8. Q. What kind of oil should be used in the air end of the compressor
and on the swab?

A. Valve oil.

49. Q. How often should the air end of the compressor be oiled?

A. No fixed rule can be given as so much depends on the condition of the
compressor, as well as the amount of work required; but in any case it
should be used sparingly.

CROSS-COMPOUND COMPRESSOR

50. Q. What do Figures 4 and 5 represent?

A. These are diagramatic views of a cross-compound compressor.

51. Q. Why is this called a cross-compound compressor?

[Illustration: Diagram of 8-1/2" Cross Compound Compressor. The
High Pressure Steam (Low Pressure Air) Piston on Its Upward Stroke
Fig. 4.]

A. Because both steam and air are compounded, that is, the steam is used
the second time before it is exhausted to the atmosphere, while the air
is compressed the second time before it is delivered to the main
reservoir.

52. Q. How many cylinders have the cross-compound compressor?

A. Four; two steam cylinders and two air cylinders.

53. Q. What is the diameter of the different cylinders?

A. The high pressure steam cylinder is 8-1/2 inches; the low pressure
steam cylinder 14-1/2 inches; the low pressure air cylinder 14-1/2
inches; high pressure air cylinder 9 inches.

54. Q. Explain the valve gear of this compressor.

A. The valve gear is the same as that of the 9-1/2 or 11 inch
compressor, only that a piston valve is used to distribute the steam
instead of a slide valve.

55. Q. Where does the steam come from that is used in the high pressure
steam cylinder?

A. Direct from the boiler.

56. Q. Where does the steam come from that is used in the low pressure
steam cylinder?

A. The steam after doing work in the high pressure steam cylinder is
exhausted into the low pressure steam cylinder, where it becomes the
working pressure of this cylinder.

57. Q. Explain the operation of this compressor.

A. When steam is first turned on, it enters the compressor at the steam
inlet (see Fig. 4) and flows through passage "a" into the reversing
valve chamber "C" and on to chambers "b" and "y" against the inner
faces of the differential pistons, causing the main valve to move to the
right. In this position of the main valve, port "g" is open to chamber
"b", thus admitting live steam to the lower end of the high pressure
steam cylinder, causing an upward movement of the piston 7. When the
piston 7 has nearly completed its up stroke, the re