McQuay MicroTech II User Manual

Page 40 OM 780-2
Condition 2 assures that the chillers that would still be on after a stage-down continue to meet the building’s flow
requirement. As an example, consider a system in which Chiller #3 is the only chiller that is part of stage 2 and not part of
stage 1. Assume that Chiller #3’s defined flow rate is 1000 gpm (63.1 L/s) and that the Decoupler Stage-Down Flow Rate
Factor is set to 1.10. If the CSM is at stage 2 and the decoupler line flow rate is slightly more than 1100 gpm (69.4 L/s),
condition 3 is satisfied. If the stage-down occurs, the flow rate from supply to return in the decoupler line drops from 1100
gpm (69.4 L/s) to 100 gpm (6.3 L/s).
Decoupler Line Flow Rate
The flow meter is used to make sure that the primary water flow through the decoupler line is greater that the primary water
flow that will be lost when the Next-OFF chiller is disabled. When the secondary loop’s demand for flow exceeds the
primary loop flow, the decoupler line temperature sensor will detect flow going backwards through the decoupler line and
the CSM will enable another chiller to increase the primary water flow. If the capacity of this newly enabled chiller is not
required, the CSM would quickly want to disable this chiller, which turns off the evaporator pump. By knowing the
decoupler line flow rate, the CSM will keep the chiller running (so that its evaporator pumps will continue to run) until the
secondary loop demand for flow decreases.
Flow Meter in the Decoupler Line
The simplest way to measure the decoupler flow rate is to place a flow meter directly in the decoupler pipe as shown in
Figure 12. The CSM reads the decoupler line flow directly and uses it to stage-down. When using this method, set the
Flow Meter Location (I/O Config screen) to “Decoupler Line”.
Flow Meter in the Common Supply Line
Another method of determining the flow rate in the decoupler line is to place a flow meter in the common supply line. The
CSM then calculates the decoupler flow rate by subtracting the primary water flow rate from the measured common supply
line flow rate (knowing that the difference of these two values must be going through the decoupler). This method is more
complicated because the CSM must determine the total primary water flow rate by adding the Chiller # Flow Rate variables
(Chiller Setup screen) of all chillers with an active flow switch. This requires that the flow through the evaporator of each
chiller was set and balanced by an experienced contractor to provide accurate primary flow rate calculation. When using
this method, set the Flow Meter Location (I/O Config screen) to “Common Supply Line”.
To set up Decoupled Sequencing Logic
1. Set the Chiller Sequencing Control Type variable to “Decoupled.”
2. Set the following variables as required:
• Chiller Stage-Up Differential
• Spare Capacity Factor
• Chiller Stage Delay Time
3. Set the Decoupler Stage-Up Temperature Differential variable as required.
4. Set the Decoupler Stage-Down Flow Rate Factor variable as required.
5. Set the Flow Meter Location variable (I/O Config screen) to indicate the position of the flow meter in the system.
6. Set the Chiller # Flow Rate variables (1 through
x
, where
x
is the number of chillers) as required.
Note: To use Decoupled sequencing logic, a chilled water supply temperature sensor, a decoupler line temperature sensor
and a flow meter must be connected to the CSM. For more information, see the “Field Wiring” section of IM 781.
Special Sequencing Logic
The CSM uses special sequencing logic to compensate for an off-line chiller. An off-line chiller is defined as a chiller that
is part of the current stage, and meets one of the following conditions:
1. The chiller was unavailable at the time the CSM’s sequencing logic would have normally enable it
2. The chiller was enabled by the CSM but did not enter the running mode
3. The chiller was enabled by the CSM and then put into Local control (the Control Source on the chiller’s unit controller
was changed from BAS network)
4. The chiller was enabled by the CSM and then lost communications with the CSM

McQuay MicroTech II Specifications

General

Brand	McQuay
Model	MicroTech II
Category	Controller
Language	English