Figure 12. Typical Primary-Secondary System - McQuay MicroTech II Operation Manual

The purpose of primary-secondary (Decoupled) systems is to maintain relatively constant flow through the chillers while at
the same time allowing variable flow to the cooling loads. Because the relationship between a building's total cooling load
and its required chilled water flow rate is seldom proportional, situations can occur in which partly loaded chillers cannot
provide enough chilled water to the secondary loop. In this instance, water flows from return to supply in the decoupler
line. As a result, supply and return water mix, and the chilled water temperature going to the cooling loads rises. The
CSM's Decoupled sequencing logic can prevent this from happening.

Figure 12. Typical Primary-Secondary System

Decoupled sequencing logic uses each chiller's load, the chilled water supply temperature, the decoupler line temperature,
and the flow rate in the decoupler line (supply to return only) to stage the chillers. A stage-up can occur for either of two
reasons: (1) to satisfy the need for additional capacity, or (2) to satisfy the need for additional flow. The values of the
sequencing variables discussed below can be viewed on the System Status screen.
Stage-Up-for-Capacity 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.
3. Conditions 1 and 2 above have been true for a period of time specified by the Chiller Stage Delay Time variable.
Stage-Up-for-Flow Control: The CSM stages-up when additional primary water flow is required. This occurs when the
following two conditions are satisfied:
1. The Decoupler Line Temperature is greater than the Chilled Water Supply Temperature by more than the Decoupler
Stage-Up Temperature Differential. (Water is flowing the wrong way through the decoupler line.)
2. Condition 1 above has been true for a period of time specified by the Chiller Stage Delay Time variable.
Stage-Down Control: The CSM stages-down when there is an excess of cooling capacity and primary chilled water flow.
This occurs when the following conditions are satisfied:
The Active Capacity of the Next-OFF chiller is less than the Spare Capacity Factor multiplied by the Sum of All Spare
1.
Capacity of All Other Running Chillers .
2. The Decoupler Line Flow Rate is greater than an adjustable percentage of the defined flow rate of the Next-OFF
chiller. The chiller flow rates are defined with the Chiller # Flow Rate variables (see Chiller Setup screen), and the
percentage is defined with the Decoupler Stage-Down Flow Rate Factor.
3. Conditions 1 and 2 above have been true for a period of time specified by the Chiller Stage Delay Time variable.
OM 780-2
Cooling Loads
Uni-directional flow meter
(supply to return)
Secondary pump
FM
Decoupler line temperature
Chilled water supply temperature
Primary pump
Chiller #1
% Load
Chiller #2
% Load
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