Dynamic compressors impart energy to the gas by velocity or centrifugal force and then convert this to pressure energy. The most common type is the centrifugal compressor. Suction gas enters axially into the eye of a rotor which has curved blades, and is thrown out tangentially from the blade circumference.
The energy given to gas passing through such a machine depends on the velocity and density of the gas. Since the density is already fixed by the working conditions, the design performance of a centrifugal compressor will be decided by the rotor tip speed. Owing to the low density of gases used, tip speeds up to 300 m/s are common. At an electric motor speed of 2900rev/min, a single-stage machine would require an impeller 2 m in diameter. To reduce this to a more manageable size, drives are geared up from standard-speed motors or the supply frequency is changed to get higher motor speeds. The drive motor is integral with the compressor assembly and may be of the open or hermetic type. On single-stage centrifugal compressors for air-conditioning duty, rotor speeds are usually about 10 000 rev/min.
Gas may be compressed in two or more stages. The impellers are on the same shaft, giving a compact tandem arrangement with the gas from one stage passing directly to the next. The steps of compression are not very great and, if two-stage is used, the gas may pass from the first to the second without any inter-cooling.
Centrifugal machines can be built for industrial use with ammonia and other refrigerants, and these may have up to seven compression stages. With the high tip speeds in use, it is not practical to build a small machine, and the smallest available centrifugal compressor for refrigeration duty has a capacity of some 260kW. Semi-hermetic compressors are made up to 7000 kW and open drive machines up to 21 000 kW capacity. There are no components which require lubrication, with the exception of the main bearings. As a result, the machine can run almost oil free.
Systems of this size require large-diameter refrigerant suction and discharge pipes to connect the components of the complete system. As a result, and apart from large-scale industrial plants, they are almost invariably built up as liquid — cooling, water-cooled packages with the condenser and evaporator complete as part of a factory-built package. The main refrigerant for packaged water chillers of the centrifugal type is R134a.
The pumping characteristic of the centrifugal machine differs from the positive displacement compressor since, at excessively high discharge pressure, gas can slip backwards past the rotor. This characteristic makes the centrifugal compressor sensitive to the condensing condition, giving higher duty and a better coefficient of performance if the head pressure drops, while heavily penalizing performance if the head pressure rises. This will vary also with the angle of the capacity reduction blades. Excessive pressure will result in a reverse flow condition, which is followed a fraction of a second later by a boosted flow as the head pressure falls. The vapour surges, with alternate forward and reverse gas flow, throwing extra stress on the impeller and drive motor. Such running conditions are to be avoided as far as possible, by designing with an adequately low head pressure and by good maintenance of the condenser system. Rating curves indicate the stall or surge limit.
Figure 4.28 Centrifugal compressor with variable geometry, showing inlet guide vanes (labelled 3) and moveable wall diffuser (labelled 4) (Carrier)
Figure 4.29 Centrifugal compressor with variable high-speed DC drive and magnetic bearings (Danfoss)
Since centrifugal machines are too big to control by frequent stopping and restarting, some form of capacity reduction must be inbuilt. The general method is to throttle or deflect the flow of suction gas into the impeller. With most models it is possible to reduce the pumping capacity down to 10-15% of full flow. The availability of low-cost inverters has led to the use of variable speed drive which offers increased centrifugal compressor efficiency. However this cannot totally replace the need for variable inlet guide vanes because of early (low head) surge arising from low flow system head requirements (Brasz, 2007). An example of a centrifugal compressor with variable geometry is shown in Figure 4.28 . A recent entry to the field of compressors is a variable speed centrifugal compressor with a DC drive motor and magnetic bearings Figure 4.29. This opens up the possibility of oil free systems.
The jet compressor is a dynamic compressor at the other end of the size scale. At present it is a subject of research work and commercial introduction has not occurred. Its use as a way of enhancing the absorption cycle has been successfully tested (Eames, 2005) .
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