Evaporator ( In a refrigeration System)

Function of evaporator:

·         In a refrigeration system, an evaporator is a device which enables a volatile liquid to vaporize for the purpose of removing heat from a refrigerated space or product.

·         It is one of the main components in a refrigeration system.

·         A refrigerant before entering the evaporator is usually a mixture of liquid and vapor due to its expansion through the expansion valve, and enter the evaporator at low temperature and pressure.

·         The liquid refrigerant vaporizes inside the evaporator to absorb heat from the object to be cooled.

·         In many cases the vapor in a superheated state is sucked into the compressor to prevent the liquid slugging.

Main Types of Evaporator:

•            Bare tube coil evaporator
•            Plate surface evaporator
•            Finned tube type evaporator

 Bare tube coil evaporator:

·         Bare tube refers the tube whose inner and outer surfaces are both smooth.

·         Bare-tube evaporators are also called prime-surface evaporators.

·         Bare-tube coils are available in a number of sizes, shapes and designs

·         Spiral bare-tube coils are often employed for liquid chilling.

·         Large ceiling-hung bare-pipe coils employing natural convection air circulation are sometimes used in frozen storage rooms and in storage coolers where the circulation of large quantities of low velocity air is desirable.

·         They are also used as either “dry” or “spray coils,” in conjunction with centrifugal blowers to provide high-velocity chilled air for blast-cooling or freezing operations.

·         The bare-tube evaporator is generally operated dry-expansion.

Bare tube coil evaporator

 

 Plate surface evaporator:

·         There are several types of plate-surface evaporators.

·         Some are construction of two flat sheets as metal so embossed and welded together as to provide a path for refrigerant flow between them.

·         This type of plate-surface evaporators has the advantages of easy cleaning and low cost in manufacturing.

·         It can be readily formed into the various shapes required to serve as structural components, for example, the walls of a household refrigerator or reach-in cooler, when it is constructed in a boxlike shape.

·         The plate construction offers some secondary heat transfer surface, but is also useful for cooling flat packaged products that contact the surface.

Plate surface evaporator

 

 Finned tube type evaporator:

·         As the name suggested, finned tube evaporators have the fins installed on the bare tube to enhance the heat transfer from the air to the refrigerant in the tube.

·         The fins here act as a secondary heat exchange surface to improve the evaporator efficiency from increasing overall evaporator surface area.

·         Finned evaporators are used extensively in residential and commercial refrigeration and air conditioning applications.

·         In order for them to be effective, there must be good thermal contact between the evaporator fins and the tubing surface.

·         This can be accomplished by several ways.

·         One method is to solder the fin directly to the tubing.

·         Another method is to slip the fin over the tubing and expand the tubing by pressure or some such means so that the fin locks onto the tubing surface.

·         A variation of the latter method is to flare the fin hole slightly to allow the fin to slip over the tube.

·         After the fin is installed, the flare is straightened and the fin is securely locked to the tube.

Finned tube type evaporator