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Ramp Loading Control — The ramp loading control
slows down the rate at which the compressor loads up. This
control can prevent the compressor from loading up during
the short period of time when the chiller is started, and the
chilled water loop has to be brought down to normal design
conditions. This helps reduce electrical demand charges by
slowly bringing the chilled water to control point. However,
the total power draw during this period remains almost
unchanged.
There are 2 methods of ramp loading with the PIC. Ramp
loading can be based on chilled water temperature or on mo-
tor load.
1. Temperature ramp loading limits the rate at which either
leaving chilled water or entering chilled water temper-
ature decreases by an operator-configured rate. The low-
est temperature ramp table will be used the first time the
chiller is started (at commissioning). The lowest tem-
perature ramp rate will also be used if chiller power has
been off for 3 hours or more (even if the motor ramp load
is selected).
2. Motor load ramp loading limits the rate at which the
compressor motor current or compressor motor load
increases by an operator-configured rate.
The TEMP (Temperature) PULLDOWN, LOAD PULL
DOWN, and SELECT RAMP TYPE may be viewed/modified
on the LID Equipment Configuration table, Config table (see
Table 2). Motor load is the default type.
Capacity Override (Table 4) — Capacity overrides
can prevent some safety shutdowns caused by exceeding
motor amperage limit, refrigerant low temperature safety limit,
motor high temperature safety limit, and condenser high pres-
sure limit. In all cases there are 2 stages of compressor vane
control.
1. The vanes are held from opening further, and the status
line on the LID indicates the reason for the override.
2. The vanes are closed until condition decreases below the
first step set point, and then the vanes are released to nor-
mal capacity control.
Whenever the motor current demandlimit set point is reached,
it activates a capacity override, again with a 2-step process.
Exceeding 110% of the rated load amps for more than
30 seconds will initiate a safety shutdown.
The compressor high lift (surge prevention) set point will
cause a capacity override as well. When the surge preven-
tion set point is reached, the controller normally will only
hold the guide vanes from opening. If so equipped, the hot
gas bypass valve will open instead of holding the vanes.
Table 4 — Capacity Overrides
OVERRIDE
CAPACITY
CONTROL
FIRST STAGE SET POINT
SECOND
STAGE
SET POINT
OVERRIDE
TERMINATION
View/Modify
on
LID Screen
Default Value Configurable Range Value Value
HIGH
CONDENSER
PRESSURE
Equipment
Service1
HCFC-22 HFC-134a HCFC-22 HFC-134a
ϾOverride
Set Point
+ 4 psig
(28 kPa)
ϽOverride
Set Point
Ͼ195 psig
(1345 kPa)
125 psig
(862 kPa)
150 to 245 psig
(1034 to 1689 kPa)
90 to 200 psig
(620 to 1379 kPa)
HIGH MOTOR
TEMPERATURE
Equipment
Service1
Ͼ200 F (93.3 C) 150 to 200 F (66 to 93 C)
ϾOverride
Set Point
+10° F
(6° C)
ϽOverride
Set Point
LOW
REFRIGERANT
TEMPERATURE
(Refrigerant
Override Delta
Temperature)
Equipment
Service1
Ͻ3° F (1.6° C)
(Above Trippoint)
2° to 5° F
(1° to 3° C)
рTrippoint
+ Override
⌬T –1° F
(0.56° C)
ϾTrippoint
+ Override
⌬T +2° F
(1.2° C)
HIGH
COMPRESSOR
LIFT
(Surge
Prevention)
Equipment
Service1
HCFC-22 HFC-134a HCFC-22 HFC-134a
None
Within
Lift Limits
Plus Surge/
HGBP
Deadband
Setting
Minimum:
T1 — 1.5° F
(0.8° C)
P1 — 75 psid
(517 kPad)
Maximum:
T2 — 10° F
(5.6° C)
P2 — 170 psid
(1172 kPad)
Minimum:
T1 — 1.5° F
(0.8° C)
P1 — 50 psid
(345 kPad)
Maximum:
T2 — 10° F
(5.6° C)
P2 — 85 psid
(586 kPad)
0.5° to 15° F
(0.3° to 8.3° C)
50 to 170 psid
(345 to 1172 kPad)
0.5° to 15° F
(0.3° to 8.3° C)
50 to 170 psid
(345 to 1172 kPad)
0.5° to 15° F
(0.3° to 8.3° C)
30 to 170 psid
(207 to 1172 kPad)
0.5° to 15° F
(0.3° to 8.3° C)
30 to 170 psid
(207 to 1172 kPad)
MANUAL
GUIDE VANE
TARGET
Control
Algorithm
Maint01
Automatic 0 to 100% None
Release of
Manual
Control
MOTOR LOAD —
ACTIVE
DEMAND LIMIT
Status01 100% 40 to 100%
у5% of
Set Point
2% Lower
Than
Set Point
31