The function of the evaporator is to absorb the heat from the space or surrounding medium which is to be cooled by means of refrigeration. The process of heat removal from the substance to be cooled or refrigerated is done in the evaporator. The liquid refrigerant is vaporized inside the evaporator (coil or shell) in order to remove heat from a fluid such as air, water etc. Evaporators are manufactured in different shapes, types and designs to suit a diverse nature of cooling requirements. Thus, we have a variety of types of evaporators, such as prime surface types, finned tube or extended surface type, shell and tube liquid chillers, etc.
Construction of Evaporator :
The evaporator as shown in the figure is the part of the refrigeration system where the refrigerant vaporizes as it picks up heat. Heated air is forced through and past the fins and tubes of the evaporator. The heat from the air is picked up by the boiling refrigerant and is carried in the system to the condenser. The evaporator is usually installed in housing under the dash panel.
Working of Evaporator:
When the air conditioning system is turned on, warm air from the passenger compartment is blown through the coils and fins of the evaporator. The evaporator receives refrigerant from the thermostatic expansion valve or orifice tube as low pressure, cold atomized liquid. As the cold refrigerant passes through the evaporator coil, heat moves from the warm air into the cooler refrigerant. When the liquid refrigerant receives enough heat, a change of state – from a low-pressure liquid into a low-Pressure vapor – takes place.
The thermostatic expansion valve or orifice tube continually meters the precise amount of refrigerant necessary to maintain optimum heat transfer, which ensures that all of the liquid refrigerants will have changed to a vapor by the time it reaches the evaporator outlet. The vaporized refrigerant then continues on to the inlet (suction) side of the compressor.
Types of Evaporator :
Evaporators are divided mainly into three groups.
1. Natural circulation evaporator
i) Evaporating pans
ii) Evaporating stills
iii) Short tube evaporator.
2) Forced circulation evaporator.
3) Film evaporator
i) Wiped Film evaporator
ii) Long Tube Evaporator
a) Climbing film evaporator
b) Falling film evaporator Types of Evaporators.
Flooded type evaporator.
Flooded type of evaporator feeds excess of liquid refrigerant so that the exit of the evaporator will be a mixture of liquid and vapor refrigerant.
In the flooded type of evaporator coil remains completely filled with liquid refrigerant as shown in the figure. The level of liquid refrigerant is maintained constant in the surge chamber by using float control. The liquid refrigerant enters into evaporator coil from the surge chamber. In evaporator coil, part of liquid refrigerant boils and converts into vapor. The vapor formed is collected at the top of the surge chamber and the remaining liquid refrigerant is returned to the surge chamber. From the top of the surge chamber, refrigerant vapors are drawn in the suction line of the compressor. In the flooded type evaporator rate of heat transfer is very high as the whole evaporator coil remains in contact with liquid refrigerant but this type of refrigerant requires a large amount of refrigerant.
Applications of flooded type evaporator:-
This type of evaporator is used for
1) Large installations, where refrigerating capacity is high.
2) In a refrigeration system where load fluctuation is higher.
3) For multi evaporator system.
The finned evaporators are the bare tube type of evaporators covered with the fins. 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, thus the bare tube evaporators are less effective. The fins on the external surface of the bare tube evaporators increase the contact surface of the of the metallic tubing with the fluid and increase the heat transfer rate, thus the finned evaporators are more effective than the bare tube evaporators.
The fins are the external protrusions from the surface of the coil and they extend into the open space. For the fins to be effective it is very important that there is very good contact between the coil and the fins. In some cases, the fins are soldered directly to the surface of the coil and in other cases, the fins are just slipped over the surface of the fins, and then they are expanded thus ensuring close thermal contact between the two. Tough the fins help increase the heat transfer, rate, adding them beyond certain numbers won’t produce any additional benefits, hence the only a certain number of fins should be applied on the external surface of the tube.
The finned evaporators are most commonly used in the air conditioners of almost all types like a window, split, packaged, and the central air conditioning systems.
Dry Expansion Evaporator
In the dry-expansion evaporator, the liquid refrigerant is generally fed by an expansion valve. The expansion valve controls the rate of flow of refrigerant to the evaporator in such a way that all the liquid is vaporized and the vapor is also superheated to a limited extent by the time it reaches the outlet end. At the inlet of the evaporator, the refrigerant is predominantly in the liquid form with a small amount of vapor formed as a result of flashing at the expansion valve.
As the refrigerant passes through the evaporator, more and more liquid is vaporized by the load. The refrigerant, by the time it reaches the end of the evaporator, is purely in the vapor state and that too superheated. Thus the evaporator in its length is filled with a varying proportion of liquid and vapor. The amount of liquid in the evaporator will vary with the load on the evaporator. The inside of the evaporator is far from ‘dry’ but wetted with liquid. All the same, this type is called the ‘dry-expansion’ system to distinguish it from the ‘flooded’ system and also probably because by the time the refrigerant reaches the evaporator outlet it is no more wet (no liquid) but dry (superheated) vapor.
Evaporating Pan : ( Steam Jacketed Kettle)
The mechanism involved in this evaporation process is conduction and convection. The material is placed in the jacketed kettle. Steam provides heat to a jacketed kettle in which the aqueous extract is placed. The raised temperature increases the tendency of the solvent molecules to escape into the vapors.
The apparatus consists of a hemispherical or shallower pan which is made of copper or stainless steel. Copper is an excellent material for the kettle, because of its good conductivity. The hemispherical shape provides the best surface for eating and vaporization. It is surrounded by a jacket with a steam inlet. Steam Jacket is also attached to a condensate outlet and vent for non-condensable gases. The kettle is also fitted with an outlet at the bottom for the discharge of the product.
The solution or suspension is placed into the kettle. Steam is allowed to enter through inlet which provides heat to solution or suspension. The condensate leaves through the outlet. On a small scale, the kettle is used and also agitation is done manually or mechanically by stirrer. The final product is collected from the bottom.
Pharmaceutical Applications :
- It is suitable for concentrating aqueous liquids.
- It is also suitable for concentrating thermostable liquors.
Advantages of the Steam jacketed kettle :
- Simple in construciton
- Easy to operate
- Maintenance and installation cost is low
- The product can be easily removed.
- Used for small scale and large scale operations.
Disadvantages of steam jacketed kettle :
- Not suitable for heat sensitive products.
- The heating area is limited and decreases in proportion to the increase in the size of the Pan.
- Used only for aqueous liquids.
- Many products produce foam.
- Open pan leads to saturation of the atmosphere and product discomfort.
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