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 16
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 15: 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 16: 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 51
for direct-drive plenum fans .
Figure 52
shows the installation
for fan array. Wiring for the transducer is field-supplied and
installed .
IM 672-11 • VISION AIR HANDLING UNIT 34
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