Diaphragm compressors are positive displacement, oil-free types. They can be driven mechanically or by hydraulic pressure. Mechanical diaphragm compressors are manufactured in smaller capacities than hydraulically actuated units and consequently overlapping of the two types occurs only over a limited range.
Mechanical units are less expensive to produce, are very compact and can be used on applications involving sub-atmospheric pressures (see Figures 15 and 16).
Hydraulically actuated units are able more easily to generate high pressure than are mechanical units as the latter are usually limited by considerations of bearing loading.
Apart from the fact that the compact form of a diaphragm unit enables it to be directly built into the electric motor or driver (or hydraulic pump in the case of an hydraulically operated machine, the main advantages offered by diaphragm compressors are:
• Complete gas tightness with only static sealing involved.
• Isolation of the gas being handled and thus the possibility of 100% oil-free compressed gas supply.
The main limitations of the type are low delivery rates (to a maximum of about 30 l/min in a single unit) and a limited compression ratio (to about 4.0). The latter limitation applies particularly to mechanically operated diaphragms where the diaphragm itself is normally of synthetic rubber. Hydraulically-operated diaphragm compressors may employ metallic diaphragms and be capable of generating much higher pressures. A further method of obtaining high pressure is to enclose the compressor and driver within a pressure vessel. The vessel is pressurised to a level equal to that present in the inlet. The same pressure is thus present on the underside of the diaphragm, reducing the pressure differential on the diaphragm when working.