A. As the low pressure air piston 9 moves up, a partial vacuum is created beneath it and air from the atmosphere enters the air inlet and passage “r” past the lower receiving valve 38 and fills the lower end of the cylinder with air at about atmospheric pressure (see Fig. 4).  In the meantime the air above the piston being compressed will hold the upper receiving valve 37 to its seat, thus preventing a back-flow of air to the atmosphere; at the same time the upper intermediate discharge valves 39 are forced from their seats, allowing the air from the low pressure air cylinder to flow through passage “u” to the high pressure air cylinder, the piston of which is now moving downward.  The air beneath the high pressure air piston 10 being compressed will hold the lower intermediate discharge valves 40 to their seats, thus preventing the air in the high pressure air cylinder flowing back to the low pressure air cylinder.  When the pressure in the high pressure air cylinder becomes slightly greater than the main reservoir pressure, the final discharge valve 42 will be forced from its seat and the air beneath the piston allowed to flow to the main reservoir through passage “w”.  On the opposite strokes of these pistons air is compressed in a similar manner, but the opposite air valves are used.

[Illustration:  Diagram of 8-1/2” Cross-Compound Compressor.  The High Pressure Steam (Low Pressure Air) Piston on Its Downward Stroke Fig. 5.]

59.  Q. How many valves are there in the air end of the compressor?

A. Ten; two upper and two lower receiving valves; two upper and two lower intermediate discharge valves; one upper and one lower final discharge valves.

60.  Q. Are the air valves all the same size?

A. No; the receiving and final discharge valves are the same size and of the size used in the 11-inch compressor, while the intermediate valves are the same as used in the 9-1/2-inch compressor.  The receiving and final discharge valves are two inches in diameter, while the intermediate valves are one and one-half inches.

61.  Q. What lift is given the different air valves?

A. All valves have 3/32-inch lift.

DEFECTS OF THE COMPRESSOR

62.  Q. What are some of the common causes for the compressor stopping?

A. Lack of lubrication; bent, worn or broken reversing rod; loose or worn reversing plate; nuts on air end of piston rod coming off; defective compressor governor; and, in addition with the cross-compound compressor, final discharge valve broken or stuck open, or packing rings in main valve pistons breaking and catching in the steam ports.

63.  Q. What will cause the piston to make an uneven stroke?

A. This may be caused by a broken or stuck open air valve, or air valves not having proper lift.  Where the piston short strokes, it is generally caused by over-lubrication of the steam end.