Demand Limit Control Option; Surge Prevention Algorithm - Carrier 19XR Start-Up, Operation And Maintenance Instructions Manual

(REMOTE TEMP [FULL RESET]). Then, enter the maxi-
mum amount of reset required to operate the chiller
(DEGREES RESET). Reset Type 2 can now be activated.
RESET TYPE 3 — Reset Type 3 is an automatic chilled wa-
ter temperature reset based on cooler temperature differ-
ence. Reset Type 3 adds ± 30° F (± 16° C) based on the
temperature difference between the entering and leaving chilled
water temperature.
To configure Reset Type 3, enter the chilled water tem-
perature difference (the difference between entering and
leaving chilled water) at which no temperature reset occurs
(CHW DELTA T [NO RESET]). This chilled water temper-
ature difference is usually the full design load temper-
ature difference. Next, enter the difference in chilled water
temperature at which the full amount of reset occurs (CHW
DELTA T [FULL RESET]). Finally, enter the amount of re-
set (DEGREES RESET). Reset Type 3 can now be activated.
Demand Limit Control Option —
control option (20 mA DEMAND LIMIT OPT) is externally
controlled by a 4 to 20 mA or 0 to 5 vdc signal from an
energy management system (EMS). The option is set up on
the RAMP_DEM screen. When enabled, 4 mA is the 100%
demand set point with an operator-configured minimum de-
mand at a 20 mA set point (DEMAND LIMIT AT 20 mA).
The auto. demand limit is hardwired to terminals J5-5 (−)
and J5-6 (+) on the CCM. Switch setting number 1 on SW2
will determine the type of input signal. With the switch set
at the ON position the input is configured for an externally
powered 4 to 20 mA signal. With the switch in the OFF
position the input is configured for an external 0 to 5 vdc
signal.
Surge Prevention Algorithm —
configurable feature that can determine if lift conditions are
too high for the compressor and then take corrective action.
Lift is defined as the difference between the pressure at the
impeller eye and at the impeller discharge. The maximum
lift a particular impeller wheel can perform varies with the
gas flow across the impeller and the size of the wheel.
A surge condition occurs when the lift becomes so
high the gas flow across the impeller reverses. This condi-
tion can eventually cause chiller damage. The surge preven-
tion algorithm notifies the operator that chiller operating
conditions are marginal and to take action to help prevent
chiller damage such as lowering entering condenser water
temperature.
The surge prevention algorithm first determines if cor-
rective action is necessary. The algorithm checks 2 sets of
operator-configured data points, the minimum load points (MIN.
LOAD POINT [T1/P1]) and the full load points (FULL
LOAD POINT [T2/P2]). These points have default settings
as defined on the OPTIONS screen or on Table 4.
The surge prevention algorithm function and settings are
graphically displayed in Fig. 19 and 20. The two sets of load
points on the graph (default settings are shown) describe a
line the algorithm uses to determine the maximum lift of the
compressor. When the actual differential pressure between
the cooler and condenser and the temperature difference be-
tween the entering and leaving chilled water are above the
line on the graph (as defined by the minimum and full load
points), the algorithm goes into a corrective action mode. If
the actual values are below the line and outside of the dead-
band region, the algorithm takes no action. When the point
defined by the ACTIVE DELTA P and ACTIVE DELTA T,
moves from the region where the HOT GAS BYPASS/
SURGE PREVENTION is off, the point must pass through
the deadband region to the line determined by the config-
ured valves before the HOT GAS BYPASS/SURGE
PREVENTION will be turned on. As the point moves from
the region where the HOT GAS BYPASS/SURGE
The demand limit
This is an operator-
38
PREVENTION is on, the point must pass through the dead-
band region before the HOT GAS BYPASS/SURGE PRE-
VENTION is turned off. Information on modifying the de-
fault set points of the minimum and full load points may be
found in the Input Service Configurations section, page 53.
Corrective action can be taken by making one of 2 choices.
If a hot gas bypass line is present and the hot gas option
is selected on the OPTIONS table (SURGE LIMIT/HGBP
OPTION is set to 1), the hot gas bypass valve can be ener-
gized. If the hot gas bypass option is not selected (SURGE
LIMIT/HGBP OPTION is set to 0), hold the guide vanes.
See Table 4, Capacity Overrides. Both of these corrective
actions try to reduce the lift experienced by the compressor
and help prevent a surge condition.
LEGEND
ECW — Entering Chilled Water
HGBP — Hot Gas Bypass
LCW — Leaving Chilled Water
P = (Condenser Psi) − (Cooler Psi)
T = (ECW) − (LCW)
Fig. 19 — 19XR Hot Gas Bypass/Surge
Prevention with Default English Settings
LEGEND
ECW — Entering Chilled Water
HGBP — Hot Gas Bypass
LCW — Leaving Chilled Water
P = (Condenser kPa) − (Cooler kPa)
T = (ECW) − (LCW)
Fig. 20 — 19XR Hot Gas Bypass/Surge
Prevention with Default Metric Settings