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CLCH-SVX010A-EN 59
Fan Inlet Airflow Measuring
System
A fan inlet airflow measuring system is available on many
centrifugal and plenum fans. Trane’s system determines
airflow using a static pressure differential.
Note: This type of system is different than a total pressure
or thermal dispersion system. As such, the
calculations will be different.
Each system comes with a differential pressure
transmitter. The minimum diameter is connected to the LO
port of the transmitter and the reference pressure point is
connected to (or actually is) the HI port of the transmitter.
Wiring
The transmitter requires 24VDC power on terminals 1 (+)
and 2 (ground) of the transmitter. When the airflow
measurement system is ordered with a factory-mounted
UC600 controller, the 24 VDC power will be supplied.
In the absence of a factory-provided UC600 controller, the
installing contractor must ensure the transmitter has
24VDC power.
Transmitter Sizing
Trane specification requires that the flow meter option
have a total accuracy of 5 percent. The total accuracy is a
combination of:
• how accurately the flow meter is in sensing airflow
• how accurately the transmitter senses the differential
pressure
• how accurately the controller translates the signal
from the transmitter to a differential pressure.
Selecting the proper transmitter is critical in order to get
accurate airflow measurements. How accurately the
transmitter senses e differential pressure is dependent on:
• the pressure range selected
• the accuracy of the selected transmitter
Trane air handlers use a 0-5 inch, 0-20 inch, or 0-30 inch
w.g. range transmitter as standard. To sufficiently cover
VAV turndown on the smallest fans with the above range,
a transmitter with an accuracy of 0.25 percent (full scale) is
used as standard. If a field-provided transmitter with a
lower accuracy is selected, the range should be chosen
closer to the actual, maximum pressure differential
expected for the application.
The transmitter outputs a signal that represents the
differential pressure which is used to calculate airflow. To
adequately calculate and display the airflow for the
smaller fans, ensure that the analog input is programmed
with enough decimal places to sufficiently represent the
pressure differential being measured.
The transmitter is factory-calibrated to the range selected
and cannot be significantly adjusted to “tighten” the range
closer to the pressure being read for the given application.
Transmitter Calibration
The transmitter is factory-calibrated to a specific pressure
range with a 0-5 inch, 0-20 inch, or 0-30 inch w.g. range
being used in most cases. To check calibration and to
adjust if necessary, consult the transmitter manufacturer
or the factory for specific procedures.
The transmitter outputs a linear, 2-10 VDC signal
representing a differential pressure measurement. With
this measurement, the airflow through the fan can be
calculated using the following equation:
Significant differences in elevation and/or temperature
will affect the density of air. For air at a constant, non-
standard density, a field-obtained K factor can be used.
Alternatively, the following equation can be used to
continuously correct the equation above:
Note: Alternative units, including SI, can be used in place
of the IP units above although the K-factor must be
converted appropriately.
Figure 67. Fan inlet airflow measuring system
CFM = K * SQRT(DP)
Where:
CFM = Airflow (ft
3
/min.) assuming a standard air
density of 0.075 lbm/ft
3
.
K = A constant factor that is unique for each fan. See
“Constant K-Factors” section on page 60 for more
information.
DP = Differential pressure (inches w.g.) being
measured by the transmitter.
ACFM = CFM * SQRT(0.075/ρ)
Where:
ACFM = Actual airflow (ft
3
/min.) corrected for non-
standard air density.
ρ = Density (lbm/ft
3
) of the air at the inlet to the fan.
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