C] set point. The favourable heating characteristic renders the

water glycerol mixture to be the most suitable test liquid.

3.2 Finite Element Analysis Results

Figure 3-3 temperature contour plot of full system

From Figure 3-3 the temperature contour plot illustrates where the maximum

temperatures exist, i.e. at the primary fluid path. Also the lowest temperatures can be

seen at the two ends furthest away from the primary fluid path.

Figure 3-4 temperature contour cut-plot of system

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Figure 3-4 illustrates the cut-plot; here it can be seen how the thermal energy

diffusion prevails throughout the water (i.e. the secondary fluid).

Figure 3-5 temperature contour plot of the heat exchanger base

Figure 3-5 represents the heat exchanger from the bottom; it can be seen the base of

the heat exchanger raises in temperature significantly. This may be an issue depending

upon the surface the heat exchanger is sitting on. However, since the heat exchanger

is propped by four narrow legs the heat transfer through the base through conduction

is greatly reduced. Figure 4.4 represents a clipped-surface cutting through the primary

fluid serpentine path. Here it can be seen a uniform thermal diffusivity pattern exists

which implies the heat exchanger is well designed.

Figure 3-6 temperature contour plot of the heat exchanger base

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4 Conclusion

It is felt that the experimental approach in completing this project was an effective

one. The reasoning behind this statement is that it was a real problem involving a heat

exchanger that is used for the characterisation of polymeric materials. Based on the

findings of this report the idle time has been cut by 50% during testing including time

needed on the machine and operator idle time. The machine on which this test rig has

to be used in conjunction with (tensile tester) is typically booked up and can be hard

to get time so it is important that while getting a time period on the machine that this

time is not wasted standing around waiting for the heat exchanger to get the liquid up

to temperature, and that as many tests can be conducted in the time available.

The experimental approach although it had many advantages also had constraints

that limited the variables, this would not have been a problem if the approach had of

been theoretical based. The mass flow could not be adjusted, nor the heat exchanger

surface area both were fixed. However it is felt that this reflects a real life scenario in

that it may not be always possible to adjust parameters that are possible to adjust

theoretical. It was also felt that adding FEA to the project added another important

engineering dynamic to the project.

In concluding to the FEA, it can be said that when the model when no cavity layer

was utilised and when the secondary fluid was water, illustrated an average

secondary fluid temperature of approximately 39°C. This was the case when the

primary fluid was constant at 42°C under steady-state conditions. This correlates well

with the experimental procedure which yielded a steady-state secondary fluid

temperature of 38°C when the primary fluid temperature was 42°C. Therefore, it can

be concluded that the FEA model is an accurate prediction model based on the applied

material properties and boundary conditions applied. With this, repetitive

experimental procedures are not essential since the numerical model can predict the

steady-state temperature of the secondary fluid once the appropriate fluid properties

have been assigned. This is particularly important in the medical device industry

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where this heat exchanger is used to control the secondary fluid at a temperature with

a close tolerance. Another advantage of the numerical model is the reduction in time

required for various fluid testing; in practice each of the experiments ran for

approximately 1 hour whereas the numerical model ran for less than 1 minute.

As a final year project for engineers with the intention of going into industry it is

felt that during class enough theoretical study and analysis is covered and that it is

important to design and conduct practical experiments logging the variable and

plotting results and analysing and trying in a practical sense to improve the

performance of devices such as heat exchangers. It is felt that this project balances

practical with theoretical knowledge obtained in lectures. It is also felt important that

leaving college as a mechanical engineer that there is a balance between practical and

theoretical knowledge as both will be needed for any mechanical engineer.