Evaporators and Chillers
● The evaporator is that part of the refrigeration system where the liquid refrigerant is evaporated. It is sometimes called the cooling coil, unit cooler, freezer coil, liquid coolers, etc.
● As the name, it is a part of the system where the liquid refrigerant is changed into a vapour by the absorption of heat is an evaporator. It is fitted on low pressure side in the circuit.
● Evaporators are classified as flooded type and dry type depending upon whether liquid refrigerant covers all heat transfer surfaces or some portion is having gas vapour being superheated.
● The evaporator with thermostatic expansion valve can be designed dry evaporator whereas evaporator with float valve will be flooded type.
Classification of Evaporators:
There are many types of evaporator, these are classified as:
(1) According to the type of construction :
(a) Bare tube coil evaporator.
(b) Finned tube or extended surface type evaporator.
(c) Plate type evaporator.
(d) Shell and tube evaporator.
(e) Shell and coil evaporator.
(f) Tube in tube evaporator.
(2) According to the manner in which liquid refrigerant is fed:
(a) Flooded evaporator
(b) Dry-expansion evaporator.
(3) According to mode of the heat transfer:
(a) Natural convection evaporator
(b) Forced convection evaporator.
(4) According to operating condition:
(a) Frosting evaporator.
(b) Non-frosting evaporator.
(c) Defrosting evaporator.
(5) According to expansion :
(a) Direct expansion evaporator.
(b) Indirect expansion evaporator.
Extended Surface Evaporator (Finned Evaporator):
● Finned coils are bare-tube coils upon which metal plate or fins have been installed.
● The fins servicing as secondary heat absorbing surfaces, have the effect of increasing decide e ata evaporator, thereby improving its efficiency for cooling air and other gases.
● With bare-tube evaporators, much of the air that circulates over the coil passes through the open space between the tubes and does not come in contact with coil surface.
● When fins are added to a coil, the fins extended out into the open space between the tubes and acts as a heat collector.
● These fins remove heat from the portion of the air that would not ordinarily come in contact with the tube surface. In some cases fins are soldered directly to the base tube.
● Fin size and spacing depend upon the particular type of application for which coil is designed.
● The size of tube determine the size of fin.
● Small tube require small fins.
● Fin spacing varies from I to 14 fins per inch depending upon operating temperature of the coil.
● The evaporator designed for low temperature application must have wide fin spacing in order to minimize the danger of restricting air circulation.
● Excessive finning may actually reduces the evaporator capacity by restricting the air circulation over the coil unnecessarily
● These evaporator are best suitable for air conditioning application where refrigerator temperature above 0°C because of rapid cooling rate.
● It is compact, occupy less space than bare tube or plate evaporator for same capacity.
● This evaporator is a finned coil type because of rapid heat transfer rate during the off cycle.
● It also maintain a relative humidity of about 90 to 95%. It requires larger evaporator surface area to make up this Loss
● It has a disadvantage when installation at the top of evaporator, may defrost and moisture flows down the evaporator surface.
● This moisture will not get sufficient time to escape before compressor lowers the temperature of the evaporator to -7°C to - 6°C range when this occurs, the frost accumulate at the lower part of the evaporator which results in block of fins gap and so free circulation of air around the coil these reduces the effectiveness of evaporator.
Bare-Tube Evaporator :
● It is simple in construction hence it is very easy to clean and defrost.
● These are constructed from steel pipe or copper tubing.
● Steel pipe used for larger evaporator and are applicable for ammonia refrigerant.
● The copper tubing is utilized for smaller evaporator for use with refrigerant other than ammonia.
● Bare-tube coil are available in a number of size and shape.
● The common shape for bare-tube coils are flat zig-zag.
● Spiral bare tube coil are often employed for liquid cooling.
● This evaporator provides less surface contact as compared to other.
● The contact surface area may be increased by extending the length of the tube.
● If the tube is too long, the liquid refrigerant completely vaporized carly leading to excessive superheating at the outlet.
● This also causes greater pressure drop between inlet and outlet of the evaporator. This reduce suction pressure to compressor.
● If diameter of tube is large, refrigerant velocity is low and specific volume of refrigerant will be greater, to the surface of the tube to allow complete vaporization. This may allow liquid refrigerant to suction line of compressor with possible damage to the compressor
● If diameter is too small, causes pressure drop due to friction and reduce efficiency.
Plate Sheet Type Evaporator :
● These are of several types.
● Some are constructed of two flat sheets of metal so embossed and welded together to provide path for refrigerant flow between two sheets.
● This type is widely used in household refrigerators and home freezers because of easy to clean, economical to manufacture.
● Another type of evaporator consists of formed coil INstalled between two metal plates that are welded together at the edges.
● This provides good thermal contact between the welded plates and coil carrying the refrigerant.
● Plate type evaporator may be used singly or in bank.
● The plate can be grouped together for ceiling mounting in holding room, freezer.
● The plate may be manifold for parallel flow or series flow of refrigerant.
● These are used for smaller capacity equipment required for peak load.
Flooded Evaporator :
● In a flooded evaporator,A constant liquid refrigerant level is always maintained.
● A float control valve is used as an expansion device which maintains constant liquid level in the evaporator.
● A accumulator placed between the evaporator and the compressor. It catches any liquid droplet that passes over and drains it back to the bottom of the evaporator.
● Due to the heat supplied by the substance to be cooled, the liquid refrigerant in the evaporator coil vaporizes and thus liquid level falls down.
● An accumulator supplies more liquid to evaporator to maintain the liquid level in evaporator. So the level in the accumulator falls down and so the float falls down and open float valve. Hence liquid level from receiver is entered into the accumulator. As liquid level in the accumulator rise float rise up and float valve closed.
