The compressed air is doubtless cooled before it gets even as far as the receiver, because so much water is tumbled over into the pipes with it, but to produce economical results the cooling should take place during compression.

Water and cast iron have about the same relative capacity for heat at equal volumes. In this water piston compressor we have only one cooling surface, which soon gets hot, while with a dry compressor, with water jacketed cylinders and heads, there are several cold metallic surfaces exposed on one side to the heat of compression, and on the other to a moving body of cold water.

But the water piston fraternity promptly brings forward the question of speed. They say that, admitting that the cooling surfaces are equal, we have in one case more time to absorb the heat than in the other. This is true, and here we come to an important class division in air compressing machinery - high speed and short stroke as against slow speed and long stroke. Hydraulic piston compressors are subject to the laws that govern piston pumps, and are, therefore, limited to a piston speed of about 100 feet per minute. It is quite out of the question to run them at much higher speed than this without shock to the engine and fluctuations of air pressure due to agitation of the water piston. The quantity of heat produced, that is, the degree of temperature reached, depends entirely upon the conditions in the air itself, as to density, temperature and moisture, and is entirely independent of speed. We have seen that it is possible to lose 21.3 per cent. of work when compressing air to five atmospheres without any cooling arrangements. With the best compressors of the dry system one-half of this loss is saved by water jacket absorption, so that we are left with about 11 per cent., which the slow moving compressor seeks to erase. We are quite safe in saying that the element of time alone in the stroke of an air compressor could not possibly effect a saving of more than half of this, or 5½ per cent. Now, in order to get this 5½ per cent. saving, we reduce the speed of an air-compressing engine from 350 feet per minute to 100 feet per minute. We must, therefore, in one case have a piston area three and one-half times that of the other in order to get the same capacity of air, and in doing this we build an engine of enormous proportions with heavy moving parts. We load it down with a large mass of water, which it must move back and forth during its work, and thus we produce a percentage of friction loss alone equal to twice or even three times the 5½ per cent. heat loss which is responsible for all this expense in first cost and in maintenance, but which really is not saved after all unless water injection in the form of spray also forms a part of the system.

It is obvious that cost of construction and maintenance have much to do with the commercial value of an air compressor. The hydraulic piston machine not only costs a great deal more in proportion to the power it produces, but it costs more to maintain it, and it costs more to run it. It is not an uncommon thing to hear engineers speak of the hydraulic piston compressor as the "most economical" machine for the purpose, but that it is so "expensive" and takes up so much room, and requires such expensive foundations that, unless persons are "willing to spend so much money," they had better take the next best thing, a high speed machine. We hear of "magnificent air-compressing engines, the largest in the country," and pilgrimages are made to see these artificial wonders when, not unlike the old pyramids, they represent a pile of inert matter - a monument to moneyed kings.

The hydraulic piston compressor has one solitary advantage, and that is, it has no dead spaces. It was conceived at a time when dead spaces were very serious conditions - were positive specters! Valves and other mechanism connected with the cylinder of an air compressor were once of such crude construction that it was impossible to reduce the clearance spaces to a reasonable point, and, furthermore, the valves were heavy and so complicated that anything like a high speed would either break them or wear them out rapidly, or derange them so that leakages would occur. But we have now reduced inlet and discharge valves and all other moving parts connected with an air cylinder to a point of extreme simplicity. Clearance space is in some cases destroyed altogether by what is, as it were, an elastic air head which is brought into direct contact with the piston. All this reduces clearance to so small a point that it has no influence of any consequence. The moving parts are made extremely simple, even arriving at a point where inlet valves are opened and closed by their natural inertia. Mr. Sturgeon, of England, has applied a most ingenious and successful inlet valve, which is opened and closed by the friction of the air piston rod through the gland.

We have, therefore, reached a point at which high speed is made possible.

Long-stroke air compressors are evidently objectionable on the basis of greater expense of construction. All the parts must be larger and heavier. The fly wheels are increased enormously in diameter and weight, and the strength of bearings must be enlarged in proportion. It is difficult to equalize power and resistance in air compressors with long strokes. The speed will be jerky, and when slow, the fly wheel rather retards than assists in the work of compression. This action tends to derange the parts and makes large bearings a necessity. The piston in a long-stroke compressor travels through considerable space before the pressure reaches a point where the discharge valve opens, and after reaching that point it has to go on still further against a prolonged uniform resistance. This makes rotative speed difficult. During the early part of the stroke, the energy of the steam piston must be stored up in the moving parts, to be given out when the steam pressure has been reduced through an early cut-off. With a short stroke and a large diameter of steam cylinder we are able to get steam economy or early cut-off and expansion without the complications of compounding.

[1]

I use material terms because they add to simplicity of expression and notwithstanding the fact that heat is vibration.

[2]

[Transcribers note: last digit illegible]

[Continued from SUPPLEMENT, No. 793, page 12677.]