COMPOUNDING OR TWO-STAGE COMPRESSION.
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 stroke of a single compressor is by the two-stage process greatly reduced.
Further compounding, for pressures above 100 pounds, becomes quite necessary to secure the advantages named hereafter; the two-stage has proved advantageous up to 500 lbs., three-stage up to 1,000 lbs., and four-stage compression up to 3,000 lbs.
Fig. 374.
As the pressures increase, however, the machines become more and more complicated, owing not only to the greater power required, but also to the heating of the air during compression. The use of water-jackets for cooling the air in the compression cylinders is general, but this does not effect thorough cooling, as only a small portion of the air in the cylinder comes in contact with the jacketed parts. This difficulty has led to the use of compound machines, in which case inter-coolers are generally used between the different stages of compression, which cause the air to shrink in volume between the stages.
Briefly summed up, the chief advantages of multi-stage over single-stage compression are:
1. Lower average temperature, resulting in lower average pressure, and permitting the compression of the same volume of air with less expenditure of energy.
2. Increased safety and ease of lubrication. When high final temperatures prevail, part of the lubricating oil vaporizes, and wear on the piston and cylinder becomes rapid. Under exceptional circumstances the combination of air and oil vapor may reach the proportions of an explosive mixture, and if the compression temperature passes its flash point damage may result. Such accidents are, however, very rare even in single-stage work; in multi-stage compression, with proper intercooling, they are impossible.
3. Greater effective capacity in free air. The final pressure in the low pressure cylinder is much lower than in a single-stage machine, and the air confined in the clearance spaces when expanded down to atmospheric pressure occupies comparatively little space. Consequently the inflow of air through the suction valves begins at an earlier point in the stroke.
4. 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.
A properly designed inter-cooler should reduce the air in the cylinders to the temperature of the outside air. The economy of compressing in several stages—or, in other words, compound compressors—is shown from the fact that in compressing air up to 100 lbs. the heat loss reaches about 30 per cent. By compressing in two stages, this loss is cut down to less than half; and in four stages, it is reduced to four or five per cent. It is evident, therefore, that the higher the pressure required the more essential is the use of compound machines.
The inter-cooler is the vital feature of the two-stage or multi-stage machine. In this construction the air is partially compressed in one cylinder; it is then passed through an inter-cooler where it is cooled and finally is compressed to the desired degree in the second or other additional cylinders.
An inter-cooler is shown in Fig. [373]. The cooling surface consists of a nest of small brass water tubes. These tubes break up the stream of air entering the cooler, while their thin walls insure rapid conduction. The receiver volume formed by the connecting pipes and inter-cooler body results in a nearly uniform discharge pressure in the low-pressure cylinder. The air being outside of the tubes encounters practically no frictional resistance, and its slow passage allows time for cooling. A pocket, with gauge glass attached, is so placed as to catch any precipitated moisture which might otherwise enter the high-pressure cylinder.
An after-cooler is shown in Fig. [374]. This serves to reduce the temperature of the air after the final compression.
The heat of compression, as may be judged from the foregoing, relating to inter and after-coolers is a feature of interest. The temperature to which it finally attains depends, 1, upon the initial temperature; 2, upon the degree of compression, or in other words, the amount of work expended upon the compression.
The extent of this heating is shown in the following table, for dry air when compression is performed with no cooling.
| Temperature of air before compression, | 60° | 90° |
| Temperature of air compressed to 15 lbs. | 177° | 212° |
| „„„„ 30 lbs. | 255° | 294° |
| „„„„ 45 lbs. | 317° | 362° |
| „„„„ 60 lbs. | 369° | 417° |
| „„„„ 75 lbs. | 416° | 465° |
| „„„„ 90 lbs. | 455° | 507° |
| „„„„ 105 lbs. | 490° | 545° |
| „„„„ 120 lbs. | 524° | 580° |
The Norwalk compound compressor is shown in outline by the cut 375. The large air cylinder on the left determines the capacity of the compressor; for illustration assume its piston at 100 square inches area; the small air cylinder can have an area of thirty-three and one-third square inches.
The small piston only encounters the heaviest pressure; at 100 pounds pressure the resistance to its advance is 3,333 pounds. The resistance against the large piston is its area multiplied by the pressure which is caused by forcing the air from the large cylinder into the smaller cylinder. In this case it is thirty pounds per square inch. But as this thirty pounds pressure acts on the back of the small piston, and hence assists the machine, the net resistance to forcing the air from the large into the small cylinder is equal to the difference of the area of the two pistons multiplied by the thirty pounds pressure. This is sixty-six and two-thirds by thirty, and equals 2,000 pounds.
Hence 2,000 pounds, the resistance to forcing the air from the larger into the smaller cylinder, plus 3,333 pounds, the resistance in the smaller cylinder to compressing it to 100 pounds, is the sum of all the resistances in the compound cylinders at the time of greatest effort. This is 5,333 pounds. By thus reducing the work to be done at the end of the stroke, more work is done in the first part, and the resistance is made nearly uniform for the whole stroke.
Fig. 375.
Note.—Arrows on the water pipes show the direction of the water circulation. When the pistons move as indicated by the arrow on the piston rod, steam and air circulate in the direction shown by arrows in the cylinders.
A—Inlet Conduit for Cold Air.
B—Removable Hoods of Wood.
C—Inlet Valve.
D—Intake Cylinder.
E—Discharge Valve.
F—Inter-cooler.
G—Compressing Cylinder.
H—Discharge Air Pipe.
J—Steam Cylinder.
K—Steam Pipe.
L—Exhaust Steam Pipe.
N—Swivel Connection for Crosshead.
O—Air Relief Valve, to effect easy starting after stopping with all pressure on pipes.
1—Cold Water pipe to Cooling Jacket.
2 and 3—Water Pipes.
4—Water Overflow or discharge.
5—Stone on end of Foundation.
6—Foundation.
7—Space to get at Underside of Cylinder.
8—Floor Line.