| for this cross flow type heat exchanger (HX) there is also a correction factor applicable. However, you could calcualte an overall heat transfer coeficient for each HX and the performance will be very predicatble based on that coefficient. Iif you are comparing the same HX to itself at different air temperatures, the rate of heat transfer will be directly proportional to the log mean temperature difference (LMTD) of the two fluids. If the coolant and air flow are representative in your test, you only need one data point to determine which HX has the better coefficient and is therefore more effective You can calculate the rate of heat tramsfer (Q) if you know the coolant flow rate, and coolant inlet and outlet temperatures using the equation Q= m c * Tin - Tout where m= the mass flow rate of the coolant, convert the gpm to a lbm/min (use the average denisty at the inlet and outlet temp to calcuate the mass flow) C is the specific heat capacity of the fluid, if you use pure water it = 1, The result Q will be in Btu/min you can then back calculate an overall heat transfer coefficient for the HX if you divide the Q you calculated by the LMTD
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