Equipment and Appliances

Most equipment and appliances are driven by electric motors, and thus the heat given off by an appliance in steady operation is simply the power consumed by its motor. For a fan, for example, part of the power consumed by the motor is transmitted to the fan to drive it, while the rest is converted to heat because of the inefficiency of the motor. The fan transmits the energy to the air molecules and increases their kinetic energy. But this energy is also converted to heat as the fast-moving molecules are slowed down by other molecules and stopped as a result of friction. Therefore, we can say that the entire energy consumed by the motor of the fan in a room is eventually converted to heat in that room. Of course, if the motor is in one room (say, room A) and the fan is in another (say, room B), then the heat gain of room B will be equal to the power transmitted to the fan only, while the heat gain of room A will be the heat generated by the motor due to its inefficiency (Fig. 29).

The power rating W_motor on the label of a motor represents the power that the motor will supply under full load conditions. But a motor usually operates at part load, sometimes at as low as 30 to 40 percent, and thus it consumes and delivers much less power than the label indicates. This is characterized by the load factor f_load of the motor during operation, which is f_load = 1.0 for full load. Also, there is an inefficiency associated with the conversion of electrical energy to rotational mechanical energy. This is characterized by the motor efficiency h_motor, which decreases with decreasing load factor. Therefore, it is not a good idea to oversize the motor since oversized motors operate at a low load factor and thus at a lower efficiency. Another factor that affects the amount of heat generated by a motor is how long a motor actually operates. This is characterized by the usage factor f_usage, with f_usage = 1.0 for continuous operation. Motors with very low usage factors such as the motors of dock doors can be ignored in calculations. Then the heat gain due to a motor inside a conditioned space can be expressed as

Heat generated in conditioned spaces by electric, gas, and steam appliances such as a range, refrigerator, freezer, TV, dishwasher, clothes washer, drier, computers, printers, and copiers can be significant, and thus must be  considered when determining the peak cooling load of a building. There is considerable uncertainty in the estimated heat gain from appliances owing to the variations in appliances and the varying usage schedules. The exhaust hoods in the kitchen complicate things further. Also, some office equipment such as printers and copiers consume considerable power in the standby mode. A 350-W laser printer, for example, may consume 175 W and a 600-W computer may consume 530 W when in standby mode.

The heat gain from office equipment in a typical office with computer terminals on most desks can be up to 47 W/m². This value can be 10 times as large for computer rooms that house mainframe computers. When the equipment inventory of a building is known, the equipment heat gain can be determined more accurately using the data given in the ASHRAE Handbook of Fundamentals.

The presence of thermostatic controls and typical usage practices make it highly unlikely for all the appliances in a conditioned space to operate at full load. A more realistic approach is to take 50 percent of the total nameplate ratings of the appliances to represent the maximum use. Therefore, the peak heat gain from appliances is taken to be

regardless of the type of energy or fuel used. For cooling load estimate, about 34 percent of heat gain can be assumed to be latent heat, with the remaining 66 percent to be sensible in this case.

In hooded appliances, the air heated by convection and the moisture generated are removed by the hood. Therefore, the only heat gain from hooded appliances is radiation, which constitutes up to 32 percent of the  energy consumed by the appliance (Fig. 30). Therefore, the design value of heat gain from hooded electric or steam appliances is simply half of this 32 percent.