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Enthalpy as it relates to free cooling in HVAC

Enthalpy is defined as the amount of internal energy within a system combined with the product of its pressure and volume. When dealing with the term in the HVAC industry, we usually assume that the process is at a constant pressure and, as such, the change in enthalpy is equal to the heat absorbed or released. At its core, the main function of an HVAC system is to transfer heat, which is a form of energy. The first law of thermodynamics, the Law of Conservation of Energy, tells us that energy can neither be created nor destroyed; energy can only be transferred or changed from one form to another. From that, we can infer that the only way to cool a space down is to remove the heat energy and transfer it somewhere else, typically outdoors. This is generally accomplished by absorbing heat from an airstream and then distributing this cool air to the area we wish to control; it helps to think of conditioned air like a sponge with the capacity to ‘soak up’ heat. The air, now laden with unwanted heat, is routed back via a Return Air pathway to start the process over again. The newly absorbed heat is ‘wrung out’ and expelled, and the cool air is distributed again in a constant cycle. This ‘wringing out’ process is mainly accomplished by mechanical cooling, such as a compressor, or free cooling provided by an air-side economizer.

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