Figure 11. Typical Primary-Only System - McQuay MicroTech II Operation Manual

Figure 11. Typical Primary-Only System

Standard sequencing logic uses each chiller's load and the chilled water supply temperature to stage the chillers. The
values of the sequencing variables discussed below can be viewed on the System Status screen.
Stage-Up Control: The CSM stages-up when additional cooling capacity is required. This occurs when the following three
conditions are satisfied:
1. All running chillers are at full load (Chiller's at Full Load status may be viewed on the System Status screen).
2. The chilled water supply temperature is greater than the System Setpoint by more than the Chiller Stage-Up
Differential.
Conditions 1 and 2 above have been true for a period of time specified by the Chiller Stage Delay Time variable. (Stage
delay timer does not start until a chiller is available to start)
Stage-Down Control: The CSM stages down when there is an excess of cooling capacity. This occurs when the following
conditions are satisfied:
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 .
2. Condition 1 above has been true for a period of time specified by the Chiller Stage Delay Time variable.
To set up Standard sequencing logic
1. Set the Chiller Sequencing Control Type variable to "Standard."
2. Set the Chiller Stage-Up Differential as required.
3. Set the Spare Capacity Factor variable as required.
4. Set the Chiller Stage Delay Time variable as required.
Note: To use Standard sequencing logic, a chilled water supply temperature sensor must be connected to the CSM. For
more information, see the "Field Wiring" section of IM 781.
Decoupled Sequencing Logic
Decoupled sequencing logic is intended for use with primary-secondary chiller systems. The Chiller Sequencing Control
Type is used to differentiate between primary-only and primary-secondary. A typical primary-secondary system is shown
in Figure 12. The distinguishing characteristics of this system are: (1) each chiller (or set of series chillers) has its own
primary pump, (2) one or more secondary pumps distribute water to the cooling loads, and (3) the secondary circuit is
hydraulically isolated from the primary circuit by a decoupler line. Evaporator piping arrangements are not distinguishing
characteristics. Parallel evaporators are shown in Figure 12, but series evaporators are also possible.
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Cooling Loads
Optional pressure-controlled loop bypass
Differential pressure transducer
DPT
Chilled water supply temperature
Primary pump
Chiller #1
% Load
Chiller #2
% Load
a0141
OM 780-2