Book - School of Science and Technology

538 Refrigeration: water chillers and heat pumps0º 60º 120º300º240º180ºFigure 19.5 Scroll compressor, operating principleHermetic piston-type compressors are widely available up to 120 kW using R22 and areused in combination to make larger capacities. Such machines are obtainable built as aweatherproof unit complete with an air-cooled condenser. A low silhouette arrangementis available for installation on flat roofs. In split systems, where the evaporator andcondenser are located apart from the compressor or each other, the length of theconnecting refrigerant lines must be kept to within practical limits.RotaryIn effect another form of piston compressor, but without valves, this type was developedfor small scale applications. The assembly is contained in a cylindrical casing and dependsfor its operation on a shaft eccentric to the cylinder carrying a rotor which when rotatedproduces the compression effect.ScrollA robust and efficient type of machine, based upon the compression effect obtained whenan involute spiral is rotated within a second fixed volute and contained within fixed platesat either face, the gas being compressed as the volume is reduced closer to the centre of thescroll, Figure 19.5. Units are available in the range 50±170 kW, operating on R134a andR407C, for either air or water cooling.ScrewCompression is produced by rotating helical screws, the seal being achieved by oil. Aretractable vane enables load variation in a simple manner over a wide range. This form ofcompressor, Figure 19.6, may be used with a wide range of refrigerants from quite small