Page 149 - Introduction to Marine Engineering

Auxiliaries 135

Second stage
delivery valve
First stage
suction valve
Second stage
suction valve First stage
delivery valve

Second stage
piston assembly

Hand drain valve

Oil pump

Figure 7.1 Two-stage air compressor

internally drilled passages in the crankshaft to both connecting rod
bearings. Cooling water is supplied either from an integral pump or the
machinery space system. The water passes into the cylinder block which
contains both stage coolers and then into the first and second stage
cylinder heads. A water jacket safety valve (Figure 7.2) prevents a
build-up of pressure should a cooler tube burst and compressed air
escape. Relief valves are fitted to the first and second-stage air outlets
and are designed to lift at 10% excess pressure. A fusible plug is fitted
after the second-stage cooler to limit delivered air temperature and thus
protect the compressed-air reservoirs and pipework.
Cooler drain valves are fitted to compressors. When these are open
the machine is 'unloaded' and does not produce compressed air. A
compressor when started must always be in the unloaded condition. This