[Illustration: Low Pressure Piston Moving Upward. High
Pressure Piston at Rest. Fig. 19.]
A. Assuming both pistons are at the bottom of their cylinders, when the
compressor throttle is opened, live steam will flow to both steam chests
"B" (see Fig. 19), and through port "o" to the under side of the piston
"T" and through port "g" to the upper side of piston "H". The steam
under piston "T" will force it upward, and when it very nearly completes
its stroke, the tappet plate "Q" will engage the button on the end of
the tappet rod "P", moving the main valve "C" to its upper position. In
this position the exhaust cavity "r" in the main valve connects port "g"
with the exhaust port "X", thus allowing steam above the piston "H" to
escape to the 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.