School of Mechanical and Design Engineering

Dublin Institute of Technology

Bachelor of Engineering Technology in
Mechanical Engineering

Laboratory 2 Plate Heat Exchanger Assignment
Robert O’Donovan Student Number: C12756051 Due Date: 24/10/2014

Lecturer: Jim Ffrench

Dublin Institute of technology Bolton Street, Dublin 1.

I. Abstract Heat exchangers are a piece of process equipment used for heat transfer between two media. The media do not come into direct contact and there is no mixing. Heat is transported from the hot medium to the cold medium by way of a heat conducting partition. In this experiment, we analysed the working principle of parallel and counter flow. We observed different fluid temperatures, fluid flow rates and how this affected the heat exchangers performance. Calculations were needed to determine the variation of the two configurations. There are some possible percentage errors that need to be considered in the experiment, these include the tube changeover from parallel to counter flow, the fluid loss will have an effect on the readings. Also if the unit is not allowed enough time to stabilise when changing the flow rates, the readings will not be accurate.

1"Vh"(L/min) Parallell"flow Counter"flow

Vs U"= U"=

3"Vc"(L/min) 2.9"W/m 2 K 3.5"W/m 2 K

The percentage difference between the U values is 20.6 %.

II. Table of Contents

1. INTRODUCTION ........................................................................................................... 1 2. THEORETICAL BACKGROUND ....................................................................................... 2 2.1. FORMULAE NEEDED .......................................................................................................... 2 3. EQUIPMENT AND PROCEDURE .................................................................................... 3 3.1. EXPERIMENT EQUIPMENT ................................................................................................... 3 3.2. EXPERIMENT PROCEDURE ................................................................................................... 3 4. RESULTS ...................................................................................................................... 4 5. SAMPLE CALCULATIONS .............................................................................................. 5 5.1. HEAT ENERGY TRANSFER .................................................................................................... 5 5.2. AVERAGE HEAT TRANSFER .................................................................................................. 5 5.3. LMTD ........................................................................................................................... 6 5.4. U (OVERALL TRANSFER COEFFICIENT) ................................................................................... 6 5.5. PERCENTAGE ERROR ......................................................................................................... 6 6. DISCUSSION OF RESULTS ............................................................................................. 7 6.1. EXPERIMENTAL RESULTS .................................................................................................... 7 6.2. COMPARISON WITH THEORETICAL VALUES ............................................................................. 7 6.3. POSSIBLE SOURCE OF ERROR ............................................................................................... 7 6.4. ACCURACY IMPROVEMENTS ................................................................................................ 8 7. CONCLUSIONS AND RECOMMENDATIONS .................................................................. 8 7.1. CONCLUSION ................................................................................................................... 8 7.2. RECOMMENDATIONS ......................................................................................................... 8 8. REFERENCES ................................................................................................................ 9 9. NOMENCLATURE ....................................................................................................... 10

III. List of Figures FIGURE 1 HEAT EXCHANGER SERVICE UNIT ........................................................................................ 3 FIGURE 2 PARALLEL FLOW GRAPH ................................................................................................... 4 FIGURE 3 COUNTER FLOW GRAPH ................................................................................................... 4 FIGURE 4 AVERAGE HEAT COEFFICIENT GRAPH ................................................................................... 5

IV. List of tables TABLE 1 PARALLEL FLOW DATA ....................................................................................................... 4 TABLE 2 COUNTER FLOW DATA ....................................................................................................... 4 TABLE 3 AVERAGE HEAT FLOW RATES ............................................................................................... 7 TABLE 4 AVERAGE U VALUES .......................................................................................................... 7

1. Introduction
The purpose of this report is to demonstrate the difference in performance between a parallel flow and counter flow configured heat exchanger. An experiment was carried out to study how the effect of fluid temperature and flow rate effected the heat exchangers performance. The logarithmic mean was calculated for both parallel and counter flow configurations as temperatures along the partition are not constant. In parallel flow, the hot and cold fluids flow in the same direction and therefore enter and exit the heat exchanger at the same end. In counter flow, the two fluids flow in oppisite directions therefore the fluids enter and exit from oppisite ends. Heat exchangers are a piece of process equipment used for heat transfer between two media. The media do not come into direct contact and there is no mixing. Heat is transported from the hot medium to the cold medium by way of a heat conducting partition. In this experiment, we analysed the working principle of parallel and counter flow. We observed different fluid temperatures, fluid flow rates and how this affected the heat exchangers performance.

