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final evaporator

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Text of final evaporator

INDEX Introduction03 Classification Of Evaporators04 Some Commonly Used Evaporators13 Short Tube Vertical Evaporator Basket Tube Evaporator Long Tube Type Evaporator Rising Film Evaporator Falling Film Evaporator Working Of Evaporator17 Thermal Design Of Evaporator18 Capacity Of Evaporator18 Factors Effecting Heat Capacity Of Evaporator19 Estimation Of Heat Transfer Area And Heat Transfer Coefficient20 Air Side Heat Transfer Coefficient Liquid Side Heat Transfer Coefficient Boiling Heat Transfer Coefficient Wilsons Plot Frosting On Evaporators25 Methods Of Defrosting Of Evaporators25 References33

ACKNOWLEDGEMENTWe wish to convey our deep gratitude to Dr. H. K. Paliwal Sir, Head of Department Mechanical Engineering (IET Lucknow), as a guide & for the encouragement, unbridled support and constructive criticism at every step of seminar. He is an avid enthusiast of the new technology and supported our decisions to take up our seminar on the topic EVAPORATORS. He not only reviewed all draft of the manuscripts but have also given much useful advice, without the active help, our task would have become very difficult.

We would also like to thanks to our friends who went out their way to provide us necessary suggestion and help.

We sincerely hope that we have not let them down.

INTRODUCTION An evaporator, like condenser is also a heat exchanger. In an evaporator, the refrigerant boils or evaporates and in doing so absorbs heat from the substance being refrigerated. The name evaporator refers to the evaporation process occurring in the heat exchanger.

Evaporation plays a major role in manufacturing variety of products in chemical process industries including food processing, pulp & paper, pharmaceutical, fertilizers etc. The evaporator is an important device in a low pressure side of a refrigeration system the liquid refrigerant in the expansion valve enters into the evaporator where it boils and changes into vapor. The function of the evaporator is to absorb heat from the surrounding location or medium which is to be cooled, by means of a refrigerator temperature of the boiling refrigerant evaporator be always be less than the surrounding medium so that the heat flows to the refrigerant. Evaporator becomes cold and remains cold due to the following reasons:

1-The temperature of the evaporator coil is low due to the low temperature of the refrigerant inside coil.2-The temperature of the refrigerant remains unchanged because any heat it absorbs is converted to the latent heat as boiling proceeds.

Classification of evaporators

According to construction:I. Bare tube evaporatorII. Finned tube evaporatorIII. Plate evaporatorIV. Shell and tube evaporatorV. Double tube evaporator

According to manner in which liquid refrigerant is fed:I. Flooded evaporator ( flooded type shell and tube evaporator)II. Dry expansion evaporator ( expansion type shell and tube evaporator)

According to mode of heat transfer:I. Natural convection evaporatorII. Forced convection evaporator

1. Bare Tube Evaporator

The simplest type of evaporator is the bare tube coil evaporator, as shown in figure. The bare tube coil evaporators are also known as Prime Surface Evaporators. Because of its simple construction, the bare tube coil is easy to clean and defrost. A little consideration will show that this type of evaporator offers relatively little surface contact area as compare to other type of coils. The amount of surface area may be increased by simply extending the length of tube, but there are disadvantages of excessive tube length. The effective length of tube is limited by the capacity of expansion valve. If the tube is too long for the valves capacity, the liquid refrigerant will be then to completely vaporize early in its progress through the tube, thus leaving top excessive superheating at the outlet. The long tube will also cause considerably greater pressure drop between the inlet and outlet of evaporator. This results in a reduced suction line pressure.

The diameter of tube in relation to tube length may also be critical .If the tube diameter is too large, the refrigerant velocity will be too low and volume of refrigerant will be too great in relation to surface area of the tube to allow complete vaporization. This, in turn, may allow liquid refrigerant to enter the suction line with possible damage to the compressor (i.e. slugging). On the other hand, if the diameter is too small, the pressure due to friction is too high and will reduce the system efficiency.

The bare tube coil evaporators may be used for any type of refrigeration requirement. Its used is, however , limited to applications where the box temperatures are under 00 C and in liquid cooling ,because the accumulation of ice or frost on these evaporators has less effect on the heat transfer then on those equipped with fins. The bare tube coil evaporators are also extensively used in house hold refrigerators because they are easier to keep clean.

