Like similar Heating, Ventilation and Air Conditioning (HVAC) appliances, an evaporator pumps a refrigerant to lower the temperature. Larger evaporators cool larger spaces. Manufacturers size evaporators in terms of tons, a unit of power equal to 12,000 British Thermal Units (BTUs) per hour. Calculate this size from the evaporator's temperature range, which describes the fluid's temperature drop, and the volumetric flow rate, measured eventually in pounds of water per hour,

Subtract the evaporator's outgoing temperature from its incoming water temperature. If you want water to enter the evaporator at 60 degrees Fahrenheit and leave at 46 degrees Fahrenheit: 60 - 46 = 14.

Multiply the answer by your proposed volumetric flow rate, measured in gallons per minute. If the evaporator must move 400 gallons per minute: 14 x 400 = 5,600.

Multiply the answer by 500: 5,600 x 500 = 2,800,000. This answer is the evaporator's size, measured in BTUs per hour.

Divide the answer by 12,000: 2,800,000 / 12,000 = 233.33. This answer is the evaporator's size in tons.