Unit installation
OMRCPMICRO 29
1. The dead band is equal to the table value times the dead band
multiplier.
2. Fan stage 3 is not available when unit capacity is less than 50%.
for each circuit and are based on the current evaporator pressure, the point on the minimum lift pressure line at
which the unit is currently operating, and the current dead band being used. The "MinCondPr" equals the
current evaporator pressure plus the point value on the minimum lift pressure line at which the unit is currently
operating. Since the pressure scale in Figure 11 is referenced to evaporator pressure, the "MinCondPr" is
represented by the minimum lift pressure line in Figure 11. "MaxCondPr" equals the current "MinCondPr" plus
the current dead band value from Figure 11. Thus, the point value on the current fan staging line represents
the "MaxCondPr". If the dead band multiplier is anything other than 1.0, the dead band value used in the
"MaxCondPr" calculation becomes the dead band value from Figure 11 multiplied by the dead band multiplier.
The fan staging pressure error is integrated as follows: Every 4 seconds the controller calculates the error.
The error is (Actual Head Pressure - MaxCondPress) or (MinCond Press - Actual Head Pressure), depending
on if the pressure is above or below the dead band. After calculating the error, the error is added to the
previous errors (totaled). Once the sum of errors exceed a maximum allowable limit, the controller will stage the
fans up or down, whichever applies. By using the head pressure algorithm, the controller brings the head
pressure back within the dead band. The cumulative pressure error is not displayed on MicroTech. The
maximum allowable limit for a stage up request is the Stage Up Error ("StageUpErr" on Menu 17). The
maximum allowable limit for a stage down request is the Stage Down Error ("StageDnErr" on Menu 17). The
two staging error set points are user adjustable, but for most applications, the default values will suffice. The
"MaxCondPr" and "MinCondPr" used for calculating the errors are displayed as items C and D on Menus 5
and 6.
Dead band multiplier
The dead band multiplier ("DeadBandMult" on Menu 17) gives some flexibility regarding the set pressure dead
bands shown in the table. This variable is provided to adjust all the dead bands up or down in 10%
increments. The dead band multiplier only dictates the response time of the condenser fans to a stage-up
request. The fan stage-down request is dictated only by the minimum lift pressure line. In general, increasing
the dead band multiplier will slow down the stageup response of the condenser fans when a change in
condenser pressure is detected. Decreasing the dead band multiplier will speed up the condenser fan stage-up
response time. If a condenser fan is repeatedly cycling on and off, for instance, the dead band could be
increased via this set point. However, in most instances, if left at the default setting of 1.0, fan operation will
be fine.
Figure 11, Minimum lift and dead band table
Minimum lift set points
In Figure 11, the lift pressure scale is referenced to
the evaporator pressure. The dead band is
graphically illustrated by the vertical distance
between the minimum lift line and the fan staging
lines. Changing one of the Minimum Lift set
points changes the slope of the minimum lift
pressure line as well as the slopes of the fan
staging lines. Flattening the curve does not widen
the dead band; the dead band distance stays the
same relative to the minimum lift pressure line and
the fan staging lines. Raising the minimum lift set
points causes the fans to stage at higher
pressures while lowering the minimum lift set
points causes the fans to stage at lower pressures.
The fan stage up and stage down errors are still
calculated in the same manner. For virtually all
applications, the default settings for "MinLift-
35%" and "MinLift-100%" should suffice. Consult
the previous information for the effects that
adjusting these set points will have on fan staging. Other considerations for adjusting the Minimum Lift Set
Points include the existence of special options such as SpeedTrol head pressure control. Guidelines for
Minimum Lift Set Points for unit with SpeedTrol follow.