The capacity of an air conditioner's compressor is measured in terms of "tons of refrigeration" (TR) because historically, it was related to the cooling capacity needed to freeze a certain amount of ice in a specific period.
The concept of "tons of refrigeration" dates back to the early days of refrigeration and air conditioning when ice was commonly used for cooling purposes. Before mechanical refrigeration systems were widely adopted, people relied on natural ice harvested from frozen bodies of water to keep food and spaces cool.
The term "ton of refrigeration" originates from the practice of measuring the cooling capacity needed to freeze one ton (2,000 pounds) of ice in 24 hours. This quantity of cooling represented a substantial amount and was a convenient unit for comparing and specifying the cooling capacity of early refrigeration and air conditioning systems.
The cooling capacity of an air conditioner's compressor is still expressed in TR even though modern air conditioners do not involve actual ice. One ton of refrigeration is equivalent to 12,000 British Thermal Units (BTUs) per hour. So, if an air conditioner has a cooling capacity of 3 tons, it can remove 36,000 BTUs of heat per hour from the conditioned space.
Though the use of TR is a somewhat antiquated unit for cooling capacity, it remains prevalent in the air conditioning industry due to its historical significance and established conventions. It's important to note that other units, such as BTUs or kilowatts (kW), are also used to express cooling capacity in modern air conditioning systems. However, TR continues to be used as a standard reference for rating the size and performance of air conditioners and heat pumps.