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Unit Functions
The calculations in this section are used in unit-level control logic
or all-circuit control logic.
EVAPORATOR DELTA T
The Delta T of the evaporator water is calculated as the
temperature of the water entering minus that leaving through all
circuits.
PENDING LWT
The slope of LWT is calculated such that the slope represents the
estimated change in LWT is immediately.
RATE OF DECLINE
The slope value calculated above will be a negative value as the
water temperature is falling. The rate of decline is calculated by
inverting the slope value and imitating it to a minimum value of
4°C/sec.
ERROR LWT
The LWT error is calculated as LWT - target LWT.
UNIT CAPACITY
Unit capacity is the Delta T of the unit operating for GPM of water.
Minimum flow rates and corresponding
Table 16.
maximum effective Delta T capacity with variable
flow rate.
"Number of
Capacity
TR
compressors"
Unit
25
1
100%
50
2
100%
75
3
100%
100
4
100%
125
5
100%
150
6
100%
175
7
100%
200
8
100%
225
9
100%
250
10
100%
CONTROLLER CALCULATIONS
Refrigerant saturation temperature
The saturated coolant temperature will be calculated from the
pressure sensor readings for each circuit.
Evaporator approach
The evaporator approximation will be calculated for each circuit.
The equation is as follows
Evaporator approximation = LWT - Evaporator saturated
temperature.
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"Nominal
"Maximum effective
Nominal
full (°F)
(Nominal
Capacity DT"
%)"
100%
10.0
96%
10.5
92%
10.9
88%
11.3
85%
11.8
81%
12.4
77%
13.0
73%
13.6
70%
14.3
63%
15.9
Capacitor approach
The capacitor approximation will be calculated for each circuit.
The equation is as follows
Capacitor approximation = Capacitor saturated temperature-
OAT.
Suction reheating
The suction superheat shall be calculated for each circuit using
the following equation:
Suction superheat = Suction temperature - Evaporator saturated
temperature.
Pumping pressure
The pressure at which a circuit will pump down is based on the low
pressure set point of the evaporator. The equation is as follows
Pump down pressure = Evaporator low pressure set point -
103KPA (15 PSI)
CIRCUIT LOGIC CONTROL
Circuit enablement
• A circuit must be enabled to start if the following conditions
are met:
• The circuit breaker is closed
• No circuit alarms are active
• The circuit mode setpoint is set to Enable
• At least one compressor is enabled to start (according to the
enable setpoints)
COMPRESSOR AVAILABILITY
A compressor is considered to be available to start if all of the
following are met:
• The corresponding circuit is enabled.
• The corresponding circuit is not in pump down.
• No cycle timers are active for the compressor.
• The corresponding circuit is not in pumping stop state -No
cycle timers are active for the compressor.
• Compressor is enabled through the enable set points.
• Compressor is not running.
CIRCUIT STATES
The circuit will always be in one of four states:
Off - The circuit is not running.
Pre-open - The circuit is preparing to start up Running - The
circuit is running
Pump off - The circuit is performing a normal shutdown.
The transitions between these states are shown in the diagram
on the next page.
65
iom-clic-cf-eng
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