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2. Theoretical Background Heat exchangers are a piece of process equipment used for heat transfer between two media. The media do not come into direct contact and there is no mixing. Heat is transported from the hot medium to the cold medium by way of a heat conducting partition. In this experiment, we analysed the working principle of parallel and counter flow. We observed different fluid temperatures with various fluid flow rates and how this affected the heat exchangers performance. As the fluid flows along the partition, the hot medium emits heat to the partition and cools down in doing so. In turn, the heated partition passes heat to the cold medium flowing along the other side of the partition. This medium is then heated. The heat transfer process at the partition can therefore be described in terms of three separate stages.

2.1. Formulae Needed Heat transfer (Hot & Cold) ������ = ������������! (������! − ������! ) Average heat transfer (Parallel & Counter flow) ������!"# = Log mean temperature difference (LMTD) ������������������������ = ∆������! − ∆������! Δ������ ������������ Δ������! ! ������! + ������! 2

Overall transfer coefficient (U) ������ = ������������∆������!"

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3. Equipment and Procedure 3.1. Experiment Equipment The heat exchanger service unit [1] has a temperature controller labelled 1. The various temperature displays are labelled 2. The various flow rate displays are labelled 3. The vessel with stirrer and coil is labelled 3. The cold-water circuit connections are labelled 5. The process schematic is labelled 6, and the hot water tank is labelled 7.

Figure 1 Heat exchanger service unit

3.2. Experiment Procedure 1. Set up the equipment and configure the experiment for parallel flow operation. 2. Collect the inlet and outlet temperatures for the listed flow rates and make a record of the data. 3. Repeat steps 1 and 2, this time configuring the equipment to counter flow. 4. Plot the results on a graph, to illustrate the typical temperature distribution for both configurations. 5. Calculate the heat transfer the 4 different flow rate configurations for both the bot and cold side. 6. Make a comparison for both configurations. 7. Calculate the average heat transfer for both configurations. 8. Calculate the LMTD for both configurations. 9. Calculate the overall transfer coefficient (U) at the various flow rates for each configuration. 10. Present the results for U on a graph 11. Discuss the results obtained and explain, with reference to the relevant theory, why there is a variation in the u values obtained.

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4. Results Vh#(L/min) 1 1 2 2 Vc#(L/min) 3 2 3 2 Thi#(1) 59.1 58.5 59.2 58.8 Tho#(3) 32.2 34.5 38.2 41.2 Parallel&Flow Tci#(4) Tco#(6) 19.8 27.5 19.5 29.8 19 31 18.9 34.4 Qh(W) 1912.9 1706.7 1493.3 1251.6 Qc(W) A547.6 A732.4 A853.3 A1102.2 Qavg(W) 682.7 487.1 320.0 74.7 LMTD(K) U(W/m 2K) 15.2 2.9 16.2 2.4 16.1 4.3 16.0 3.5

Table 1 Parallel flow data

Parallel Flow 80 Temperatue (°C) 60 40 20 0 0 0.5 1 1.5 2 2.5 3 3.5 V(L/min) Figure 2 Parallel flow graph

1,3 hot 2,3 hot 2,2 hot 1,3 cold 1,2 cold 2,3 cold 2,2 cold 1,2 hot

Vh#(L/min) 1 1 2 2

Vc#(L/min) 3 2 3 2

Thi#(1) 59.2 59 58.4 58.7

Tho#(3) 38.5 36.2 31.5 29.1

Counter(Flow Tci#(6) Tco#(4) 35.7 18.7 32.3 18.5 29.7 18.4 27.6 18.3

Qh(W) 1478.9 1629.0 1921.9 2114.8

Qc(W) 1208.9 981.3 803.6 661.3

Qavg(W) 1343.9 1305.1 1362.7 1388.1

LMTD(K) U(W/m 2K) 18.7 3.5 17.9 4.3 18.1 2.4 17.5 2.9

Table 2 Counter flow data

Counter Flow 70 60 Temperature (0C) 50 40 30 20 10 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Figure 3 Counter flow graph

2,2 Hot 2,3 Hot 1,2 Hot 1,3 Hot 1,3 Cold 1,2 Cold

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Average Heat Transfer Coefficient (U-­‐Value) W/m2K 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0.0 0.5 1.0 1.5 V (L/min) Figure 4 Average heat coefficient graph

Vc=3 L/min (Parallel) Vc=2 L/min (Parallel) Vc=3 L/min (Counter) Vc=2 L/min (Counter) 2.0 2.5

5. Sample Calculations 5.1. Heat energy transfer ������ = ������������! (������! − ������! ) ������ = . 017 4.183 ∗ 10^3 59.1 − 32.2 = 1913 W Where: Q = Heat transfer rate ṁ = Mass flow rate Cp = Specific heat energy of water (������! − ������! ) = Temperature difference

W/m2K

5.2. Average heat transfer

������������������������ =

������������ + ������������ ������

Qavg = (1912.9 – 547.6) / 2 = 682.7 W Where: Q = Heat transfer rate Qh = Avg hot flow rate Qc = Avg cold flow rate

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5.3. LMTD ������������������������ =

∆������! − ∆������! Δ������ ������������ Δ������! !

