Introduction
Since the advent of the worldwide energy crisis, a great deal has been written about ways of reducing energy consumption. However a major source of conservation has been almost universally overlooked: the potential energy source in the waste heat produced by air compressors. Compressors, irrespective of type, are potential sources of the substantial recovery of energy (heat) and the information that follows should be carefully studied. It must be noted that there is no accepted measure of efficiency of a given recovery system and each installation must be considered individually, preferably in co-operation with the supplier. When planning to install a compressed air system, full consultation with compressor suppliers and heater and ventilating consultants should take place so that the air compressor is treated as part of the "Total Energy System".
Energy Conservation
The following points affect the power consumed and should be discussed with the supplier.
Oil Grade (Oil Flooded Units)
Oil grade has an influence on performance. High viscosity tends to improve air volume flow but increase the power. Too low a viscosity has the opposite effect. The choice at the moment is rather a subjective one and consultation with the compressor suppliers, as well as those of the downstream equipment, is advisable. Therefore using synthetic lubricants ensure that they comply with the compressor manufacturer's instructions and are compatible with all downstream equipment. The long intervals between oil changes permitted by synthetic lubricant manufacturers may lead to neglect of essential servicing of compressors, including filter changes, unless the maintenance schedule is strictly followed.
Control Systems
Most compressors run for much of their lives at less than full output. All are fitted with some form of control system which varies the volume of air delivered to suit demand so that a suitable delivery pressure is maintained. From an energy point of view it is important to understand the effect on power consumption so that the correct system of each application is used. An electrically driven compressor running for lengthy periods either partially or totally unloaded may still consume up to 75% of its full load power because of the low power factor and the lower efficiency of the motor that results.
Modulating and Offloading
The most common forms of control on air compressors are either modulating or offloading. Some compressor sets fit both and have a selector switch. With offloading, the compressor continues to run without compressing air. Both systems use a butterfly or other form of inlet throttling valve. When modulating, the opening of the valve is controlled to meet demand so that the delivery pressure is maintained constant. The alternative is offloading where the inlet valve is either fully open or fully closed. This needs an air receiver or large capacity piping system otherwise hunting may occur.
Stop/Start
In this case the compressor motor is stopped when a set pressure is achieved. The compressor will start up again after demand has caused a drop in pressure. This also requires an air receiver to prevent the compressor starting and stopping too frequently.
Automatic Dual Control
A refinement of the control system just described is automatic stop/start. This is a time which stops the set if it has been running offload for 10 minutes. This interval is chosen as most motor manufacturers recommend not more than 6 starts per hour. When demand returns the compressor restarts.
Variable Speed Control
The ideal control for positive displacement compressors is by varying speed. Variable speed electric motors have been expensive and their control systems bulky, however compact and lower cost variable speed controls are now available and in the long term this may become the standard for air compressor. These newer controls also provide a 'Soft Start' feature, thus allowing the compressor to be stopped completely and started again more than 6 times per hour.
Multi-Set Control
Particularly in installations where there is a wide fluctuation in demand, there is a strong case for using several smaller compressors with a master control. When one of the sets is stopped it uses not electricity and is not subject to war so that for example on a 2 set installation when only one set is running, power consumption must be 50% or less. Various forms of multi-set controls are available from pneumatic or electrical to sophisticated microprocessor controlled systems which elect the best mode of running to optimise power consumption; equalize the running hours etc.
Leaks
Compressed air leaks are not dangerous as with electrical leaks, nor are they messy as are hydraulic leaks, but they are a blatant waste of energy. It is a salutary experience to walk through a factory when there is no demand for compressed air but the compressors are working and to listen to the leaks. A quick check on how much compressed air is being wasted is made by timing when the compressor is on load and when off load (see testing the System). Leaks may also indicate potential sources of uncontrolled release of stored energy.
Pressure Drops in Pipes
Generally a ring main is the best arrangement for a factory compressed air system. The size of the pipe should give a maximum air velocity below 6 metres per second. For example, this means that for a 100 kW 265 L/s compressor set, the air main should be minimum of 80 mm bore. Justification of large air mains is seen when the pressure drop and power losses per 100 metres of air main for the 100 kW 7 bar set are tabulated as below:
| Pipe Nominal Bore mm | Pressure Drop per 100 metres in bar | Equivalent Power Loss in kW |
|---|
| 40 | 1.8 | 9.5 |
| 40 | 0.65 | 3.4 |
| 65 | 0.22 | 1.2 |
| 80 | 0.04 | 0.2 |
| 100 | 0.02 | 0.1 |
Heat Recovery
It is estimated that air compressors consume between 10 and 15% of all power used in a typical industrial plant. It is feasible to recover between 80 and 90% of the power input to the compressor with very little capital investment and with a very high rate of return on investment. Since the annual running cost of a compressor set approximates to its capital cost and normally over 90% of the energy consumed is rejected as unused heat, it is evident that heat recovery dramatically reduces the cost of using compressed air.
Heat Recovery from Air Cooled Compressors
A 100 kW set rejects 93 kW to the oil and air coolers and all this energy is available as heat in the cooling air and can be used for factory heating during part of the year. It is important that the compressors are sited so that the maximum use is made of the cooling air leaving the compressor set for factory heating. This air at the temperature of 50°C+ is ideal for factory heating. If necessary, this air can be ducted so that it can be diverted outside in summer. As packaged sets have low noise levels in the order of 75dB(A) at 1 metre, siting does not often cause problems. Additionally, it is not necessary when more than one compressor is involved to site the sets together. One could be new a source of high demand.
Heat Recovery from Water Cooled Compressors
When water cooling is used, then by use of an appropriate water/water heat exchanger the energy can be used to heat water for domestic or process use, thus allowing heat recovery during the whole year.
Special Arrangements for Heat Recovery
Special units are now available for connection to air cooled, oil flooded compressors which emit energy recovery during the whole year. The cooling oil is caused to by-pass the normal oil cooler and to enter an oil to water heat exchanger which extracts the heat and cools the oil. The hot water passes through a water to water heat exchanger in an insulated hot water accumulator thus heating the water for domestic of process use. Should the demand for hot water be less than the energy provided by the compressor oil, then special controls divert all or part of the oil through the normal oil cooler. To determine real values of heat recovery from a compressor, it is proposed that recoverable energy means energy that can be put into an energy recovery system and used for a purpose. Energy lost to the environment surrounding an air compressor is not considered recoverable. Based on this definition, losses that cannot be recovered include:
- Radiation and cooling air leakage from the compressor after it has been halted.
- Energy that cannot be recovered from the compressed air.
The first category is straightforward but the second is very complex. Laboratory tests, however, indicate that approximately 80% of the total energy used is recoverable as heat. Even larger savings are possible if the compressor is driven by an internal combustion engine. In such cases the exhaust heat from the engine is also recovered. In addition to oil or petrol fueled engines, units are available with engines modified to use natural gas which avoids the need for fuel storage.
