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01/14/08 Selection Considerations for Evaporator Coils
Forced Circulation Air Coolers
Forced circulation air units consist essentially of a coil type evaporator with either:
This provides forced convection of air across the cooling coil surface and expels it into the space being cooled. Fan and coil evaporator units primarily cool and dehumidify the controlled air space.
Evaporator Coil Selection Considerations
Primary selection is based on the following:
This will provide the information for which materials can be used in the construction of the unit
Liquid Feed System
This type of system requires a Hand Expansion valve or metering device, which controls the flow as liquid enters the coil. The liquid is pumped to the coil(s) using a mechanical pump from a low-pressure vessel. This type of coil is a liquid overfeed unit and requires NO superheat of vapor leaving the coil. The liquid feed ratio can be 2 to 10 times that of direct expansion. The coil capacity will be 10% to 15% higher than Direct Expansion.
This type of system requires a surge drum to provide static head and store the liquid refrigerant at the evaporator temperature. Gravity allows liquid to completely wet the internal coil surface providing excellent heat transfer. A vapor line from the coil outlet returns the wet vapor to the surge drum.
Phillips Liquid Overfeed
This type of system requires a pumper drum or pump trap and high-pressure vapor to pump the liquid refrigerant into the coil. A Hand Expansion valve or metering device controls the flow as liquid enters the coil. This type of coil is a liquid overfeed unit and requires NO superheat of vapor leaving the coil.
In most systems a certain amount of oil is constantly circulating throughout the system. It is therefore imperative that coil design and operation are completed in a manner to prevent oil from being trapped in the coil. Failing to return oil to the compressor at the same rate as it leaves will cause irreparable damage.
The most popular is the direct drive propeller fan. In some situations multiple fans are used on one evaporator to achieve required air circulation.
Heat transfer from the prime surface (or tube) is increased significantly by attaching fins or extended (secondary) surface. Fins provide a compact heat transfer surface with much lower material cost and evaporator size requirements.
Fin efficiency is the ratio of “actual heat transfer from the fin” to the “heat transferred if the entire fin were at the base temperature”.
Selection Of Fin Spacing
Fin spacing for forced convection depends on many considerations:
Evaporators collect frost when the coil surfaces are below freezing. Air flow blockage and frost insulation greatly reduce heat transfer and coil capacity.
Finned Tube Heat Transfer
Heat transfer for finned coils follow three (3) basic forms of heat movement:
Most refrigeration evaporator’s use forced convection (fan motor) passing a large volume of air past the finned surface.
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