As refrigeration technicians, we deal with heat energy on a daily basis. The equipment we install, service, and maintain is designed to transfer heat energy from an area where it is not wanted to an area where it does not matter. For example, in a walk-in cooler, the equipment is transferring the heat energy from the product to outside of the cabinet. By removing this heat energy from the product, we keep it at its desired temperature.
So, what is heat energy and how does transferring it from one body to another reduce or keep the product at its desired temperature? According to molecular theory, all substances are made up of molecules that are in constant motion. It is heat energy that causes this molecular activity. As heat energy is removed from a substance, its molecular activity will decrease; as heat energy is added, its molecular activity will increase. The intensity of this molecular activity affects the temperature of the substance. As the molecular activity decreases, the temperature of a substance will decrease, and as the molecular activity increases, the temperature of a substance will increase. So temperature is really an indication of the intensity of the heat energy within a substance. We lower the temperature of a product by slowing down its molecular activity, and we accomplish this by reducing its heat energy.
Heat energy always travels from a high level to a lower level, generally from a higher temperature to a lower temperature. The greater the temperature difference, the greater the exchange of heat energy. Refrigeration systems create sufficient temperature differences to allow the movement of the heat energy from the product to the outside of the cabinet. For example, in a walk-in cooler, a product at 45°F will transfer its heat energy to the air circulating within the cabinet at 35°. This air, in turn, transfers its heat energy to the refrigerant flowing through the evaporator at 20°. The refrigerant with this absorbed heat energy travels to the compressor, where it is compressed to increase its pressure and temperature. It then flows on to the condenser where the refrigerant comes in contact with the condensing medium (either air or water) at a lower temperature and transfers its heat energy to the condensing medium and outside of the cabinet.
The unit of heat energy commonly used in our trade is the Btu. A Btu, as most of you know, is defined as the amount of heat energy exchanged that will cause 1 pound of water to change 1° in temperature. We rate our systems in how much heat exchange will occur typically in a one-hour period. For example, a 12,000-Btuh system will transfer 12,000 Btu of heat energy from the product to be cooled if it operates for one hour. When discussing the capacity of a system, sometimes a technician (or system literature) will drop or omit the “h” in “Btuh” or the “/hr.,” in “Btu/hr.” from the stated capacity, as it is generally assumed to be a one-hour period. However, I believe it is always better to include the “h” or “/hr.” in our discussions so we remember the system’s capacity is based on the amount of heat energy transferred in a unit of time.
Publication date: 11/7/2016