Written Written Written Why do we need cooling?The body core temperature is around 37°C and that of skin is 33.7°C but the body constantly converts chemical energy to heat. Hence to maintain this temperature we need to dump that heat in surrounding environment. Mechanism of heat rejection :Radiation : Q = constant * {(body temp in K)^4 - (surrounding temp in K)^4}Convection Q = h * Area of body *{(body temp) - (surrounding temp)}Evaporation Q = he * Area of body *{(body vapour pressure) - (surrounding vapour pressure)}   Out of which convection and evaporation play major role as temperature difference between the body and the surrounding is very small to carry out significant radiative heat transfer.  Convective heat transfer on the other hand plays important role. It actually moves heated air from your skin and replaces it with cool surrounding air. In natural convection this happens due to density difference between hot air and cold air.    Evaporative heat transfer takes place as water from the body evaporates through pores in skin and takes away the heat of evaporation.

The driving force for this mechanism is vapour pressure of water in surrounding air. Why air circulation is important?Convective heat transfer coefficient "h" in the equation depends on the velocity of air. Roughly it is related to velocity V as h=8*V^0.6. Evaporative mass transfer coefficient also varies roughly the same. Hence by increasing velocity of air around the body, we can dump more heat. This reduces the temperature difference required for heat transfer. Logically one can increase the set point of AC to higher temperature (27-28°C instead of 24-25°C) and still have same level of comfort. This will help AC to run the compressor for less amount of time.About the heat rejection from fan Fan motors are much more efficient compared to compressor. Efficiency of a typical 50 W fan motor is around 80-85% hence the heat loss is hardly 10 W, which is very small.(Human body dissipates 50 to 80 W even while sleeping)So practically there will not be load addition due the fan heat loss. But turning on fan will only work on living things only as it does not actually reduce the temperature.