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
Air issuing from the submerged perforated pipes breaks up the waves
GENERAL VIEW OF A SUN POWER PLANT IN EGYPT
CLOSE-UP OF ONE OF THE REFLECTORS SHOWING THE BOILER
The fact that compressed air on expanding is cooled, makes it an ideal power for use in mining machinery. Steam is inconvenient because of the difficulty of transmitting it to the machines without loss of heat, because it would heat the machines so that they cannot be handled readily and because the exhaust steam would fill the mine with an impenetrable fog; electricity is dangerous in mines that are apt to contain explosive gases, because the wires are liable to be broken and cause sparks by short circuiting, and because sparks are likely to form between the brushes and commutators of the motors; but compressed air has none of these objections, and, furthermore, the discharge from the machines furnishes the operators with ample supplies of fresh cool air which drives out disagreeable and dangerous gases.