EVAPORATORS AND TYPES

HVAC
EVAPORATOR AND TYPES

1. HVAC EVAPORATOR AND TYPES ABHINAND SANTHOSH ANANTHAKRISHNAN ANDREA SUSAN BINOY HALA GAFOOR MEENAKSHY M S

2. EVAPORATOR ◉ The purpose of the evaporator is to receive low-pressure, low temperature fluid from the expansion valve and to bring it in close thermal contact with the load. ◉ The refrigerant takes up its latent heat from the load and leaves the evaporator as a dry gas. ◉ In most cases air or a liquid is first cooled, and this is then used to cool the load. e.g., in a cold-room air is cooled and this air cools the stored produce and carries away heat leaking through the structure; in a water chiller, water is circulated to cool the load, etc.

3. WORKING HEAT ABSORBER. HOT AIR PASSES OVER THE FINS OR COILS. WHEN PASSING ACROSS THE METAL, IT ABSORBS HEAT THROUGH THE SURFACE TO THE REFRIGERANT. WHY LOW TEMPERATURE? LOW TEMPERATURE IS MAINTAINED REFRIGERANT BOILS AND PHASE CHANGE OCCURING FROM LIQUID TO VAPOUR. IMPORTANT TO KEEP THE TEMP OF THE EVAPORATOR COIL ABOVE FREEZING POINT. IN SUPERHEATING PHASE FULLY VAPOUR.

4. CLASSIFICATION OF EVAPORATOR BASED ON MODE OF HEAT TRANSFER: NATURAL CONVECTION EVAPORATOR FORCED CONVECTION EVAPORATOR FORCED CONVECTION EVAPORATOR: Bare Tube Evaporators Finned tube evaporators Plate evaporator Tube in tube evaporator Shell and coil evaporator Shell and tube evaporator Flooded type Direct expansion type ( DX)

5. Bare Tube Evaporators The bare tube evaporators are made up of copper tubing or steel pipes. The copper tubing is used for small evaporators where the refrigerant other than ammonia is used, while the steel pipes are used with the large evaporators where ammonia is used as the refrigerant. The bare tube evaporator comprises of several turns of the tubing, though most commonly flat zigzag and oval trombone are the most common shapes. The bare tube evaporators are usually used for liquid chilling. In the blast cooling and the freezing operations the atmospheric air flows over the bare tube evaporator and the chilled air leaving it used for the cooling purposes.

6. Plate Type of Evaporators the coil usually made up of copper or aluminum is embedded in the plate so as so to form a flat looking surface. Externally 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. The plate type heat exchangers can be easily formed into various shapes as per the requirement. Thus in the household refrigerators and the deep freezers, where they are used most commonly, they can be converted into the box shape to form the closed enclosure, where various food can be kept in the frozen state. The plates can also be welded together that can be used the larger evaporators of higher capacities.

7. Finned Evaporators In this type of evaporator the outer surface area is surrounded by thin metallic plates called fins. Fins are thin metallic plates, usually aluminum or copper, securely attached or banded to the evaporator tubes, with bare tube evaporators. These fins increase the heat transfer efficiency of the evaporator. Finned evaporators are used in air cooling application.

8. Shell and Coil Evaporator 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. They have thermal capacity for application for high but infrequent loads. They are used for chilling of drinking water and other aspects where cleanliness is a factor, e.g. Bakeries and Photographic Laboratories.

9. Shell and Tube evaporators Shell and Tube evaporators consist of a cylindrical shell in which there are a number of straight tubes arranged in parallel, held in place by end plates. When in operation, refrigerant is expanded in the tubes with chilled liquid in the shell. Shell diameters range from 150 mm to 1.5 m, numbers of tubes from less than fifty to several thousand. Tube diameters range from 16 mm to 50 mm with tube lengths 1.5 m to 6 m. Steel tubes are used for ammonia and copper tubes used for other refrigerants. Copper tubes give high heat transfer coefficients.. They are used for both Dry and Flooded applications.

10. SHELL AND TUBE TYPE EVAPORATOR FLOODED TYPE FLOODED EVAPORATOR IS ALWAYS FILLED WITH LIQUID REFRIGERANT. THE REFRIGERANT FLOWS THOUGH THE SHELL AND WATER PASSES THROUGH THE TUBES. THE REFRIGERANT REMAINS IN THE LIQUID STATE AS IT PASSES FROM THE INLET TO THE OUTLET. REQUIRES LARGE AMOUNTS OF REFRIGERANT TO AVOID SUPERHEATING. GRAVITY CAUSES THE REFRIGERANT TO FLOW DOWN INTO THE SYSTEM. HIGHER COP (COEFFICIENT OF PERFORMANCE). DX TYPE (DIRECT EXPANSION) DOES NOT ALLOW THE EVAPORATOR COIL TO BE FULL OF LIQUID REFRIGERANT. THE REFRIGERANT FLOWS THROUGH TUBES AND WATER PASSES THROUGH THE SHELL. THE REFRIGERANT IS LIQUID AT FIRST BUT LATER BECOMES GAS BY THE TIME IT REACHES THE OUTLET. REQUIRES LESS AMOUNT OF REFRIGERANT FOR THE SAME COIL VOLUME AS FLOODED USUALLY 1/3 OR 1/4. PRESSURE DIFFERENCE BETWEEN THE OUTLET AND INLET CAUSES THE REFRIGERANT TO FLOW. LOWER COP.

