4.8 - CAPACITY CONTROL
This function adjusts the number of active compressors and loaders to keep the leaving water temperature at the
setpoint. The
precision
with which this is achieved depends upon the
capacity of the water loop, the water flow
rate, the load and the number of capacity control steps available
. The control continuously takes account of the
temperature error with respect to the control point, the rate of change of the leaving fluid temperature and the delta
of temperatures between the entering and leaving fluid in order to determine the optimum time to add or subtract
steps of capacity.
Note: a too important number of starts of a particular compressor will automatically cause a limitation of the
compressors starts. This will affect the precision of the water loop temperature control.
4.8.1 - Logic description
4.8.1.1 - Basic logic
To operate the capacity control task uses the following inputs:
w
EWT
Entering water temperature
w
LWT
Leaving water temperature
w
CTRL_PNT
Control point
w
Cooling z_multiplier Z Multiplier (range: 1.0 to 6.0)
w
Heating z_multiplier Z Multiplier (range: 1.0 to 6.0)
The control calculates the SMZ: load/unload factor. It is compared to the z_multiplier to determine when to add or
remove stage:
w
If (sum/z) > +z_multiplier -> stage = +1 (add a compressor)
w
If (sum/z) > +0.6 and if a loader is available (loader is OFF) -> stage = +1 (add a loader)
w
If (sum/z) < - z_multiplier -> stage = -1 (remove one compressor)
w
If sum/z) < -0.6 and loader is avaialble (loader is ON) -> stage = -1 (turn off one loader)
Note: z_multiplier doesn't apply to loaders.
4.8.1.2 - Z Multiplier calculation
The z_multiplier factor allows to indirectly increase the compressor on/off time delay. In consequence, it can be
adjusted (increased) to avoid compressors short cycling. This function requires the following Service configuration:
v
Auto Z Multiplier sepoint (zm_spt). Default to 6.
v
Cooling and heating Z Multiplier max value (hc_zm). default to 6.0.
This function adapts the z_multiplier to the unit compressor cycles number (the cycle number shall be based on
the compressor which has been the most started during an hour). The significant period for compressors cycling
shall be of one hour.
If the cycle counter exceeds the setpoint (zm_spt), then the z_multiplier value shall be increased based on the
cycle number (PID loop). The function shall not take care of the loaders cycling.
The calculated value of the z_multiplier shall not exceed the configured hc_zm.
If the unit returns to normal operating condition with a cycles counter going down then the calculated z_multiplier
shall progressively return to its normal value (zm = 1.0). The rate at which the calculated z_multiplier returns to
normal shall be lower than the rate at which it is increased.
The setpoint value shall allow to adapt the auto z multiplier calculation to the installation. The setpoint ranges
from 4 to 6.
During normal operation, each modification of the unit capacity is followed by a dead time when no capacity
modification is possible. The duration of this dead time is 90 seconds.
4.8.1.3 - Overrides
CARRIER SA
USER'S GUIDE FOR
30GX&HX PRO_DIALOG 4 V.2
ECG-UG-02-002
Rev: Original
Page: 38/98
©
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ARRIER
S
.
A
NOT
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DISCLOSED
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PERSONS
OUTSIDE
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ORGANIZATION
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