A much better plan is to arrange the compressor to run at a speed sufficient to give a slight reserve of air above what is needed and to have it running constantly during the whole working period.
This may seem at first sight an arbitrary attitude to take up, but it is the result of considerable experience, and need not be considered as extravagant when it is borne in mind that air compressors of more than 40 to 50 cubic feet per minute capacity are seldom employed for such work, and so the maximum power absorbed is light.
The compressor illustrated in Fig. 71 is designed and made by the Airostyle and Lithos, Ltd., and has a number of well thought out refinements, both as to accessibility and form of valve bodies and valves, and also with regard to the filtration of the air and the lubrication of the cylinder. Ring oiler bearings are employed for the crankshaft, and the compressor throughout is as carefully built as a gas engine. The makers claim that an absolutely pure supply of air is delivered by this compressor at all speeds up to 350 r.p.m.
Water cooling is employed and very ample cooling surface is provided, not only for the cylinder but also the cylinder cover is cooled, and has the valves located in pockets in same.
An air compressor is shown in Fig. 72, and another one, which is water cooled, in Fig. 73. These are made by the DeVilbiss Co.
In Fig. 74 are illustrated various forms of air compressors; a pressure gauge, safety valve and drain cock are furnished with each receiver. These tanks have welded seams and are tested to 200 lbs. pressure.
Fig. 75.—Aerograph Air Pump and Tank Combined.
Fig. 76.—The Aerograph Type of Air Pump for Larger Installations.