counter flow and parrallel flow exchanger

8/13/2019 Counter flow and parrallel flow exchanger

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1.0 ABSTRACT

The objective of conducting this experiment is to demonstrate the effect of flow rate variation

on the performance characteristics of a counter-flow concentric tube heat exchanger. There are

two types of flow which are parallel flow and counter flow. For every flow, the procedure is the

same but the arrangements of valves are different in order to change the direction of flow. Thevariable that needs to change is the volumetric flow rate of the hot fluid and all six readings of

the temperature are recorded for every changing. Using the data, the heat exchanger performance

factors such as power emitted, power absorbed, power lost, efficiency, logarithmic mean

temperature difference and overall heat transfer coefficient are calculated. The effect of changing

the volumetric flow rate of the hot fluid on each of these heat exchanger performance factors are

discussed.


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2.0 INTRODUCTION

Heat is the transfer of energy from one system to surrounding when they have different

temperature. Based on 2nd

law of thermodynamics, heat is transferred in the direction of

decreasing temperature. This law gives information that heat flows from high temperature to low

temperature. Heat is basically exchanged by two method which mainly convection and

conduction processes. Conduction is when there are contacts between the bodies and convection

is when there is no contact between them and heat transfer through movement of air. In this

experiment, student will conduct an experiment of heat exchange between two fluids that has

different initial temperature. At the end of the experiment, the results between counter flow and

parallel flow that flows in the concentric tube heat exchanger machine will be differentiated.

A heat exchanger is a system which thermal energy is transferred from one fluid toanother. The types of heat exchangers that are to be tested in this experiment are parallel flow

heat exchanger and counter flow heat exchanger. Heat exchanger is built for efficient heat

transfer form one medium to another. A metal wall separate the fluid flows so that they will not

mix or may be in direct contact. Heat exchangers are widely used in space heating, refrigeration,

air conditioning, power plants and many more. One of the most common heat exchanger is the

radiator in a car where it transfers heat to air flowing through the radiator. The variable that

affect the performance of a heat exchanger are the fluid physical properties, fluid mass flow rate,

inlet temperature of fluid, physical properties of heat exchanger materials, the area of heat

transfer surfaces and the ambient conditions. The way that a heat exchanger works is when the

cold water entering the heat exchanger inlet gaining heat and the hot water losing heat before

both of this water exit the exchanger.

The primary advantage of a concentric configuration, as opposed to a plate or shell and

tube heat exchanger, is the simplicity of their design. As such, the insides of both surfaces are

easy to clean and maintain, making it ideal for fluids that cause fouling. Additionally, the heat

exchanger robust build means that they can withstand high pressure operations. Common heat

exchanger works under turbulent conditions which at low flow rates in order to increase the heat

transfer coefficient, and hence increase the rate of heat transfers.


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3.0 THEORY

A heat exchanger is equipment where heat exchange takes place between two fluids that enter and

exit at different temperature. The primary design objective of the equipment may be either to remove heatfrom hot fluid or to add heat to cold fluid. In parallel flow or concurrent flow, hot and cold fluids flow in

the same direction, thus entering and exiting the heat exchanger on the same end. Meanwhile in counter

flow or counter current flow, hot and cold fluids flow in the opposite directions, thus entering and exiting

the heat exchanger from opposite ends.

Figure 1: Parallel flow and Counter flow configurations

In a heat exchanger, the temperature difference between the hot fluid and cold fluid may vary

along the length of the heat exchanger as shown in the Figure 3 below. This is due to the fact that the hotfluid temperature decreases as it transfers heat to the cold fluid, while the cold fluid temperature

increases. As shown in the Figure below, for parallel or co-current flow arrangement, the temperature

difference is maximum at the inlet and decreases slowly towards the outlet. Accordingly, the heat transfer

rate is maximum at the inlet and minimum at the outlet.

For counter flow arrangement, the difference between temperatures of hot and cold fluid, and

consequently the heat transfer rate at any location usually maximum at hot fluid inlet end, point 2. The

temperature difference decreases dramatically compared to parallel flow arrangement as we move

towards the hot fluid exit. The mean temperature difference is not simply taken as the difference between

average bulk temperature of hot fluid and cold fluid but being calculated based on the formula given.


