Evaporators are heat transfer surfaces in which the volatile liquid evaporates when it receives evaporative heat from space or cooling products. Due to the wide variety of mechanical refrigeration applications, evaporators are designed and manufactured in a variety of types, shapes and sizes.
Evaporators are divided in terms of building, refrigerant liquid feeding mode, operating conditions, air flow (or liquid) method, type of refrigerant controller and application.
Construction of evaporators
Evaporators are divided into three categories in terms of construction.
Tube and plate evaporators are sometimes classified as simple evaporators because their entire surface is partially in contact with evaporative refrigerant. But in finned evaporators, the primary levels of heat transfer are merely tubes and blades due to lack of direct contact with the refrigerant material are called secondary heat transfer surfaces that absorb heat from the ambient air and direct it to the refrigerant carrier tubes.
However, the performance of tube and plate evaporators at all temperatures and in various designs is satisfactory, they are usually used for cooling liquids and air in environments where the space temperature is lower than 1 ° C and preventing the formation of flake on the evaporator surface is not possible. The formation of flake on these evaporators does not affect their capacity as much as the finned evaporators. In addition, most simple evaporators, especially the plate type can be easily cleaned and the flake accumulated on them can be cleaned without interrupting the frosting process and endangering the quality of brush-cooled products or crushing.
1) Tube evaporators
Tube evaporators are usually made of copper or steel tubes. Steel tubes are used in large evaporators and ammonia-working evaporators, and copper tubes are used in smaller evaporators that work with refrigeration with chlorofluor compounds. These evaporators are made in different sizes, shapes and designs for different applications and are from common designs of flat zigzag and elliptical spiral. Spiral coils are often used to cool liquids.
Large tube coils that cool with natural air flow are often used in frozen storage rooms and cold storage to cool large amounts of air at low flow. These coils are also used as dry or sprayed with water with centrifugal blowers to supply high-speed cold air in wind freezing.
2) Plate evaporators
Plate evaporators have different types. Some of them are made of two metal plates that grooves for passing refrigerant are made. These plate evaporators are used due to the economical formability and ease of cleaning in cold storage and freezers. Other types of plate evaporators consist of tubes located between two metal plates. Metal plates are welded together at the end. In this evaporator, the space between the plates is filled or empty with eutectic solution for heat contact between plates and tube of refrigerant carrier, so that atmospheric –out pressure causes good thermal contact between the tubes and plates.
Evaporators containing eutectic solution are especially useful when storage capacity is needed, and are most often installed in refrigerated trucks either horizontally or vertically on their roofs or walls and connected to the central refrigeration system when stopping in the parking lot. So there will be the stored refrigerant capacity for cooling the next day’s work in the eutectic solution. In this type of evaporators, the temperature of the plates is controlled by the melting point of the eutectic solution.
Plate evaporators are used individually or in group. The plates may be connected in series or in parallel. Plate evaporators are used in multilevel freezing chambers. These evaporators are also widely used as internal walls in freezers, showcase cold storage , ice cream cold storage , and carbonate tanks. Plate evaporators are especially useful for cooling liquids in installations where abnormal peak load conditions occur intermittently, because the formation of flake on the surfaces of the plates at low loads provides storage refrigeration capacity that helps the equipment of refrigeration to impose a burden on courier conditions, because it enables the use of equipment with less capacity than required for courier and saves the system’s initial and current costs.
3) Finned evaporators
Finned coils are a type of tube evaporators that fins are placed on them as secondary level of absorbing heat on the tubes to enhance the external surface and thus improve the cooling efficiency of the air with other gases. Much of the air passes through it without contact with the surface of the cold coil in tube evaporators, but by adding fins to the tube, the heat exchange surfaces penetrates into the open space between the tubes and act as heat collectors and directs heat of the air, which usually not contact with primary surfaces and guides to the tube.
The fins are obviously effective when there is good thermal contact between them and the tube. For this purpose, the fins sometimes form tube on itself, and sometimes after they are placed on the tube, by pressure or other factors, the tube will be widened, so the fins penetrate the body of the tube surface and a good thermal contact is created. Sometimes they heat the fins to be expanded and they can slip on the tube. In this method, after placing the fins on the tube, the heat is removed to fins to be completely locked into the tube.
The spacing and size of the fins depend to some extent on the type and diameter of the tube, and may vary from 40 to 500 numbers per meter depending on the evaporator temperature.
Since the formation of flake on air cooling coils operating at low temperatures is unavoidable, flake formation restricts the conduit between the fins and slowing the air flow to the coil, to minimize the possibility of restriction of air flow in evaporators designed for low temperature applications, the distance of fins should be more and the number of possibility of air flow limitation in evaporators designed for low temperature applications, the distance of fins to be more and their number to be lower (about 80 to 200 fins per minute). But in air conditioning coils and other applications where the surface temperature of the coil is higher than freezing temperatures because of lack of forming flake, the fins are arranged even with intervals of 1.8 meters.
In coils where airflow is gravitationally done and with natural displacement, to reduce the coil’s resistance to airflow, the distance of fins is selected more than fan coils. Considering that external fins affect external level of coil, adding fins not only increases the capacity of the evaporator but also reduces it by unnecessary restriction of airflow.
Because the capacity of the finned evaporators due to flake formation is affected more than other types, they are suitable for air coolers whose coil surface temperature is higher than zero degrees Celsius. If these evaporators are used in low temperature applications, it should be possible to de-flake at specified intervals.
Finned coils have higher thermal surfaces than simple evaporators and occupy less space at the same capacity than tube and plate evaporators, and therefore their use saves considerable space. This issue of using them is suitable for devices with forced air flow.