What is meant by Evaporators?

The evaporator is the heat exchanger where the heat is removed from the system by the boiling of the

refrigerant in the evaporator. Thus it can be said that the heat can be removed from air, water or any other process coolant. The evaporator may be

refrigerant cooling coils in an air stream (Air Handling Unit – AHU) for air conditioning, shell and tube heat exchangers with refrigerant in the tube or shell sides, plate heat exchangers or refrigerant coils submerged in water or brine tanks. The coils are generally of two types: (a) Direct Expansion, wherein regulated quantity of refrigerant liquid is allowed to enter through the expansion valve and the vapour get sucked by the compressor or (b) Flooded, wherein the entire shell or large coil is flooded with refrigerant liquid by gravity from a surge drum or by pumping, the vapour from the boiling liquid move back into the drum and then get absorbed by the compressor. The drum gets liquid refrigerant from a liquid receiver due to the pressure difference. In flooded evaporators, the heat transfer surface is usually wetted with liquid refrigerant. Flooded evaporators usually accumulate lubricating oil and suitable arrangements are provided to drain the oil. Evaporators are designed with parallel tubes to maximize heat transfer area, minimize pressure drop and also ensure adequate refrigerant velocity to facilitate the return of lubricating oil back to the compressor. The oil film formation on heat transfer surfaces can significantly reduce the heat transfer rate. A specific cooler is used when water is required very close to freezing temperature, which comprises vertical tubes or plates, with water trickling down in film form on one side and boiling refrigerant on the other side. The water collects in a tank below. The heat transfer in the evaporator depends largely on the surface area, the fluids involved, the turbulence in the fluid streams and the operating temperature and pressure. The heat transfer coefficients may show variations depending on the evaporator design and refrigerants used. The benefits of Plate Heat

Exchangers (PHHEs) over conventional shell and tube heat exchangers are smaller refrigerant volumes (5 to 20%) due to low internal holding volume, easy access for maintenance and inspection, easier process of heat transfer area by addition of plates, high heat transfer coefficients resulting from the intense turbulence in the plate channels and reduced fouling tendencies and higher reliability. The plate heat exchanger can be designed to have closer approach temperatures i.e. higher evaporating temperature to achieve higher COP. PHEs have been in use for secondary heat transfer to products (especially in food and pharmaceutical industries) for a long time. More recently, PHEs have been also used as evaporators and condensers in refrigeration systems in place of shell and tube heat exchangers. Larger heat transfer area in the evaporator leads to a better heat transfer from the coolant to the refrigerant, resulting in higher refrigerant temperature and pressure and less energy consumption. The Heat transfer area can be increased in heat exchangers by providing more tubes or larger number of smaller tubes or by surface area enhancement techniques like the use of grooved tubes, finned tubes, helical wire inserts in tubes etc. At certain limits, the coefficient of heat transfer can be increased by operation at higher process coolant velocities.