In the first century of development, compressors for higher capacity were made larger, 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 tend to be multicylinder, with bores not larger than 175 mm and running at higher shaft speeds. Machines of four, six and eight cylinders are common. These are arranged in a multibank configuration with two, three or four connecting rods on the same throw of the crankshaft to give a short, rigid machine (see Figure 4.4).
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, of two, three or four cylinders (see Figure 4.5).