exhaust, at the same time steam is admitted through port “s” to the under side of piston “H”, forcing it upward.  As this piston very nearly completes its stroke, the tappet plate “L” (see Fig. 20) engages the button on the tappet rod “P”, moving the main valve “A” to its upper position.  Exhaust cavity “r” now connects port “o”, which leads to the lower end of the cylinder at the right, with the exhaust port “X”, thus allowing the steam under piston “T” to escape to the exhaust, at the same time steam is admitted through port “V” to the upper end of the cylinder at the right, on top of piston “T”, forcing it downward; as it very nearly completes its stroke, the tappet plate “Q” engages the shoulder on the tappet rod “P”, moving the main valve “C” to its lower position.  The exhaust cavity “r” in the valve now connects port “s” with the exhaust port “X”, allowing steam below piston “H” to escape to the exhaust, and at the same time steam is admitted to the top of this piston, forcing it down, thus completing a cycle of the compressor.

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

A. As the piston in the low pressure cylinder “D” moves up (see Fig. 19), a partial vacuum is formed below it, and air flowing through the strainer passes downward through the air passage, then past the lower receiving valve “W” into the lower end of the cylinder, filling it with air at about atmospheric pressure.  In the meantime the air that is being compressed above the piston holds the receiving valve “U” to its seat, and lifts the upper intermediate discharge valve “K” from its seat, allowing the air to pass from the low to the high pressure cylinder “F”.  The high pressure piston now moving up causes a partial vacuum to be formed below it, and air from the atmosphere flows past the lower receiving valve “N”, filling this end of the cylinder with air at about atmospheric pressure.  The air above the piston being compressed, holds the upper intermediate valve “K” and receiving valve “J” to their seats and lifts the upper final discharge valve “M”, allowing the air to pass to the main reservoir.  The action is the same on the down stroke, only air is compressed in the opposite end of the cylinders and the opposite air valves are used.

166.  Q. What should be the lift of the different air valves?

A. In the No. 1 and No. 2 compressors all valves should have 1/16-inch lift; in the No. 5 and No. 6 all valves should have 3/16-inch lift.

167.  Q. If a receiving valve to the low pressure air cylinder breaks or sticks open, what effect will it have on the compressor, and how may it be located?

A. No air will be compressed in the low pressure cylinder, as the piston moves toward the defective valve, and may be located by noting the movement of the low pressure piston, as it will be much quicker toward the defective valve than the opposite stroke.  Air will blow back to the atmosphere as the piston moves toward the defective valve, and may be detected by holding the hand over the strainer.