Why Are Compressors Measured in Tons?

When we look at compressors, there are a lot of numbers going around, but one of the bigger ones is the Tons. We sell compressors in all manner of weight-ratings, from less than a ton to hundreds of tons. The thing is though, this doesn’t mean we need a crane and a massive truck to load the compressor before it ships out. Compressor tonage is not actually a measure of weight. In fact, it is the result of some weird and convoluted math and history.

 

Old Fashioned AC

Before we had the modern air conditioner, there were just a handful of ways to actually cool a room or a building. You could open a window, sit in front of a fan, use an evaporative cooler, or get a block of ice. That’s right, once upon a time we didn’t just have “ice boxes”, we had ice-conditioning too. The precursor to modern refrigeration was massive chunks of ice, usually cut from frozen lakes in the north and hastily delivered anywhere cooling was needed. You would go down to your local ice house and buy however much your fridge or cooling system needed.

For building-scale cooling, there would be a block of ice essentially placed in a special cabinet in the air ducts and fans would blow air over it. The ice would remove heat from the air and melt. The air would be cooled and circulated around the room or building to lower the overall temperature. During World War II, this is how they would cool President Roosevelt’s rail car. At the time there wasn’t an Air Force One or Marine One, but we did have a bullet proof train car, with ice-cooling for the president. Even up to 1940, there just wasn’t a way to make a small air conditioner, so ice remained the go-to option.

 

The 1 Ton Standard

Somewhere along the line, someone thought to measure cooling capacity in tons of ice. That’s a pretty reasonable thought for large-scale enterprises. If you’re a butcher, you probably would order ice by the ton throughout the year. Any large-scale cooling solution would need to use tons or suffer the consequences.

We already have a way of measuring thermal energy, which is the BTU, British Thermal Unit. It takes about 143 BTUS to melt one pound of ice. There are 2000 pounds in a ton. So one ton of ice should be equivalent to 286,000 BTUs of cooling capacity. When we look at the specs for any air conditioner though, these numbers don’t really line up.

Let’s look at a Goodman GSX140181. It’s rated at 1.5 tons and 18,000 BTU. We’ve already covered tough, a single ton is 286,000 BTU. Well, there is one final step in the calculation: time. The one ton standard is per-hour and there are 24 hours. If we divide that 286,000 into 24, we get 11,917 BTU/hr. For roundness sake we’ll say 1 ton = 12,000 BTU. 1.5 *12,000 = 18,000 BTU.

The usage of tonnage for compressors and other AC components was not meant as a way to measure capacity directly but rather relate it to a quantity the world would already be familiar with: ice-cooling. We can consider this to be an analog of Horse Power. When steam engines, gas engines, and other such developments came along, the way to relate the equipment was how many horses it could replace. With the first air conditioners and refrigeration systems, we had to say how much ice it could replace.

 

Errors in the Measurement

There is however, one big caveat to bear in mind. The label may say one-ton, but that is under utterly ideal circumstances. Air conditioner performance is impacted by the design and the environment. If conditions are not ideal, you will not get that full 1-ton capacity. Truth be told, conditions are rarely ideal. In most cases, you’ll need an experienced HVAC contractor and lots of numbers to work out the best capacity for any new installations.

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