Compressor

“ FIG. 18.—HYDRAULIC AIR COMPRESSOR.
The illustration shows the general type of this compressor, though it has been subject to much modification in different places. In America, a plunger is used instead of a piston, and as it always moves in water the result is more satisfactory. The piston, or plunger, moves horizontally in the lower part of a U shaped cylinder. Water at all times surrounds the piston, and fills alternately the upper chambers. The free air is admitted through a valve on the side of each column and is discharged through the top. ”

“ A modification of the reciprocating pump was manufactured by the Sanitary Devices Manufacturing Company in which [145] light-weight poppet valves placed in the heads of the cylinder were used, as indicated in Fig. 79. Curves of the watts per cubic foot and efficiency of this type of compressor are shown in Fig. 80. It will be noted that this compressor shows a better efficiency than the air compressor at all degrees of vacuum and it is the best reciprocating pump that the writer has ever tested. ”

“ When the compressor is unloaded, it is evident that the function of the air piston is merely to force the compressed air through the discharge valves and passages from one end to the other until more compressed air is required, this being indicated by a fall in the receiver pressure. ”

“ The office of the air compressor is to store up air under high pressures, which can be utilized at a greater or less distance, without sustaining any loss by condensation in the pipes, as is the case of carrying steam in pipes long distances. ”

“ 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. ”

“ If W is the net disposable force on the shaft of the engine which works the compressor, v1 the volume of air at the compressor, p1. given by the compressor, and at the temperature of the surrounding air, and p0 the atmospheric pressure, the efficiency of the compressor, assuming the air to expand according to Boyle's law, is given by the well-known formula-- . Let p2 be the value to which the pressure is reduced by the loss of pressure at the end of the conduit, and v2 the volume which the air occupies at this pressure and at the same temperature ”

“ The two-stage or multi-stage system of air compression is used generally for high pressure work. The system is most usefully employed between 40 and 120 pounds gauge pressure. For the moderate working pressure of 90 to 100 lbs., the two-stage compression has demonstrated its efficiency chiefly for the reason, that the heat generated in the last half of the stroke75 of a single compressor is by the two-stage process greatly reduced. ”

“ The heat generated by compression in the cylinder, C, is absorbed by a spray of water from the pump, H, the vapor being carried along with the air through the pipe, R, to the chamber, Y, where it is separated, and falling to the bottom is circulated, as just described, by the pump, J. X is a small auxiliary air compressor, to obtain the necessary compression to start the engine, and is worked from the boiler, W. In future engines this compressor will be superseded by a specially designed injector, which will produce the necessary pressure at a considerable reduction in cost. ”

“ They are simple when they have but one cylinder, and duplex when they have two cylinders. A straight line or direct acting compressor is one in which the steam and air cylinders are set tandem. An indirect acting compressor is one in which the power is applied indirectly to the piston rod of the air cylinder through the medium of a crank. Mr. W. L. Saunders writes in regard to direct and indirect compression as follows:—
“The experience of American manufacturers, which has been more extensive than that of others, has proved the value of direct compression as distinguished from indirect. ”

“ In driving the Mont Cenis tunnel but579 16 per cent of the power developed was available, and up to 1880 the efficiency was extremely low; but to-day as high as 80 per cent is obtained. The air compressor is simply a force pump with ingenious devices to overcome the loss of energy. For ordinary use the air is compressed to a pressure of from 60 to 80 pounds per square inch. This is done in a single cylinder for low pressures, but for high pressures two cylinders are used. From the compressor the air is conducted to a reservoir, from which it is piped to the machine which it is to run. ”

“ The gas compressor is worked by a water motor, hot air or gas engine; a low pressure water motor may be efficiently driven by water from the main, but with large installations it is more economical to drive the compressor by a gas engine. To overcome the intermittent flow of gas caused by the stroke of the engine, a regulator on the floating bell principle is placed after the compressor; the pressure of gas in the apparatus governs automatically the flow of gas to the engine. ”

“ One such two-cycle motor that has been designed especially for automobile use employs a separate air compressor driven by the engine itself and used as the clutch and variable speed transmission of the car. The amount of pressure generated in the compressor is dependent upon the resistance offered to its operation—or, in other words, it increases with additional load carried by the motor. The compression, or compressed air, rather, is carried directly from the compressor to the cylinders of the motor, being admitted at the proper time by a rotary valve driven by the crank shaft. ”

“ A very ingenious apparatus for compressing air was invented in the earliest years of the iron age to furnish a continuous blast of air for the Catalan forges. This compressor, known as a “trompe,” can hardly be termed a machine because it contains no moving parts except water, which is the motive power, and the air which it traps and compresses. To understand its operation we must look into a peculiar property of water flowing out of a reservoir into a pipe or nozzle. There is a converging motion that tends to contract the jet of water just after it leaves the pipe. ”

“ These check valves lift alternately, and cushion on water; and as the compressed air is forced into the pipe connecting with the receiver, without a possibility of any of it escaping back into the receiving chambers, it is claimed that there is the smallest possible loss of power, and that the machine will give fully 90 per cent of steam power expended in the shape of compressed air. The compressor is compact in form, strongly made, simple in construction, and not liable to get out of order. ”

“ A. By use of the duplex governor the main reservoir pressure may be controlled at two different predetermined pressures; as when running along the excess or low pressure head controls the compressor, at the low pressure—usually ninety pounds—this being sufficient to keep the brakes released and fully charged; whereas, in lap position, as following a brake application, the maximum or high pressure head controls the compressor at the maximum pressure used—generally 130 pounds—this for a prompt release and quick recharge of the brakes. ”

