16 • MultiFlex CUB-II and CUB-TD I&O Guide
026-1705 Rev 7 06-APR-2010
6.2. Condenser Fan Control
6.2.1. Temperature Differential (TD)
Strategy
Temperature differential strategy attempts to
keep a minimum amount of difference between
the temperature of the refrigerant and the ambi-
ent temperature.
The CUB-II and CUB-TD temperature differ-
ential 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-tun-
ing of the drop leg temperature measurement.
In TD condenser control operation, when the
TD rises above
TD se (TD deadband /
2)
, an increase in fan capacity is called for. Simi-
larly, 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.
6.2.1.1.
TD Control Fail-Safes
TD control operates as explained in
Section
6.2.1.
as long as plenum and drop leg tempera-
ture inputs are available and the drop leg temper-
ature remains within a minimum/maximum
range configured by the user. Otherwise, TD
control uses various fail-safe modes to maintain
proper operation of condenser control.
Drop Leg Temperature Not Available
In the event drop leg temperature is not
available (due to probe failure), all fan stages
will be staged on.
Drop Leg Temperature Above Maximum Drop Leg
Setpoint
If the drop leg temperature (DropLeg Temp)
rises above the maximum drop leg temperature
setpoint (DropLeg Max), an increase will be
called for until DropLeg Temp drops below
DropLeg Max. This protects against the possibil-
ity of the DropLeg 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
(DropLeg Min) a decrease will be called for
until DropLeg Temp rises above DropLeg Min.
This protects against the possibility of the
DropLeg 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 DropLeg Min and Max set-
points.
6.2.2. Pressure Control Strategy
Pressure control strategy uses Cut In/Cut Out
setpoints for control based on discharge pres-
sure. The CUB-II can control a maximum of two
fans using this strategy. This strategy is not
available in the CUB-TD.
Traditional strategy condenser fan control
simply compares the value of the discharge pres-
sure transducer to a set of Cut In/Cut Out set-
points for each fan. If the discharge pressure
rises above the Cut In setpoint for a fan, that fan
will turn ON and will remain active until the dis-
charge pressure falls below the fan’s Cut Out set-
points.
Condenser fans may be programmed with a
minimum ON time to prevent short-cycling.
