Daikin Skyline OAH003GDAC Installation And Maintenance Manual page 42

Optional Piezometer Point Airflow Measurement Device
A Piezometer point is an option with EC fans to measure
airflow through the fan. The device consists of a piezometer
point mounted in the throat of the funnel and a static pressure
tap mounted near the inlet of the funnel. The pressure drop is
measured from the tap located near the inlet of the funnel to
the piezometer point in the throat. The inlet tap is connected
to the high-pressure side of the transducer and the piezometer
point is connected to the low-pressure side.
A transducer is always factory supplied when the Piezometer
Option is selected with an EC Fan.
installation for EC fans.
See the equations and factors required to calculate flow using
the piezometer point:
Non-Standard Density Method
The following equation is used to measure the flow for
non-standard density:
ACFM = C1 × A × √(ΔP/ρ)
where: A = Inlet funnel throat area (square feet) - from
ΔP = The differential in static pressure from the piezometer ring
and the inlet pressure tap (inches w.g.)
ρ = Air density (pounds mass/cubic foot)
C1 = Value from Table 20
Standard Density Method
The equation can be simplified by assuming standard density
and assuming funnel dimensions match the drawing dimensions.
Table 20 shows the factor (F) for each fan size and type. The
equation then becomes the following:
For standard air (ρ = 0.075 lb/ft3):
ACFM = F × √ (ΔP)
where: F = factor from Table 21
ΔP = The differential in static pressure from the piezometer ring
and the front pressure tap (inches w.g.)
Table 20: ECM Factors for Free and Ducted Inlet —
Non-Standard Density Method
ECM Size C1 Free Inlet
355
450
Table 21: ECM Factors for Free and Ducted Inlet —
Standard Density Method
ECM Size Free Inlet F
355 1372.74
450 2226.07
IM 777-18 • SKYLINE OUTDOOR AIR HANDLER
Figure 76 shows the
Table 21
C1 Ducted Inlet
774.47 774.47
783.31 783.31
Ducted Inlet F Area A
1372.74 0.485
2226.07 0.778
Figure 79: Piezometer Transducer
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