Twin City Airflow Measuring System Using Piezometer Ring - ES-105 - Manual

One of the following equations is used to measure
the flow:
ACFM = C1 * A * (∆P/ρ)
where: A = Actual inlet funnel throat area (square
feet) - from tables on pages 2 and 3
∆P = The differential in static pressure from
the piezometer ring and the front
pressure tap (inches w.g.)
ρ = Air density (pounds mass/cubic foot)
C1 = Value from Table 1 below
The equations can be simplified by assuming standard
density and assuming funnel dimensions match drawing
dimensions. The following tables show the factor (F) for
each fan size and type. The equation then becomes the
following:
For standard air ( ρ = 0.075 lb/ft 3 ):
ACFM = F ∆P
where: F = factor from tables on pages 2 and 3
∆P = The differential in static pressure
from the piezometer ring and the
front pressure tap (inches w.g.)
©2014 Twin City Fan Companies, Ltd.
Twin City Fan now offers an air measuring device as an
option on centrifugal fans. It is based on the principle
of a flow nozzle. The inlet cone of the fan is used as
the flow nozzle. By measuring the pressure drop through
the inlet cone, the flow can be calculated. The system
consists of a piezometer ring mounted in the throat and
a static pressure tap mounted on the face of the inlet
cone. A differential pressure transducer and a digital
display can be provided. The display must be capable
of performing the square root function in order to read
out in CFM directly. By testing fans in the laboratory,
Twin City Fan was able to determine flow coefficients for
various fan types. The flow coefficients were combined
to give the equations listed below by fan type. Based
on testing performed in Twin City Fan’s laboratory, the
accuracy of the system was determined to be +/- 5%.
The pressure drop is measured from the tap located
on the face 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 con-
nected to the low-pressure side.
Measurement of Airflow
The equations below are accurate for flow estimation for flows from 40% to 100% of wide-open volume. According
to testing done previously at Twin City Fan, several factors affect the accuracy of this method of determining flow.
The equations below assume the following:
• There are no vanes or other obstructions in or near the inlet
• Even flow entering the funnel (no pre-swirl)
• Standard wheel to inlet cone overlap
• Accurate determination of air density at the inlet
• Free inlet (consult Twin City Fan for ducted inlet factors)
Non-Standard Density Method Standard Density Method
Product
C1
Free Inlet
C1*
Ducted Inlet
EPFN/EPQN (Arr. 1 or 4), Sizes 122-165
EPLFN/EPLQN, Sizes 122-165
753.06 794.06
EPFN/EPQN (Arr. 1 or 4), Sizes 182-982
EPLFN/EPLQN, Sizes 182-982
MPLFN/MPLQN,MPLFS/MPLQS
692.03 740.14
EPF/EPQ (Arr. 3), Sizes 122-165 726.39 765.94
EPF/EPQ (Arr. 3), Sizes 182-982 667.52 713.93
MPQN, Sizes 122-165 753.06 794.06
MPQN, Sizes 182-490 692.03 740.14
APF/APQ 753.06 794.06
BC/BCS/BAF SWSI 735.42 786.56
BC/BCS/BAF DWDI 1470.84 1573.12
TSL/TFE 735.42 753.56
QSL/QFE/QIFE/TVIFE 696.00 735.83
BAE-SW ** 720.40 735.80
BAE-DW** 1440.80 1471.60
RB (Std. Inlet Bell) 913.33 997.49
HIB/RTF 727.94 785.07
HAF 790.17 760.92
VBC/VAF 1414.00 NA
* Values for ducted C1 factors are based on
duct diameter matching standard inlet collar
diameter.
** BAE sizes smaller than 182 use BC\BCS\BAF
Factors.
ES-105
September 2014
Airflow Measuring System
Using Piezometer Ring
INSTALLATION, OPERATION & MAINTENANCE MANUAL
Table 1: C1 Values