Due to the fact that refrigeration was initially produced by ice, the degree of heat removal in a cooling operation was expressed in terms of the pounds or tons of ice required by the unit of time, usually per day.
One pound of ice has been found to absorb 144 BTU when it melts. A ton of ice, therefore, absorbs 200 x 144 = 288,000 BTUs. When a ton of ice melts in 24 hours, the ratio is 288,000 / 24, that is, 12,000 BTUs per hour, or 12,000 / 60 = 220 BTUs per minute.
This relationship has been officially designated as a ton of refrigeration (TR), and is the basis for the classification of all refrigeration machinery.
