t them from closing properly, which in turn can lead to
These carbon like deposits can become extremely hot on valves vapour mixtures, leading to possible fires and explosions in
never be run with the air intake filter removed.
Air temperature
Second stage
16
bar Third stage barent of a failure of a bursting disc a thicker one must not be used The above figures are for a salt water temperature of about
16°C.
Final air temperature at exit from the after-cooler is generally at or below atmospheric temperature.ain factors govern the choice of lubricant for the cylinders of air compressor, these are:
Fitted after each cooler is a drain valve, these are essential. To
Operating temperature, cylinder pressures and air condition. emphasise, if we consider
30
m3 of free air relative humidity 75%temperature being compressed every minute to about
10
bar, about of water would be obtained each minute.cts oil viscosity and deposit formation. If the temperature is Drains and valves to air storage unit must be open upon starting
this results in low oil viscosity, very easy oil distribution, low up the compressor in order to get rid of accumulated moisture. .
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Cylinder Pressures
If these are high the oil requires to have a high film strength ensure the maintenance of an adequate oil film between the rings and the cylinder walls.
Air Condition
Air contains moisture that can condense out. Straight mineral would be washed off surfaces by the moisture and this could t o excessive wear and possible rusting. T o prevent this compounded oil with a rust inhibitor additive would be used. Compounding agents may be from 5 to 25% of non-mineral oil, which is added to a mineral oil blend. Fatty oils are commonly added to lubricating oil that must lubricate in the presence of water, they form an emulsion which adheres to the surface to be lubricated.
Two Stage Air Compressor
Most modem diesel engines use starting air at a pressure of about 26 bar and to achieve this a two stage type of compressor would be adequate. These compressors are generally of the reciprocating type, with various possible arrangements of the cylinders, or they could be a combination of a rotary first stage followed by a reciprocating high pressure stage. This latter arrangement leads to a compact, high delivery rate compressor.
Fig. 124. shows a typical two-stage reciprocating type of air compressor, the pressures and temperatures at the various points would be approximately as follows:
Air temperature
pressure Before the coolers After the coolers
First stage 4 bar 130°C 35°C
Second stage 26 bar 130°C 35°C
Compressor Valves
Simple suction and discharge valves are shown in Fig. 125. These would be suitable diagrams for reproduction in an examination. Modem valves are somewhat more streamlined and lighter in order to reduce friction losses and valve inertia. Materials used in the construction are generally:
NCILLARY SUPPLY SYSTEMS 203
FIG 124
2 STAGE AIR COMPRESSOR
to
outlet
stage
I
Valve Seat0.4% carbon steel hardened and polished working surfaces. Valve
Nickel steel, chrome vanadium steel or stainless steel, hardened and ground, then finally polished to a mirror
Spring
Hardened steel. (N.B. all hardened steel would be tempered). Valve leakages do occur in practice and this leads to loss of efficiency and increase in running time.
Effects of Leaking Valves
1. First stage Suction
Reduced air delivery, increased running time and reduced pressure in the suction to the second stage. If the suction valve leaks badly it may completely unload the compressor.
2. First Stage Delivery
With high pressure air leaking back into the cylinder less air can be drawn in, this means reduced delivery and increased discharge
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relatively large compared to the compressor unit
that the number of starts per unit time is not too great. It FIG 127
remembered that the starting current for an electric motor is VALVE
double the normal running current. inlet
Constant Running Control
This method of control is the one most often used. The
runs continuously at a constant speed and when the desired pressure is reached the air compressor is unloaded in some way that no air is delivered and practically no work is done in compressor cylinders.
The methods used for compressor unloading vary, but that commonly used is on the suction side of the compressor. If compressor receives no air then it cannot deliver any. Or if the taken in at the suction is returned to the suction no air will delivered. In either case virtually no work would be done in compressor cylinder o r cylinders and this would provide economy compared to discharging high pressure air to the atmosphere through a relief valve.
Fig. 127. shows diagrammatically a compressor unloading valve fitted to the compressor suction. When the discharge air pressure
reaches a desired value it will act on the piston causing the spring disc in this
loaded valve close shutting off the supply of air to the the is
compressor. in the
An alternative method of unloading the compressor, while pressure can fall, the continuing to run it, is to hold the suction valve open. During of the
periods of unload the suction valve plates are held open by pins
operated by a relay valve and piston, not unlike that shown in Fig. FIG 128
127. When the pressure in the air reservoir falls to a preset level, AIR DRAIN
the unloading piston is vented and springs withdraw the pins holding allowing the suction valve to operate normally. Fig. 125.
Automatic Drain
Fig. 128. shows an automatic air drain trap which functions in a near similar way to a steam trap.
With water under pressure at the inlet the disc will lift, allowing the water to flow radially across the disc from A to the outlet B. When the water is discharged and air now flows radially outwards from A across the disc, the air expands increasing in velocity ramming air into C and the space above the disc, causing the disc
208 REED'S MOTOR ENGINEERING KNOWLEDGE
through which the air slowly leaks to outlet.
Obviously this gives an operational frequency to the and closing of the disc which is a function of various factors, size of groove, disc thickness, volume of space above the disc, It is therefore essential that the correct trap be fitted to the system to ensure efficient and effective operation.