Air compressors, for starting air systems, are invariably of the reciprocating type. Although of slightly loss volume through-put than rotary, the reciprocating unit is more easily capable of developing the pressures required for starting air systems. Compression should, for maximum efficiency, follow the isothermal law, but in practice it is more closely aligned to the adiabatic curve, with the result that the delivery temperature is somewhat higher than is really desirable.
This high temperature has several undesirable side effects, not the least of which is to cause the temperature of the delivery valves to rise and encouraging their fouling up as the oil and dust bake out on the high temperature zones. Air at a high temperature is also less dense, so for a given volume there is a reduction in mass. To limit these problems, as far as is practicable, stage compression is often restored to. The benefit of this is that each stage is subjected to a low compression ratio so that terminal temperatures are limited and work input is also reduced. Stage compression also allows intercooling to take place between the stages so that the compression can be made to follow the isothermal curve more closely. This too limits the work input required so that either the compressor drive motor can be reduced or compression is quicker. Interstage cooling also causes condensation to occur so that some of the moisture may be drained out after the cooler, the net effect being that drier air is delivered to the receiver. The clearance volume is very important in the efficient operation of a compressor. Too large a clearance and the air trapped there at the end of a compression and delivery stroke will expand back to suction conditions before a fresh charge can be drawn in. This can dramatically reduce the volumetric efficiency of the compressor.
The clearance volume should be kept as small as is safely possible; too small and collision between the piston top and the cylinder cover may result. This problem may occur when bearing wear down takes place in the crankshaft system. The slack so generated may allow the piston to ‘throw’ itself up and collide with the cover. This dangerous condition is particularly apparent when the compressor is running unloaded (compression is not taking place and there is little or no resistance to piston movement). During compressor overhaul, the engineer should always check bearing clearances as well as bumping clearances (i.e. distance between piston top and cylinder cover at TDC).
The stage compression (and the relatively low compression ratios thereby available) also reduces the amount of air trapped in the clearance volume and hence again improves the volumetric efficiency over single stage compression. Compression in a single stage would, for a given clearance volume result in a large mass of air being trapped due to the high pressure (pv=mRT) with its consequent expansion greatly reducing the effective suction.
The valves used in both the suction and delivery sides of a compressor are of the plate type. These offer low inertia coupled with large area for flow for only a small lift. The result is rapid opening with minimum resistance to flow. (In a compressor running at 600 rev/min, the operating time for a delivery valve is in the region of 0,025 s).
The plates are subjected to shock loading both on opening and closing. To reduce this as much as possible the opening is cushioned, either by shaped pockets that reduce the impact of full opening or by ‘cushion’ plates that take the burnt of the impact and in so doing offer some protection to the valve plate itself. The impact on closing cannot be avoided and may in time lead to crazing of the sealing surfaces with cracking eventually taking place. For this reason, plates that have worn after subsequent lapping in should not be reversed; otherwise the rate of failure would be very rapid. The lapping in itself should be to obtain as smooth a surface as possible. A mirror-like finish will offer less drag to the air flow, improving efficiency and keeping the temperature of the air down.
The pipes from the compressor to the receiver should also be smooth and have as few restrictions as possible, allowing the air to flow freely to the receiver and entrain with it any solids, liquids etc. These can be separated put in the bottle relatively easily and should not be allowed to accumulate in the pipes where they would cause an obstruction (and possibly corrosion). For similar reasons, the portings in the valve bodies should be as contoured as possible, permitting the air to flow easily through the valve. Ports that are not adequately radiussed will cause turbulence within the air flow, acting not only as a restriction to flow but also tending to heat up the air and consequently the valve and plates.
The gum like deposits that appear on the valves is a combination of lubricating oil from the compressor, oil, and dust from the atmosphere. To limit this build up, piston lubrication should be kept to the minimum necessary to prevent wear (and sufficient to resist air leakage). Similarly, the suction filter should always be kept clean and, depending upon the type, slightly moist with oil.
Leaky valves will cause the compressor to run for longer periods before fully charging the receivers, as well as raising the temperature of the discharge air. Faulty valves are usually indicated by the above and by a change in interstage pressures relevant to the discharge pressure. The sooner they are rectified the better, as unfavourable conditions may occur if the compressor is run at length.
Amongst other things, leaky valves can result in the recycling of compressed air so that the temperature of the air continues to rise, and may develop to a point where any oil fuel vapour present is carried to its self-ignition temperature and detonation occurs. It is best to present this arising by regular inspection of running pressures and temperatures coupled with regular maintenance of the valves.
Lubrication of the piston has always been a problem. The use of self-lubricating materials (PTFE coatings ect.) is of great benefit. Where oil has to be used then the minimum commensurate with safety should be applied. If specific lubricating boxes are used for the piston then the choice of oil can be made to match as closely as possible the working conditions in the cylinder. If, however, some form of splash lubrication from the crankshaft system is adopted then a compromise will have to be made. The oil should be able to spread around the cylinder as well as possible to ensure maximum lubrication from minimum oil. The oil should also resist being washed off by the incoming wet air (second stage). An extension of this is that the oil should be able to protect the cylinder, rings etc. from corrosion during idle periods. Finally, the oil should have a low coking tendency so that gumming up of the valve plates does not readily occur. This latter requirement contradicts the view that the oil should have a high flash point. High flash point oils have, by nature, a high carbon content, which not only adversely affects valve carbonisation but also implies that the oil is viscous and may not spread easily around the rings and cylinder.
Air cooling is achieved in multi tube heat exchangers circulated with water. The straight tubes are easier to keep dean and offer small resistance to air flow. Should one perforate however, the pressure of the air would immediately be imposed upon the whole of the coolant passages (water being incompressible). This would put such a strain upon the relatively weak casings that rupture would almost certainly result were it not for the large area relief devices fitted to the water casing. These may take the form of spring loaded valves or bursting discs. In either case they offer, on lifting, a rapid release of volume and would drop the pressure quickly enough to protect the casing from damage. Nevertheless always check during overhaul that these devices are still operational.