Due to the fact that in early stages refrigeration was produced by ice, the rate of removal of heat in a cooling operation was expressed in terms of tons of ice required per unit time, usually per day (24 hours).

IN F.P.S system the latent heat of fusion of ice is 144B.T.U./lb. which means that one lb. of ice absorbs 144BTU Of heat when it melts. One short ton (2000lbs.) of ice, therefore absorbs 2000*144 or 288,000 BTU of heat. In other words when one ton of ice melts in 24 hour it will produce cooling effect at the rate of 288,000/24

or 12.000BTU per hour or 12,000/60=200BTU per minute. This rate of cooling has been designated as a ton of refrigeration. So a ton of refrigeration is not a unit of mass but it is a measurement of the rate of heat transfer.

In MKS system of units 72000 kcal per 24 hours or 3000 kcal per hour or 50 kcal per minute rate of cooling means one ton refrigeration. Hence a machine having its capacity to produce cooling effect of 200 BTU/min or 50kcal/min is classed as one ton machine.

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