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.
[Illustration: 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.
[Illustration: 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.