Air Conditioning System Basics


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Air conditioning systems are everywhere in our lives – at home, in our vehicles, at work, in manufacturing facilities – yet most people give them little thought other than setting a comfortable temperature until they stop working or malfunction in some other way.

Air conditioning systems are available in many types and sizes ranging from small window units to enormous industrial systems. This article will describe the basics of the vapor compression air conditioning systems that are found in residential and commercial applications.

The diagram below shows a ductless split air conditioning system. While the diagram shows a specific type of system, the components are common in some form in all vapor compression systems.

A vapor compression air conditioning system includes the following major components:

  1. Compressor   Refrigerant enters the compressor as a low pressure vapor. The compressor increases the refrigerant pressure. The refrigerant is heated by the compression process.
  2. Condenser   The high pressure and high temperature refrigerant from the compressor passes through coils in the condenser that are cooled by a fan. The refrigerant changes phase to a liquid as it is cooled. The fan exhausts the heat from the condenser coils to the exterior environment, removing heat from the air conditioning system. The refrigerant exits the condenser as a cool high pressure liquid.
  3. Expansion Valve   The refrigerant passes through an expansion valve after it leaves the condenser. The expansion valve reduces the refrigerant pressure as it flows through the valve. The refrigerant becomes cold as the pressure is dropped and it begins to return to a vapor state.
  4. Evaporator   The cold refrigerant from the expansion valve passes through coils in the evaporator. The warm interior air that is to be cooled by the air conditioning system is directed over the evaporator coils by a blower and exhausted to the interior spaces, either directly or by a duct system. The refrigerant fully returns to a vapor state as it is warmed in the evaporator while it removes heat from the interior air. The refrigerant then returns to the compressor to complete the refrigerant loop.

In addition to cooling the interior air, removal of moisture from the air can be an important function of air conditioning systems. This is accomplished in the evaporator when water vapor in the interior air condenses into a liquid as it passes over the cold evaporator coils. That liquid condensate drips into a collection system that routes the condensate outside the conditioned space. The cool air exiting the evaporator contains less water vapor than the air entering the evaporator because of the condensate that is formed and removed.


Common components of a ductless split air conditioning system.

Vapor compression systems can be used to heat an interior space by reversing the refrigerant flow so that the interior coil becomes the condenser and the exterior coil becomes the evaporator. Heat pumps achieve this by adding a reversing valve to the refrigerant system.

Vapor compression air conditioning systems are generally reliable and function for many years so long as they are properly maintained. That maintenance includes regular air filter changes, coil cleaning, condensate system cleaning, and refrigerant system inspections.

Common problems with air conditioning systems include refrigerant leaks, stalled compressors, clogged condensate drains, dirty coils, and long term corrosion. The sizing of air conditioning systems and their moisture removal capacities are often questioned when interior spaces experience moisture related problems.

Evaluation of air conditioning system problems often involves consideration of the air conditioning system maintenance and any mechanical problems with the system in addition to the design and sizing of the system. As an example, inadequate maintenance rather than incorrect sizing of a system can result in elevated moisture in a building when the condensate collection system becomes obstructed. The air conditioning system will not remove moisture from a building interior if the condensate does not flow to the building exterior. Instead, the condensate can evaporate back to the interior air or overflow the condensate collection system and cause localized water damage. Condensate collection systems must be inspected and cleaned annually to prevent obstruction by microbial growth that forms in the collection system and drain piping.

In coming blogs, I will describe specific components and design attributes of air conditioning systems as well as common problems they can cause.

John Phillips, senior consulting engineer at Warren, has more than 30 years of crane and heavy equipment experience and more than 19 years of experience in forensic engineering.  A licensed professional engineer in South Carolina, North Carolina, Georgia, Louisiana and Ohio, he’s NCEES registered both as a model engineer and with The United States Council for International Engineering Practice, USCIEP. John has designed crane systems, supervised installation, tested and certified lifting equipment even serving as a project engineer for maintenance and certification of nuclear weapon lifting and handling systems. John is a certified fire and explosion investigator and fire and explosion investigator instructor by the National Association of Fire Investigators. John is a member of the American Society of Materials and American Society of Testing and Materials, as well as a voting member of ASTM Ships & Marine Forensic Sciences, Forensic Engineering, and Performance of Buildings committees.

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