Application

Sequence of Operation
BTU Calculations
32
Siemens Industry, Inc.
Application Note, App 2928
140-1302
2015-07-07
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 [➙ 33] shows how this value is used to sequence the
cooling flow and the supply air temperature setpoint. The loop is tuned by adjusting the
values of the feedback gain points that are visible in the database (ROOM P GAIN;
ROOM I GAIN), and the sample interval point, LOOP TIME.
The output of the room temperature loop, TEMP LOOPOUT, reflects the load
requirements. The value of TEMP LOOPOUT is a supply air temperature expressed as
“degrees above or below the room temperature setpoint if the supply flow is at 100%”.
If the supply flow is less than 100% of the currently active air supply airflow maximum,
DISCH STPT is adjusted to an amount greater than TEMP LOOPOUT by a
corresponding percentage.
Example
CTL STPT = 70°F: OCC.UNOCC = OCC
If
TEMP LOOPOUT =
and
SUP AIR VOL =
then
DISCH STPT =
Formula for DISCH STPT:
CTL STPT +
(TEMP LOOPOUT x
100% ÷ SUP AIR VOL)
10°F
OCC SUP MAX
80°F
70° + (10° x 100% ÷ 100%)
10°F
0.5 ´ OCC SUP MAX
90°F
70° + (10° x 100% ÷ 50%)
–5°F
0.25 ´ OCC SUP MAX
50°F
70° + (–5° x 100% ÷ 25%)
0°F
any flow
70°F
70° + (0° x 100% ÷ x%)