©2025 Copeland LP.
026-1803 R13 Supervisor I&O User Guide 9 - 56
9.40 TD Control
9.40.1 Overview
The TD Control application controls fans sequentially based
on the temperature differential (TD) of the condenser. When
an increase is called for, the next fan will turn on when the
time since the last fan state change is greater than the Fan On
Delay Time setpoint (or will turn on immediately if the last
change was more than the Fan On Delay). Additional fans will
be staged on at Fan On Delay intervals while an increase in
capacity is called for. When a decrease is called for, the next
fan will stage off when the time since the last fan state change
is greater than the Fan Off Delay Time setpoint (or will turn off
immediately if the last change was more than Fan Off Delay).
Additional fans will be staged off at Fan Off Delay intervals
while a decrease in capacity is called for.
9.40.2 Temperature Differential (TD)
Strategy
TD strategy attempts to maintain a constant difference (i.e.,
TD setpoint) between the temperature of the refrigerant and
the ambient temperature. The TD setpoint allows you to set
the constant that is to be maintained.
TD strategy uses drop leg temperature for the refrigerant
temperature and plenum temperature for the ambient
temperature. The formula for determining the TD control
value is:
TD = (dropleg temp) + (dropleg offset*) - plenum temp.
* DropLeg Offset is a user-configured parameter to allow fine-tuning
of the drop leg temperature measurement.
In TD condenser control operation, when the TD rises above
TD setpoint + (TD deadband / 2), an increase in fan capacity is
called for. Similarly, when the TD drops below TD setpoint -
(TD deadband / 2), a decrease in fan
capacity is called for. TD can control up to four condenser
fans.
9.40.3 TD Control Fail-Safes
TD Control operates as long as plenum and drop leg
temperature inputs are available and the drop leg
temperature remains within a minimum/maximum range
configured by the user. Otherwise, TD control uses various
fail-safe modes to control the condenser fans.
In the event drop leg temperature is not available (due to
probe failure), all fans will be staged on.
Drop Leg Temperature Above Maximum Drop
Leg Setpoint
If the Drop Leg temperature (Drop Leg Temp) rises above the
maximum drop leg temperature setpoint (Drop Leg Max), an
increase will be called for until Drop Leg Temp drops below
Drop Leg Max. This protects against the
possibility of the Drop Leg Temp rising too high as a result of
high plenum temperature.
Drop Leg Temperature Below Minimum Drop
Leg Setpoint
If the Drop Leg temperature drops below the minimum drop
leg temperature setpoint (Drop Leg Min) a decrease will be
called for until Drop Leg Temp rises above Drop Leg Min. This
protects against the possibility of the Drop Leg Temp falling
too low as a result of low plenum
temperature.
Plenum Temperature Not Available
If plenum temperature is not available (due to probe or
communication failure), control will be based on the Drop
Leg Min and Max setpoints.
9.40.4 Configuration
TD Control allows the user to operate a condenser at its
designed temperature differential and saves energy by
maintaining optimal condenser TD with the least amount of
fans on as possible.
Standard discharge pressure control may attempt to
operate the condenser below its designed TD and turn on
more fans than necessary.
Set the number of condenser fans to be controlled in the
Num Cond Fans field under Setup (4 is the default value).
9.40.5 Setpoints
TD setpoints are configured to allow control of the
condenser at its designed TD during swings in ambient
temperature.
For the Drop Leg Minimum setpoint, if the drop leg
temperature falls below this setpoint, condenser fans will be
cycled OFF regardless of the value of the TD setpoint.
For the Drop Leg Maximum setpoint, it is the maximum
allowable value of the drop leg temperature. If the drop leg
temperature rises above this setpoint, the condenser fans are
cycled ON regardless of the value of the TD.
The Drop Leg Offset setpoint is an adjustment entered, if
needed, after measuring liquid subcooling during operation
of an individual condensing unit.
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