Evaporators

Coil generally of two types: (a) Direct Expansion (DX) type, which is regulated by the amount of liquid refrigerant allowed to enter through the expansion valve and vapors get absorbed by the compressor or (b) Flooded type, in which the whole shell or a large coil flooded with liquid refrigerant gravity separator or pumping, boiling liquid vapour return a breather and then get absorbed by the compressor. Separator receives the liquid refrigerant with liquid receiver due to the difference of the pressures. In flooded evaporators, all the heat exchanger surface is wetted with liquid refrigerant. Flooded evaporators, tend to accumulate lubricants and related activities are envisaged for the oil drainage.

Evaporators are designed for parallel pipeline to maximize heat transfer area, minimize pressure drop, and also ensure adequate refrigerant speed to return lubricating oil back to the compressor. The formation of an oil film on the surfaces of heat exchange can significantly reduce the intensity of heat.

Another type of evaporator, which is used when the need water very close to freezing, say 0.5C, is Baudelot cooler, which is consists of vertical pipes, plates or water deception down as a film on the one side and refrigerant boiling point on the other side. Water is collected in the tank below.

Heat transfer in the evaporator depends on the surface area of fluids involved in a turbulent fluid flows, temperature and presssure. Heat transfer coefficients in the range from 1500 to 5000 W/m2K, depending on the design of the evaporator (refrigerant boiling point in the tube or in the way) and refrigerants used.

Advantages of plate heat Exchangers (PHEs) compared with conventional shell-and-tube heat exchangers less refrigerant volume (from 5 to 20%) due to low domestic holding volume, easy access for inspection and maintenance, simple increase of the heat transfer area by adding plates, high heat transfer coefficients in the result of intense turbulence in the plate channels and reduction of fouling trends and higher reliability. PHEs can be designed to closer approach temperatures, i.e. the higher the boiling point in order to reach higher the COP. PHEs are used for secondary heat transfer products (especially food and pharmaceutical products) for a long time. In recent years, PHEs were also used as evaporators and condensers of refrigerating systems.

The greater the area of heat transfer in the evaporator leads to a better heat transfer fluid or material being cooled by the refrigerant, hence higher refrigerant pressure and temperature and lower power consumption. Heat transfer area can be increased in heat exchangers, providing more tubes or a larger number of smaller tube or surface area to strengthen the use of corrugated pipes (fig.2.15), finned tubes, spiral wire inserts tubes (fig.2.16) and other heat transfer Coefficient can also be increased by increasing the velocity of the fluid within certain limits. Compromises must be made depending on the pressure drops, energy and material cost...