One of the more interesting suggestions made in recent years for improving the efficiency of compressed air generation is the closed loop or two level system.

The conventional compressed air system generates air at about 7 bar gauge pressure and when its energy has been extracted exhausts it to the atmosphere. As has previously been pointed out, much of the energy content of the compressed air is wasted through rejection of heat to the atmosphere and through the inability of most output devices to use the air expansively. The new development generates air at a much higher pressure than the conventional 7 bar and releases it, not to the atmosphere, but to an intermediate pressure, so that the pressure differential remains at 7 bar. A typical two level system might be one in which the main operating pressure was 12bar and the discharge pressure was 5 bar; the 7 bar differential is thereby retained. The 5 bar pressure would then be carried back to the compressor inlet by a separate line.

It is not immediately obvious why this is better than discharge to atmosphere. The reason lies in the thermodynamics of the compressor. The ratio of compression in this example is (12 + 1)/(5 + 1) = 2.2, instead of 8, which makes for a more efficient use of energy in the compressor. The power needed to drive a compressor is primarily dependent on the pressure ratio, but the energy available at the point of use is dependent on the pressure difference between inlet and exhaust. As indicated in the chapter on Energy Generation, most tools and other output devices work as if they were “hydraulic”, ie they are unable to use the compressive energy of the air. There is an advantage in salvaging some of that energy and putting it back into the compressor, a principle that is adopted in the two level system.

The theory of two level systems has been developed by the Fluid Power Centre (FPC) at Bath University under a contract from the British Compressed Air Society and the Department of the Environment. So far as is known, no company has yet ventured to install a practical embodiment of such a system, but because of its theoretical importance for power savings it is worth discussing it in detail in this volume. The BCAS are anxious to cooperate with a potential user. Unfortunately, until a company is found willing to invest in its practical development, it will remain an interesting academic exercise. Much of the theoretical treatment presented here relies on the analysis done by FPC and is necessarily provisional.

The main reason for considering two level working is its improved efficiency, but there

are other advantages too:

• The absence of discharge to atmosphere reduces noise and eliminates exhaust pollution.

• In the operation of cylinder actuators, there is increased damping due to the higher density of the air, and a shorter delay time between operation of the valve and the start of actuator motion.

• Less heat has to be disposed of by cooling

• The size of the moisture removal and filtration equipment is much reduced.

There are, of course, counterbalancing disadvantages which up to now have put off potential users:

• Compressors designed to operate in this mode are not yet available, but since the design is no more difficult than that of the second stage of a conventional compressor, they could be made available if the demand was there.

• Tools are not available. This is the more fundamental difficulty: there is little industrial experience in designing tools which operate with a back pressure at the exhaust. Simple rotary tools would appear to need only minor modifications, but the limited studies that have been made on percussive tools with a back pressure (mostly for use underwater at depth) indicate that a radical design change would be needed for satisfactory operation.

An extra hose would have to be used to return the exhaust. This would make tools less easy to handle.

• A double main would have to be installed, to take the exhaust air back to the compressor inlet and to provide a lower operative pressure for those cases in which the high pressure would be unsatisfactory, as for example for a blow gun or for pneumatic transport.

Some of these problems have been tackled by the Fluid Power Centre at Bath, but their final resolution awaits an order for the equipment.

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