valves for air conditioning and refrigeration. One example are the valves related to piston compressors. 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. The smallest compressors have spring steel reed valves, both inlet and outlet in the cylinder head and arranged on a valve plate.  Above a bore of about 40 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 . 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. Although intended to handle only dry gas, liquid refrigerant or traces of oil may sometimes enter the cylinder and must pass out through the discharge valves. These may be arranged on a spring-loaded head, which will lift and relieve excessive pressures. Some makes also have an internal safety valve to release gas pressure from the discharge back to the suction inlet. An alternative valve design uses a conical discharge valve in the centre of the cylinder head, with a ring plate suction valve surrounding it. This construction is used in compressor bores up to 75 mm. Valve and cylinder head design is very much influenced by the need to keep the volumetric clearance (q.v.) to a minimum.

(Ref.: Refrigeration and air-conditioning by Trott and Welch)