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Introduction:

The main idea of the experiment is to explore and distinguish the difference between a parallel flow and counter flow in a pipe heat exchanger. The pipes are made of copper and the system used to conduct the experiment could be interchangeable between parallel and counter flow. The experiment was conducted with the help of the Hampden 6-pass heat exchanger demonstrator and the manual as well. For each parallel pipe heat exchanger, the total heat transfer (hot and cold), logarithmic mean temperature difference, overall heat transfer coefficient, number of transfer units, and heat exchanger effectiveness was found. After that the surface temperature of the pipe was plotted against the distance of the pipe. Finally, the comparison of the two systems was done. The experiment exposed the students to the practical aspect of what was learned in the heat transfer class.

Experimental Methods:

To conduct the experiment, the following steps was done in the laboratory:

For parallel flow:

· Starting with all the valves closed to insure the best results.

· Open the following valves for the cold water and hot water for parallel flow:

Cold water valves: CV-1, CV-5, CV-8, CV-12, CV-14

Hot water valves: HV-1, HV-2, HV-7, HV-8, HV-13, HV-14

· Turn the three-way valve to parallel position for parallel flow.

· Use NV-1 and NV-2 to set the flow rate to the desired value (2 gal/min).

· Using MX-1 to set the hot water side temperature to limit the max hot water.

· Leave the system running for 10 min to allow stabilize.

· After that take the reading from the dial by changing to each section of the pipe.

· Use table 3-1 in the manual for parallel flow

· Using TC-26 or TC-27 panel jack, use a probe could to measure the surface temperature.

· The thermocouple is very sensitive and great care should be taken when using it.

· Record the readings of the distance associated using table 3-2 for parallel flow

For counter flow:

· Starting with all the valves closed to insure the best results.

· Open the following valves for the cold water and hot water for counter flow:

Cold water valves: CV-1, CV-5, CV-6, CV-12, CV-14

Hot water valves: HV-1, HV-2, HV-7, HV-8, HV-13, HV-14

· Turn the three-way valve to parallel position for counter flow.

· Use NV-1 and NV-2 to set the flow rate to the desired value (2 gal/min).

· Using MX-1 to set the hot water side temperature to limit the max hot water.

· Leave the system running for 10 min to allow stabilize.

· After that take the reading from the dial by changing to each section of the pipe.

· Use table 4-1 in the manual for parallel flow

· Using TC-26 or TC-27 panel jack, use a probe could to measure the surface temperature.

· The thermocouple is very sensitive and great care should be taken when using it.

· Record the readings of the distance associated using table 4-2 for counter flow

Theory:

In this double pipe heat exchanger experiment, the parallel flow as well as the counter flow was conducted. The difference in the flow direction could result in efficiency variance.

To find the overall heat transfer q the equation could be found from the following:

Ta and Tb are the respective fluid temperatures; however, the equation could be found using:

Where U is the overall coefficient of heat transfer, A is the surface area of heat transfer and delta T is the log mean temperature difference. The coefficient could be found using the average heat transfer divided by the area multiplied by the log mean temp.

In other world the heat transfer could be found for the cold and hot pipe (inner and outer). The equation could be written as:

Ideally the cold and hot pipe should have the same value where in the experiment they are not equal to each other due to some energy loss in the system.

The logarithmic mean temperature difference could be found from the equation:

Where T1 and T2 is different for parallel flow and counter flow.

Figure (1): A schematic of parallel and counter flow

For the parallel flow:

For the counter flow:

To find the effectiveness of the double pipe heat exchangerthere are two equations one associated with the parallel and the other with the counter.

Finally, the number of transfer units could be found using the equation below:

When for the hot and cold compared which ever value is smaller will be the

Results and Discussions:

From the data obtained form the lab, total heat transfer (hot and cold), logarithmic mean temperature difference, overall heat transfer coefficient, number of transfer units, and heat exchanger effectiveness was found.

Overall heat transfer:

Table (1): The main results for the overall heat transfer

Ideally the how and cold heat transfer should be the same, but since there is minimal rate of energy loss in the system a small difference in values can be seen.

Logarithmic mean temperature difference:

Table (2): The main results for the Logarithmic mean temperature difference

Overall heat transfer coefficient:

Table (3): The main results for Overall heat transfer coefficient

The overall heat transfer coefficient could be found from the heat transfer equation. Since the Area of heat transfer is given and log mean temp diff. is known, the overall coefficient can be known.

Number of transfer units:

Table (4): The main results for Number of transfer units

The used in both the parallel flow and the counter flow was the hot . Since when compared with the cold is had a smaller value.

Heat exchanger effectiveness:

Table (5): The main results for Heat exchanger effectiveness

Assuming the mass flow rate is constant and the specific heat at constant pressure is constant as well, when compared, parallel flow was less effective that counter flow.

Plot of parallel and counter flow surface temperature vs pipe distance:

Plot (1): The Surface Temperature (C ) VS the distance of the pipe (in) for parallel flow

Plot (2): The Surface Temperature (C ) VS the distance of the pipe (in) for counter flow

Comparing the two plots it is noticeable that the parallel flow surface temperature is gradually declining and it forms a unified linear line. On the other hand, the counter flow had a surface temperature that is fluctuating ranging from 44 C to 38 C.

Conclusion and Recommendation:

All in all, the results obtained was close to the expectations. However minimal variance in numbers occurred like the heat transfer hot and cold values. When calculated the parallel flow had an efficiency of 18.9% in comparison with the counter flow which had 19.5%. The logarithmic mean temperature difference was about the same for the parallel flow and the counter flow which equaled to 40.The plot of the surface temperature of the outer pipe (hot) against the distance of the pipe was drawn. The results showed that the parallel flow was a consistent decrease with distance, where the counter flow had a fluctuating surface temperature. There might be a small percentage of error due to the equipment used like the thermocouple.

Reference:

1. Bergman, T. L. Lavine, A. S. Incropera, Dewitt, D. P., 2011, Introduction to Heat Transfer,

6th Edition, John Wiley & Sons, New York.

Attachment:

Excel sheet for the parallel flow:

Excel sheet for the parallel flow:

Surface T VS Distance

DISTANCE (IN)

Temperature(C)

Surface T VS Distance

Diatance (IN)

Tenperature (C)

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