versing 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 passage "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 button 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
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).