11. HIGHER INSTALLATION COST. MAINTENANCE IS NOT AS EASY AS DX TYPE. HIGHER HEAT TRANSFER RATE MORE EFFICIENT DUE TO WETTED SURFACE. USUALLY USED FOR LARGER PLANTS. IN CASE REFRIGERANT LEVEL DECREASES THE FLOAT VALVE ALLOWS MORE LIQUID REFRIGERANT INTO THE SYSTEM. LIQUID REFRIGERANT TRAPPED IN ACCUMULATOR IS RE-CIRCULATED THROUGH EVAPORATOR. SURGE CHAMBER IS PROVIDED TO PREVENT LIQUID FROM GOING TO THE COMPRESSOR AND ONLY ALLOWING VAPOUR. LESS INSTALLATION COST. EASIER MAINTANENCE. LOWER HEAT TRANSFER RATE. LESS EFFICIENT USED IN DOMESTIC APPLICATIONS. EXPANSION VALVE CONTROLSTHE RATE OF FLOW OF LIQUID REFRIGERANT SUCH THAT ALL THE IQUID REFRIGERANT IS VAPOURISED BY THE TIME IT REACHES THE OUTLET. REFRIGERANT DOES NOT RECIRCULATE WITHIN EVAPORATOR DUE TO SUPERGEATING. SURGE CHAMBER NOT REQUIRED AS THE LIQUID REFRIGERANT GETS CONVERTED TO VAPOUR COMPLETELY.

12.   BARE TUBE COIL EVAPORATOR   FINNED TUBE EVAPORATOR PLATE EVAPORATOR SHELL AND TUBE EVAPORATOR SHELL AND COIL EVAPORATOR WORKING The atmospheric air flows over the bare tube evaporator and the chilled air leaving it used for the cooling purposes   Fins extend into the spaces between the tube and act as heat collector.   During the plate evaporator process, liquid is pumped between thin plates with the heating medium on the mating surfaces. The product is then evaporated with the vapor generated forming a high velocity core.   The main function of Shell & Tube Type Evaporator is to act as a heat exchanger in a vessel form. This is mainly done to cool closed circuit for re-circulating fluid flow.  The refrigerants passed through the expansion device that passed into tubes and evaporates to produce a cooling effect.   They have one or more spiral shaped, bare tube coils enclosed in a welded steel shell. EFFICIENCY Low efficiency   more effective than bare tube evaporators   Very efficient    high efficiency MATERIAL Copper tubing or steel pipes     copper with aluminium fins pressed on it  stainless steel  Al, stainless steel, carbon steel non ferrous copper alloy   COMPARISON

13. ADVANTAGES  Easier to keep clean The fins on the external surface of the bare tube evaporators increases the contact surface of the of the metallic tubing with the fluid and increase the heat transfer rate   1.Simple and Compact in size 2.Heat transfer efficiency is more 3.Can be easily cleaned 4.No extra space is required for dismantling 5.Capacity can be increased by introducing plates in pairs 6.Leaking plates can be removed in pairs, if necessary without replacement 7.Maintenance is simple 8.Turbulent flow help to reduce deposits which would interfere with heat transfer   1.Less expensive as compared to Plate type coolers 2.Can be used in systems with higher operating temperatures and pressures 3.Pressure drop across a tube cooler is less 4.Tube leaks are easily located and plugged since pressure test is comparatively easy 5.Tubular coolers in refrigeration system can act as receiver also. 6.Using sacrificial anodes protects the whole cooling system against corrosion 7.Tube coolers may be preferred for lubricating oil cooling because of the pressure differential    .compact and light weight .High efficiency .Low installation costs .Flexible design .Low maintenance Low pressure drop   BARE TUBE COIL EVAPORATOR   FINNED TUBE EVAPORATOR PLATE EVAPORATOR SHELL AND TUBE EVAPORATOR SHELL AND COIL EVAPORATOR

14. DISADVANTAGES When the fluid (air or water) to be chilled flows over the bare tube evaporator lots of cooling effect from the refrigerant goes wasted since there is less surface for the transfer of heat from the fluid to the refrigerant. The fluid tends to move between the open spaces of the tubing and does not come in contact with the surface of the coil   Tough the fins help increase the heat transfer, rate, adding them beyond certain numbers won’t produce any additional benefits, hence only certain number of fins should be applied on the external surface of the tube.   1.Initial cost is high since Titanium plates are expensive 2.Finding leakage is difficult since pressure test is not as ease as tube coolers 3.Bonding material between plates limits operating temperature of the cooler 4.Pressure drop caused by plate cooler is higher than tube cooler 5.Careful dismantling and assembling to be done 6.Over tightening of the clamping bolts result in increased pressure drop across the cooler 7.Joints may be deteriorated according to the operating conditions 8.Since Titanium is a noble metal, other parts of the cooling system are susceptible to corrosion   1.Heat transfer efficiency is less compared to plate type cooler 2.Cleaning and maintenance is difficult since a tube cooler requires enough clearance at one end to remove the tube nest 3.Capacity of tube cooler cannot be increased. 4.Requires more space in comparison to plate coolers       BARE TUBE COIL EVAPORATOR   FINNED TUBE EVAPORATOR PLATE EVAPORATOR SHELL AND TUBE EVAPORATOR SHELL AND COIL EVAPORATOR

15. APPLICATION Usually used for chilling purpose Used in air conditioners of almost all types like window, split, packaged and the central airconditioning systems Used as the partitions in the freezers, frozen food display cases, ice cream cabinets, soda fountains and others Used in large refrigeration and central air conditioning system Used for cooling drinking water in stainless steel tanks to maintain sanitary conditions and also bakeries and photographic laboratories   BARE TUBE COIL EVAPORATOR   FINNED TUBE EVAPORATOR PLATE EVAPORATOR SHELL AND TUBE EVAPORATOR SHELL AND COIL EVAPORATOR