The following observation are recorded for steam condenser.

Condenser vacuum = 680 mm (Hg)

Barometer reading = 760 mm (Hg)

Condenser temperature = 35°C

P_{sat@35° C} = 42 mm Hg

What will be the vacuum efficiency?

Explanation

Vacuum Efficiency – It is defined as the ratio of actual vacuum recorded by gauge inside the condenser to the ideal vacuum.

\(Vacuum\;efficiency = \frac{{actual\;vacuum}}{{ideal\;vacuum}}\)

Actual vacuum = Barometric pressure – Actual absolute pressure of the condenser.

Actual vacuum = Pb – (Ps + Pa) 

Ideal vacuum = Barometric Pressure – Absolute pressure of steam.

Ideal vacuum = Pb – Ps

where

P_b = Barometric process, P_s = Absolute pressure of steam, P_a  = Absolute pressure of air

Calculation:

Given:

Actual vacuum = condenser vacuum = 680 mm (Hg)

Barometric reading = 760 mm (Hg)

Absolute pressure head of steam = 42 mm (Hg)

Vacuum efficency is:

 \({\eta _v} = \frac{{Actual~vacuum}}{{Ideal~vacuum}} = \frac{{680}}{{760 - 42}} = 94.7\% \)