250+ TOP MCQs on Condenser and Vacuum Efficiency and Answers

By engineer / May 26, 2024

Answer: a
Clarification: The correct expression for condenser efficiency is –
η_condenser = (frac{Rise , in , temperature , of , cooling , water}{Temp. , corresponding , to , vacuum , in , the , condenser-Inlet , temperature , of , cooling , water})
It can also be written as
η_condenser = (frac{Rise , in , temperature , of , cooling , water}{Temp. , corresponding , to , the , absolute , pressure , in , the , condenser-Inlet , temperature , of , cooling , water} )

7. The inlet and outlet temperatures of cooling water to a certain condenser are recorded to be 30°C and 40°C respectively. If the absolute pressure in the condenser is given to be 0.11 bar, determine efficiency.
a) 45.21%
b) 75.65%
c) 95.32%
d) 56.46%
Answer: d
Clarification: Given, t_wi = 30°C, t_wo = 40°C
From steam tables corresponding to 0.11 bar
t_s = 47.71°C
Therefore,
η_condenser = (frac{Rise , in , temperature , of , cooling , water}{Temp. , corresponding , to , the , absolute , pressure , in , the , condenser-Inlet , temperature , of , cooling , water} )
(%)η_condenser = (frac{t_{wo} – t_{wi}}{t_s-t_{wi}}) x 100
Substituting the values,
η_condenser = (frac{40 – 30}{47.71-30}) x 100 = 56.46%.

8. In a condenser, the temperature of steam entering in the condenser is observed to be 30°C. The condenser vacuum is recorded to be 670 mm of Hg. If the barometer reading is 760 mm of Hg, determine the vacuum efficiency.
a) 92.01%
b) 84.25%
c) 31.65%
d) 67.51%
Answer: a
Clarification: Actual vacuum = 670 mm of Hg
Using steam tables, At 30°C
Partial pressure of steam, P = 0.04242 bar = 0.04242/0.001333 mm of Hg
= 31.82 mm of Hg
Now, Maximum obtainable vacuum = 760 – 31.82 = 728.18 mm of Hg
Vacuum efficiency = η_vacuum = (frac{Actual , vacuum}{Maximum , obtainable , vacuum} )
= 670 / 728.18
= 0.9201 or 92.01%.

9. The outlet temperature of cooling water to a condenser is observed to be 42°C. Determine the inlet temperature of cooling water if the absolute pressure inside the condenser is 0.1 bar. Take condenser efficiency as 76%.
a) 12.36°C
b) 29.87°C
c) 36.21°C
d) 40.65°C
Answer:
Clarification: Given, t_wo = 42°C, η_condenser = 0.76, P = 0.1 bar
Using steam tables, corresponding to 0.1 bar
t_s = 45.83°C
We know that,
η_condenser = (frac{t_{wo} – t_{wi}}{t_s-t_{wi}} )
Substituting the values,
0.76=(frac{42- t_{wi}}{45.83-t_{wi}} )
Solving the equation for t_wi we get,
t_wi = 29.87°C.

10. The following data refers to a steam condenser –
Inlet temperature of cooling water = 36°C
Absolute pressure inside the condenser = 0.12 bar
efficiency = 78%
Determine the outlet temperature of cooling water.
a) 42.37°C
b) 39.12°C
c) 46.49°C
d) 35.21°C
Answer: c
Clarification: Given, t_wi = 36°C, P = 0.12 bar, η_condenser = 0.78
Using steam tables, corresponding to 0.12 bar
t_s = 49.45°C
We know that,
η_condenser = (frac{t_{wo} – t_{wi}}{t_s-t_{wi}} )
Substituting the values,
0.78 = (frac{t_{wo} – 36}{49.45-36} )
Solving for two we get,
t_wo = 46.49°C.