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PDH Course M244
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c. Screw compressors are available in capacities ranging from 20 to 1000 tons and even higher from few manufacturers. d. Screw compressors have an ability to operate at compression ratios of as high as 30 which can be varied for optimum load adjustment. e. The factors favoring screw compressors is their compact size, lightweight, quite & vibration free operation and high energy efficiency both in full and part load operation. f. The major drawback is their high cost. For smaller loads, reciprocating machines are less expensive to purchase and for large loads centrifugal machines cost less.
4) Scroll Compressors a. Scroll compressors have been used in commercial practice for systems that have capacity less than 30 TR. On such small sizes, these do not affect the life cycle economics drastically. COMPRESSOR HOUSING The assembly compressor and drive motor are known as the compressor unit. There are three main types , depending on the way the unit is built. 1) Open systems Open systems when the compressor and the motor are easily identifiable and the operating mode is by direct or belt drive; they present the problem that the rotating shaft may not be gastight. Open-type compressors can be used in all commercial refrigeration plants except small ones. They are particularly useful when ammonia is the refrigerant fluid. 2) Semi-hermetic or accessible units Semi-hermetic units are those where the compressor and the drive motor are mounted in the same crankcase and hence the electric motor operates in a refrigerant atmosphere. Semi-hermetic compressors are recommended for medium-capacity installations and should operate with halocarbon refrigerants. 3) Hermetically sealed or non-accessible units Hermetically sealed units are those where both elements are enclosed in a completely gastight steel casing, eliminating any risk of refrigerant leakage, but making any repair in the unit practically impossible. Hermetic compressor systems should be used for very small refrigerating capacities. Hermetic compressors and large semi-hermetic (above about 8 kW motor power) compressors are usually suction-gas cooled i.e. the refrigerant cools the motor before compression. This reduces the capacity of the compressor. Externally cooled types, where the vapor passes directly into the cylinders, are usually above 8% more efficient than equivalent suction cooled models. Open-type compressor units have several advantages over the other two types, making their installation advisable particularly in large refrigeration plants where skilled labor and maintenance personnel a re employed. First, the electric motor is not in contact with the refrigerant vapor so the heat dissipa ted neither increases its temperature nor is it discharged by the compressor. Second, they are driven by standar d electric motors which are easily replaced, and even the winding can be easily repaired. Third, the electric motor does not contaminate or affect the refrigerating circuit. Lastly, they are less sensitive to system contamination so the erection of the refrigerating circuit does not demand extreme care and skilled operators.
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PDH Course M244
www.PDHonline.org
c. Screw compressors are available in capacities ranging from 20 to 1000 tons and even higher from few manufacturers. d. Screw compressors have an ability to operate at compression ratios of as high as 30 which can be varied for optimum load adjustment. e. The factors favoring screw compressors is their compact size, lightweight, quite & vibration free operation and high energy efficiency both in full and part load operation. f. The major drawback is their high cost. For smaller loads, reciprocating machines are less expensive to purchase and for large loads centrifugal machines cost less.
4) Scroll Compressors a. Scroll compressors have been used in commercial practice for systems that have capacity less than 30 TR. On such small sizes, these do not affect the life cycle economics drastically. COMPRESSOR HOUSING The assembly compressor and drive motor are known as the compressor unit. There are three main types , depending on the way the unit is built. 1) Open systems Open systems when the compressor and the motor are easily identifiable and the operating mode is by direct or belt drive; they present the problem that the rotating shaft may not be gastight. Open-type compressors can be used in all commercial refrigeration plants except small ones. They are particularly useful when ammonia is the refrigerant fluid. 2) Semi-hermetic or accessible units Semi-hermetic units are those where the compressor and the drive motor are mounted in the same crankcase and hence the electric motor operates in a refrigerant atmosphere. Semi-hermetic compressors are recommended for medium-capacity installations and should operate with halocarbon refrigerants. 3) Hermetically sealed or non-accessible units Hermetically sealed units are those where both elements are enclosed in a completely gastight steel casing, eliminating any risk of refrigerant leakage, but making any repair in the unit practically impossible. Hermetic compressor systems should be used for very small refrigerating capacities. Hermetic compressors and large semi-hermetic (above about 8 kW motor power) compressors are usually suction-gas cooled i.e. the refrigerant cools the motor before compression. This reduces the capacity of the compressor. Externally cooled types, where the vapor passes directly into the cylinders, are usually above 8% more efficient than equivalent suction cooled models. Open-type compressor units have several advantages over the other two types, making their installation advisable particularly in large refrigeration plants where skilled labor and maintenance personnel a re employed. First, the electric motor is not in contact with the refrigerant vapor so the heat dissipa ted neither increases its temperature nor is it discharged by the compressor. Second, they are driven by standar d electric motors which are easily replaced, and even the winding can be easily repaired. Third, the electric motor does not contaminate or affect the refrigerating circuit. Lastly, they are less sensitive to system contamination so the erection of the refrigerating circuit does not demand extreme care and skilled operators.
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