Evaporator

evaporators and its types

1. Definition :  The evaporator is a heat transfer system, and is that part of a refrigeration cycle in which liquid refrigerant is evaporated for the purpose of removing heat from the refrigerated space or product.

2.  Responsible for absorbing heat into the refrigeration system  The evaporator is maintained at a temperature that is lower than the medium being cooled  The capacity of any evaporator or cooling coil is the rate at which heat will pass through the evaporator walls from the refrigerated space to the vaporizing liquid  Removes both latent and sensible heat from the air in the refrigerated box  Latent heat is in the form of moisture  Sensible heat reduces air and product temperature.

3. Types Of Evaporators Based on their construction the various types of evaporators are: 1. Bare Tube Evaporators : a) The bare tube pipes evaporators are made up of copper tubing or steel b) The copper tubing is used for small evaporators refrigerant other than ammonia is used where the c) the steel pipes are used with the large ammonia is used as the refrigerant. evaporators where d) the atmospheric air flows over the bare tube evaporator chilled air leaving it used for the cooling purposes.. and the e) The bare tube evaporators are usually used for liquid chilling

4. 2) Plate Type of Evaporators a) the coil usually made up of copper or aluminum is embedded in the plate so as so to form a flat looking surface. b) the plate type of evaporator looks like a single plate, but inside it there are several turns of the metal tubing through which the refrigerant flows . c) advantage of the plate type of evaporators is that they are more rigid as the external plate provides lots of safety. d) This type of evaporator is widely used in domestic refrigeration. e) The plates can also be welded together forming the bank of the plate type of evaporators that can be used the larger evaporators of higher capacities.

5. 3) Finned Evaporators a) Are the bare tube type of evaporators covered with the fins b) he fluid tends to move between the open spaces of the tubing and does not come in contact with the surface of the coil, thus the bare tube evaporators are less effective . c) the finned evaporators are more effective than the bare tube evaporators. d) are most commonly used in the air conditioners of almost all types like window, split, packaged and the central air conditioning systems.

6. 4) Shell and Tube types of Evaporators a) Are used in the large refrigeration and central air conditioning systems. b) comprise of large number of the tubes that are inserted inside the drum or the shell. c) Depending on the direction of the flow of the refrigerant in the shell and tube type of evaporator, they are classified into two types: dry expansion type and flooded type of evaporator . 1. Dry expansion evaporator : In dry expansion evaporators the refrigerant flows along the tube side and the fluid to be chilled flows along the shell side. 2. Flooded evaporator : In case of the flooded type of evaporators the refrigerant flows along the shell side and fluid to be chilled flows along the tube.

7. 5) SHELL AND COIL EVAPORATOR 1. Shell and Coil evaporators have one or more spiral shaped, bare tube coils enclosed in a welded steel shell. They are generally dry expansion type with refrigerant in coils and chilled liquid in the shell. 2. 3. They have thermal capacity for application for high but infrequent loads. 4. They are used for chilling of drinking water and other aspects where cleanliness is a factor, e.g. Bakeries and Photographic Laboratories.

8. According to mode of heat transfer   Natural Convection Evaporators : a) are often used in applications where low speed air desirable or damage dehydration product possible . b) The greater the temperature difference will produce higher rates of air circulation, because the difference between the density of warm and cold air increases. c) Cold air is denser than warm air. Therefore, the natural circulation evaporators, usually mounted high above the floor die refrigerated Cabinet.

9. Forced convection evaporator : a) High velocity air over a transfer. closely finned coil can increase heat b) The use of forced draught can reduce the overall size and weight of the evaporator which can reduce purchasing and installation costs. c) Forced draught also gives: -   more rapid cooling   even temperatures   good humidity control   easy defrosting. The fins on a forced draught evaporator are very closely spaced which gives a high capacity rating.

10. FACTORS AFFECTING THE HEAT TRANSFER CAPACITY OF AN EVAPORATOR MATERIAL TEMPERATURE DIFFERENCE VELOCITY OF REFRIGERENT THIC K NESS OF THE E V APOR A T OR COIL W ALL CONTACT OF SURFACE AREA

11. T YPES OF EVAPORATORS According to type of construction Bare tube coil evaporator Finned tube evaporator Plate evaporator Shell and tube evaporator Shell and coil evaporator Tube in tube evaporator According to the manner in which liquid refrigerant is fed Flooded evaporator Dry expansion evaporator

12. According to the mode of heat transfer Natural convection evaporator Forced convection evaporator According to operating condition Frosting evaporator Non-frosting evaporator Defrosting evaporator

13. Bare tube coil evaporator Prime surface evaporator Easy to clean and defrost s/f contact area is less Limited applications Finned tube evaporator Over the bare tube metal fins are fastened Shape, size, spacing can be adapted for better rate of heat transfer Extended surface evaporators

14. Plate evaporator The bare coils are either welded on the plate or between the two plates which are welded together Used in household refrigerators, beverage cooler, ice cream cabinets Shell and tube evaporator Contraction is same as shell and tube type of condenser Available in flooded as well as dry expansion type Baffle plates are provided for good turbulence of liquid Capacity 2TR to 250TR

15. Shell and coil evaporator Generally dry expansion evaporators for chilling water Used for small capacity 2TR to 10TR Restricted to operate above 5 degree calicoes to prevent freezing problems Tube in tube evaporator Double tube evaporator Refrigerant can flow in outer pipe and liquid to be cooled can flow in inner pipe The flow of refrigerant can be parallel or counter

16. Natural convection evaporators Low velocity and min. hydration is require Velocity of air depends upon temp. difference Circulation of air around coil depends upon its size shape and location The coil should occupy 2/3 rd of width of the path & 3/4 th the length of the box Forced convection evaporators Air is forced over refrigerant coils Fins are provided to increase heat transfer rate More efficient than natural convection evaporators Require less cooling surface and high evaporator pressure can be used which save power input to the compressor

17. Frosting evaporators Operates below 0°C The frost forms on the evaporator comes from the moisture of the air Cooling efficiency is decreases until the ice and frost is removed Non-frosting evaporators Operates above 0°C therefore frost does not forms on evaporators Temp. close to cooling 0.6°C to 1°C RH from 75-80 % in the cabinet This keeps the food fresh & stops shrinking in weight

18. D EFROSTING EVAPORATORS Frost creates on the coils on the coils when the compressor is running & melts after the compressor shuts off Temp. of about -7°C to -6°C It also keeps high RH of about 90% to 95%