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Fan Inlet Airflow Measuring System
A fan inlet airflow measuring system (piezometer) 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.
Figure 86. Fan inlet airflow measuring system
High
port
"+"
Wiring
The transmitter requires 24VDC/24VAC 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-mounted UC600 controller, the
installing contractor must ensure the transmitter has
24VDC/24VAC power.
Transmitter Sizing
The 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 itself 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.
CLCH-SVX013B-EN
Selecting the proper transmitter is critical in order to get
accurate airflow measurements. How accurately the
transmitter senses the differential pressure is dependent
on:
•
•
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
Low
adequately calculate and display the airflow for the
port
"-"
smaller fans, ensure that the analog input is programmed
with enough decimal places to sufficiently represent the
pressure differential being measured.
Note: The transmitter is factory-calibrated to the range
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:
CFM = K * SQRT(DP)
Where:
CFM = Airflow (ft
lbm/ft
K = A constant factor that is unique for each fan. See
Factor," p. 70
DP = Differential pressure (inches w.g.) being measured by the
transmitter.
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:
ACFM = CFM * SQRT(0.075/ρ)
Where:
ACFM = Actual airflow (ft
density.
ρ = Density (lbm/ft
Note: Alternative units, including SI, can be used in place
the pressure range selected
the accuracy of the selected transmitter
selected and cannot be significantly adjusted to
"tighten" the range closer to the pressure being
read for the given application.
3
/min.) assuming a standard air density of 0.075
3
.
for more information.
3
/min.) corrected for non-standard air
3
) of the air at the inlet to the fan.
of the IP units above although the K-factor must be
converted appropriately.
Start-Up
"Constant K
69
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