● As the refrigerant absorb heat from warmer substance, the vapour formed by vaporizing the liquid in the coil being lighter, rises up and passes on to the top of the accumulator from where it is supplied to the suction side of the compressor
● The baffle plate arrests any liquid droplets or particles present in the vapour.
● The advantage of the flooded evaporator is that the whole surface of the evaporator refrigerant under all the load conditions. liquid
● Thus, it gives high heat transfer rate than dry expansion evaporator of the same size.
Applications :
1. These are used in the chemical and food processing industries.
2. These are used in comfort and process air cooling installation.
3. These are found in cold storage boxes and freezers.
Dry Expansion Evaporator :
● In dry expansion evaporator the liquid refrigerant passes through the evaporator coil or pipe is limited and is completed vaporized when it reaches to the end of the evaporator so that only vapour should be enter in the suction line of compressor hence thermostatic expansion valve or capillary is used.
● Dry expansion coolers have low friction loss. These are simple in design and tube not subject to freezing or rupture. Overall heat transfer is low and low initial cost.
● Dry-expansion coolers are generally used halo-carbon refrigerant.
Comparison between flooded type evaporator and Dry expansion evaporator:
Dry expansion evaporator → Flooded type evaporator
1. Limited amount of refrigerant is feed to evaporator → Evaporator coil is filled with excess amount of refrigerant
2. Rate of heat transfer is less → Rate of heat transfer is very high
3. It is used for small capacity installations → It is used for large capacity installations
4. It is used with constant load applications → It can be used with fluctuating load applications
Shell and Tube type Evaporator :
● The shell and tube evaporator is similar as shell and tube condenser. It consist of shell, tube sheets and tubes, waler boxes and refrigerant connection. In the smaller size the shell may be standard pipe but welded shell are used for large size.
● Two pass horizontal shell and tube evaporator is equipped with enclosed water box and mounted in horizontal position.
● The tube sheet usually 1 inch thick are welded to the shell and drilled to receive tube.
● The tubes are inserted through their respective tube sheet hole and welded to provide gas tight joint.
● The refrigerant vapour from the compressor expanded in the tube and chilled water is circulated through the shell.
● If the evaporator is operated flooded then water is circulated through the tube and liquid refrigerant is circulated through the shell
● The height of liquid refrigerant in the shell is controlled by float valve.
● Many times liquid refrigerant spray through the nozzle into the shell and water is circulated through the tube.
● The liquid refrigerant is collected at the bottom of the shell and is re-circulated by pump
● The heat transfer rate is less and used for small unit.
Classification of Chillers
The chillers are classified as:
1. Direct expansion chillers.
2. Direct expansion cooling coil for air with forced convection.
3. Direct expansion coil for air blast freezer.
4. Flooded chillers
D-X Chillers :
● The number of passes on refrigerant side have unequal tube distribution. The first pass will have minimum number of tubes and last pass the maximum.
● The number of tube goes on increasing progressively in each pass.
● The liquid enter the D-X chiller has minimum volume and hence required minimum space. The liquid progressively get converted into vapour and volume increases.
● The number of tube in successive passes therefore goes in increasing. The total pressure drop on refrigerant side is normally limited to 1°C.
● The second fluid passes through the shell which is water or brine travels through number of baffles in a zig-zag manner.
● The baffle spacing has to be properly design to achieve required heat transfer and pressure drop on the fluid side has to be within allowable limit about 8m of w.g. This type of chiller uses tube of thinner gauge compared to flooded chiller.
● The total charge in D-X type is much lower than flooded system. D-X chiller are like positive displacement system since all the refrigerant entering the inlet is positively forced out to the outlet.
● The refrigerant is mixed with oil and oil returned through the suction pipe along with the refrigerant vapours back to the compressor.
● For proper oil return, the pipe sizes for suction and discharge have to be designed within allowable velocity limit. The D-X chiller are used right from about I TR to 10000 TR capacity.
Flooded Chillers :
● Flooded chiller are mainly used in ammonia refrigeration although they are also used with halocarbon mainly for petrochemical application.
● In this type the refrigerant is in shell and water or brine passes through the tubes.
● The flooded chillers are not fully tubed. A surge drum is used in conjunction with the flooded chillers.
● The length of the surge drum is 3/4 of the length of the chiller and diameter of the surge drum is approximately half the shell diameter of the flooded chillers.
● The tube we are of thicker gauge as they are normally finned.
● Integrally finned tubes from 19 fpi to 41 fpi are commonly used in halocarbon refrigerant. The tube distribution in various passes on the secondary fluid side is eqal.
● In flooded chiller oil return became a problem. On the shell side 2-3 outlets are provided with shut off valve for taking oil rich mixture through oil rectification system and returning the vapour to the suction side and oil being returned to the compressor crank case.
● In case of ammonia flooded coolers, the oil gets accumulated at the bottom of the shell as the same is heavy and can be drained while the system is in running condition.
● The oil return in the halocarbon system became more serve when evaporating temperature became lower. The oil rectification system must be incorporated even in case of water chilling application.
● These are designed for liquid level of about 60 to 70% of the shell diameter.
● The tube are submerged in a bath of liquid refrigerant and hence achieve a high 'u' value thereby giving a compact design.
● In ammonia system, pump recirculation methods are commonly adopted for low temperature job such as blast freezers, frozen storage, ice hardening rooms, and many dairy application.
● The gravity feed system are commonly used for standard cold storages for fruit, vegetable, potatoes, etc.
