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:

• Prime surface tubing only
• Extended surface tubing

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.
Many names are used to describe this type of equipment, such as:

• Blower Coil
• Unit Cooler
• Air Cooler
• Air Unit
• Evaporator

Evaporator Coil Selection Considerations

Primary selection is based on the following:

• Type Of Refrigerant Used

This will provide the information for which materials can be used in the construction of the unit

• Copper tube and aluminum fin (Freon, Glycol, Water only)
• Aluminum tube and fin (Ammonia, Freon, Glycol, Water)
• Stainless steel tube and aluminum fin (Ammonia, Freon, Glycol, Water, CaCl2)
• Stainless steel tube and fin (Ammonia, Freon, Glycol, Water, CaCl2)
• Galvanized steel tube and fin (Ammonia, Freon, Glycol)
• Type Of Refrigerant Feed
• Direct Expansion
• Top feed mechanically pumped liquid recirculation
• Bottom feed mechanically pumped liquid recirculation
• Gravity flooded
• Phillips liquid overfeed system
• Operating Conditions And Temperature
• Room temperature requirements
• Air flow in cfm & external static requirement
• Air throw (horizontal distance)
• Fin space (specific to moisture retention in product or frosting of coil)
• Defrost method
• Space Available For Unit And Location
• This will provide unit dimensions (length x width x height)
• Air throw required for total air movement in the given space.
• Details For Selection
• Capacity - Cooling or Freezing load
• Number of coils required for the space
• Type of application –        flower cooler, loading dock, freezer, blast freezer
• Type of defrost – air, electric, hot gas, water
• Electrical – Voltage, Amps
• Coil operating weight and support system
• Noise level of fans

Liquid Feed System

Direct Expansion:
In this type of system a capillary tube or thermostatic expansion valve controls or meters the liquid feed to the evaporator.  This type is often referred to as a DX Coil Feed.
This type of system presents a complex distribution problem.  The coil surface should cool effectively and uniformly throughout the coil tubing with even refrigerant distribution.  Distribution of refrigerant needs to allow enough liquid refrigerant into the coil to ensure that complete vaporization occurs within the coil.
Approximately 10% to 15% of coil surface is required to superheat the vapor.  This type of liquid feed system is the lowest in total heat transfer.

Re-circulated Liquid

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.

Gravity Flooded

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.

Design Considerations

Oil Circulation

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.

Fan Type

The most popular is the direct drive propeller fan.  In some situations multiple fans are used on one evaporator to achieve required air circulation.
A unit with a centrifugal fan complete with belt drive is necessary in some applications with high external static pressure.
Draw- through and Blow-through fans are available on many evaporators.  Blow-through design is used in low discharge air velocity and should be limited to 40 foot air throw applications.  Draw-through provides better air flow across the coil/fin area and will provide greater throw capability.  For extended air throw requirements discharge air nozzles or long throw adapters (24” or 36” length) are often used.
ie:  30” dia. 100ft., 36” dia. 120 ft., 42” dia. 140 ft. approx. @ 600 to 700 fpm face velocity.

Extended Surface

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

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”.
The Heat Transfer Co-efficient is increased by air velocity between the fins.  One of the limiting factors is the boundary layer formed on each fin as the air passes.

Selection Of Fin Spacing

Fin spacing for forced convection depends on many considerations:

• Frost formation and frost fouling
• Air velocity
• Pressure condensate
• Coil to air temperature difference
• Drainage
• Cost and weight
• Fin spacing ranges from 1 fin per inch to 14 fins per inch

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:

• Evaporation               – of frost during defrost cycle
• Convection                – of air moving through the evaporator
• Condensation           – of moisture in the air during the cooling cycle

Most refrigeration evaporator’s use forced convection (fan motor) passing a large volume of air past the finned surface.