Figure No. 01 Bare Tube Evaporator

2 Finned Type Evaporators

These evaporators are used for cooling and dehumidifying the air directly by the refrigerant flowing in the tubes. Similar to fin-and-tube type condensers, this evaporator consists of coils placed in a number of rows with fins mounted on it to increase the heat transfer area. Various fin arrangements are used. Tubes with individual spiral straight fins or crimpled fins welded to it are used in some applications like ammonia. Plate fins accommodating a number of rows are used in air conditioning applications with ammonia as well as synthetic refrigerants such as fluorocarbon based refrigerants.

The liquid refrigerant enters from top through a thermostatic expansion valve as shown in Fig. 02 This arrangement makes the coil return to compressor better rather than feeding refrigerant from the bottom of the coil. When evaporator is close to the compressor, a direct expansion coil is used since the refrigerant lines are short, refrigerant leakage will be less and pressure drop is small. If the air-cooling is required away from the compressor, it is preferable to chill water and pump it to air-cooling coil to reduce the possibility of refrigerant leakage and excessive refrigerant pressure drop, which reduces the COP.

The fin spacing is kept large for larger tubes and small for smaller tubes. 50 to 500 fins per meter length of the tube are used in heat exchangers. In evaporators, the atmospheric water vapour condenses on the fins and tubes when the metal temperature is lower than dew point temperature. On the other hand frost may form on the tubes if the surface temperature is less than 0oC. Hence for low temperature coils a wide spacing with about 80 to 200 fins per m is used to avoid restriction of flow passage due to frost formation. In air-conditioning applications a typical fin spacing of 1.8 mm is used. Addition of fins beyond a certain value will not increase the capacity of evaporator by restricting the airflow. The frost layer has a poor thermal conductivity hence it decreases the overall heat transfer coefficient apart from restricting the flow. Therefore, for applications in freezers below 0oC, frequent defrosting of the evaporator is required.

Figure no.02 Finned Type Evaporators

3. Plate Surface Evaporators

These are also called Bonded Plate or Roll-Bond type evaporators. Two flat sheets of metal (usually aluminum) are embossed in such a manner that when these are welded together, the embossed portion of the two plates makes a passage for refrigerant to flow. This type is used in household refrigerators. Figure shows the schematic of a roll-bond type evaporator. In another type of plate surface evaporator, a serpentine tube is placed between two metal plates such that plates press on to the tube. The edges of the plates are welded together. The space between the plates is either filled with a eutectic solution or evacuated. The vacuum between the plates and atmospheric pressure outside presses the plates on to the refrigerant carrying tubes making a very good contact between them. If eutectic solution is filled into the void space, this also makes a good thermal contact between refrigerant carrying tubes and the plates. Further, it provides an additional thermal storage capacity during off-cycle and load shedding to maintain a uniform temperature. These evaporators are commonly used in refrigerated trucks. Fig 03 shows an embedded tube, plate surface evaporator.

Figure no. 03aFigure no.03 b

Figure no. 03 Plate Surface Evaporators

4. Shell and Tube type Evaporator

The shell-and-tube type evaporators are very efficient and require minimum floor space and headspace. These are easy to maintain, hence they are very widely used in medium to large capacity refrigeration systems. The shell-and-tube evaporators can be either dry type or flooded type. As the name implies, a shell-and-tube evaporator consists of a shell and a large number of straight tubes arranged parallel to each other. In dry expansion type, the refrigerant flows through the tubes while in flooded type the refrigerant is in the shell. A pump circulates the chilled water or brine

Figure no. 04 Shell and Tube type Evaporator

5. Double Tube Type Evaporator

This consists of two concentric tubes, the refrigerant flows through the annular passage while the liquid being chilled flows through the inner tube in counter flow. One design is shown in Fig. 05 in which the outer horizontal tubes are welded to vertical header tubes on either side. The inner tubes pass through the headers and are connected together by 180o bends. The refrigerant side is welded hence there is minimum possibility of leakage of refrigerant. These may be used in flooded