TITLE.PM5

44 ENGINEERING THERMODYNAMICS

dharm
M-therm/th2-2.pm5

Fig. 2.25

= 13640 × 9.79

761

1000

550

+ 1000

F

H

I

K = 13640 × 9.79 (0.761 + 0.55)

= 175.065 × 10^3 N/m^2 = 175.065 kPa = 1.75 bar. (Ans.)

Example 2.7. A U-tube mercury manometer with one arm open to atmosphere is used to

measure pressure in a steam pipe. The level of mercury in open arm is 97.5 mm greater than that

in the arm connected to the pipe. Some of steam in the pipe condenses in the manometer arm

connected to the pipe. The height of this column is 34 mm. The atmospheric pressure is 760 mm

of Hg. Find the absolute pressure of steam. (Poona University, Nov. 1997)

Solution. Equating the pressure in mm of Hg on both

arms above the line XX (Fig. 2.26), we get

pabs. + pwater = pHg + patm.

Now, pwater = 136.^34 = 2.5 mm of Hg.

∴ pabs + 2.5 = 97.5 + 760

or pabs = 97.5 + 760 – 2.5

= 855 mm of Hg.

= 855 × pHg × g × 10–5 bar

= 1000855 (m) × (13.6 × 1000) (kg/m^3 )

× 9.81 × 10–5

= 1.1407 bar. (Ans.)

Example 2.8. A U-tube manometer is connected to a gas pipe. The level of the liquid in the

manometer arm open to the atmosphere is 170 mm lower than the level of the liquid in the arm

connected to the gas pipe. The liquid in the manometer has specific gravity of 0.8. Find the

absolute pressure of the gas if the manometer reads 760 mm of Hg.

(Poona University, Dec. 2000)

Fig. 2.26