Thermal Engineering Questions and Answers – Condenser and Vacuum Efficiency

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This set of Thermal Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Condenser and Vacuum Efficiency”.

1. What type of pump can be used for obtaining maximum vacuum in condensers?
a) Air pump
b) Centrifugal pump
c) Steam pump
d) Gear pump
View Answer

Answer: a
Explanation: Air pump can be used to obtain a desired level of vacuum in the condensers. It extracts air and other non-condensable gases. Air pumps are classified into wet air pumps and dry air pumps. Wet air pumps are used to remove a mixture of condensate and non-condensable gases while dry air pump removes the air only.
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2. What instrument is used to measure vacuum in the condensers.
a) Thermometer
b) Vacuum gauge
c) Barometer
d) Pressure transducer
View Answer

Answer: b
Explanation: Vacuum measurement in condensers is done using vacuum gauge. In case of condensers, vacuum pressure refers to the pressure below atmospheric pressure. For calculation purposes the barometric reading is taken to be 760 mm of Hg.

3. Which of the following is the correct definition of vacuum efficiency?
a) It is the ratio of actual vacuum to atmospheric pressure
b) It is the ratio of atmospheric pressure to actual vacuum
c) It is the ratio of maximum obtainable vacuum to actual vacuum
d) It is the ratio of actual vacuum to maximum obtainable vacuum
View Answer

Answer: d
Explanation: Vacuum efficiency is defined as the ratio of the actual vacuum to the maximum obtainable vacuum.
Mathematically,
ηvacuum = \(\frac{Actual \, vacuum}{Maximum \, obtainable \, vacuum} \)
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4. Which of the following expressions is the correct one for calculating the vacuum efficiency of a condenser?
a) \(\frac{Actual \, vacuum}{Absolute \, pressure \, of \, steam-Barometer \, Pressure} \)
b) \(\frac{Actual \, vacuum}{Barometer \, Pressure-Absolute \, pressure \, of \, steam} \)
c) \(\frac{Barometer \, Pressure-Absolute \, pressure \, of \, steam}{Actual \, vacuum} \)
d) \(\frac{Absolute \, pressure \, of \, steam-Barometer \, Pressure}{Actual \, vacuum} \)
View Answer

Answer: b
Explanation: Vacuum efficiency is the ratio of the actual vacuum to the maximum obtainable vacuum.
Therefore,
ηvacuum = \(\frac{Actual \, vacuum}{Maximum \, obtainable \, vacuum} \)
but Maximum obtainable vacuum = Barometer pressure – Absolute pressure of steam
\(\frac{Actual \, vacuum}{Barometer \, Pressure-Absolute \, pressure \, of \, steam} \)

5. Which of the following is the correct definition of condenser efficiency?
a) It is the ratio of the difference between the outlet and the inlet temperatures of cooling water to the difference between the temperature corresponding to the vacuum in the condenser and inlet temperature of cooling water
b) It is the ratio of the difference between the temperature corresponding to the vacuum in the condenser and inlet temperature of cooling water to the difference between the inlet and outlet temperature of cooling water
c) It is the ratio of the difference between the atmospheric pressure and the absolute pressure inside the condenser to the difference between the atmospheric pressure and the absolute partial pressure of the steam
d) It is the ratio of the difference between the actual pressure inside the condenser and the partial pressure of the steam to the atmospheric pressure
View Answer

Answer: a
Explanation: Condenser efficiency is defined as the ratio of the difference between the outlet and the inlet temperatures of cooling water to the difference between the temperature corresponding to the vacuum in the condenser and inlet temperature of cooling water.
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6. Which of the following is the correct expression for calculating condenser efficiency?
a) \(\frac{Rise \, in \, temperature \, of \, cooling \, water}{Temp. \, corresponding \, to \, vacuum \, in \, the \, condenser-Inlet \, temperature \, of \, cooling \, water} \)
b) \(\frac{Rise \, in \, temperature \, of \, cooling \, water}{Inlet \, temperature \, of \, cooling \, water- Temp. \, corresponding \, to \, vacuum \, in \, the \, condenser} \)
c) \(\frac{Inlet \, temperature \, of \, cooling \, water-Temp. \, corresponding \, to \, vacuum \, in \, the \, condenser}{Rise \, in \, temperature \, of \, cooling \, water} \)
d) \(\frac{Temp. \, corresponding \, to \, vacuum \, in \, the \, condenser-Inlet \, temperature \, of \, cooling \, water}{Rise \, in \, temperature \, of \, cooling \, water} \)
View Answer

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

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%
View Answer

Answer: d
Explanation: Given, twi = 30°C, two = 40°C
From steam tables corresponding to 0.11 bar
ts = 47.71°C
Therefore,
ηcondenser = \(\frac{Rise \, in \, temperature \, of \, cooling \, water}{Temp. \, corresponding \, to \, the \, absolute \, pressure \, in \, the \, condenser-Inlet \, temperature \, of \, cooling \, water} \)
(%)ηcondenser = \(\frac{t_{wo} – t_{wi}}{t_s-t_{wi}}\) x 100
Substituting the values,
ηcondenser = \(\frac{40 – 30}{47.71-30}\) x 100 = 56.46%.
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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%
View Answer

Answer: a
Explanation: 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 = ηvacuum = \(\frac{Actual \, vacuum}{Maximum \, obtainable \, vacuum} \)
= 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
View Answer

Answer: b
Explanation: Given, two = 42°C, ηcondenser = 0.76, P = 0.1 bar
Using steam tables, corresponding to 0.1 bar
ts = 45.83°C
We know that,
ηcondenser = \(\frac{t_{wo} – t_{wi}}{t_s-t_{wi}} \)
Substituting the values,
0.76=\(\frac{42- t_{wi}}{45.83-t_{wi}} \)
Solving the equation for twi we get,
twi = 29.87°C.
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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
View Answer

Answer: c
Explanation: Given, twi = 36°C, P = 0.12 bar, ηcondenser = 0.78
Using steam tables, corresponding to 0.12 bar
ts = 49.45°C
We know that,
ηcondenser = \(\frac{t_{wo} – t_{wi}}{t_s-t_{wi}} \)
Substituting the values,
0.78 = \(\frac{t_{wo} – 36}{49.45-36} \)
Solving for two we get,
two = 46.49°C.

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