Air Conditioning Evaporators

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This is the third and final of three articles concerning the basics of air conditioning systems.  This article concerns the evaporator, a part of the air conditioning system that is usually unseen inside the air handling unit. The evaporator, in concert with a fan, supplies cool air to a building interior.

The evaporator is the component of a vapor compression air conditioning system where the desired work of the system, the removal of heat and moisture from the interior space, is accomplished.  This is done by a fan that circulates the interior air across the cold evaporator coil.  The air temperature is reduced as it passes over the coil and moisture in the air is removed when it condenses on the coil and drains off as condensate.

Condensation

Engineers refer to temperature reduction from air passing over the evaporator coils as sensible heat reduction, while the moisture removal function is referred to as latent heat reduction.  Both sensible and latent heat are forms of energy that can be desirable to remove from a building interior.  A change in latent heat involves the energy transfer associated with a change in phase without a change in temperature, in this case water vapor in the air changes phase to a liquid condensate when latent heat is removed from the air.

Sources of sensible heat in a building interior are obvious; outside temperature, solar heating, heat producing equipment, etc.  Sources of latent heat include the infiltration of humid outside air, cooking, bathing, and other human activities.

The evaporator coil of a vapor compression air conditioning system.

The evaporator coil of a vapor compression air conditioning system.

Evaporators are sized to provide the sensible heat removal (cooling) capacity as well as the latent heat removal capacity needed for a particular building.  A key sizing criteria for an evaporator is the sensible heat ratio, which specifies how much latent heat is removed for a given amount of temperature reduction.  In most cases the sensible heat ratio is determined geographically by the air conditioning distributor when they supply evaporators with an appropriate latent heat capacity for the local environment.  Sometimes different sensible heat ratios are selected by the air conditioning system designer to accommodate unusual moisture removal requirements.

Although mechanically simple, evaporators require proper installation and maintenance to attain their designed temperature and moisture removal capacities.  A loss of cooling capacity is usually apparent from increased air conditioning run times or increased interior temperatures and can result from a dirty filter, dirty coils, or an inoperative evaporator fan.  Improper moisture removal is more difficult to recognize.

Cleaning an evaporator coil.

Cleaning an evaporator coil.

A dirty coil can trap moisture or raise the coil surface temperature enough that the amount of water vapor that condenses on the coil is reduced.  A condensate drain line that is obstructed by biological growth or animals will not remove the condensed water vapor to the building exterior, instead that condensate will accumulate in the air handling unit that contains the evaporator coil or the drain pan beneath the coil where it can return to the building interior by evaporation, cause short cycling of the air conditioning system by activating a condensate overflow switch, or cause water damage by leaking to the building interior.

A condensate drain that has been obstructed by nesting animals.

A condensate drain that has been obstructed by nesting animals.

In general, annual inspection and cleaning of the evaporator coil and the condensate drain system is required.  That work includes flushing the drain system to ensure proper flow as well as checking the operation of any condensate drain pumps or overflow switches.  Condensate drain systems tend to become obstructed by biological growth or nesting animals because of their low flow rates as well as extended inactive periods when the air conditioning system is not operating.  A building owner can periodically inspect condensate drains where they exit a building to ensure that they are flowing, although the presence of some flow does not always evidence that proper flow is present.

A condensate drain that is flowing well.

A condensate drain that is flowing well.

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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