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Figure 2: Temperature distribution for counter flow heat exchanger

Figure 3: Temperature distribution for parallel flow heat exchanger

The overall heat transfer coefficient, although very important in heat exchanger analysis, can also

be difficult to obtain experimentally. This coefficient depends primarily on fluid convection and wall

conduction resistances as well as resistances caused by deposits and chemical reactions known as fouling

which take place on the surface of the heat exchanger during normal operation. It may also depend on

whether or not fins are used; as we have seen in an earlier experiment, fins will decrease the overall

resistance by increasing the total area available for heat transfer.


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The equations for calculating the performance characteristics: power emitted, power absorbed, power lost,

efficiency (), logarithmic mean temperature difference (Tm), and overall heat transfer coefficient (U).

The efficiencyfor the cold medium is:

100,,

,,

incinh

incoutc

c

TT

TT

The efficiencyfor the hot medium is:

100,,

,,

incinh

outhinh

h

TT

TT

The mean temperature efficiencyis:

2

hc

mean

The power emittedis given below (where hV is the volumetric flow rate of the hot fluid):

outhinhphhh TTCVEmittedPower ,,

The power absorbedis given below (where cV is the volumetric flow rate of the cold fluid):

incoutcpccc TTCVAbsorbedPower ,,

The power lostis therefore:

AbsorbedPowerEmittedPowerlostPower

The overall efficiency() is:

100EmittedPower

AbsorbedPower


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The logarithmic mean temperature difference(Tm) is:

incouth

outcinh

incouthoutcinh

m

TT

TT

TTTT

T

T

TTT

,,

,,

,,,,

2

1

21

lnln

Theoverall heat transfer coefficient (U) is:-

Where the surface area () for this heat exchanger is 0.067m2


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4.0 OBJECTIVES

1. To demonstrate the effect of flow rate variation on the performance characteristic of heatexchanger.

2. To study the working principle of parallel flow and counter flow heat exchangers.3. To study the effect of fluid temperature on counter flow heat exchanger performance


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5.0EQUIPMENT

Figure 4:Concentric Tube Heat Exchanger Figure 5:Schematic diagram of heat exchanger

Figure 6:Valve and heat insulator Figure 7:Volumetric flow rate meter

Hot temperature

Cold temperature

Valve

Insulation


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Figure 8:Power supply switch Figure 9:Temperature control/thermostat

Hot water inlet

temperature

Hot water outlettemperature

Cold water inlet

temperature

Cold water outlet

temperature

Hot water middle

temperature Cold water middle

temperature


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6.0 PROCEDURES

Counter Flow Heat Exchanger

1. The valve was set up according to the schematic diagram of counter flow heatexchanger.

2. The hot water inlet temperature, Th,inis set to 60oC with the decade switch.3. The cold water volumetric flow rate, Vcis set to run at a constant 2000 cm3/min.4. Then, the hot fluid volumetric flow rate, Vhis set to 1000 cm3/min.5. The temperature readings of hot water inlet, Th,in, hot water middle, Th,mid, hot water

outlet, Th,out, cold water inlet, Tc,in, cold water middle, Tc,midand cold water outlet, Tc,out

are recorded after 5 minutes.

6. Step 4 and 5 are repeated by changing the value to 2000 cm 3/min, 3000 cm3/min and4000 cm

3/min.

Parallel Flow Heat Exchanger

1. Set up the valve according to the schematic diagram of parallel flow heat exchanger2. Repeat the whole experiment from step 2-6.3. All of the data are recorded in proper table.


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7.0 RESULTS

1. Counter Flow Heat Exchanger:

Vh Th,in

(C)

Th,mid

(C)

Th,out

(C)

Tc,in

(C)

Tc,mid

(C)

Tc,out

(C)(cm

3/min) (m3/s)

1000 1.6667 x 10-5 62 51 46 27 30 34

2000 3.3333 x 10-5 60 53 50 27 32 37

3000 5 x 10-5 59 54 51 27 33 38

4000 6.6667 x 10-5 58 53 51 27 33 38

Table 1.1:Temperatures for counter-flow heat exchanger

Vh Power

Emitted

(W)

Power

Absorbed

(W)

Power

Lost

(W)

Efficiency

()

(%)

T1

(C)

T2

(C)

Tm

(C)

U

W/(m2.C)

(cm3/min) (m3/s)

1000 1.6667 x 10-5 1096.2997 971.8214 124.4783 88.65 28 19 23.21 624.94

2000 3.3333 x 10-5

1371.6898 1388.3164 -16.6266 101.21 23 23 0.00 0.00

3000 5 x 10-5

1646.5229 1527.1480 119.3749 92.75 21 24 22.47 1014.54

4000 6.6667 x 10-5

1921.5114 1527.1480 394.3634 79.48 20 24 21.94 1038.93

Table 1.2:for counter-flow heat exchanger


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2. Parallel Flow Heat Exchanger:

