The function of evaporator is to absorb heat from surrounding location or medium which is to be cooled, by means of a refrigerant.
Evaporator is also known as cooling coil, a chilling coil or freezing coil. The evaporator cools by using from the refrigerants latent heat of vaporization to absorb heat from the medium being cooled.
Capacity of evaporator, Q = UA (T2 – T1) W
Factors affecting the heat transfer capacity of an evaporator are :
3.) Velocity of refrigerants
4.) Thickness of evaporator coil walls
Types of Evaporator :
Bare Tube Coil Evaporators :
- It is also known as Prime Surface Evaporators. This type of evaporator offers little surface contact area. These type of evaporators may be used for any type of refrigeration requirement.
- Its use however limited to applications where the box temperatures are under zero degree Celsius and also in household refrigerators because they are easier to keep clean.
Finned Evaporators :
These are also called as extended surface evaporators. These are designed for air conditioning application wheel the refrigerator temperature is above zero degree Celsius. It will defrost itself on the off cycle when the temperature of the coil is near zero degree Celsius.
Plate Evaporator :
The plate evaporators are generally used in household refrigerators, home freezers, ice cream cabinets etc.
Flooded Evaporator :
In flooded evaporator a constant liquid refrigerant level is always maintained. A float control valve is used as an expansion device which maintains the constant liquid level in the evaporator. In order to prevent liquid refrigerant to enter into the compressor an accumulator is generally used with the flooded evaporator. The advantage of flooded evaporator is that the whole surface of evaporator coil is in contact with liquid refrigerant under all load conditions. These are used in chemical and food processing industries.
Dry Expansion Evaporator :
Dry expansion evaporators are not really dry at all. They simply used relatively little refrigerants as compared to flooded evaporators having the same coil volume. They are usually one-fourth to one-third filled with liquid refrigerant.
Since the medium to be cooled comes in direct contact with evaporator surfce, in flooded and dry expansion evaporators, therefore they are called as direct expansion evaporators.
Frosting Evaporator :
The frosting evaporators always operate at temperature below zero degree Celsius. This means that the coil frosts continually when in use. The frost which from grows in thickness the coil or cooling efficiency decreases until the ice and frost is removed.
Non Frosting Evaporator :
The non frosting evaporator operate at a temperature above 0°C at all times. Therefore, the frosts does not form on the evaporator. The big advantage of a non-frosting evaporator is its operation at temperature close to freezing (6°C to 1°C). This permits to maintain a relative humidity from 75 to 85 percent in the cabinet.
Since non-frosting evaporators have greater area, therefore they are heavy than the frosting evaporators. They also requires baffles to direct the air flow in one direction past the coils. Coil efficiency increased as the baffles speed up air conditioning process.
Defrosting Evaporators :
A defrosting evaporator is one in which frost accumulates on the coil when the compressor is running and smelts after the compressor shuts off. The evaporator remains at a temperature of about (-7°C to 6°C).
A defrosting evaporator is usually a finned coil because of the necessity of rapid heat transfer during the off cycle. It also keeps a high relative humidity of about 90 to 95 percent
Method of Defrosting an evaporator :
- Manual defrosting method
- Pressure control defrosting method
- Temperature control defrosting method
- Water defrosting method
- Simple hot gas defrosting method
- Automatic hot gas defrosting method
- Thermo bank defrosting method
- Reverse cycle defrosting method
- Electric defrosting method