Introduction
I get some strange phone calls. I recently received one from a customer who wanted to know how many “tons of air conditioning” he needed to cool some equipment he had purchased from my company. After I assisted this customer with his problem, he asked me if I knew where this strange unit came from. Here is the story I told him.
Background
This unit of air conditioning is a real fossil. It goes back to the first days of the air conditioning/refrigeration industry. The ton came into use by the refrigeration industry because early refrigerators were used to make ice. The ton represents the amount of cooling capacity needed to make 1 ton of ice per day. In the US, home air conditioners are usually rated in BTUs/hour, and commercial refrigeration units are usually rated in terms of tons of ice frozen per day.
The temperature of liquid water reduces by roughly 1 °C for every calorie per gram. When liquid water is at 0 °C and you continue to extract heat, water begins to undergo a phase transition from liquid to ice. The energy required to make this transition is referred to as heat of fusion (symbol ΔH°m). The heat of fusion for water is 79.72 cal/g.
We can compute the energy required for one ton of water to transition from liquid to ice as shown in Equation 1.
| Eq. 1 |  |
We usually say that 12000 BTUs per hour equals on 1 ton of refrigeration per day. Equation 2 illustrates this calculation.
| Eq. 2 |  |
Conclusion
I must admit that I find the units of measure used in the US confusing. I wish things were different, but I am a realist. All I can do is try to shed some light on the subject.
I think it’s so important to optimize your home with the best unit that is going to save you money and save on the cost of energy. I really enjoyed this post and feel it shed light on a subject that is really important right now.