TSI Incorporated 8372, Operation And Service Manual

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Appendix A
Back Pressure
It is commonly known that an air capture hood may induce a back pressure.
In general, back pressures are caused by restrictions in the flow path as well
as frictional pressure losses. In order to improve accuracy and sensitivity,
all air capture hoods incorporate a contracted flow section. The contracted
section restricts the flow through the hood which induces a back pressure. A
hood having a more abrupt contraction than the A
CCU
B
ALANCE
Plus may
induce greater back pressure due to the turbulence caused by the abrupt
contraction. In addition any apparatus within the hood, such as supporting
poles and the flow manifold, also induces some frictional pressure drop.
Back pressure may cause slight errors in the flow measurements. Figure 18
shows the pressure drop through the A
CCU
B
ALANCE
Plus over its entire flow
range. As shown in the figure, the back pressure is only 0.05 in. H
2
O at a
flow rate as high as 1000 CFM.
Figure 18: Pressure Drop Through the A
CCU
B
ALANCE
P
LUS
If you wish to make back pressure corrections, you must first determine the
back pressure correction factor, Cb. The back pressure correction factor, Cb,
can be determined as follows: