Experiments with two level operation
Some of the experiences with various kinds of compressors and output machines are worth listing as a measure of the achievements so far.
I. There will always be a certain amount of leakage from the system even if all unnecessary leaks are eliminated. No tool can be perfectly sealed and it is expected that there will always be some usage of air from the low pressure line. This loss is taken care of by having a make-up compressor in the low pressure line.
2. Most of the conventional types of displacement compressors have been examined.
With some modifications, it appears that they can all be used. With rotary displacement machines the built-in pressure ratio would need to be correct for the pressure difference between high and low. Throttling of the inlet would be an inappropriate method of control, so an onload/offload system seems the obvious choice.
3. Cylinder actuators present the least problem, provided that the pressure rating is adequate. In conventional operation, choking the exhaust is used to give speed stability; it appears that two level systems are equally suitable in this respect.
4. Percussive tools. Redesign is necessary, but tests so far performed on a chipping hammer indicate that this would present no major problem. Ordinary percussive tools are very leaky. The air escapes to atmosphere anyway, so there is no motivation to reduce the leakage rate. In a two level system, leakage would be much more serious, so FPC have modified their chipping hammer to ensure that leakage to atmosphere is practically eliminated.
5. Rotary motors show a Joss in power through inlet losses, which could be remedied by increasing port sizes.
6. Circulation of the lubricating oil is inevitable in a closed system. An oil-flooded compressor would seem to be an appropriate choice, since it could easily cope with the extra oil. Oil-free compressors would probably need to have an oil separator at their inlet. FPC claim to have had no problem in this respect.
General conclusions on two level systems
There are problems to be overcome, but none appears to be insuperable. The rewards in terms of fuel economy are considerable. FPC claim that a reasonable figure for energy savings is of the order of 40%. There will be capital savings in the compressor, aftercooler and dryer, only partly offset by the extra cost of running a second high pressure mains line. The concept is a challenge to an enterprising factory manager who is considering a new installation. BCAS are willing to give technical assistance to any company who is interested in pursuing the idea.