“ The new air compressor herewith illustrated may be operated by steam or water power, and is available for work in mines, tunnels, or quarries, for driving rock drills, coal cutters, and hauling and pumping engines, working mining pumps, for use in factories, and in fact for all service where a safe and efficient power is required. The construction of the machine, the capacity of which differs according to the amount of power required, will readily be understood from the illustration. ”

“ The capacity of each compressor is 230 cubic feet.
MAIN LINE, PIPING AND WIRING FOR BLOCK AND INTERLOCKING SYSTEM, SHOWING JUNCTION BOX ON COLUMN
The motor-driven air compressors [Pg 142] are controlled by a governor which responds to a variation of air pressure of five pounds or less. When the pressure has reached a predetermined point the machine is stopped and the supply of cooling water shut off. When the pressure has fallen a given amount, the machine is started light, and when at full speed the load is thrown on and the cooling water circulation reëstablished. ”

“ Horse-Power.—Having shown how the quantity of air can be computed we are now in a position to estimate the horse-power of the air-compressor necessary to operate the tube. I say estimate, because we have to take into consideration the efficiency of the air-compressor, and that is not an absolutely fixed quantity: it varies with different types of machines and with their construction. When working with pressures of less than ten pounds, the friction of the machine is an important factor. ”

“ As every operating engineer is well aware, all machinery is kept in better condition and runs more economically, when it is housed in proper environment and receives skilled attention. The compressor, in this [Pg 66] case, need not be erected near the work, but may be placed some distance away, preferably in the power station, as the pipe line connecting the conveyor with the machine will have a very small bore, compared with the air pipe to the exhauster on a suction plant, and will also be inexpensive to erect and maintain. ”

“ It is economical to combine the pressure [Pg 85] and suction systems by arranging the air compressor to draw air from the vacuum receiver into the compressor cylinders whence it is returned to the pressure line.
Automatic valves keep the pressures in the pressure and vacuum sides of the system within pre-determined limits. “Make up” air is admitted by the automatic opening of an atmospheric valve when the pressure side of the system is low and the vacuum side high, so that the pump is deprived of sufficient air to operate the system efficiently. ”

“ The air delivered by a two-stage or multi-stage compressor is dryer than that furnished by a single cylinder. Under constant pressure the power of air to hold watery vapor decreases with its temperature, and during its passage through the inter-cooler much of the original moisture in the air is precipitated. Consequently less trouble is experienced from condensation in the discharge pipe. ”

“ Most artificial ice is made by what is called the compressor system. It is called the compressor system because the ammonia with which che freezing is done is first compressed by means of powerful steam pumps. These pumps then force it through pipes over which cold water is running. This cold water bath still further condenses the gas to a liquid state, which drives out all the heat there is in it. The illustration shows how water is kept playing on the pipes containing the compressed ammonia. ”

“ A. If an intermediate discharge valve breaks or sticks open, no air will be compressed by that end of the compressor where is located the defective valve, as the air will simply flow back and forth from the high to the low pressure cylinders; no air will be taken in from the atmosphere through the strainer as the pistons move from the defective valve.
169. Q. If a final discharge valve breaks, what effect will it have on the compressor?
A. Main reservoir air will be free to return to the high pressure cylinder as the high pressure piston moves from the defective valve ”

“ If the compressor is situated in such a position that most of the heat of compression is delivered at the jet, there is little to be gained by reheating. In most cases, however, the air has returned to normal temperature by the time it reaches the point where it is to be used, and, if a suitable air heater is installed at this point, the volume may be increased greatly by a comparatively small expenditure. ”

“ In the left-hand corner, at the back, is a large steam-engine, while a smaller one occupies the corner diagonally across. Shafts, wheels, and belting run to two large air-pumps and three steel compressors, each about the size and shape of a small travelling trunk, and used respectively for compressing oxygen, nitrous oxide, and ethylene. A fourth compressor, or double compressing chamber, is cylindrical in form, and is wrapped in thick white flannel. This is the source of the liquefied oxygen. The system which Professor Dewar has followed is not novel in its general principles, as he explains. ”

“ It is simply necessary to add enough weight to the moving parts, that is, to the piston, piston rod, fly wheel, etc., to cut off early in the stroke and secure rotative speed with the most economical results and with the cheapest construction. It is obvious that the theoretically perfect air compressor is a direct-acting one with a conical air cylinder, the base of the cone being nearest the steam cylinder. ”

“ With an electrically driven compressor, speed control is fixed within quite limited bounds and if direct expansion [51] system is used more power is purchased than needed for reasons already mentioned.
(10) The total cost for operating pumps and calcium taken collectively will be far less than the uncontrollable ammonia leakage. The coil attendants can be less skilled.
(11) Brine temperature 25 degrees below zero, Fahr., can be carried, sufficiently low for all purposes. Rooms can not be controlled at as exact temperatures with direct expansion as with brine, owing to its sensitiveness. ”

“ The steam end and valve motion are the regular Deane pattern, assuring positive operation. The air cylinder is provided with a water jacket.
Fig. 372.
A power wall or post air compressor is shown in Figs. 370-372. The machine is single acting and is recommended where little space is available, as it can be bolted to the wall or to a post, or on the under side of the ceiling. The crank shaft and connecting rod are of cast steel. ”

“ For small installations, single-stage reciprocating machines, driven directly by steam engines or by electric motors are, no doubt, the most suitable. In the case of large plants, using the air continuously in a number of air jets, where the load factor is high, it is certainly more important to install a two or three-stage compressor, owing to the greater economy of working. The larger capital expenditure will be compensated by the considerable saving of energy. As compared with single-stage compression to 100 lbs. gauge pressure, a saving of 20 per cent. ”