Heat Conduction Lab. The objective of this lab was to learn and understand the principles of heat transfer in a shell and tube heat exchanger. Necessary materials were provided in the lab to investigate the parallel and counter-current flow heat exchange

In order to obtain a suitable relationship between the mean coefficient of heat transition and the mass flow rates of the hot cold mediums, it can be theorized that if the average mass flow rate of the two mediums were to be calculated, an individual could obtain a linear relationship; essentially removing the experimental error of fluctuating mass flow rates for both mediums. In Figure #3 & #7, the plot of mean mass flow rate of the hot and cold mediums versus the mean coefficient of heat transition is shown, and a linear relationship can be observed, proving the linearly proportional relationship between mass flow rate and heat transitions.

It can be observed that the temperature profile of the Parallel Flow heat exchanger follows the theoretical temperature profile as shown in Figure #3 in the lab manual [3]. An individual can extrapolate, from the temperature profile (refer to Figure #4 – Appendix A), the idea that as the hot water travelled the length of the heat exchanger, its temperature declined. Conversely it can also be deduced that the cold water temperature increased as it passed through the heat exchanger. The generic idea behind this is that as the flows of the hot and cold water progressed through the length of the heat exchanger, the temperature differential decreased, indicating a linear heat exchange increase between the two flows.

In Figure #8 – Appendix A, which corresponds to Counter Flow in the heat exchanger, it can be discerned that the temperature of the hot water flow decreased and the temperature of the cold water flow increased as each of them advanced through the length of the heat exchanger; although the temperature differential was uniform over the length of the heat exchanger. This helps in minimizing the thermal stresses along the length of the heat exchanger. [1]

The mean coefficients of heat transition for all five runs in Parallel Flow were noted to be between a maximum value of 1484.47 W/K*m2 and a minimum value of 976.45 W/K*m2 (refer to Table #14); these values fall between the theoretical range for overall heat transfer coefficient for water-to-water heat exchangers of 850 - 1700 W/K*m2 [2]. The mean coefficients of heat transition for counter Flow (Range: 961.89 – 1473.46 W/K*m2, refer to Table #29) also lie in the theoretical range of 850 - 1700 W/K*m2. This revelation is very beneficial when discerning whether the experiment was successful or not.

From the efficiency calculations for Parallel Flow in the heat exchanger (refer to Table #15), it can be observed that the heating efficiency is 91% and the cooling efficiency is 109.8%. Almost similarly the efficiency calculations for Counter Flow in the heat exchanger (refer to Table #30) show that the heating efficiency is 88.7% and the cooling efficiency is 112.6%. These are very close to ideal but several different types of errors (discussed below) prevent the efficiency from reaching its maximum value.

Air bubbles in the heat exchanger prior to the start of the experiment revealed physical imperfections in the experimental apparatus, since air bubbles are a direct cause of a leak (leak of air in to the system and heat out from the system) in the proposed closed system of experimental setup, it can be discerned that there may have been air-to-water heat exchange which could justify the un-idealistic efficiency values. Air bubbles also cause the flow to develop into a turbulent flow and thus increase the heat transfer rate; this causes uncertainty towards the integrity of results, as the higher than ‘’true value’’ of the heat transfer coefficients may have been calculated.

Heat loss to the environment is also a concern since the ambient temperatures can provide a heat sink for the heat gained by the cool water flow at the start of its progression through the pipe until the end of the length of the heat exchanger. This can result in a lower temperature differential and indicate a lower than ‘’true value’’ heat transfer coefficient.

The deposition of particulates over time in a heat exchanger can increase and hamper the performance of the heat exchanger. The deposits add another coating on top of the surface of the pipes that prove to be a barrier for heat exchange and cause an increase in the temperature differential and in turn a decrease in the heat transfer rate.

Conclusion

It was identified that the mean coefficient of heat transition was linearly proportional to the mean mass flow rate of the hot and cold flows. It was also detected that the temperature differential decreased as each of the hot and cold flow progressed through the length of the heat exchanger in both Parallel Flow & Counter flow, hence the heat transfer increased; therefore the temperature differential can be said to have an inversely proportional to the heat transfer.

The range for the mean coefficients of heat transition for Parallel Flow & Counter Flow were observed to fall between the theoretical range for the overall heat transfer rates for water-to-water heat exchangers.

The heating & cooling efficiency of the Parallel & Counter flow were observed to be very sufficient but physical imperfections in the experimental apparatus, heat loss to the environment and deposition of particulates over time, may have limited the efficiency from reaching its optimum value.

It can be said that a firm understanding of the heat transfer in the shell and tube heat exchangers was gained.

