Volumetric efficiency Vh = Actual suction volume Vx / Theoretical suction volume Vs

For greater efficiency air compression should be isothermal as this requires the minimum work input.

In practice Isothermal compression is not possible, an ideal Isothermal cycle requires sufficient time to allow all the required heat to be transferred out of the cylinder, practicality dictates that the piston must have a relatively high speed to give a reasonable output.

 

Cylinder cooling on a single stage compressor gives better efficiency but there is a limitation in the surface area to cylinder volume that can be used for cooling effect, but multistage compressors with an efficient extended surface interstage cooler gives cycle improved compression efficiency better approaching that of the isothermal. In theory the greater the number of stages the closer the curve will approach the ideal isothermal compression curve, however there is an increase in cost, complexity, and the law of diminishing returns limit the number.

Compression in stages has the following advantages;

1. The compression ratio at each stage is lower and so the final temperature is lower. This reduces problems with lubrication
2. . The machine is smaller and better balanced
3. water can be drained off at each stage
4. Compression better approaches the ideal isothermal

It is important that the compressor clearance volume is kept small as possible in order to improve overall volumetric efficiency as the air trapped in this space must expand to below suction pressure before new air can enter, this is an effective loss of stroke.

A clearance is required in order to prevent the piston striking the cylinder cover when starting or stopping off load. The clearance volume is sometimes referred to as the 'Bump Clearance'.

Crankcase lubrication
Lubrication of the crankcase in a compressor does not pose any specific problems and normally consist of splash lubrication with pressurised oil being fed to shell bearings. Where drip cylinder lubrication is used, this should be kept to a minimum conducive with liner wear. A standard mineral oil similar to that used in the main engine may be used, although due to carbon deposits, higher quality oils are generally used with the most effective being specifically designed synthetics which have allow a considerable reduction in maintenance but are costly.

Mineral oils contain a blend of lighter elements such as paraffin's, and heavier elements such as asphaltenes. During compression the lighter elements are vaporised leaving the heavy ends, these coat the piston rings and discharge valves in combination with oxidised oil deposits. These deposits also coat passage ways and coolers resulting in higher interstage air temperatures. Deposits on discharge valves cause them to become sticky and leak resulting in hot air being drawn back into the cylinder for recompression. This increases the temperature and hence causes greater oxidation and deposits, and so the condition deteriorates with increasing rapidity.

Temperature can become very high, this may result in oily deposits at the discharge valves carbonising. Eventually this carbon could glow red and cause detonation. It is more likely, however, that oily deposits will be carried over to the air receiver and air start manifold to be ignited by blowpast at the cylinder air start valve.

Deposits at piston rings cause leakage allowing oil to enter the cylinder from the crankcase thus increasing the danger it is essential that crankcase lubrication be kept to a minimum compatible with an acceptable wear rate. Regular maintenance will minimise oily deposits build up and hence the risk of explosion