Unit Functions
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IOM 1207-6 • TRAILBLAZER
®
MODEL AGZ CHILLERS
Unit Functions
The calculations in this section are used in unit level control
logic or in control logic across all circuits.
Evaporator Delta T
The evaporator water delta T is calculated as entering water
temperature minus leaving water temperature.
LWT Slope
LWT slope is calculated such that the slope represents the
estimated change in LWT over a time frame of one minute.
Pulldown Rate
The slope value calculated above will be a negative value
as the water temperature is dropping. A pulldown rate is
calculated by inverting the slope value and limiting to a
minimum value of 0°C/min.
LWT Error
LWT error is calculated as LWT – LWT target.
Unit Capacity
For applying unit capacity limits, an estimate of total unit
capacity is needed. Unit capacity will be based on the
estimated circuit capacities.
The unit capacity is the number of compressors running (on
circuits that are not pumping down) divided by the number of
compressors on the unit.
Control Band
The Control Band defines the band in which unit capacity will
not be increased or decreased.
Constant Evaporator Flow
If Variable Evaporator Flow set point is set to No, the control
band is calculated as follows:
• Four compressor units: Control Band = Full Capacity
Evap Delta T Set Point * 0.35
• Six compressor units: Control Band = Full Capacity Evap
Delta T Set Point * 0.25
Variable Evaporator Flow
When the unit set point for Variable Evaporator Flow is set
to Yes, the control band increases as capacity decreases to
account for the decrease in flow. It is assumed that the flow
will vary to maintain the full capacity evaporator temperature
delta at part load conditions. The control band is limited at each
capacity step to a maximum value that corresponds to the
minimum flow for that capacity step.
Since evaporator flow is represented by the set point Full
Capacity Evaporator Delta T, the calculations of the control
band for variable flow applications are explained in terms of
delta T also. The term ‘Effective Full Capacity Delta T’ means
the approximate temperature delta that would be observed with
the unit running at full capacity for the given flow. ‘Nominal flow’
means the flow that is needed for a 5.56°C (10°F) delta T at full
unit capacity.
Table 50: Minimum Flows and Corresponding Maximum
Effective Full Capacity Delta T with Variable Flow
Number of
Compressors
Unit
Capacity
Minimum Flow
(nominal %)
Max Effective Full
Capacity DT
4
100%
62.5%
8.9 °C (16 °F)
75%
55%
10.1 °C (18.2 °F)
50%
47.5%
11.7 °C (21.1 °F)
25%
40%
13.9 °C (25 °F)
6
100%
62.5%
8.9 °C (16 °F)
83.3%
58%
9.59 °C (17.3 °F)
66.7%
53.5%
10.39 °C (18.7 °F)
50%
49%
11.35 °C (20.4 °F)
33.3%
44.5%
12.49 °C (22.5 °F)
16.7%
40%
13.9 °C (25 °F)
For variable evaporator flow, the Control Band is calculated as
follows:
1.
Effective Full Capacity Delta T = (Full Capacity Evap
Delta T* 100) /Unit Capacity
2.
If above value is more than the Max Effective
Full Capacity dT listed in the table above for the
corresponding unit capacity, it is set equal to the value in
the table.
3.
Effective Full Capacity Delta T with the limit applied is
then multiplied by 0.35 for units with four compressors
and by 0.25 for units with six compressors. This gives
the total control band for the unit configuration and actual
unit capacity.
Staging Temperatures
If the unit is configured for use without glycol:
When the LWT target is more than half the Control Band above
3.9°C (39.0°F)
• Stage Up Temperature = LWT (Control Band/2)
• Stage Down Temperature = LWT target – (Control
Band/2)
If the LWT target is less than half the Control Band above
3.9°C (39.0°F)
• Stage Down Temperature = LWT target – (LWT target -
3.9°C)
• Stage Up temperature = LWT Control Band –
(LWT target – 3.9°C)
If the unit is configured for use with glycol, the compressor
staging temperatures are calculated as shown below:
• Stage Up Temperature = LWT (Control Band/2)
• Stage Down Temperature = LWT target – (Control
Band/2)
The Start up and Shutdown temperatures are referenced from
