![]() Regardless of their different sizes and applications, their function is the same: to reduce vapor refrigerant’s volume resulting in a pressure increase. Reciprocating, rotary, scroll, screw, and centrifugal compressors are the five standard types technicians see. There are many different types of compressors used in HVAC systems. That stage is considered the heart of the refrigeration circuit. However, in this article, the spotlight is the compression stage. All of these stages are essential to accomplish the job of moving heat. The refrigeration cycle consists of four stages: compression, condensation, expansion, and evaporation, as shown in Figure 1. Most air conditioning and refrigeration systems use compression refrigeration to manipulate temperatures and therefore move heat. We often call these heat gains, and these gains always occur based on a temperature differential. Those are heat sources, so they will affect the quantity of heat to remove from the environment. So, if you want to cool down a room where many people work or where equipment heats up, you need to consider this heat when selecting the system in addition to the heat entering through the walls, floor, ceiling windows, etc. In air conditioners, BTUs measure the quantity of heat that an air conditioning unit can remove from a space per hour. Usually, in HVAC systems, the amount of heat is measured in BTUs, which corresponds to the amount of energy it takes to change 1 pound of water by 1☏. That means that the glass has much less heat content, even though they have the same temperatures. However, the glass of water has a much lower volume than the swimming pool. In other words, the substances have the same temperature. If we dip a 50☏ glass of water into a 50☏ pool, we do not transfer heat between them because there is thermal equilibrium. Two substances have the same temperature if their average molecular speed is equal. Molecules can have different velocities, but the temperature is the average value of all their speeds. ![]() The velocity increase results in a temperature increase. When we heat a glass of water in a microwave, its molecules start to move more quickly. Substances are composed of many moving molecules that change their speed as they release or gain heat. Many in our industry can misunderstand the differences between temperature and heat, although these are related.
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