References

[1] Breese, J. (2005, January 01). Predicting the Performance of Heat Exchangers. Retrieved November 18, 2011, from HPAC Engineering: http://hpac.com/heating/predicting_performance_heat/

[2] Cengel, Y. A., & Ghajar, A. J. (2011). Heat and Mass Transfer: Fundamentals & Applications (4th ed.). The McGraw-Hill Companies.

[3] Equipment for Engineering Education, Instruction & Operation. (1998). Heat Transfer Lab Manual. Hamburg, Germany.

Appendix A – Tables & Figures

Parallel Flow:

Table 1: Observed Data for Parallel Flow

Table 2: Inlet Temperature and Interpolated Data for the Hot Water Leg

Table 3: Outlet Temperature and Interpolated Data for the Hot Water Leg

Table 4: Inlet Temperature and Interpolated Data for the Cold Water Leg

Table 5: Outlet Temperature and Interpolated Data for the Cold Water Leg

Table 6: Calculated Data for Hot Water Inlet

Table 7: Calculated Data for Hot Water Outlet

Table 8: Calculated Data for Cold Water Inlet

Table 9: Calculated Data for Cold Water Outlet

Table 10: Exchanged Heat Fluxes & Mean Heat Transfer Rate

Table 11: Change in Inlet & Outlet Temperatures and Log Mean Temperature

Table 12: Mean Coefficient of Heat Transition

Table 13: Mean Mass Flow Rate at Inlet & Outlet of Hot Water and Cold Water

Figure 1: Mean Mass Flow Rate vs. Mean Coefficient of Heat Transition for the Hot Water Leg in Parallel Flow

Figure 2: Mean Mass Flow Rate vs. Mean Coefficient of Heat Transition for the Cold Water Leg in Parallel Flow

Table 14: Mean Mass Flow Rate in Parallel Flow

Figure 3: Mean Mass Flow Rate vs. Mean Coefficient of Heat Transition in Parallel Flow Heat Exchanger

Table 15: Efficiency Calculations of Heating & Cooling

Figure 4: Temperature Profile of Cold & Hot Water for Parallel Flow Heat Exchanger

Counter Flow:

Table 16: Observed Data for Counter Flow

Table 17: Inlet Temperature and Interpolated Data for the Hot Water Leg

Table 18: Outlet Temperature and Interpolated Data for the Hot Water Leg

Table 19: Inlet Temperature and Interpolated Data for the Cold Water Leg

Table 20: Outlet Temperature and Interpolated Data for the Cold Water Leg

Table 21: Calculated Data for Hot Water Inlet

Table 22: Calculated Data for Hot Water Outlet

Table 23: Calculated Data for Cold Water Inlet

Table 24: Calculated Data for Cold Water Outlet

Table 25: Exchanged Heat Fluxes & Mean Heat Transfer Rate

Table 26: Change in Inlet & Outlet Temperatures and Log Mean Temperature

Table 27: Mean Coefficient of Heat Transition

Table 28: Mean Mass Flow Rate at Inlet & Outlet of Hot Water and Cold Water

Figure 5: Mean Mass Flow Rate vs. Mean Coefficient of Heat Transition for the Hot Water Leg in Counter Flow

Figure 6: Mean Mass Flow Rate vs. Mean Coefficient of Heat Transition for the Cold Water Leg in Counter Flow

Table 29: Mean Mass Flow Rate in Counter Flow

Figure 7: Mean Mass Flow Rate vs. Mean Coefficient of Heat Transition in Counter Flow Heat Exchanger

Table 30: Efficiency Calculations of Heating & Cooling

Figure 8: Temperature Profile of Cold & Hot Water for Counter Flow Heat Exchanger

Appendix B – Sample Calculations

Sample Calculations for the Mean Coefficient of heat transition (km):

Hot Water Leg:

Cold Water Leg:

                                                                   

Appendix C – Formulas Used

The equation used to interpolate density (kg/m3) and specific heat (J/kg*K) at a certain temperature from Table #1 in the lab manual is stated below [2]:

                                                 (1)

The equation used to calculate the mass flow rate (kg/s) from volumetric flow rate (L/min) is stated below:

                                                                                        (2)

The equation used to calculate the heat transfer rate (W) is stated below:

                                                                                (3)

The equation used to calculate the heat flux exchanged is stated below:

                                                                        (4)

The equation used to solve for the mean heat transfer rate (W) is stated below:

                                                                        (5)

The equation used to calculate the change in temperature (°C) is stated below:
                                                                                (6)

The equation used to solve for the log mean temperature (K) is stated below:

                                                                        (7)

The equation to calculate the mean coefficient of heat transition (W) is given below:

                                                                                (8)

The efficiency equations for the heating and cooling of the fluid are stated below:

                                                                  (9 & 10)

Appendix D – Miscellaneous

Table 31: Material Values for Water at 1.013 bar