To attain a higher capacity, compressors were made larger during the first century of development, having cylinder bores up to 375 mm and running at speeds up to 400 rev/min. The resulting component parts were heavy and cumbersome. To take advantage of larger-scale production methods and provide interchangeability of parts, modern compressors are multi-cylinder, with bores not larger than 175 mm and running at higher shaft speeds. Machines of four, six and eight cylinders are common. Figure 4.6 shows a large reciprocating machine built with a welded steel crankcase, the largest of this type being suitable for up to 1000 kW refrigeration.
Cylinders are commonly arranged in banks with two, three or four connecting rods on the same throw of the crankshaft to give a short, rigid machine. This construction gives a large number of common parts — pistons, connecting rods, loose liners and valves — through a range of compressors, and such parts can be replaced if worn or damaged without removing the compressor body from its installation. Compressors for small systems will be simpler, consisting of two, three or four cylinders (see Figure 4.7).
Figure 4.6 Cut away view of industrial size multi-cylinder compressor (Grasso)
Figure 4.7 Cut away view of small commercial four-cylinder compressor (Bitzer)
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