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3.9.2 Illustrative Example 2
ОглавлениеThe height of a liquid column of mercury is 2.493 ft. Assume the density of mercury is 848.7 lb/ft3 and atmospheric pressure is 2116 lbf/ft2 absolute. Calculate the gauge pressure in lbf/ft2 and the absolute pressure in lbf/ft2, psia, mm Hg, and in H2O.
Solution. Expressed in various units, the standard atmosphere is equal to:
| 1.0 | Atmospheres (atm) |
| 33.91 | Feet of water (ft H2O) |
| 14.7 | Pounds force per square inch absolute (psia) |
| 2116 | Pounds force per square foot absolute (psfa) |
| 29.92 | Inches of mercury (in Hg) |
| 760.0 | Millimeters of mercury (mm Hg) |
| 1.013 x 105 | Newtons per square meter (N/m2) |
The density equation is describing the gauge pressure in terms of the column height and liquid density is:
(3.9)
where Pg = gauge pressure, ρ = liquid density, g = acceleration of gravity, h = column height, and gc = conversion constant. Thus,
The pressure in lbf/ft2 absolute is:
The pressure in psia is;
The corresponding gauge pressure in psi is:
The pressure in mm Hg is:
Finally, the pressure in in H2O is:
The reader should note that absolute and gauge pressures are usually expressed with units of atm, psi, or mm Hg. This statement also applies to partial pressures. One of the most common units employed to describe pressure drop is inches of H2O, with the notation in H2O or IWC (inches of water column).
