Lesson test: Isentropic efficiency

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Isentropic efficiency

1.[2p]
For an adiabatic steady-flow device with one inlet and one outlet, what does the entropy balance require?

$s_{2} \geq s_{1}$ , with equality only if the device is reversible

Correct

$s_{2} = s_{1}$ always, since no heat crosses the boundary

The answer is: $s_2 \geq s_1$, with equality only if the device is reversible

$s_{2} \leq s_{1}$ , since work is extracted

The answer is: $s_2 \geq s_1$, with equality only if the device is reversible

Nothing, unless the device is also reversible

The answer is: $s_2 \geq s_1$, with equality only if the device is reversible

2.[3p]
Steam enters a turbine at $3$ MPa and $350$ degrees Celsius with $s = 6.7450$ kJ/(kg K) and exhausts to $10$ kPa where $s_{f} = 0.6492$ and $s_{f g} = 7.4996$ . What is the isentropic exit quality?

CorrectNot quite: 0.813

3.[2p]
The isentropic work of that turbine is $980.0$ kJ/kg. At an isentropic efficiency of $0.87$ , what is the actual work, in kJ/kg?

CorrectNot quite: 852.6

4.[3p]
Air at $295$ K is compressed from $100$ to $800$ kPa. With $k = 1.400$ , what is the isentropic exit temperature, in kelvin?

CorrectNot quite: 534

5.[3p]
For that compressor, $w_{s} = 240.6$ kJ/kg. At an isentropic efficiency of $0.82$ and $c_{p} = 1.005$ kJ/(kg K), what is the actual delivery temperature, in kelvin?

CorrectNot quite: 587

6.[2p]
A real adiabatic turbine exhausts wetter than the ideal one operating between the same inlet state and exhaust pressure.

True

The answer is: False

False

Correct

7.[2p]
Why is $w_{r e v} = - \int v d p$ the reason vapour power cycles condense before pumping?

The work scales with specific volume, and a liquid's is thousands of times smaller than a vapour's

Correct

The integral is only valid for liquids

The answer is: The work scales with specific volume, and a liquid's is thousands of times smaller than a vapour's

Condensing removes the entropy generated in the turbine

The answer is: The work scales with specific volume, and a liquid's is thousands of times smaller than a vapour's

Pumps have higher isentropic efficiencies than compressors

The answer is: The work scales with specific volume, and a liquid's is thousands of times smaller than a vapour's

8.[3p]
Match each device to how its isentropic efficiency is defined.

Turbine
Compressor
Nozzle
Throttling valve

ideal work over actual work
actual over ideal exit kinetic energy
actual work over ideal work
undefined, since ideal work is zero

Show the answer

Turbine: actual work over ideal work Compressor: ideal work over actual work Nozzle: actual over ideal exit kinetic energy Throttling valve: undefined, since ideal work is zero

9.[3p]
Which of these are honest limits on isentropic efficiency as a measure?

Select all that apply

It is not comparable between a turbine and a compressor on the same duty

Correct

A multistage turbine's overall value exceeds its stage values, because of the reheat effect

Correct

It says nothing about a device whose ideal work is zero

Correct

It can exceed one for a well-designed machine

The answer is: It is not comparable between a turbine and a compressor on the same duty, A multistage turbine's overall value exceeds its stage values, because of the reheat effect, It says nothing about a device whose ideal work is zero

1.[2p] For an adiabatic steady-flow device with one inlet and one outlet, what does the entropy balance require?

2.[3p] Steam enters a turbine at 3 MPa and 350 degrees Celsius with s=6.7450 kJ/(kg K) and exhausts to 10 kPa where sf=0.6492 and sfg=7.4996. What is the isentropic exit quality?

3.[2p] The isentropic work of that turbine is 980.0 kJ/kg. At an isentropic efficiency of 0.87, what is the actual work, in kJ/kg?

4.[3p] Air at 295 K is compressed from 100 to 800 kPa. With k=1.400, what is the isentropic exit temperature, in kelvin?

5.[3p] For that compressor, ws=240.6 kJ/kg. At an isentropic efficiency of 0.82 and cp=1.005 kJ/(kg K), what is the actual delivery temperature, in kelvin?

6.[2p] A real adiabatic turbine exhausts wetter than the ideal one operating between the same inlet state and exhaust pressure.

7.[2p] Why is wrev=-∫vdp the reason vapour power cycles condense before pumping?

8.[3p] Match each device to how its isentropic efficiency is defined.

9.[3p] Which of these are honest limits on isentropic efficiency as a measure?

1.[2p]
For an adiabatic steady-flow device with one inlet and one outlet, what does the entropy balance require?

2.[3p]
Steam enters a turbine at $3$ MPa and $350$ degrees Celsius with $s = 6.7450$ kJ/(kg K) and exhausts to $10$ kPa where $s_{f} = 0.6492$ and $s_{f g} = 7.4996$ . What is the isentropic exit quality?

3.[2p]
The isentropic work of that turbine is $980.0$ kJ/kg. At an isentropic efficiency of $0.87$ , what is the actual work, in kJ/kg?

4.[3p]
Air at $295$ K is compressed from $100$ to $800$ kPa. With $k = 1.400$ , what is the isentropic exit temperature, in kelvin?

5.[3p]
For that compressor, $w_{s} = 240.6$ kJ/kg. At an isentropic efficiency of $0.82$ and $c_{p} = 1.005$ kJ/(kg K), what is the actual delivery temperature, in kelvin?

6.[2p]
A real adiabatic turbine exhausts wetter than the ideal one operating between the same inlet state and exhaust pressure.

7.[2p]
Why is $w_{r e v} = - \int v d p$ the reason vapour power cycles condense before pumping?

8.[3p]
Match each device to how its isentropic efficiency is defined.

9.[3p]
Which of these are honest limits on isentropic efficiency as a measure?