ton of refrigeration
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The ton of refrigeration is defined as exactly 12 000 BTU per hour, which is 3516.85... watts. The value is exact given the exact International Table BTU. It measures a rate of heat removal, not a mass, despite the name.
Watch out: A ton of cooling is a power, not a weight, and it is unrelated to the equipment's mass. Note also that it derives from the US short ton, so it does not scale from the metric tonne. A residential three-ton air conditioner removes 36 000 BTU/h, about 10.5 kW.
| 1 ton | 3,516.8528 W |
The unit comes from the ice trade. It is the cooling rate a mechanical plant needs to match the effect of melting one short ton of ice over 24 hours, using a latent heat of fusion of about 144 BTU per pound: 2000 lb times 144 BTU/lb is 288 000 BTU per day, or 12 000 BTU per hour. Selling refrigeration in tons let customers compare a new machine directly against the ice deliveries it replaced.
About the ton of refrigeration
Before mechanical refrigeration, cooling was a delivery service: ice was cut from northern lakes in winter, stored in insulated houses and shipped south. When compressor plants arrived, the obvious way to sell them was to state how much ice delivery they made unnecessary. Melting one short ton of ice absorbs about 288 000 BTU, and doing that steadily over a day is 12 000 BTU/h, so a machine rated at "one ton" replaced a ton of ice per day. The unit outlived the trade it was invented to compete with.
It remains the standard capacity unit for North American air conditioning and chillers. A residential system is typically 1.5 to 5 tons, a rooftop unit 5 to 50 tons, and a large building chiller several hundred to a few thousand tons. The conversions to remember are 1 ton = 12 000 BTU/h = 3.517 kW, and for a rough sanity check on efficiency, a modern chiller consumes somewhere near 0.5 to 0.6 kW of electricity per ton of cooling delivered. That ratio, kW per ton, is the industry's efficiency metric, and unlike most efficiency figures a lower number is better.