i. Static pressure measurements at strategic points in a system provide
invaluable information on the performance of the system. These measurements are
neither difficult to obtain nor do they require expensive or delicate instrumentation.
(1) Coefficient of entry. Estimation of airflow, using static pressure, provides
a fairly accurate estimation of flow rate of an exhaust opening if the coefficient of entry
can be determined. (The coefficient of entry is the ratio of actual flow and theoretical
flow.) Coefficients of entry for various hoods are given in Figure 4-10. Measurements
are made between one and three diameters of straight duct from the throat of the
exhaust inlet (point where the hood is connected to the branch duct). It is advisable to
take multiple readings 90 degrees apart. The flow rate in cfm can then be calculated
using this formula:
Q = 4005 Ce A √ SPh
Where: Q = Rate of flow in cfm (see Figure 4-11)
Ce = Coefficient of entry
A = Cross-sectional area of duct in square feet
SPh = Average static pressure reading in inches of water (in the hood).
(2) Static pressure. Static pressure comparisons provide a means of either
continuously or periodically monitoring the performance of a system. It is true that
additional information may be required, but strategically located static pressure
measurements can detect dirty filters, dented exhaust hoods, and changes in fan static
pressure. The placement of such devices across a filter can determine the need for
shaking, cleaning, or maintenance.