Heat exchanger calculation issues
Thread Content
The company requires an assessment and calculation of the heat exchanger. Is this task about calculating the heat transfer coefficient of a heat exchanger? This raises a question: the heat flux of the heat exchanger is the same as the cooling requirement. For a constant heat load, if, during the selection of the heat exchanger, a larger one is chosen, then according to the formula Q=KS*temperature difference, doesn’t the heat transfer coefficient decrease as a result? Is this heat transfer coefficient calculated using the film coefficients inside and outside the tube, or is it determined by applying the formula Q=KS*temperature difference?
OP, what you’re doing here is conducting a calculation related to the manufacturing process; generally, this is done after the equipment has been designed, by using the actual data obtained from the equipment to determine whether it meets the requirements of the process.
For a heat exchanger in operation, it is sufficient to use 2 measured flow rates and 4 measured temperatures for calculation, and this approach yields accurate results.
The inlet and outlet temperatures of the cold and hot streams are measured; Q is calculated based on the flow rate of the cold or hot stream. Then, using the formula Q = KS * logarithmic mean temperature difference * temperature difference correction factor, the overall heat transfer coefficient K is determined
In the calculation sheet, K is calculated using the film coefficient of the selected heat exchanger based on the flow rates on the cold and hot sides, while S is the area of the selected heat exchanger, with a certain margin included. The temperature difference is constant; therefore, the value of Q calculated in this way will be greater than the actual flow rate required. If calculated in reverse, using heat, area, and temperature difference, the actual value of K that is obtained will be smaller than the K value given in the calculation book. Can the K value be compared in this way to reflect the heat transfer situation? Is it possible to conduct such calculations? If that’s not the case, then what aspects need to be compared with the theoretical values when calculating the heat exchanger’s performance?
This post was last edited by wiseboy on 2015-7-21 at 13:06; the actual K value corresponds to the actual flow rate and temperature； The theoretical K corresponds to the designed flow rate and temperature. This cannot be compared. Besides, comparing K values is meaningless. The general comparison items are: 1. Have the actual flow rate and temperature reached the designed values? ； 2. Whether the actual heat output meets the designed value ； 3. Sometimes, it is necessary to calculate the heat transfer efficiency. However, the heat exchange efficiency is not necessarily the higher the better, as its specific value is often determined by the process conditions – it should neither be too high nor too low!
The calculation of heat transfer efficiency is based on the principles given in relevant standards; it is determined by using the value of WCp that is lowest, and by taking the actual temperature rise divided by the temperature difference between the inlet temperature of the substance being cooled or heated and that of the other heat source. Is there another way to calculate it?
I’ve learned it.* . . . . . . . . . . . .