The Traveling Engineers' Association Part 31

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?

[Ill.u.s.tration: 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 pa.s.sage "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 reversing plate 18, which is attached to the top of this piston, comes in contact with a shoulder on the reversing rod 21, forcing it upward, carrying with it the reversing valve 22, the movement of which closes port "m", at the same time opens port "n", filling chamber "D" with live steam from chamber "C" and pa.s.sage "a". This balances the pressure on the two sides of the large piston of the differential pistons, and the pressure acting against the inner side of the small piston causes the main valve to move to the left (see Fig. 5). The main valve moving to the left closes port "g" to the live steam and at the same time connects this port with port "f" leading to the lower end of the low pressure steam cylinder, causing an up stroke of the low pressure steam piston 8. In the meantime port "c", which leads to the upper end of the high pressure steam cylinder, is open to chamber "y", allowing live steam to flow down on top of the high pressure steam piston 7, forcing it downward. As the high pressure steam piston about completes its downward stroke, the reversing plate 18 engages the b.u.t.ton on the lower end of the reversing rod 21, pulling the rod and reversing valve 22 down, closing port "n" and at the same time connecting port "m" and "l" through the exhaust cavity "q", thus allowing the steam in chamber "D" to escape to the exhaust. The pressure being removed from the outer face of the large differential piston, the main valve will again move to the right, opening port "g", admitting live steam beneath the piston 7, and at the same time connecting the upper end of the high pressure steam cylinder through port "c", chamber "h" and port "d" to the upper end of the low pressure steam cylinder, causing a downward movement of the low pressure steam piston; the steam below this piston will now be free to escape to the exhaust through port "f", chamber "i" and port "e". Thus it will be seen that the steam used in the high pressure steam cylinder is live steam from the boiler, while the steam used in the low pressure steam cylinder is the exhaust steam from the high pressure steam cylinder.

58. Q. Explain the operation of the air end of the compressor.

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 pa.s.sage "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 pa.s.sage "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 pa.s.sage "w". On the opposite strokes of these pistons air is compressed in a similar manner, but the opposite air valves are used.

[Ill.u.s.tration: 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.

64. Q. What are some of the common causes for the compressor running hot?

A. The overheating of the compressor may be due to any one of the following causes: Running at high speed; working against high pressure; packing rings in air piston badly worn; air cylinder worn; defective air valves; air pa.s.sages or air discharge pipe partially stopped up; leaky piston rod packing; lack of lubrication.

65. Q. What will cause the compressor to run slow?

A. This may be caused by leaky air piston packing rings; final discharge valves leaking, or air pa.s.sages partially stopped up. A defective governor may also cause the compressor to run slow.

66. Q. What will cause the compressor to run very fast and heat, and not compress any air?

A. This may be caused by the strainer becoming clogged with ice or dirt, preventing air entering the cylinder.

67. Q. If, when steam is first turned on, the piston makes a stroke up and stops, where would you look for the trouble?

A. The shoulder on the reversing rod may be worn; the opening in the reversing plate too large to engage the shoulder on the reversing rod; loose reversing plate studs preventing the piston traveling far enough to reverse the compressor, or the main valve stuck in its position at the right.

68. Q. If the piston makes a stroke up and a stroke down and stops, where is the trouble?

A. This may be caused by a loose reversing plate, or the b.u.t.ton on the lower end of the reversing rod worn or broken off, or the nuts off the piston rod in the air end, or the main valve stuck in its position at the left.

69. Q. What will cause the piston to make a quick up stroke?

A. This may be caused by a broken or stuck open upper receiving or lower discharge valve.

70. Q. What will cause the piston to make a quick down stroke?

A. Lower receiving or upper discharge valve broken or stuck open.

71. Q. If a receiving valve breaks or sticks open, how may it be located?

A. The air will flow back to the atmosphere as the piston moves toward the defective valve and may be detected by holding the hand over the strainer.

72. Q. If a receiving valve in a cross-compound compressor breaks, what may be done?