VALVES

Piston compressors may be generally classified by the type of valve, and this depends on size, since a small swept volume requires a proportionally small inlet and outlet gas port. Small compressors have spring steel reed valves for both inlet and outlet arranged on a valve plate and the differing pressures kept

VALVES

Figure 4.8 Reed valve plate (Emerson Climate Technologies)

Separated by the cylinder head (Figure 4.8). Above a bore of about 80 mm, the port area available within the head size is insufficient for both inlet and outlet valves, and the inlet is moved to the piston crown or to an annulus surrounding the head. The outlet or discharge valve remains in the central part of the cylinder head. In most makes, both types of valve cover a ring of circular gas ports and so are made in annular form and generally termed ring plate valves (Figure 4.9). Ring plate valves are made of thin spring steel or titanium, limited in lift and damped by light springs to assist even closure and lessen bouncing.

VALVES

Figure 4.9 Ring plate valves (Grasso)

Although intended to handle only dry gas, droplets of liquid refrigerant or oil may sometimes enter the cylinder and must pass out through the discharge valves. On large compressors with annular valves, these may be arranged on a spring-loaded head, which will lift and relieve excessive pressures.

Valve and cylinder head design is very much influenced by the need to keep the clearance volume to a minimum. A valve design which achieves a small clearance volume uses a conical discharge valve in the centre of the cylinder head, with a ring-shaped suction valve surrounding it (Figure 4.10). The suc­tion gas enters via passageways within the ‘sandwich’ valve plate. The piston has a small raised spigot which fits inside the ring-shaped suction valve. When the conical discharge valve lifts, high-pressure gas passes into the cylinder head. This construction is used in compressor bores up to 75 mm.

VALVES

Figure 4.10 Conical discharge valve and sandwich type valve plate (Emerson Climate Technologies)

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