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. small ac units portableRoughly it is related to velocity V as h=8*V^0.6. ac unit breakdownEvaporative mass transfer coefficient also varies roughly the same. does a portable ac unit need a windowHence 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. So turn it off while leaving that room. To conclude always use fan with AC as long as the room is occupied. You are here » Spring and Summer Energy-Saving TipsEfficient blade design lowers energy use. Designed by engineers from the Florida Solar Energy Center, the Gossamer Wind ceiling fan moves more air with less energy than any other model. When I was a young backpacker traveling through India, Sri Lanka, and Thailand in the 1970s, I couldn’t afford air-conditioned hotels or restaurants. In these tropical conditions, I became quite accustomed to the benefits of Casablanca-style fans. Although a fan can’t lower the temperature of the air, it can make people feel cooler.

Moving air accelerates the rate at which perspiration evaporates from your skin. The evaporation process requires heat, so increased evaporation means that more heat is leaving your body. Moving air also removes heat from your body by convection — at least as long as the air temperature is below 100°F or so. Once the air temperature significantly exceeds your body temperature, however, fans tend to raise rather than reduce your body temperature (unless you are very wet). Fans use electricity and their motors give off heat, so it’s important to use them sparingly and appropriately. Buy an efficient model — ideally a Gossamer Wind fan. (Emerson Climate Technologies is the best current source for the Gossamer fan; Emerson sells the fan under the Midway Eco brand (model CF955BS). Choose a fan with a long enough down-rod. A ceiling fan works best when it’s located 7 or 8 feet above the floor. To make it easier to turn off the fan when you leave the room, the fan should be controlled by a wall switch, not a pull chain.

Remember that ceiling fans make the most sense in a house without air conditioning; if your house has AC, resist the temptation to turn on the cooling. If no one is in the room, turn off the fan. Once the air temperature exceeds 100°F, turn off the fan and take a shower. Ceiling fan energy use varies by model. Watt draw ranges from 15 to 30 watts on low speed, 30 to 50 watts on medium speed, and 50 to 100 watts on high speed. According to a paper by Danny Parker, Michael Callahan, Jeffrey Sonne, and Guan Su, the average Florida home has 4.3 ceiling fans and consumes about 800 kWh per year to operate these fans. A central air conditioner requires far more power — on the order of 2,000 to 5,000 watts. Alas, many people never turn off their ceiling fans. Even in homes with six or seven fans, the fans are often left humming for 24 hours a day. But a fan in an empty room isn’t cooling the chairs or beds; rather, its motor is giving off heat. Seven 60-watt fans give off 420 watts continuously.

In 24 hours, the waste heat from these fan motors is equivalent to ten hours’ operation of a 1,000-watt hair dryer. Ceiling fans were invented for use in buildings without air conditioning, like the Asian hotels I visited. Used in this traditional manner, a fan improves occupants’ comfort while using much less energy than an air conditioner. In warmer areas of the U.S., it has become common to install ceiling fans in air-conditioned homes. According to ceiling fan manufacturers, these fans save energy two ways: The first point is true, of course: the longer that you can resist the temptation to turn on the AC, the better. If ceiling fans make this possible, you’ll save energy. As far as I know, however, no researcher has ever found a correlation between ceiling fan installation and reduced use of air conditioning. Ceiling Fans Are Evil How to Buy an Energy-Efficient Ceiling Fan High-Tech Ceiling Fans for Low-Tech Cooling GBA Encyclopedia: Fans and natural cooling

The second point — the theory that ceiling fan owners choose a higher AC thermostat setpoint — has been called into question by researchers in Florida. According to a 1996 paper (“Are Energy Savings Due to Ceiling Fans Just Hot Air?”) by P. James, Jeffrey Sonne, R. Vieira, Danny Parker, and M. Anello, “Data from 386 surveyed Central Florida households suggests that although fans are used an average of 13.4 hours per day, no statistically valid difference can be observed in thermostat settings between households using fans and those without them.” It’s unlikely that any energy savings from ceiling fans installed in a house with air conditioning will ever be high enough to justify the cost of their installation. Arnie Katz, an energy expert at Advanced Energy in Raleigh, N.C., received an inquiry from a homeowner who was debating the installation of seven ceiling fans (at an installed cost of $250 each) in a new home. Katz replied, “The most optimistic estimates I’ve seen on energy savings from ceiling fans peg the air conditioning savings at about 15%, assuming people do raise the thermostat setting and only run the fans when people are in the room, and taking into account the cost of energy used by the fan itself.

At current interest rates, the $1,750 will cost you about $12 a month, or $144 a year, for thirty years on your mortgage. In order to break even, your air conditioning bill needs to be $960 per year. If your bills are that high, you’ve got some major problems that ceiling fans aren’t going to solve — problems like inadequate or poorly installed insulation, leaky ducts, a poorly performing air conditioner, or a very leaky house.” Many ceiling fan marketers claim that ceiling fans can save energy in the winter as well as the summer. Here’s the basis of the claim: during the winter, hot air rises. A ceiling fan helps destratify the air in the room, so that the “wasted” heat hanging out near the ceiling is brought down to floor level. Because of this mixing, the furnace or boiler will turn on less frequently, thereby saving energy. The only problem with the claim is that no researcher has ever been able to confirm it. The “destratification savings” myth was debunked 17 years ago, when an article in the June 1993 issue of Consumer Reports pointed out that there is no evidence to back the claim that destratification saves energy.

“In theory, ceiling fans can also save energy in the winter,” Katz wrote. “To date, I’ve seen no research that demonstrates this actually happens in the real world.” Needless to say, the use of ceiling fans (whether blowing air upward or downward) during the winter can actually make occupants less comfortable — since blowing air has a cooling effect. If you’ve been operating your ceiling fans during the winter, run this experiment: turn off the fans and see if you are more comfortable or less comfortable. If you actually need to run the fan to feel comfortable, something is seriously wrong with your heat distribution system or the integrity of your home’s thermal envelope. Instead of using electricity to operate your ceiling fans during the winter, it’s time to diagnose the source of the comfort problem — perhaps poorly located registers or a leaky ceiling — and solve the actual problem. Once you’ve installed central air conditioning in your home, it’s hard to pretend it’s not there.