McQuay seasoncon ALP-067A, Installation And Maintenance Data

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Low Pressure Control
The low pressure control is a single pole pressure switch that closes
on a pressure rise. It senses evaporator pressure and is factory set
to close at 60 PSIG and automatically open at 25 PSIG.
The control
has an adjustable range of 20 in. of Hg.
to 100 PSIG and an adjusta-
ble differential of 6 to 40 PSIG. To check the control (unit must
be running),
move the pumpdown switch(es) (PSl and PS2) to the "man.
pumpdown" position. As the compressor pumps down condenser pressure
will rise and evaporator pressure will drop. The lowest evaporator
pressure reached before cut-out is the cut-out setting of the control.
By moving the pumpdown switch(es) (PSl and PS2) to the "auto. pump-
down"
position, evaporator pressure will rise. The highest evapora-
tor pressure reached before compressor re-start is the cut in setting
of the control.
Fantrol
Head Pressure Control
--
Fantrol is a system for progressively turning on or off condenser
fans when they are no longer required. This is done to reduce con-
denser capacity (typically in low outdoor ambient temperatures) and
is accomplished by a combination of pressure and temperature actuated
controls. The first fan (No.
11) is started by its contactor when
the first compressor in the unit starts. The second fan (No. 12) is
controlled by a pair of parallel wired pressure switches, one of
which senses condenser pressure in refrigerant circuit No. 1, and
one which senses pressure in refrigerant circuit No. 2. The third
fan (No. 13-3 fan units only) is controlled by a pair of parallel
wired temperature switches,
one of which senses condenser air inlet
temperature for refrigerant circuit No. 1 and one for circuit No. 2.
Pressure temperature control set points are indicated below.
WITH COMPRESSOR WITH COMPRESSOR
To check the cut-in points of the controls,
the unit must initially
be off. With the unit prepared for start up according to the proce-
dures outlined in this bulletin,
move pumpdown switches (PSl & PS2)
to the "auto. pumpdown" position. Evaporator pressure will begin to
rise and the compressor(s) should start with fan No. 11 starting im-
mediately. After the compressor(s) starts,
observe condenser pres-
sure as it rises. When the condenser pressure reaches approximately
270 PSIG, contactor Ml2 should pull in to start fan No. 12. On 3
fan units, fan No.
13 should start via contactor Ml3 whenever the
ambient air at the condenser inlet reaches 80F.
It may be difficult to check the cut-out point of fan No. 13 (on 3
fan units) at the instant it happens, but it should be off whenever
the ambient air at the condenser inlet is below 70F.
To check the
cut-out point of fan No. 12,
some means of reducing the load on the
unit must be available or the fan operation and condenser pressure(s)
must be observed as the load drops off naturally. When the condenser
pressure drops to approximately 170 PSIG,
contactor Ml2 should drop
out to turn off fan No. 12.
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