Application

Sequence of Operation
Temperature Control Loops
30
Siemens Industry, Inc.
Application Note, App 2926
140-1308
2015-07-07
Room CO2
RM CO2 displays the CO
2
value in units of parts-per-million (PPM). RM CO2 (from the
digital 2200/2300 room units) can be used with PPCL in the PTEC/ATEC controller or
unbundled for control or monitoring purposes.
Room RH
RM RH displays the relative humidity value in percent. RM RH can be used for PPCL
in the PTEC or unbundled for control or monitoring purposes.
RM RH displays the relative humidity value in percent.
Temperature Control Loops
Room Temperature Loop
The room temperature loop operates in both heating and cooling modes. This loop
reads the room temperature control point, CTL TEMP, and then controls the room
temperature to the value of CTL STPT by generating TEMP LOOPOUT. TEMP
LOOPOUT is then used in the BTU calculations in order to determine the value of the
DISCH STPT and, during VAV operation, to determine the value of TEMP CTL VOL.
See
BTU Calculations – VAV Mode and BTU Calculations – Constant Volume Mode .
Supply Temperature Loop
The supply temperature loop is a heating loop which operates at all times. The heating
loop generates the point VALVE CMD which drives the heating valve in order to
maintain the discharge temperature set in DISCH STPT. See
BTU Calculations – VAV
Mode
and BTU Calculations – Constant Volume Mode .
BTU Calculations
NOTE:
BTU Compensation vs. Room Temperature Sensor (RTS) Control – In applications
that do not include BTU Compensation, the heating loop modulates the heating valve
directly using RTS control.
In applications that do include BTU Compensation, the heating valve is controlled
indirectly in that the BTU Compensator raises or lowers the discharge air temperature
setpoint based on the temperature requirements of the room and supply/exhaust
airflow quantities, followed by the modulation of the heating valve to meet the
discharge setpoint. Although this may seem a slower and more involved method of
temperature control, it can actually provide faster and more accurate response
because it is a feedforward system instead of a feedback system. With BTU
Compensation, some temperature error can be anticipated and corrected in advance.
BTU Calculations – VAV Mode
During VAV operation, the controller adjusts the supply airflow and the supply air
temperature setpoint as necessary to maintain CTL TEMP at CTL STPT. The room
temperature PID loop calculates the value of TEMP LOOPOUT. The Figure
Temperature Control Sequence [➙ 32] shows how this value is used to sequence the
cooling flow and the supply air temperature setpoint. The loop is tuned by adjusting the