I am not a mechanical engineer; but I have physics background. I want to lift about 250 watts of heat from a metal block. At the same time I have to maintain its temperature at 30 deg Cel. For this I want to use water-air heat exchanger (HE). Heat exchanger will have inlet & outlet pipes. Hot water coming from metal block will enter HE. Fan inside HE will pass the heat from water to ambient air. The water thus cooled will come out of HE through outlet and will cool the metal block. This is closed loop. I want to know what should be water flow rate. I have to pick some ready-made HE available in the market. Hence, I tried using following equation: q = f * c * (T_hot - T_cold) where q= heat to be removed from metal block f = water flow rate through inlet & outlet of HE c = specific heat capacity of water T_hot = temperature of water entering HE = 30 deg C T_cold = temperature of water coming out of HE Ambient temperature is 22 deg C. Questions: Is this correct approach ? In this equation, contact area between water and metal block does not come in the picture. Why is this so? Does working/functioning of HE depend on temperature difference between inlet temperature & ambient temperature ? Any help will be highly appreciated. Thanks.