Page 148 - Introduction to Marine Engineering

Chapter 7

Auxiliaries

Machinery, other than the main propulsion unit, is usually called
'auxiliary' even though without some auxiliaries the main machinery
would not operate for long. The items considered are air compressors, heat
exchangers, distillation equipment, oil/water separators, sewage treatment plants
and incinerators.

Air compressor

Compressed air has many uses on board ship, ranging from diesel
engine starting to the cleaning of machinery during maintenance. The
air pressures of 25 bar or more are usually provided in multi-stage
machines. Here the air is compressed in the first stage, cooled and
compressed to a higher pressure in the next stage, and so on. The
two-stage crank machine is probably the most common, and one type is
shown in Figure 7.1.
Air is drawn in on the suction stroke through the first-stage suction
valve via the silencer/filter. The suction valve closes on the piston
upstroke and the air is compressed. The compressed air, having reached
its first-stage pressure, passes through the delivery valve to the
first-stage cooler. The second-stage suction and compression now take
place in a similar manner, achieving a much higher pressure in the
smaller, second-stage cylinder. After passing through the second-stage
delivery valve, the air is again cooled and delivered to the storage system.
The machine has a rigid crankcase which provides support for the
three crankshaft bearings. The cylinder block is located above and
replaceable liners are fitted in the cylinder block. The running gear
consists of pistons, connecting rods and the one-piece, two-throw
crankshaft. The first-stage cylinder head is located on the cylinder block
and the second-stage cylinder head is mounted on the first: each of the
heads carries its suction and delivery valves. A chain-driven rotary-gear
pump provides lubricating oil to the main bearings and through
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