������������������������ =

( 59.1 − 32.2 − 27.5 − 19.8 ) 59.1 − 33.2 ������������ 27.5 − 19.8 ������������������������ = 15.2 ������

5.4. U (Overall transfer Coefficient)

������������������������ = ������������∆������������������ 682.7 = U(.04)(15.2) = 1122.9 (W/m2K)

Where: Qavg = Average heat flow rate U = Average heat transfer coefficient A = Area within heat exchanger ΔT = Log mean temperature difference

5.5. Percentage Error

% ������������������������������ =

������������������������������������������ − ������������������������������������������������ ×100 ������������������������������������������������ 2.9 − 2.4 ×100 2.4

% ������������������������������ =

% ������������������������������ = ������. ������ %

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6. Discussion of Results 6.1. Experimental Results

From the data in Table 1 and Table 2, the general characteristics of parallel flow and counter flow can be observed. With the Parallel flow configuration, the exit temperature of the hot fluid must have a higher value than the exit temperature of the cold fluid. This is supported by the data taken. With the counter flow configuration, the exit temperature of the hot fluid must be higher than the exit temperature of the cold fluid, and this is also supported by the data. From the calculations, for overall effectiveness, it indicates that the counter flow configuration is more effective than the parallel flow configuration. This can be seen in Table 3 and Table 4. The percentage difference for the U values is 20.6 %.

1"Vh"(L/min) Parallell"flow Counter"flow
1"Vh"(L/min) Parallell"flow Counter"flow

Vs Q= Q=
Vs U"= U"=

3"Vc"(L/min) 682.7"W 1343.9"W
3"Vc"(L/min) 2.9"W/m 2 K 3.5"W/m 2 K

Table 3 Average heat flow rates

Table 4 Average U values

6.2. Comparison with Theoretical Values In theory Q hot = Q cold. According to the analysis and calculations, there is a larger than expected discrepancy between them. Factors such as thermal conductivity and thermal resistance should have been taken into consideration when calculating the values. These would have contributed to the discrepancy between the calculated data and the theoretical values.

6.3. Possible Source of Error Sources of error in this experiment include measurement error, heat transfer to the surrounding air, cooling due to evaporation and fluid loss when changing from parallel flow to counter flow configuration. Measurement error is likely to be the most significant as the unit needs time to stabilise to give accurate readings.

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6.4. Accuracy Improvements Accuracy could be improved by insulating the unit; this would reduce error due to heat transfer and evaporation. Conducting the experiment over a longer period could reduce the measurement error; this will give the unit time to stabilise and be more precise. Better instrumentation would also improve the accuracy.

7. Conclusions and Recommendations

7.1. Conclusion After calculating the heat transfer rate at the four different flow rate combinations for each of the configurations. The data shows that the counter flow configuration is more efficient in transferring heat between two mediums. The graphs show how the mean coefficient of heat transition increases with increasing flow rate. This is due to the fact that higher flow rates result in greater heat transfer. The experimental data shows that with counter current the outlet temperature of the heated medium is higher than the outlet temperature of the cooled medium. With the parallel flow configuration the outlet temperature is always lower than the inlet temperature. In theory Q hot = Q cold according to the analysis and calculations, there is a larger than expected discrepancy between them. Factors such as thermal conductivity and thermal resistance should have been taken into consideration when calculating the values. These would have contributed to the discrepancy between the calculated data and the theoretical values. Sources of error in this experiment include measurement error, heat transfer to the surrounding air, cooling due to evaporation and fluid loss when changing from parallel flow to counter flow configuration. Measurement error is likely to be the most significant as the unit needs time to stabilise to give accurate readings.

7.2. Recommendations Recommendations for improving the accuracy of the experiment include; 1. Improving the units insulation would give a higher efficiency 2. Better instrumentation would improve accuracy. 3. Let the cold tap run before carrying out the experiment to allow the temperature to become constant.

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8. References

[1] Lentfer, H. (2014) Wl 110 Heat Exchanger Supply Unit. Available at: http://www.gunt2e.de/s5218_1.php [Accessed 18 October 2014].

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9. Nomenclature

∆T = change in temperature (oK) LMTD = Log mean temperature difference (oK) ln = natural log ṁ = mass flow rate (kg/s) % = Percentage Q = heat transfer (W) Qavg = average heat transfer (W) T = temperature (oK)

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