Unit installation

28 OMRCPMICRO
Head Pressure Control
Air-cooled units only (ALRs & WHRs with remote condensers)
For each circuit, the first stage of condenser fans is wired in parallel with the compressor output so that they
are energized with the first compressor stage. The fans for each circuit are controlled independently, but use
the same Head Pressure Set Points for staging. The transducers responsible for measuring condenser
pressure are mounted on the head of the lead compressors' discharge. Refer to the sensor location charts
towards the back of this manual for specific locations.
Each circuit has three additional digital outputs available for refrigerant head pressure control. Each output
will energize an additional bank of condenser fans with each bank consisting of 1 or 2 fans, depending on the
size of the unit.
The number of condenser fan stages per circuit controlled by MicroTech head pressure control is the number
of fan contactors per circuit minus 1 since the first fan comes on with the compressor. The number of fan
stages for each unit size is as follows:
ALR-035, 040D/E.............................................................1
ALR-050D/E......................................................................2
ALR-060 thru 185D/E .....................................................3
Air-cooled unit EERs are maximized by not allowing condenser fan stage 3 (the last fan stage) to operate when
the unit capacity is below 50%.
Above 50% capacity, unit EER is maximized by energizing as many condenser fan stages as possible.
Lift pressure
The minimum acceptable lift pressure is determined by the expansion valve. "Lift Pressure" is defined as the
difference between the saturated condensing pressure and the saturated evaporator pressure—or the minimum
differential pressure to be maintained across the expansion valve. The "MinLift-35%" and the "MinLift-100%"
set points on Menu 17, "Head Pres Spt" reflect the desired lift pressures at 35% and 100% circuit cooling
capacity. At low circuit capacities, it is desirable to maintain a minimum lift of 85 psig. The first fan stage
comes on with the first stage of compressors and the cooling capacity is roughly 33-35%. That is when the
MicroTech starts evaluating the necessity of bringing on the second stage of fans. The Minimum Lift-35% set
point is the minimum differential pressure to be maintained across the expansion valve when the unit is
running at 35% circuit capacity. At higher tonnage capacities, a higher lift pressure must be maintained to
accommodate the increased flow through the expansion valve. The Minimum Lift-100% set point is the
minimum differential pressure to be at 100% capacity. At 100% circuit cooling capacity, a lift pressure of about
140 psig should be maintained to provide proper flow through the TXV. This is the default setting for the
"MinLift100%" setting. Even though the adjustable range for this item is 120-180, 140 should be the maximum
used for virtually all cases to prevent high pressure alarms.
The lift pressure between 35% and 100% circuit capacity cooling is linear as shown in Figure 11. The Minimum
Lift -35 % and -100% endpoints define a minimum lift pressure line which provides the MicroTech controller
with the correct minimum lift pressure to be maintained at any unit operating capacity.
Fan staging logic
In the MicroTech logic, there are set dead band values which define pressure differential ranges above the
minimum lift pressure within which no condenser fan staging will occur. The dead band table is shown in
Figure 11. The dead band value used is based on the current circuit capacity and current number of condenser
fans in operation as shown in the table. The dead band range lies entirely above the minimum lift pressure line.
In Figure 11, the dead bands are graphically illustrated as the vertical distance between the minimum lift
pressure line and the fan staging lines. No additional fan staging occurs if the head pressure is within the
dead band zone. If the head pressure moves outside the dead band, the controller integrates the pressure error
over time. The error calculation is based on minimum condenser pressure ("MinCondPr") and maximum
condenser pressure ("MaxCondPr") values which MicroTech calculates internally. The values are calculated