Quincy Compressor 50093-103 - Manual (Page 38)

32
The control system remains in this state until the vacuum
level in the plant system increases to 20” HgV, at which
point the vacuum regulator permits more vacuum to be
applied to the inlet valve air cylinder causing it to
gradually close the inlet valve until the amount of air
permitted into the vacuum pump is equal to the amount of
air being leaked into the plant vacuum system through
use. If vacuum system usage continues to decrease
causing the level to increase to 23” HgV, the vacuum
switch trips, opening the normally closed contacts and
stopping power flow to the solenoid. This causes the
solenoid to shift to the normally open position. This
permits enough air flow (regulated by a 0.033" orifice) from
the inlet valve air cylinder through the solenoid to the
vacuum pump inlet to close the inlet valve completely and
keeps the vacuum level in the system from rising above
the maximum set point of 23”HgV.
The 0.025" orifice is sized to permit the air cylinder to open
when the vacuum signal from the vacuum regulator or
solenoid valve ceases, while still restricting air flow into
the control piping network to a level such that the vacuum
regulator remains effective during operation. The 0.033"
orifice is sized to work together with the 0.025" orifice
such that when the solenoid valve opens, only enough
vacuum is provided at the air cylinder to close it. This
enhances the responsiveness of the control system. The
0.078" orifice keeps the air cylinder from oscillating.
The vacuum pump is now running with a fully closed inlet
valve. When vacuum level in the system drops to the
lower set point, 20" HgV in this case, the switch closes,
shifting the solenoid valve to the closed position and
permitting the inlet valve to open. This cycle continues as
required by vacuum system usage, and a vacuum level of
20" to 23" HgV is maintained in the vacuum system.
If the vacuum pump stops during operation, either
manually from the panel, or automatically due to a high
temperature condition, the inlet valve will not act as a
check valve to prevent air from entering the vacuum
system through vacuum pump.
What has been described here is the operation of the
control system over a 20" to 23 ' HgV vacuum range, with
lower set point of 20' HgV, an upper set point of 23' HgV,
and a 3' HgV differential The control system is designed to
operate with an upper set point of 10" HgV to 29.6" HgV at
sea level barometer (29.92 in. Hg).
Auto Dual with Modulation
The Auto/Dual system offers two choices of controlling
the Quincy QSVI Vacuum pump. With the selector switch
in the “Continuous Run” position, the vacuum pump
operates continuously matching demand with a differential
pilot valve controlling the position of the inlet valve.
When maximum system vacuum is reached, the vacuum
switch opens, closing the inlet valve and, although the
vacuum pump continues to run, no more vacuum will be
produced. When the Auto Dual mode is selected, the
vacuum pump will also perform as above, however, a solid
state timer is activated when the vacuum switch contacts
open. This timer is adjustable within a ten (10) minute
range. When the timer reaches the end of its delay, the
vacuum pump will automatically shut down and assume a
"stand-by" mode. Upon a drop in vacuum pressure, the
vacuum switch contacts close, restarting the vacuum
pump automatically. The timer should be set, during unit
start-up, for a minimum of ten (10) minutes.
Should the plant vacuum level drop causing the vacuum
switch contacts to close during the unloaded/timing mode,
the vacuum pump will continue to operate, resetting the
timer and instructing the inlet valve to re-open.
When operation in the Auto mode allows the vacuum
pump to start and stop more than five (5) time per hour,
select the Continuous Run mode and allow the unit to
operate continuously. Excessive motor starts will shorten
motor life.