Cooling load is defined as the rate at which heat has to be removed from a space to maintain a constant temperature. A room or office accumulates heat continuously from sources such as people, equipment, lights and solar radiation from the sun. Air conditioners cool by continuously removing this heat as it accumulates at the rate required to maintain the temperature set by the thermostat. For this reason, the cooling load is calculated in units of BTU/hr, which represents the speed heat is removed.

Determine the floor area in square feet (ft^2) for the space in question. Calculate the area by multiplying the length by the width. For example, if you have an area 20 feet long and 30 feet wide, your area is 600 square feet (20 x 30).

Estimate the cooling load factor, or CLF, for your type of space using the following as a guide: residential/apartment, CLF is 1.0; office, CLF is 1.2; classroom, CLF is 1.5; and assembly, CLF is 2.5. CLF is in units of CFM/SF which is cubic feet per minute per square feet.

Calculate the air cooling requirements in cubic feet per minute, or CFM, using this formula: floor area x cooling load factor. For example, if you are calculating the cooling load for a classroom with a cooling load factor of 1.5 from Step 2 and the floor area of 600 square feet from Step 1, your CFM will be CFM = 600 ft^2 x 1.5 CFM/ft^2 = 900 CFM.

Calculate the total cooling load, or TCL, using the formula TCL = 1.08 x CFM x "T where "T is the interior design temperature minus cooling coil temperature. The interior design temperature is the thermostat temperature and cooling coil temperature is the temperature measured at the cooling coil of the AC unit. As an example, if the interior design temperature is 75 degrees F, cooling coil temperature is 55 degrees F and CFM is 900 CFM, then TCL = 1.08 x CFM x "T = 1.08 (900) (7555) = 19,440 BTU/hr.
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