Tham khảo tài liệu 'refrigeration and air conditioning 3 e part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Table Model No. PLE08 Suction Condensing at Temperature Pressure 30 C 35 c Capacity Power Capacity -50 -45 -40 -35 -30 40 C 45 c Power Capacity Power Capacity Power 54 Refrigeration and Air-Conditioning Compressors 55 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 Figure . The main refrigerant for packaged water chillers of the centrifugal type are R123 and R134a. 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. There are no components which require lubrication with the exception of the main bearings. As a result the machine can run almost oil-free. 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 .
