The Norwalk Compoand Air-Compressor is shown in outline in Fig. 8. The lettering on the cut refers to the several parts as follows: A, inlet conduit for cold air ; B, removable hoods of wood C. inlet valve: D, intake cylinder; E, discharge-valve; intercooler; a, compressing eylinder : H. discharge air-pipe : J. steam-cylinder; K steam-pipe ; L. exhaust steam-pipe; swivel connection for crosshead; 0, air relief-valve, to effect easy starting after stopping with all pressure on the pipes; 1, cold-water pipe to cooling jacket ; 2 and 3, water-pipe ; 4. water overflow or discharge; 5, stone on end of foundation ; 0, foundation ; 7. space to get at underside of cylinder; 8, iloor-line. Arrows on the water-pipes show the direction of water circulation. When pistons move as indicated by the arrow on the rod. steam and air circulate in direction shown by arrows in the cylinders. The air is admitted to the cylinder by valves of the well-known Corliss steam-engine pattern, which have a posi tive movement from the main shaft. The port is large, is clear of obstructions, and opens di rectly into the cylinder. The action of the forces in a compound air-compressor are described as follows: The large air-cylinder on the left determines the capacity of the compressor, and for illustration its piston is taken at 100 sq. in. area. The small air-cylinder in the center can have an area of 33i sq. in. The small piston only encounters the heaviest pressure, and at 100 lbs. pressure the resistance to its advance is 3,333 lbs. The resistance against the large piston is its area multiplied by the pressure which is caused by forcing the air frorn the large cylinder into the smeller cylinder, which is in this case 30 lbs. per sq. in. But as this 30 lbs. pressure acts on the back of the small piston and hence assists the machine. the net resistance of forc ing the air from the large into the small cylinder is equal to the difference of the area of the two pistons multiplied by the 30 lbs. pressure. This is (1t by 30 and equals 1.099 lbs. Hence 1,999 lbs. the resistance to forcing the air from the large into the smaller cylinder plus 3,333 lbs. the resistance in the smaller cylinder to compressing it to 100 lbs. is the sum of all the resistances in the compound cylinders at the time of greatest effort. This is 5,331 lbs. The time of greatest effort is at the end of the stroke, or when the engine is passing the center. In the single machine this resistance would be 10,000 lbs., hence in the compound machine the maximum strains are less by over 46 per cent, or nearly one half. By thus reducing the work to be done at the end of the stroke, more work is (lone in the first part of the stroke, and the resistance is made nearly uniform for the whole stroke.
Wider hrirelion_.—The practice of injecting water into the air-cylinders of compressors is now generally discontinued by American makers. The relative advantages and disadvan tages of this water injection are thus summed up by William L. Saunders, in his pamphlet. on Compressed-Air Production (1691): Two systems are in use by which the heat of compression is absorbed, and the difference between one and the other is so distinct that air-compressors are usually divided into two classes: 1, wet compressors ; 2, dry compressors. A wet. com pressor is that which introduces water directly into the air-eylinder (luring compression. A dry compressor is that which introduces no water into the air during compression. Wet com pressors may be subdivided into two classes: 1. those which inject water in the form of a spray into the cylinder during compression ; 2, those which use a wnter-piston fur forcing the air into confinement. The injection of water into the cylinder is usually known as the Col lation idea. Compressors built on this system have shown the highest isothermal results tht is, by means of a finely divided spray of cold water the heat of compression has been absorbed to a point, where the compressed air has been discharged at a temperature nearly equal to that at which it was admitted to the cylinder. The advantages of water injection during compression are as follows; 1. Low temperature of air during compression. 2. In creased volume of air per stroke, due to filling of clearance spaces with water and to a enhl-air cylinder. 3. Low temperature of air immediately after compression, thus condens ing moisture in the air-receiver. 'I. Low temperature of eylinder mid calves, thine main taining peeking, eta. 5. Economical results, due to compressien of moist air (see Table III). The first advantage is by far the most important one, and is really the only excuse for water injection in air-compressors. The percentage of work of compression which is con verted into heat and loss when no cooling system is used is as follows: Compressing to 2 atmospheres, loss 9.2 per cent ; to 3, 15-0 per cent ; to 4, 19.6 per cent ; to 5, 21-3 per cent ; to 6, 240 per cent ; to 7, 26.0 per cent ; to 8, 27.4 per cent. We see that in compressing air to five atmospheres, which is the usual practice, the heat loss is 21.3 per cent, so that, if we keep down the temperature of the air during compression to the isothermal line, we save this loss. The best practice in Anteriea has brought this heat loss down to 3.6 per cent (old Ingersoll injection air-compressor), while in Enrope the heat loss has been reduced to 1.6 per cent. Introducing water into the air-cylinder in any other way. except