McQuay MicroTech II User Manual

OM 780-2 Page 101
CAUTION
If a BAS is coordinating a free cooling strategy in which it changes cooling tower setpoints as it changes the network
command, the CSM should be set up to be Unoccupied during any period when free cooling is possible. If this is not
done, chillers could start and operate with extremely low condenser water temperatures.
Stage-Up Status
Whenever the common Chilled Water Supply Temperature is higher than the System Setpoint by more than the Chiller
Stage-Up Differential, the CSM controls the chilled water capacity by sequencing its chillers. As the CSM sequences
chillers on and off, it “stages-up” and “stages-down.” If the sequence order is set properly, each successive stage has more
capacity than the preceding stage. Additional capacity could be in the form of one added chiller (typical), or a chiller swap
(in which the replacement chiller has more capacity than the one that is stopped). To allow you to monitor the stage-up
operation the following variables are provided on the System Status screen:
• Next-ON Chiller
• Number of Chillers Running
• Chillers at Full Load
• Chiller Stage Delay Timer
• Stage-Up Inhibit Source
• Current Row
• Current Row Capacity (Tons)
• Max Row Capacities (Tons)
• And for Decoupled Systems (Chiller Sequence Control Type = Decoupled, Chiller Seq screen)
• Decoupler Line Temp
• Chilled Water Supply Temp + Decoupler Stage Up Differential
Whenever stage-up is not inhibited, the next-on chiller will be enabled when the number of chillers running equals the
chillers at full load. Decoupled systems will also stage-up if the decoupler line temp is greater than (chilled water
supply temp plus the decoupler stage-up differential). All stage-ups require that the chiller stage delay time has
expired since the last stage-up or stage-down. See the “Sequencing Logic” section of this document for more detail on
stage-up.
If multiple rows are used, the CSM will move to a higher row when:
• All the available chillers in the current row are enabled (Next-ON Chiller In This Row variable will read “No
Additional Chillers In This Row To Enable”)
• All the enabled chillers in the current row are at full load
• The Current Row Capacity is less than the Maximum Row Capacity of a higher row
• The chiller stage delay time is exceeded
The CSM calculates maximum row capacity by multiplying the Chiller Availability (Misc screen) by the chiller’s Tonnage
(an operator input on the Chiller Setup screen) of every chiller assigned to a row and adding all these values together.
Stage-Down Status
To allow you to monitor the stage-down operation the following variables are provided on the System Status screen:
• Next-OFF Chiller
• Spare Capacity x Spare Capacity Factor
• Next-OFF Active Capacity
• Chiller Stage Delay Timer (see Stage-Up Status)
• And for Decoupled Systems (Chiller Sequence Control Type = Decoupled, Chiller Seq screen)
• Decoupler Line Flow Rate
• Next-OFF Chiller’s Flow x Decoupler Stage Down Flow Rate Factor

McQuay MicroTech II Specifications

General

Brand	McQuay
Model	MicroTech II
Category	Controller
Language	English