McQuay seasoncon ALP-067A Installation And Maintenance Data page 22

Sequence of Operation
The following sequence of operation is typical for ALP Seasoncon air
cooled condensing unit operation.
unit, but where components that apply to the fourth compressor are
referred to, the equivalent components for the third and second com-
pressors are indicated in parentheses.
With the control circuit power on, control stop switch Sl closed,
and manual pumpdown switches PSl and PS2 closed ("Auto." position),
115 volt power is applied through control circuit fuse Fl to the com-
pressor crankcase heaters HTRl through HTR4, (HTR3, HTR2), and also
to the contacts of low pressure switches LPl and LP2.
When the remote time clock, ambient thermostat, manual shutdown
switch and/or evaporator fan interlocks energize the thermostatic
circuit, and provided that high pressure controls HP1 and 2 and com-
pressor motor protectors MPl through PIP4 (MP3, MP2) do not sense an
alarm condition, safety relays R5 through R8 (R7, R6) are energized
closed applying power to the temperature control thermostat.
point the unit will operate automatically in response to the thermo-
stat.
On a call for cooling, the temperature control thermostat TCl ener-
gizes liquid line solenoid valve SVl, opening the valve and allowing
refrigerant to flow into the evaporator.
builds up, low pressure control LPl closes, energizing low pressure
relay R9 which closes to energize compressor contactor Ml, starting
compressor number 1.
denser fan relay R17. Closing its contacts and providing power to
condenser fan motor contactors Mll, 12 & 13 on 3 & 4 compressor units,
or Mll, & 12 on 2 compressor units.
If additional stages of cooling are required, temperature control
thermostat TCl energizes liquid line solenoid valve SV2 after time
delay relay TDll has sequenced closed, to initiate the same starting
sequence in refrigerant circuit number 2.
On 3 and 4 compressor units, if additional cooling is still required,
the third and fourth stages of temperature control thermostat TCl
energize the third and fourth compressors after time delay relays
TD12 and TD13 have sequenced closed.
Pumpdown Cycle
As the temperature control thermostat is satisfied, it opens its con-
tacts, de-energizing liquid line solenoid valve SVl, causing the
valve to close.
erant from the evaporator to the condenser, the low pressure control
LPl opens, shutting down the compressor and condenser fan motors.
Should a closed solenoid valve allow refrigerant to leak to the low
side of the refrigerant circuit during unit "off" time, the buildup
in pressure will cause the low pressure control to close, energizing
the low pressure relay and starting the compressor for pumpdown.
NOTE: Models ALP-067AS and 078AS have single refrigerant circuits
but dual pumpdown switches (PSl & PS2).
these units down, it is recommended that both pumpdown
switches be moved to the "man.pumpdown" position simultane-
ously.
page 22
Closing relay R9 contacts also energizes con-
When the compressor has pumped most of the refrig-
It is written for a 4 compressor
As refrigerant pressure
To manually pump
At this