How Do You Calculate External Heating and Cooling in Pinch Technology?

[cabellos2]

Im just looking through a past exam paper and I am having trouble understanding the method to solve this problem>


Q. In a process there are 4 fluid streams with properties shown

STREAM 1 - Type = Hot, Thermal Capacity = 0.2kW/K, Initial Temp = 350, Final Temp = 50
STREAM 2 - Hot, 0.4, 250, 50
STREAM 3 - Cold, 0.1, 20, 200
STREAM 4 - Cold, 0.3, 100, 300

Calculate the rate of external heating and cooling required if a heat recovery system is used with minimum allowable temperature between streams being 25K

Calculate the Pinch temperatures of the hot and cold streams.



How do I go about this? Id be very grateful if someone could run through the stages of this question. Thanks

 

[Valkarie]

.The first step is to calculate the amount of heat exchanged between the four streams. This can be done by multiplying the thermal capacity (kW/K) of each stream by the temperature difference between the initial and final temperatures. The total amount of heat exchanged will then be the sum of the heat exchanged in each stream. For example, for Stream 1: 0.2 x (350 - 50) = 60 kW After calculating the total amount of heat exchanged between all four streams, the next step is to calculate the Pinch temperatures of the hot and cold streams. This can be done by dividing the total amount of heat exchanged by the thermal capacity of the hot and cold streams respectively. For example, if the total amount of heat exchanged is 200 kW, and the thermal capacity of the hot stream is 0.4 kW/K, then the Pinch temperature of the hot stream would be 500 K (200/0.4). Once you have calculated the Pinch temperatures of the hot and cold streams, you can then calculate the rate of external heating and cooling required if a heat recovery system is used with a minimum allowable temperature between streams being 25K. This can be done by subtracting the Pinch temperature of the cold stream from the Pinch temperature of the hot stream, and then subtracting the minimum allowable temperature (25K). This will give you the rate of external heating and cooling required. For example, if the Pinch temperature of the hot stream is 500 K and the Pinch temperature of the cold stream is 200 K, then the rate of external heating and cooling required would be 275 K (500-200-25).

 

[Mene]

I would approach this problem by using the principles of Pinch Technology, which is a method used to optimize the energy usage in a process. The first step would be to plot the temperature vs. heat flow diagram for the four streams given in the problem. This will help visualize the heat transfer between the streams and identify the pinch point, which is the minimum temperature difference between the hot and cold streams.

Next, I would calculate the minimum temperature difference between the hot and cold streams, which is given as 25K in the problem. This will help determine the pinch temperature for both the hot and cold streams.

To calculate the rate of external heating and cooling required, I would use the principles of heat transfer and energy balance equations. This will involve determining the heat transfer rate between the hot and cold streams at the pinch point, and then adjusting the heat transfer rates for the streams outside of the pinch point to meet the minimum temperature difference requirement.

Finally, I would use the calculated heat transfer rates to determine the rate of external heating and cooling required for the process.

Overall, solving this problem requires a good understanding of heat transfer principles and the application of Pinch Technology methodology. I would recommend reviewing these concepts and practicing similar problems to gain a better understanding of the process.