AIR SPRINGS AND CUSHIONS
UNLIKE water, air is a highly compressible, elastic fluid, and as such furnishes an excellent medium for the storage of energy. It acts just like a clock spring into which energy may be introduced and stored by winding or compressing the spring. The energy remains locked up in the spring and when the spring is released, it gives back just as much energy as was put into it, except for slight frictional and heat losses. Air is a much better spring than steel or any other metal because it never loses its elasticity from fatigue and because it has an enormous capacity for the storage of energy. It possesses one serious drawback, however. Much of the energy that is expended in compressing it is converted into heat. If the air were to be used immediately and without transmitting it to a distance, there would be no advantage in extracting the heat, but heat cannot be stored in air for long and it would gradually escape from the storage reservoir or air receiver and from the pipes leading the air to the machines that it was to operate. As the heat escaped it would lower the pressure of the air and hence much of the energy would be lost.
HEAT OF COMPRESSION
In Chapter V we described a hydraulic system of compressing air and noted that one of the advantages of this system is that it delivers air cooled to the temperature of the water. This, of course, does not mean that there is no loss due to heat. The air bubbles as they are compressed are cooled by the water that compresses them; in other words, the heat of compression passes off into the water, making the water warmer than it would otherwise be. Heat is not produced without expenditure of energy and a certain proportion of the water power is thus wasted.
In most cases air is compressed by steam or electrically driven compressors, and in such machines the heat due to compression is a serious matter. The air cylinders are water-jacketed to carry off the heat. But air is a poor conductor; it acquires heat faster than it can give it off to the water surrounding the cylinders. In compound air compressors the air compressed in one cylinder is cooled before being passed on to the next cylinder, where it is further compressed. The heat loss in compressing air in a single stage up to 100 pounds gauge pressure is about 30 per cent. Air that enters a compressor at the normal pressure of the atmosphere and with an initial temperature of 60 degrees F., if not cooled will become heated to 415 pounds gauge pressure. The higher the initial heat of the air, the greater the rise of temperature. If a volume of air be subjected to 294 pounds gauge pressure, it will occupy about one-tenth of its former volume. If the air was introduced into the compressor at zero, it would acquire a temperature of about 650 degrees; if introduced at 60 degrees, it would show an increase of about 800 degrees; and, if started at 100 degrees, it would show an increase of 900 degrees in passing through the compressor. This shows the advantage of compressing the air in stages and cooling the air between stages.
Sometimes the heat developed is sufficient to produce a disastrous explosion. Air is noncombustible, but the oils used to lubricate the compressor are vaporized by the heat and when mixed with air form a powerful explosive. Care has to be taken that none but high-grade oil with a high flash point be used in the compressor and that the temperature of compression does not rise to near the flash point lest the vaporized oil be ignited.
That fire can be produced by sudden compression of air has long been known in the Philippine Islands. The natives use a small air tube with a close-fitting plunger. Combustible matter is placed in the bottom of the tube, and on striking the plunger a sharp blow this is ignited.
When compressed air is used in an air motor it expands and in so doing absorbs heat. The more rapidly it expands, the more heat it absorbs. This heat it extracts from the motor and from the atmosphere into which it escapes, and it is a common occurrence to find a thick coating of frost around the exhaust port. This is due to condensation of moisture in the atmosphere or in the compressed air itself, which, because of the rapid extraction of heat, is converted into snow. On cold days enough frost may be produced in the exhaust pipe to clog it and interrupt the operation of the motor, and frost sometimes clogs the air lines leading to the motor.
WHARF ON THE PACIFIC COAST PROTECTED BY A PNEUMATIC BREAKWATER