Page 190 - Mechanical Engineers' Handbook (Volume 2)

9 Flow Rate 179
1.38 P
calib calib
R AIR calib
T
(25 14.7)(144)
1.38 0.279
(53.3)(460 70 F)
W calib (1.44)(1.38)(.075) 0.149 lb /s
m
0.585
W (0.149) 0.216 lb /s
USE m
0.279
W W
SCFM (USE) USE USE 2.877 SCFM
0.075
STD,AIR
This correction equation was derived under the assumption that the float was held up
only by the pressure force (i.e., buoyancy and viscous drag were both negligible). When
metering liquids of high density, the buoyant force must be accounted for, and if the viscosity
of the fluid exceeds the viscosity immunity ceiling for a given meter, then the viscous drag
must be taken into account. Precautions for dealing with these two cases are covered by the
manufacturer’s instructions. A meter purchased for gas service cannot be converted to liquid
service by the preceding equation, or vice versa; a new calibration is required.
Variable-area meters are generally limited in accuracy to 1or 2% of full-scale
reading.
It is important to measure the actual density of the fluid flowing in the meter to ensure
that the calibration conditions are properly met. Pressure and temperature must be measured
at the meter, just upstream of the float.
Irreversibilities involved in the mixing of the annular jet introduce losses in pressure
that are nearly independent of flow rate and roughly equal to the pressure required to hold
up the float. More accurate meters generally have higher losses (up to 3 or 4 psi used with
air).

9.5 Laminar Flowmeters

Commercial laminar flowmeters consist of a matrix or core of small-diameter passages ar-
ranged so that the pressure drop across this core can be measured. The meter must be sized
properly so the flow in these passages will remain laminar, even at the highest rated flow.
If the flow remains laminar, then the pressure drop is linearly related to the volume flow
rate through the core. General commercial practice is to provide flow-straightening sections
upstream and downstream of the core and measure the pressures in the space between the
flow straighteners and the core, as shown in Fig. 28.
Laminar flowmeters produce a pressure difference, usually 0–8 in. H O, which must be
2
measured using an appropriate auxiliary instrument. The meter responds to volume flow;
hence it is also necessary to measure the density of the fluid flowing. If the composition is
known, density can be calculated knowing temperature and pressure at the inlet.
The pressure drop across a well-designed laminar flowmeter is linearly proportional to
the volume flow rate (ACFM) multiplied by the viscosity of the fluid flowing. The pressure
drop is independent of density.
Each meter is accompanied by a calibration curve, which typically reads:
Air Flow in Cubic Feet per Minute at 70 F and 29.92 Inches of Mercury Absolute vs.
Pressure Drop, Inches of Water.