Counter flow and parrallel flow exchanger

TABLE OF CONTENT 1.0 ABSTRACT..……….……….2 2.0 INTRODUCTION..……….3 3.0 THEORY……..………...…4 4.0 OBJECTIVES..……….…...8 5.0 EQUIPMENT…..……….………9 6.0 PROCEDURES………..………..11 7.0 RESULTS………..………...12 8.0 SAMPLE CALCULATION………..………14 9.0 DISCUSSION………..………..………17 10.0 CONCLUSION………..………18 11.0 REFERENCE ………..………..……19 12.0 APPENDIX………..………..…20

2 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. The variable 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.

3 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 to another. 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.

4 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 heat from 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 hot fluid 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.

5 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.

6 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 efficiency for the cold medium is:

100









T

T

T

T



The efficiency for the hot medium is:

100

_







T

T

T

T



2











The power emitted is given below (where Vh is the volumetric flow rate of the hot fluid):







The power absorbed is given below (where Vc is the volumetric flow rate of the cold fluid):







The power lost is therefore:

Absorbed Power Emitted Power lost Power  

The overall efficiency () is:

100   Emitted Power Absorbed Power 

7 The logarithmic mean temperature difference (Tm) is:



 











The overall heat transfer coefficient (U) is:-

8 4.0 OBJECTIVES

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

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

9 5.0 EQUIPMENT

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

10

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

Hot water inlet temperature

Hot water outlet temperature

Cold water inlet temperature

Cold water outlet temperature

Hot water middle

temperature Cold water middle

11 6.0 PROCEDURES

Counter Flow Heat Exchanger

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

2. The hot water inlet temperature, Th,in is set to 60oC with the decade switch.

3. The cold water volumetric flow rate, Vc is set to run at a constant 2000 cm3/min.

4. Then, the hot fluid volumetric flow rate, Vh is 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,mid and cold water outlet, Tc,out

are recorded after 5 minutes.

6. Step 4 and 5 are repeated by changing the value to 2000 cm3/min, 3000 cm3/min and 4000 cm3/min.

Parallel Flow Heat Exchanger

1. Set up the valve according to the schematic diagram of parallel flow heat exchanger 2. Repeat the whole experiment from step 2-6.

12 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) (cm3/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 (ƞ) (%) ΔT1 (°C) ΔT2 (°C) ΔTm (°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}

13

2. Parallel – Flow Heat Exchanger:

Vh Th,in (°C) Th,mid (°C) Th,out (°C) Tc,in (°C) Tc,mid (°C) Tc,out (°C) (cm3/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 (ƞ) (%) ΔT1 (°C) ΔT2 (°C) ΔTm (°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

14

8.0 SAMPLE CALCULATION

From table A-9 (Properties of saturated water): AtTc, in=27 ºC Vc=2000 cm³/min = 2000 cm³/min × 1 min/60 s × 1 m³/100³ cm = 3.3333 x 10-5 m3/s ρc=997 + 2(996-997)/(30-25) = 996.6 kg / m³ Cpc =4180 + 2(4178 - 4180)/(30 - 25) = 41792 J/kg.K. AtTh, in= 62 ºC ρh=983.3 + 2(980.4 - 983.3) / (65 - 60) = 982.14 kg / m³ Cph= 4185 + 2(4187 - 4185 ) / (65 - 60) = 4185.8 J/kg.K As = 0.067 m2

a) Power Emitted = Vh ρh Cph (Th, in - Th, out)

= (1.6667 x 10-5)(982.14)(4185.8)(62 - 46)

= 1096.2997 W

b) Power Absorbed = Vc ρc Cpc (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

15 d) Overall Efficiency (ƞ) = (Power Absorbed / Power Emitted) x 100

= (971.8214 / 1096.2997) x 100 = 88.65 %

e) Logarithmic Mean Temperature Difference, ΔTm =

( – ) ( )

i) For Counter – Flow Heat Exchanger:

ΔT1 =Th, in – Tc, out = 62 - 34 = 28 °C ΔT2= Th, out – Tc, in = 46- 27 = 19°C ΔTm = = 23.21°C

ii) For Parallel – Flow Heat Exchanger:

ΔT1 =Th, in – Tc, in = 60 - 28 = 32 °C ΔT2= Th, out – Tc, out = 47-33 = 14°C ΔTm = = 21.77°C

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

U =971.8214 / (0.067 x 23.21) = 624.94W/(m2.°C)

17 9.0 DISCUSSION

18 10.0 CONCLUSION

19 11.0 REFERENCES

1. MEC 551 Thermal Engineering, McGraw-Hill,2013, ISBN 978-112-130510-6 2. 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.Convective

heat and mass transfer. Vol. 3. New York: McGraw-Hill, 1993.

4. Bejan, A. "Concept of irreversibility in heat exchanger design: Counterflow heat exchangers for gas-to-gas applications." J. Heat Transfer;(United States) 99.3 (1977).

5. Mozley, J. M. "Predicting dynamics of concentric pipe heat

20 12.0 APPENDICES