Application

Sequence of Operation
BTU Calculations
34
Siemens Industry, Inc.
Application Note, App 2928
140-1302
2015-07-07
BTU Calculations – Constant Volume Mode
During the Constant Volume Mode, the BTU Compensator operates as follows:
During Constant Volume operation, the controller adjusts the supply air temperature
set point as necessary to maintain CTL TEMP at CTL STPT. The room temperature
PID loop calculates the value of TEMP LOOPOUT. This value is used to adjust the
value of the supply air temperature set point. The loop is tuned by adjusting the values
of the feedback gain points, ROOM P GAIN and 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 set point 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%)
CTL STPT = 70°F: OCC.UNOCC = UNOCC
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
UOC SUP MAX
80°F
70° + (10° x 100% ÷ 100%)
10°F
0.5 ´ UOC SUP MAX
90°F
70° + (10° x 100% ÷ 50%)
–5°F
0.25 ´ UOC SUP MAX
50°F
70° + (–5° x 100% ÷ 25%)
0°F
any flow
70°F
70° + (0° x 100% ÷ x%)
While the actual number of BTUs is not explicitly calculated, DISCH STPT varies as
the supply flow varies in order to maintain a constant quantity of heat entering the
room. (An example of when the supply flow can vary during CV mode is when it varies
in order to maintain proper room pressurization when the fume hood sash position
changes.)
This module also limits TEMP LOOPOUT to values that generate supply air
temperature set points that are less than DISCH MAX and greater than DISCH MIN.