Vh Th,in

(C)

Th,mid

(C)

Th,out

(C)

Tc,in

(C)

Tc,mid

(C)

Tc,out

(C)(cm

3/min) (m3/s)

1000 1.6667 x 10-5 60 50 47 28 32 33

2000 3.3333 x 10-5 60 52 50 28 33 36

3000 5 x 10-5 59 53 51 28 34 37

4000 6.6667 x 10-5 58 53 52 28 34 38

Table 2.1:Temperatures for parallel-flow heat exchanger

Vh Power

Emitted

(W)

Power

Absorbed

(W)

Power

Lost

(W)

Efficiency

()

(%)

T1

(C)

T2

(C)

Tm

(C)

U

W/(m2.C)

(cm3/min) (m3/s)

1000 1.6667 x 10-5 891.6251 694.1582 197.4669 77.85 32 14 21.77 475.83

2000 3.3333 x 10-5 1371.6898 1110.6531 261.0367 80.97 32 14 21.77 761.32

3000 5 x 10-5 1646.5229 1249.4847 397.0382 75.89 31 14 21.39 872.04

4000 6.6667 x 10-5 1647.0098 1388.3164 258.6934 84.29 30 14 20.99 987.03

Table 2.2:for parallel-flow heat exchanger


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8.0 SAMPLE CALCULATION

From table A-9 (Properties of saturated water):

AtTc, in=27 C

Vc=2000 cm/min

= 2000 cm/min 1 min/60 s 1 m/100 cm

= 3.3333 x 10-5m

3/s

c=997 + 2(996-997)/(30-25)

=996.6 kg / m

Cpc =4180 + 2(4178 - 4180)/(30 - 25)

=41792 J/kg.K.

AtTh, in= 62 Ch=983.3 + 2(980.4 - 983.3) / (65 - 60)= 982.14 kg / m

Cph= 4185 + 2(4187 - 4185 ) / (65 - 60)

=4185.8 J/kg.K

As = 0.067 m2

a) Power Emitted = VhhCph(Th, in- Th, out)= (1.6667 x 10

-5)(982.14)(4185.8)(62 - 46)

= 1096.2997 W

b) Power Absorbed = VccCpc(Tc, out- Tc, in)= (3.3333 x 10

-5)(996.6)(4179.2)(34 - 27)

= 971.8214 W

c) Power Loss = Power Emitted Power Absorbed= 1096.2997 - 971.8214

= 124.4783 W


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d) Overall Efficiency () = (Power Absorbed / Power Emitted) x 100= (971.8214 / 1096.2997) x 100

= 88.65 %

e) Logarithmic Mean Temperature Difference, Tm = ( )

i) For Counter Flow Heat Exchanger:T1 =Th, inTc, out

= 62 - 34

= 28 C

T2= Th, outTc, in

= 46- 27

= 19C

Tm =

= 23.21C

ii) For Parallel Flow Heat Exchanger:T1 =Th, inTc, in

= 60 - 28

= 32 C

T2= Th, outTc, out

= 47-33

= 14C

Tm =

= 21.77C


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iii) Overall Heat Transfer Coefficient, U = Power Absorbed / (As. Tm)

U =971.8214 / (0.067 x 23.21)

= 624.94W/(m2.C)


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9.0DISCUSSION

[Refer next page]


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10.0CONCLUSION

[Refer next page]


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11.0 REFERENCES

1. MEC 551 Thermal Engineering, McGraw-Hill,2013, ISBN 978-112-130510-62. Thermodynamics an engineering approach,Yunus A.Cengel,Michael A.Boles

,McGraw-Hill,2011,ISBN 978-007-131111-3

3. Kays, William Morrow, Michael E. Crawford, and Bernhard Weigand.Convectiveheat and mass transfer. Vol. 3. New York: McGraw-Hill, 1993.

4. Bejan, A. "Concept of irreversibility in heat exchanger design: Counterflow heatexchangers for gas-to-gas applications."J. Heat Transfer;(United States)99.3

(1977).

5. Mozley, J. M. "Predicting dynamics of concentric pipe heatexchangers."Industrial & Engineering Chemistry48.6 (1956): 1035-1041.


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12.0APPENDICES

[Refer next page]

counter flow and parrallel flow exchanger

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