Shipboard air compressors may be centrifugal, rotary, axial flow screw, or reciprocating. The more common reciprocating type of air compres-sor is generally selected for capacities from 100 to 800 cfm with outlet pressure ratings of 125, 600, and 3,000 psi, up to 5,000 psi. The rotary lobe type of air compressor is selected for capacities up to 8,800 cfm and for pressures of no more than 20 psi. The centrifugal type of air compressor is selected for capacities of 800 cfm or greater (up to 2,100 cfm in a single unit) and for pressures up to 125 psi. The axial flow screw type of air compressor is selected for capacities up to 100 cfm and for pressures of no more than 125 psi.

Most general-service air compressors aboard ship are the reciprocating type. In the recipro-cating air compressor, the air pressure is increased by the use of one or more cylinders. This is very much like the compression that takes place in an internal-combustion engine.

RECIPROCATING AIR COMPRESSORS

All reciprocating air compressors are similar in design and operation. The following discussion describes the basic components and principles of operation of low-, medium-, and high-pressure reciprocating air compressors.

CYLINDER ARRANGEMENT

Most low-pressure reciprocating air com-pressors are of the two-stage type. They have either a vertical V (fig. 14-1), a vertical W (fig. 14-2), or a vertical in-line arrangement

Figure 14-2.-Low-pressure reciprocating air compressor (vertical W configuration).

of cylinders. The V-type and in-line com-pressors have one cylinder for the first (lower pressure) stage of compression and one cylinder for the second (higher pressure) stage of compression. The W-type compressor has two cylinders for the first stage of com-pression and one cylinder for the second stage. The vertical W cylinder arrangement is shown in view A of figure 14-3. Notice that the pistons in the lower-pressure stage (1) have larger diameters than the pistons in the higher-pressure stage (2).

Medium-pressure air compressors are gen-erally the two-stage, vertical, duplex, single-acting type. Many medium-pressure compres-sors have differential pistons. This type of piston is used in machines designed for more than one stage of compression during each stroke of the piston. (See view A of fig. 14-3.)

Most high-pressure compressors are motor-driven, liquid-cooled, four-stage, single-acting units with vertical cylinders. View B of figure 14-3 shows the cylinder arrangements for the high-pressure air compressors installed in Navy ships. Small-capacity, high-pressure air systems may have three-stage compressors. Large-capacity, high-pressure air systems may be equipped with four-, five-, or six-stage compressors.