IM 777-8 • Skyline Air Handler 28 www.DaikinApplied.com
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Optional Piezometer Ring Airflow Measurement Device
Piezometer rings are available as an option on direct drive
plenum fans to measure airflow though the fan. The device
consists of a piezometer ring 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 ring in the throat.
The inlet tap is connected to the high-pressure side of the
transducer and the piezometer ring is connected to the low-
pressure side.
Below are the equations and factors required to calculate flow
using the piezometer ring:
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 Table 2
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 1 below
Standard Density Method
The equation can be simplified by assuming standard
density and assuming funnel dimensions match the drawing
dimensions.
Table 17
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 2
ΔP = The differential in static pressure from the piezometer ring
and the front pressure tap (inches w.g.)
Table 16: DDPL Factors For Free and Ducted Inlet—
Non Standard Density Method
Product
C1 Free Inlet
C1 Ducted Inlet
DDPL Size 11-16
753.06
794.06
DDPL Size 18-44
692.03
740.14
Table 17: DDPL Factors For Free and Ducted Inlet—
Standard Density Method
DDPL Size
Free Inlet F
Ducted Inlet F
Area A
11 and 12
944.92
996.36
0.344
15
1206.40
1272.08
0.439
16
1518.58
1601.26
0.552
18
1821.92
1948.58
0.721
20
2185.80
2337.76
0.865
22
2713.93
2902.60
1.074
24
3285.02
3513.39
1.300
27
3997.61
4275.53
1.582
30
4945.21
5289.01
1.957
33
5968.62
6383.56
2.362
36
7290.21
7797.03
2.885
40
8869.55
9486.16
3.510
44
10827.92
11580.68
4.285
Optional Transducer for Piezometer Rings
A transducer is available for Piezometer rings. Factory
mounting locations for the fan transducer is shown in
Figure 45
for direct-drive plenum fans.
Figure 46
shows the installation
for fan array. Wiring for the transducer is field-supplied and
installed.
