Application

Sequence of Operation
Control Loops
7
Siemens Industry, Inc. Application Note, App 2520 140-1217
2015-05-06
Control Loops
The terminal box is controlled by two Proportional, Integral, and Derivative (PID)
control loops; a temperature loop and a flow loop.
The temperature loop is a cooling loop. The active temperature loop maintains room
temperature at the value in CTL STPT. See
Control Temperature Setpoints .
Temperature Loop – A cooling loop that maintains room temperature at the value in
CTL STPT. See
Control Temperature Setpoints .
The temperature loop generates cooling loopout which is then used to generate FLOW
STPT. FLOW STPT is the result of scaling the cooling loopout to the appropriate range
of values determined by CLG FLOW MIN and CLG FLOW MAX. In order to scale it,
the loopout is multiplied by the range (MAX – MIN) and then added to the minimum
setpoint.
When CLG FLOW MIN ≠ 0 CFM, FLOWSTPT ≠ CLG LOOPOUT, the minimum flow
setpoint is (CLG FLOW MIN/CLG FLOW MAX) × 100% flow. And, FLOW STPT is
[CLG LOOPOUT × (100% – minimum setpoint)] + minimum setpoint.
Example
If CTL FLOW MIN = 200 cfm, and CLG FLOW MAX = 1000 cfm, the minimum flow setpoint is (200
cfm/1000 cfm) × 100% flow = 20%.
When CLG LOOPOUT is 0%, FLOW STPT = 20% flow.
[0% × (100% – 20%)] + 20% = 20%
This ensures that the airflow out of the terminal box is no less than CTL FLOW MIN.
When CLG LOOPOUT is 50%, FLOW STPT = 60% flow.
[50% × (100% – 20%)] + 20% = 60%
When CLG LOOPOUT is 100%, FLOW STPT = 100% flow.
[100% × (100% – 20%)] + 20% = 100%
Flow Loop – The flow loop maintains FLOW STPT by modulating the supply air
damper, DMPR COMD. The flow loop maintains the airflow between CTL FLOW MIN
and CTL FLOW MAX.
To enhance stable flow control, an advanced algorithm is used to calculate a
controllable setpoint as the value approaches zero cfm (lps).
You can set CLG FLOW MIN equal to, but not greater than, CLG FLOW MAX. If the
minimum and maximum values are set equal, the flow loop becomes a constant
volume loop and loses its ability to control temperature.
FLOW is the input value for the flow loop. It is calculated as a percentage based on
where AIR VOLUME is between 0 cfm and CTL FLOW MAX. This percentage is
referred to as % flow.
 If AIR VOLUME = 0 cfm, FLOW is 0% flow.
 If AIR VOLUME = CTL FLOW MAX, FLOW is 100% flow.
The low limit of FLOW STPT is the percentage that corresponds to the volume given in
CTL FLOW MIN. This percentage can be calculated as:
(CTL FLOW MIN/CTL FLOW MAX) × 100% flow
The flow loop ensures that the supply air will not be less than CTL FLOW MIN.