Question

Hot water enters a counter flow heat exchanger at 950C.This hot water is used to heat a cool stream of water from 8 to 400C.The flow rate of the cool water is 1.2 kg/s, and that of the hot water is 2.7 kg/s. The overall heat transfer coefficient is 850 W/m² 0C. What is the area of the heat exchanger and its effectiveness?

Answer 1

The area of the heat exchanger is approximately 22.56 m², and its effectiveness is approximately 0.87.

To calculate the area of the heat exchanger and its effectiveness, one can use the effectiveness-NTU (Number of Transfer Units) method. The equation for the heat exchanger effectiveness (ε) is given by:

ε= ​1−e ^−NTU⋅(1+Cr)/1+Cr

Once NTU and Cr are determined, they can be substituted into the first equation to find ε. Then, the heat exchanger area (A) can be calculated using the rearranged form of the NTU equation:

A= NTU⋅C min/U

Given the values for the temperatures, flow rates, and overall heat transfer coefficient, the area of the heat exchanger is approximately 22.56 m², and its effectiveness is approximately 0.87. This indicates the efficiency of the heat exchange process in transferring thermal energy between the hot and cold streams in the counterflow heat exchanger.