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Micropanel
The microprocessor discharge pressure
cutout is backed-up by the mechani-
cal high pressure cutout located in
each refrigerant circuit.
High Discharge Pressure Unload Point:
D I S C H A R G E
U N L O A D
=
2 7 5 . 0
The discharge pressure unload point is used to avoid a
high pressure cutout shutdown by unloading a com-
pressor, if its discharge pressure approaches the cutout
value. The chiller can then continue to run automati-
cally at reduced capacity until the cause of the exces-
sive pressure is resolved (e.g. dirty condenser coils) or
cleared (e.g. high ambient temperature).
For the first 60 seconds of operation, discharge pres-
sure limiting is disabled. After this time, if discharge
pressure exceeds the programmed limit, a 1 second
unload pulse will be sent to the slide valve of the af-
fected compressor every 5 seconds until the discharge
pressure drops below the programmed limit.
Reloading will take place when discharge pressure
has dropped 58 PSIG (4 bar) below the programmed
threshold.
Typically the unload point should be set 18 to 30 PSIG
(1.2 to 2 bar) below the discharge pressure cutout set-
ting. The microprocessor will accept a range of pro-
grammable values between 200 and 400 PSIG (13.8
and 27.52 bar).
Low Suction Pressure Cutout:
S U C T I O N
P R E S S U R E
C U T O U T
=
4 4
The low suction pressure cutout protects the evapora-
tor from damage due to ice build up caused by opera-
tion at low refrigerant suction pressure.
After the compressor starts, and the pump down cycle
is completed (pump down to cutout or 30 seconds,
whichever comes first), suction pressure is monitored
as long as the compressor runs. For the first 270 sec-
onds of running, suction pressure can be lower than
the programmed cutout, but must be greater than:
PROGRAMMED X
CUTOUT
70
P R E S S U R E
P S I G
P S I G
RUN TIME / 3 +10
100
For example, if the programmed cutout = 44
PSIG (3 bar) and the run time = 60 seconds
NEW CUTOUT = 3 x
60/3 + 10 = 13.2 PSIG (0.9 bar)
This cutout value increases with time, until after 270
seconds, it equals the programmed cutout value. If suc-
tion pressure falls below the calculated cutout value
before 270 seconds, the system will be shutdown.
After 270 seconds, a transient timer system prevents
short term fluctuations in suction pressure from caus-
ing shutdown as follows: If suction pressure drops be-
low the cutout point, a 90 second transient timer starts.
During the 90 second time period, the suction pressure
must be greater than:
PROGRAMMED
X
100 - TRANSIENT TIME REMAINING
CUTOUT
For example, if the programmed cutout = 44 PSIG
(3 bar) and the timer has run 30 seconds
NEW CUTOUT = 3 x
100 - 60
100
This cutout value increases with time, until after 90
seconds, it equals the programmed cutout value. If the
suction pressure rises to more than 5 PSIG (0.3 bar)
above the programmed cutout value during the 90 sec-
ond time period, the timer will be reset.
If the suction pressure does not rise to more than 5 PSIG
(0.3 bar) above the cutout, the timer will remain at zero
and if the pressure then falls below the cutout again, the
system will shutdown on a low pressure fault.
If the Dip Switch (S1) on the AMB board is set for
'WATER' cooling mode, the cutout is programmable
between 44 and 70 PSIG (3.03 and 4.83 bar). In this
mode, a setting of 44 PSIG (3.03 bar) is recommended.
If the Dip Switch (S1) on the AMB board is set for
'GLYCOL' (Brine) cooling mode, the cutout is pro-
grammable between 5 and XX PSIG (0.34 and 4.83
bar). In this mode, the cutout should be set to the satu-
rated refrigerant pressure equivalent to 18°F (10°C) be-
low the temperature of the chilled liquid.
High Ambient Temperature Cutout:
H I G H
A M B I E N T
C U T O U T
=
1 3 0 ° F
The high ambient temperature cutout is used to select
the ambient temperature above which the chiller may
FORM 201.25-NM1 (302)
100
100
= 17.6 PSIG (1.2 bar)
T E M P
YORK INTERNATIONAL
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