These indicator cards show at a glance that heat is responsible for the important air losses, and that so far as the design of the compressing engine is concerned, we have attained a point very near perfection.  All the devices, past, present and future, on which inventors spend so much time, and in the development of which capitalists are innocently inveigled, aim to save this six per cent. loss! We hear a good deal about “Centrifugal Air Compressors,” “Rotaries,” “Plunger Pumps,” etc., designs involving expensive complications without any heat advantage, and which seem to be based upon the “iridescent dream” of a large loss in the present method of compressing air.  Here we have a simple engine, compact and complete in itself, capable of high speed without injury, constructed on the basis of our best steam engine practice, which produces compressed air power at a loss of only six per cent.

Clearance is not taken into consideration in the foregoing figures, but clearance is very much more of a bete noir in theory than in practice.  The early designers, as shown in the “Dubois-Francois” illustrations, Figs. 3 and 4, regarded clearance loss as a very serious matter.  Even at the present time some air compressor manufacturers admit water through the inlet valves into the air cylinder, not so much for the purpose of cooling as to fill up the clearance space.  A long stroke involving expensive construction is usually justified by the claim that a large saving is effected by reduced clearance loss.  Let us see what the effect of this clearance is.  Assuming that we have an air compressor which shows an isothermal pressure line, there would be some loss of power due to clearance space, because we would have a certain volume of air upon which work was done and heat produced, that heat having been absorbed and the air being retained in the cylinder and not serving any useful purpose.  But let us assume that we have a compressor which shows an adiabatic pressure line.  We now have the air in the clearance space acting precisely as a spring, compressed at each stroke, retaining its heat of compression, and giving it out against the air piston at the point when the stroke is reversed.  There is no loss of power in such a case as this, but, on the contrary, the air spring is useful in overcoming the inertia of the piston and moving parts.  The best air compressors give a result about midway between the isothermal and the adiabatic, and the net loss of power directly due to clearance is so small as to be practically unworthy of consideration.

It must not be inferred from the preceding remarks that the designer of an air compressor may neglect the question of clearance.  On the contrary, it is a very important consideration.  If we assume a large clearance space in the end of an air cylinder of a compressor which is furnishing air at a high pressure, we may readily conceive that space to be so large, and that pressure so high, that the entire volume of the cylinder would be filled by the air from the clearance space alone, and the compressor would take in no free air and would, of course, produce no compressed air.