Table 10 — Terminal Service Configuration Screen
DESCRIPTION | DEFAULT | POINT NAME |
COOLING |
|
|
Terminal Type | 1 | TERMTYPE |
Primary Inlet Size |
|
|
Inlet Diameter | 6.0 in. | RNDSZ |
Inlet Area | 0.0 in. | SQA |
Probe Multiplier | 2.443 | PMF |
Calibration Gain | 1.000 | CAL_GAIN |
Offset | 0 cfm | OFFSET |
Damper |
|
|
Proportional Gain | 30.0 | KP |
Integral Gain | 5.0 | KI |
Derivative Gain | 0.0 | KD |
Starting Value | 20 % | STARTVAL |
CW Rotation | Close | DMPDIR |
Pressure Independent | Yes | PRESIND |
HEATING |
|
|
Heat Type | 0 | HEATTYPE |
VAV Central Heating | Yes | CENHEAT |
Heating |
|
|
Proportional Gain | 8.0 | KP |
Integral Gain | 3.0 | KI |
Derivative Gain | 0.0 | KD |
Starting Value | 80 F | STARTVAL |
Ducted Heat | Yes | DUCTHEAT |
Maximum Temperature | 110 F | MAXTEMP |
Number of Electric Heat Stages | 1 | STAGES |
Heat On Delay | 2 | HONDEL |
Fan Off Delay | 2 | FNOFFD |
Normal | HEATYPE | |
SPT Trim | 0.0 F | SPTTRIM |
SAT Trim | 0.0 F | SATTRIM |
Remote Contact Configuration | Close | RMTCFG |
→Probe Multiplier — This configuration is used to input a factor for the velocity pressure probe installed in the terminal inlet. Most inlet probes will have some aerodynamic characteristics that will affect the differential pressure output from the probe. The formula used by the ComfortID™ controller for calculat- ing the airflow (cfm) is based on measuring velocity with a Pitot tube probe. A PMF (Pitot measurement factor) is required in the calculation for different probes. Because various probe characteristics are different, the PMF is used to determine the correct airflow based on the type of probe installed. The PMF will compensate for the difference between
The default PMF value of 2.273 is the correct value to use when the zone controller is used with a Carrier probe in a Carrier air terminal. For terminals and probes supplied by other manufacturers, the PMF must be calculated and entered into the zone controller configuration in order to correctly measure airflow.
To determine the correct PMF value, there are several meth- ods depending on the data supplied by the terminal manufac- turer. The manufacturer may supply a “K factor” or may sup- ply a chart of velocity pressure vs. airflow for the terminal. The K factor is the actual airflow velocity at a velocity pressure reading of 1 in. wg for the probe. This value is in ft/min and can be used to calculate the PMF. When the K factor is entered into the following equation, it is compared to the value of 4005, which is the K factor for a Pitot tube probe:
PMF = (4005/K FACTOR)2
If a chart is supplied by the manufacturer instead of the K factor, then the K factor can be calculated from the chart using the following formula:
K FACTOR = (cfm at
As an example, an air terminal with an
that gives an airflow value of 820 cfm at a velocity pressure reading of 1 in. wg. To determine the PMF for the terminal:
1.Determine duct area.
radius of duct = diameter of duct/2 radius =
radius =
Area of circular duct = Πr2 Area = 3.14159 x 42 Area = 3.14159 x 16 Area =
Area must be in ft2
2.Determine K factor.
K factor = (820 cfm/0.34906 ft2) K factor = 2349 fpm
3.Determine PMF.
PMF = (4005 fpm/2349 fpm)2 PMF = 2.907
Another way to determine the probe constant for a probe without documentation is to measure the velocity pressure with a Magnahelic gage. Open the damper and adjust the static pres- sure or open the damper until you have one inch of velocity pressure on the Magnahelic gage. Measure the total CFM of air being delivered. The CFM just measured divided by the inlet area in square feet should equal the K factor for the formula. Now use the K factor that was empirically derived to determine the probe multiplier.
Probe Multiplier: Range | 0.250 to 9.999 |
Default Value | 2.443 |
Calibration Gain — Air terminal testing by industry standards is done with straight duct, upstream of the terminal. Since some applications do not get installed in this manner, the actual air- flow from the terminal at balancing may not equal the reading from the zone controller.
39 | 303 |
