McQuay MicroTech II User Manual

Page 44 OM 780-2
Stage-down with Series-Piped Chillers
Series-pipe chiller pairs will be disabled together unless they are the last two chillers enabled. In the example shown in
Table 11, when all four chillers are enabled, the Next OFF chiller will be chiller 4. When system conditions allow a stage-
down to occur, both chiller 3 and 4 will be disabled at the same time. Now only chillers 1 and 2 are enabled and the Next
OFF chiller will be chiller 2. When system conditions allow a stage-down to occur, only chiller 2 will be disabled.
The CSM adjusts the stage-down conditions when it will be disabling chiller pairs together. The first stage-down condition
is:
1. The Active Capacity of the Next-OFF chiller is less than the Spare Capacity Factor
multiplied by the Sum of All Spare
Capacity of All Other Running Chillers
.
If the Next-Off chiller is series-piped with another enabled chiller, the Active Capacity of the Next-Off chiller is re-
calculated as the sum of the active capacity of the Next-OFF chiller plus the active capacity of the chiller paired with the
Next-OFF chiller in the series-piped configuration. Also, the Sum of All Spare Capacity of All Other Running Chillers is
re-calculated as the sum of all running chillers except the Next-OFF chiller and the chiller paired with the Next-OFF chiller
in the series-piped configuration.
To sequence series-piped chillers
1. Set the “This Chiller Is Series-Piped With Chiller #” variable of each chiller that has it’s evaporator piped in series
with another chiller. These variables are on the Chiller Setup screen.
2. Set the Flow Rate variable of each chiller in a series-piped pair equal to that of the other chiller making up the pair.
These variables are on the Chiller Setup screen.
3. On the Load Limiting screen, set the Load Balancing Flag to “On” and set the following variables as required:
• Load Balancing Capacity Difference Limit
• Load Balancing Start Delay Time
4. Place all chiller numbers into a Load Balancing Group # on the Load Limiting screen. Each pair of chillers should
have it’s own Load Balancing Group # so that only the two chillers creating the series-piped pair are load balanced
together.
5. On the Chiller Seq screen, set the Sequence Number of all chillers as required. The Sequence Number of series-piped
chiller pairs will typically be the same. Make sure that both chillers of a series-piped pair always exist in the same
row.
6. When the chiller unit controller of either chiller in a series-piped pair gets enabled, it must enable the primary water
flow through the evaporator of both chillers. The flow enable device (pump or valve) must be wired in parallel with
both chiller unit controllers so that either chiller can start the evaporator water flow.
Stage-Up Inhibiting
Stage-up inhibiting limits loading by preventing further stage-ups. If stage-up inhibiting is on, the CSM is able to stage-
down, but it is not able to stage-up when a normal stage-up would otherwise occur. If stage-up is not inhibited, normal
sequencing control occurs. Stage-up inhibiting does not prevent a forced stage-up from occurring when an enabled chiller
goes off-line.
Stage-up inhibiting does not directly influence the loading of individual chillers, and it cannot actively reduce the system-
wide load. It can only prevent more capacity—in the form of additional chillers—from being added to the system. There
are two types of stage-up inhibiting:
• Daily Method
• Network Method
The system can use either type or both types at the same time. The Stage-Up Inhibit Source value on the System Status
screen tells you which (if any) method of Stage-Up Inhibiting is active.
Daily Method
With the Inhibit Stage-Up After Time variable, you can specify a particular time after which no more stage-ups occur. For
example, if the chiller system shuts down at 9:00 p.m., the operator may want to prevent more capacity from being added to
the system after 8:15 p.m. In this instance, the operator could set the Inhibit Stage-Up After Time to “8:15 PM”.
To set up daily stage-up inhibiting control
1. Set the Inhibit Stage-Up After Time as required. Normal stage-ups do not occur after this time.

McQuay MicroTech II Specifications

General

Brand	McQuay
Model	MicroTech II
Category	Controller
Language	English