[Illustration:  Diagrammatic View, Down Stroke Fig. 3.]

In the meantime the air above the piston, being compressed, will hold the upper receiving valve 86a to its seat, and when the pressure is slightly greater than that in the main reservoir, this pressure acting under the upper discharge valve 86c, will lift this valve from its seat and now the air will be free to flow through passage “G” to the main reservoir connection.  On the down stroke the action is similar, air is taken in through the upper receiving valve 86a, while the air below the piston is being compressed and forced past the lower discharge valve 86d, to the main reservoir. (See Fig. 3.)

46.  Q. What lift should the air valves have?

A. All valves should have a lift of three thirty-second of an inch.

47.  Q. At what speed should the compressor be run to obtain the best results?

A. At 100 to 120 single strokes per minute.

48.  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.