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3 Flow Rates and Pressure Measurements
a)
b)
Pointer
Chain
Partial vacuum
Vacuum chamber
Spring
Pivot
Axle
Fig. 3.1 a Mercury barometer and b aneroid barometer
Exercise 3.1 The atmospheric pressure at the topographic surface level measured
by a barometer is 765 mm Hg. The static pressure generated by a fan is 60,000 Pa.
Estimate the reading of this same barometer at the bottom of a 750 m shaft, ventilated
from the surface by the fan. Omit friction losses in your calculations.
Solution
First, units are converted:
765 mm Hg =
1 atm
760 mm Hg
101,300 Pa
1 atm
= 101,966 Pa
Next, the pressure of the static column of air in the shaft is calculated:
P = ρgh = 1.2
kg
m 3 9.8
N
kg
750 m = 8820 Pa
By adding the pressure generated by the fan, the absolute pressure at the bottom
of the shaft is obtained, thus:
101,966 Pa + 8820 Pa + 60,000 Pa = 170,786 Pa
3 Flow Rates and Pressure Measurements
a)
b)
Pointer
Chain
Partial vacuum
Vacuum chamber
Spring
Pivot
Axle
Fig. 3.1 a Mercury barometer and b aneroid barometer
Exercise 3.1 The atmospheric pressure at the topographic surface level measured
by a barometer is 765 mm Hg. The static pressure generated by a fan is 60,000 Pa.
Estimate the reading of this same barometer at the bottom of a 750 m shaft, ventilated
from the surface by the fan. Omit friction losses in your calculations.
Solution
First, units are converted:
765 mm Hg =
1 atm
760 mm Hg
101,300 Pa
1 atm
= 101,966 Pa
Next, the pressure of the static column of air in the shaft is calculated:
P = ρgh = 1.2
kg
m 3 9.8
N
kg
750 m = 8820 Pa
By adding the pressure generated by the fan, the absolute pressure at the bottom
of the shaft is obtained, thus:
101,966 Pa + 8820 Pa + 60,000 Pa = 170,786 Pa