ABSTRACT
Reciprocating Machines The reciprocating machine is represented by the conventional engine type of machine with pistons and valves, and also by diaphragm types where the compressing element, either piston or cam, is separated from the compressed gas by a flexible membrane. The latter design is used where contamination of the compressed gas is undesirable. There are also certain types of piston machines which, when fitted with carbon or PTFE rings and special seals, can also be used for the compression of highly inflammable or toxic gases, or to provide ‘clean’ compressed air. The diaphragm compressor is used for small flows and at relatively low pressures because of the limiting strength of the membranes, whereas the piston type machine can be found working at pressures up to 40 000 bar. The rotational speed of these machines can vary from a few hundred to several thousand revolutions per minute. From this, it will be seen that the acoustic signature of a positive displacement compressor can vary from a slowrunning piston compressor to that of a multi-cylinder high speed compressor, as shown in Fig 1. Between these two extremes is a multiplicity of shapes, sizes and considerations, all of which have in
common certain principal sources of noise. These are inlet noise, noise radiating from the valve chest and crankcase, outlet noise, motor noise and noise from the motor cooling fan.
