McQuay MicroTech II User Manual

Page 80 OM 780-2
Loop Bypass Valve Control
The CSM can control the position of a loop bypass valve to maintain the differential pressure at the Loop Differential
Pressure Setpoint. This type of control is typically used in primary-only systems, but it can also be effectively used in
primary-secondary systems. In either case, the loop bypass valve control method is the same. A loop bypass valve must be
a normally closed (NC) valve (a closed valve prevents flow from bypassing the cooling loads). When the CSM opens the
valve it increases the voltage signal.
Configuration 7 (Figure 36) shows a typical primary-only system. If there is no bypass valve, the Pump Control Option
variable and the Chilled Water Loop Modulation Control Option variable should both be set to “None.”
In a primary-secondary system, loop bypass valve control can be used with any pump logic (single-pump, lead/standby,
sequencing constant speed pumps or sequencing variable speed pumps) when exact pressure control is required; however, it
cannot be used in the same application with pump speed control. Typical primary-secondary systems that use a bypass
valve are shown in Configurations 1, 2 and 3.
The CSM uses a proportional-integral (PI) control loop to generate an analog valve position signal, which it sends to the
valve via an analog output (AO 1 on Remote D). The Differential Pressure Bypass Valve Position variable (Flow screen)
shows the current value of this signal.
PI Control Process
When the pressure is above the Loop Differential Pressure Setpoint, the control loop increases the valve position, which
opens the valve and increases the bypass flow from supply to return. When the pressure is below the Loop Differential
Pressure Setpoint, the control loop decreases the valve position, which closes the valve and reduces the bypass flow. The
position can modulate between 0% (low signal) and 100% (high signal).
The PI control loop has four adjustable variables that are used for pump speed control: (1) Loop Differential Pressure
Deadband, (2) Loop Differential Pressure Propband, (3) Loop Differential Pressure Sample Time, (4) Loop Differential
Pressure Integral Time. The Loop Differential Pressure PI Function time plot is provided on the Load Flow Control screen
to assist in tuning the PI loop.
To set up loop bypass valve control
1. Set the Chilled Water Loop Modulation Control Option variable to “Chilled Water Loop Bypass Valve.”
2. For primary-only applications, set the Pump Control Option variable to “None.”
3. Commission Remote I/O Module D on the Device Addressing screen. Set the Loop Bypass Valve AO Zero variable
on the I/O Config screen.
4. Set the following variables as required:
• Loop Differential Pressure Setpoint
• Loop Differential Pressure Deadband
• Loop Differential Pressure Propband
• Loop Differential Sample Time
• Loop Differential Integral Time
Scheduling
The CSM’s control mode can be scheduled for occupied operation with any of nine methods (listed highest too lowest in
priority):
1. CSM internal weekly scheduling (priority = 16)
2. CSM internal holiday scheduling (priority = 16)
3. CSM internal special-event scheduling (priority = 16)
4. CSM internal timed override (priority = 16)
5. Optimal Start (priority = 13)
6. External time clock (priority = 11)
7. Modbus BAS scheduling (priority = 10)

McQuay MicroTech II Specifications

General

Brand	McQuay
Model	MicroTech II
Category	Controller
Language	English