Why is the cooling power of air-conditioning measured in tons?

Imagine this: a contractor told your grandma that he would install a new air conditioning (A/C) system in her room. As he was explaining, he mentioned that the place required a four-ton unit. Confused, your grandma said, “Oh my! How will you bring something so huge in the house?”

Grandma’s reaction was completely normal. Most people do not recognize the unit of measurement A/Cs use. When someone says that an A/C is four tons, it does not refer to its weight. Instead, tons measure how much heat your cooling device removes from a place in an hour.

Before refrigerators were invented, people would cut ice from the ponds during winter. They will then store it to cool buildings and preserve food for summer. The ice was sold in tons. When Willis Carrier first invented A/Cs, he used the term ton to express their capacity because people were more familiar with it. People use tons to compare the cooling effect of A/Cs to the effect of melting a ton of ice.

Willis Carrier 

Willis Carrier is an American engineer who invented the modern air conditioning system. He first developed an A/C unit in 1902 and then founded an HVAC company in 1215. He called it Carrier Corporation, which focuses on the manufacturing and distributing heating, ventilation, and air conditioning appliances.

Carrier graduated at Cornell University with a degree in engineering. Two years later, New York City suffered air quality problems. One of the most affected was Sackett-Wilhelms Lithographing and Publishing Company in Brooklyn. Paper stock there would absorb moisture from the summer air making ink application difficult.

Offering a solution, Carrier submitted drawing proposals of an air conditioning with a humidity control system. The authorities recognized his ingenuity, and after several four years of testing, the U.S. granted him a patent. Carrier invented the world’s first spray-type A/C equipment. Its purpose was to humidify or dehumidify air, heat water, and then cool it.

Air Conditioning

Air conditioning is a part of a central heating and cooling system that draws out heat energy from the house and transfers it outside. Recall the law of conservation of energy. It says that energy will never be created nor destroyed but can only be converted from form to another.

Now, if you have some unwanted energy in the house, like too much heat in your room, you know that you cannot get rid of it. The only thing you can do is move into another place or convert the heat into another form. Yes, this is where air conditioners come in.

A/C uses the physics of phase conversion. When a liquid converts to gas, it absorbs heat. AC engineers invented unique technology to force air to evaporate and condense in coils through heat repeatedly.

Four pieces ice cubes start to melt

Tons

One British Thermal Unit (BTU) is the amount of heat water should absorb to increase its temperature by one degree Fahrenheit. In the Imperial unit, that value is 143 BTUs. Lighting a kitchen match creates about one BTU of heat. For a ton of ice to melt in a day, it must absorb at least 12,000 BTUs of heat per hour. In a practical sense, one ton is equal to 12,000 BTU/h or 3.5 kilowatts.

There is a science to solve the BTUs of heat. Remember the law of physics? When the ice reaches its melting point at 32 degrees Fahrenheit, it does not change its temperature. It would only shift from a solid into a liquid. Since the liquid’s melting point is at 32 degrees Fahrenheit, you would put 143 BTU for it to start melting.

Now, to melt a ton of ice, multiply 143 BTUs to the weight of tons in pounds (which is 2000 lbs). 143 BTU/lb x 2000 lbs = 286,00 BTU. You need 286,000 BTU to melt the ton of ice completely. If it melts uniformly in a day, divide 286,000 BTUs to 24 hours. You get 11 917 BTU/hr or 12,000 BTU/hr.

However, when the water is in liquid form, it would only take one BTU to raise the temperature to one degree. This is true until the liquid changes into a gas. For it to reach the boiling point at 212 degrees Fahrenheit, you would need 180 BTUs. The boiling point is when liquid starts changing into gas.

When the liquid is gaseous, A/Cs condense the air, thereby removing the heat. When the heat is removed, it releases cooled air into the room. During this process, you would notice droplets of water are produced at the back panel of your appliance.

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