Effect of the excess heat exchange area of the heat exchanger on wall temperature
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I recently completed a course project on heat exchangers, and there was a part related to expanding the capacity by 1.1~1.2 times, which means increasing the heat exchange area to 1.1~1.2 times its original value. The heat exchange area is given by f0 = Q0/(K*tm). The teacher wants to know what the total heat transfer amount Q0 will be after this expansion, and whether it meets the new requirement of Q11 = 1.1~1.2Q0. It seems from the teacher’s words that K remains unchanged, the temperature of the hot water inlet t1 stays the same, the temperature of the cold water inlet t2 also stays the same, the flow rate remains constant, and Q0 is assumed to remain unchanged as well. Increasing the heat exchange area changes the effective average temperature difference tm; it is said that the outlet temperatures of the cold and hot fluids can be calculated through iteration, in order to determine whether the total heat exchange amount meets the requirements. My problem is that I don’t know how to use this 1.1-fold increase to calculate the two new outlet temperatures. The teacher said iteration is needed, but I can’t find the relevant relationship. Please help me out{:1_90:}
This post was last edited by adog8207 on 2018-1-13 23:41. In the design of heat exchangers, a margin in area is taken into consideration. In actual operation, the actual heat load still satisfies Q=KS△t; due to the excess heat exchange area, △t will decrease, resulting in a heat load that is slightly higher than the designed value. When calculating the actual heat transfer load of a heat exchanger, iteration is required; it can be calculated as follows: Given conditions: the flow rates of the hot and cold fluids remain constant, the inlet temperatures (T1, t1) remain unchanged, and the heat transfer coefficient is assumed to be constant, with the heat transfer area being the actual area determined through selection. 1) The flow rates of the hot and cold fluids remain constant, the inlet temperatures (T1, t1) remain unchanged, and the heat transfer coefficient is assumed to be constant ； 2) Based on the previous design calculations, the outlet temperature of the cold side (or hot side) is assumed, the heat load of the heat exchanger is calculated, and the outlet temperature of the other side is determined through heat balance calculations ； 3) Calculate the heat transfer temperature difference △t based on the inlet and outlet temperatures of the hot and cold media ； 4) Calculate the heat load using Q=KS△t, where the heat exchange area is the actual area after selection (for your calculation, this area is increased to 1.1~1.2 times its original value). 5) Compare the heat loads obtained from the two calculations to see if they are the same (within an acceptable range of deviation); if not, it is necessary to reset the outlet temperature on the cold side (or hot side) and repeat steps 3 through 5 until the heat loads are equal.
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