It is cooling coil in a refrigeration system. Normally its temperature is much lower than 0 o C. The liquid refrigerant enters into the evaporator and is converted into vapor by taking latent from the items to be cooled. The items to be cooled surround the evaporator. The evaporator inlet is connected to the outlet of the expansion valve. The evaporator outlet is connected to the inlet of the compressor.
Types of Evaporators
The classification is based on vapor condition, frost formation, construction and secondary refrigerant used.
(a) Vapor Condition- These evaporators are on vapor condition at the inlet of compressor. These are of two types.
(i) Dry evaporators
These are used in small (≤ 3 tons) and medium refrigeration plants (≥ 3 and ≤ 50 tons). These are used with hermetic sealed compressors and commercial refrigeration units. In these refrigerant becomes dry saturated vapor just before the outlet of the evaporator and super heated vapors enters the compressor. The cooling effect is contributed mainly by latent heat. These are used in fridges, window air conditioners, water coolers, cinema houses, hospitals etc. This type of evaporator uses expansion valve capillary tube in domestic units and thermostatic expansion valve in commercial units.
(ii) Flooded Evaporators
These are used with large cooling capacity industrial refrigeration units (≥50 tons). In these, refrigerant remains in liquid and vapor state at the just outlet of the evaporator. This liquid vapor mixture enter into a cylindrical unit called the accumulator which sends only the vapors to the compressor and liquid is sent back at the inlet of the evaporator. In this type, entire evaporator is used more effectively for causing cooling effect by the latent heat only. These are used in big industries and refineries having large cooling requirements. This type of evaporator uses low side and high side float valves as expansion devices.
(b) Based on Frost – These are frosting, defrosting type and non frosting types.
(i) Frosting type—Used in frozen food cabinets
(ii) Defrosting type – Used in fridges. Defrosting was manual in earlier days. Presently it is automatic by using a heater in the evaporator with a timer. Say after every 24 hours, compressors becomes off and heater is on for a fixed period (say ten minutes). The heater goes off and compressor automatically restarts. The evaporator becomes frost free.
(iii) Non frosting type – when the evaporator is working above freezing point of water i.e. in water coolers.
(c) Evaporators based on shape of construction
(i) Bare tube type –Used in ice plants for cooling of the brine solution.
(ii) Plate and tube type – Used in fridges
(iii) Finned tube type – Used in window air conditioners and water coolers
(iv) Shell and Tube type – Used for getting chilled water (between 5-90C) in central air conditioning plants. Chilled water is sent to the various rooms. Each room is having a fan coil unit. In this, chilled water flows through the coil and air is blown over it with a blower. The cooled air cools the room. The hot water returns back to the shell and tube type evaporator. Refrigerant flows through the tubes and water is there in the shell. This type of evaporator is also called WATER CHILLER.
(v) Shell and coil Type — Used in ice plants for the cooling of the brine solution much below 00C. Sodium chloride solution does not freeze till -290C and calcium chloride brine solution do not freeze till -550C. These solutions are called antifreeze solutions. There are many more anti-freeze solutions. Any one of these can be used as a secondary refrigerant in the ice plant.
(d) On the basis of secondary refrigerant
(i) Direct system- Cools air which further cools the products placed in contact with air as in case of a fridge or window air conditioner.
(ii) Indirect System-Cooling coil cools water or brine. . This cooled brine cools the products as in case of ice plants. Chilled water is used in central air conditioning plants.