in the form of a spray. has hut little effect in cooling the air during compression. On the contrary, it is a most fal lacious system, beemcse it introduces all the disadvantages of water injection without its iso thermal influence. Water. by mere surface contact with the air, takes up but little heat, while the air, having a chance to increase its temperature, absorbs water through the affinity of air for moisture, and thus carries over a volume of saturated hot air into the receiver and pipes, which on cooling, as it always does in transit, deposits its moisture and gives trouble through water and freezing, It is therefore of much importance to bear in mind that, unless water can be introduced during compression, to such an extent as to keep down the temperature of the air in the cylinder, it had better not be introduced at all. If too little water is intro
duced into an air-cylinder during compression, the result is warm, moist air; and if too much water is used. it results in a surplus of power required to move a body of water which renders no useful service. Table 11 (p. 20) deduced from Zahner's formula gives the quantity of water which should be injected per cubic foot of air compressed in order to keep the temperature down to 104° F. Objections to water injections are as follows: 1, Impurities in the water, which, through both mechanical and chemical action, destroy exposed metallic surfaces. 2. Wear of cylinder, piston, and other parts, due directly to the fact that water is a bad lubricant, and, as the density of water is greater than that of oil. the latter floats on the water and has no chance to lubricate the moving parts. 3. Wet air arising from insufficient quantity of water and from inefficient means of ejec tion. 4. Mechanical complications connected with the water-pump, and the difficulties in the way of proportioning the volume of water and its temperature to the volume, tempera tore. and pressure of the air. 5. Loss of power required to overcome the inertia of the water. 6. Limit:Ulnas to the speed of the compressor, because of the liability to break the cylin der head-joint by water confined in the clearance spaces. 7. Absorption of air by water." \I r. John Darlington. of England. gives the following particulars of as modern air-com pressor of Eurolican type: " Engine, two vertical cylinders, steam jacketed with Meyer's ex pansion gear. Cylinders, 16.0 in. diameter, stroke 09.4 in, : compressor, two cylinders, diameter of piston. 23-0 in. ; stroke, in. revolutions per minute, 30 to 40 ; piston speed, 39 to 53 in. per second : capaeity of cylinder per revolution. 20 cubic ft. ; diameter of valves, viz., four inlet and four outlet, 51 in. ; weight of each inlet valve, 8 lbs. ; outlet. 10 lbs.; pressure of air, 4 to 5 atmospheres. The diagrams taken of the engine and compressor show that the work expended in compressing one pubic metre of air to 421 effective atmos pheres was 38,128 lbs. According to Boyle and Mariotte's law it would be 37.534 lbs., the difference being, 51I4 or a toss of I-6 per cent. (Jr if compressed without abstraction of heat, the work expended would in that ease have been 48,158. l'iie volume of air compressed per revobition was 0.5654 pubic metre. For obtaining this mensure of compressed air, the work expended was 21,557 lbs, The work done in the steam-cylinders, from indicator dia grams, is shown to have been 25,205 lbs., the useful effect being 85,1 per cent of the power expended. The temperature of air on entering the cylinder was 50° F., on leaving 62'F., or an increase of 12° F. Without the water-jacket and water injection for cooling the tem perature, it would have been 302° F. The water injected into the cylinders per revolution was 0.81 gallon." We have in the foregoing a remarkable isothermal result. The heat of compression is so thoroughly absorbed that the thermal loss is only 1.6 per cent.; but the loss by friction of the engine is 14-5 per cent, and the net economy of the whole system is no greater than that of the best American dry compressor, which loses about one half the theoreti cal loss due to heat of compression, but which makes up the difference by a low friction loss.
The wet compressor of the second class is the. water-piston compressor. In America, a plunger is used instead of a piston, and as it always moves in water the result is more satis factory. 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. The movement of the piston causes the water to rise on one side and fall on the other. As the water falls the space is occupied by free air, which is compressed when the motion of the piston is reversed and the water column raised. The discharge-valve is so proportioned that sonic of the water is carried out after the air has been discharged, Hence there are no clearance losses. Hydraulic piston compressors are subject to the laws that govern piston-pumps, and are therefore limited to a piston-speed of about 100 ft. per minute, It is 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. 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. but in which the friction loss alone is greater than the heat loss which is responsible for all the expense in first cost and in main tenance of such a compressor, but which really is not saved unless water injection in the form of spray forms a !tart of the system.
tr,sr Saunders, in his pamphlet, gives the following useful tables relating to the compression of air :