Lesson test: The steady-flow devices

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The steady-flow devices

1.[3p]
Match each device to the form the energy equation takes for it.

Nozzle
Turbine
Throttling valve
Mixing chamber

$w = h_{1} - h_{2}$
$h_{1} + V_{1}^{2} / 2 = h_{2} + V_{2}^{2} / 2$
$h_{1} = h_{2}$
${\dot{m}}_{1} h_{1} + {\dot{m}}_{2} h_{2} = {\dot{m}}_{3} h_{3}$

Show the answer

Nozzle: $h_{1} + V_{1}^{2} / 2 = h_{2} + V_{2}^{2} / 2$ Turbine: $w = h_{1} - h_{2}$ Throttling valve: $h_{1} = h_{2}$ Mixing chamber: ${\dot{m}}_{1} h_{1} + {\dot{m}}_{2} h_{2} = {\dot{m}}_{3} h_{3}$

2.[3p]
Air enters a nozzle at $200$ degrees Celsius and $30$ m/s and leaves at $100$ degrees Celsius. With $c_{p} = 1.005$ kJ/(kg K), what is the exit velocity, in m/s?

CorrectNot quite: 449

3.[3p]
Saturated liquid R-134a at $800$ kPa, $h_{f} = 95.47$ kJ/kg, is throttled to $132.8$ kPa where $h_{f} = 25.49$ and $h_{f g} = 212.91$ kJ/kg. What quality results?

CorrectNot quite: 0.329

4.[2p]
Throttling an ideal gas leaves its temperature unchanged.

True

Correct

False

The answer is: True

5.[2p]
Why does a pump cost so much less work than a compressor over the same pressure range?

Reversible steady-flow work is $\int v d p$ , and a liquid's specific volume is thousands of times smaller

Correct

Pumps are built to much higher isentropic efficiencies than compressors

The answer is: Reversible steady-flow work is $\int v\,\mathrm{d}p$, and a liquid's specific volume is thousands of times smaller

A liquid absorbs the work as internal energy rather than as pressure

The answer is: Reversible steady-flow work is $\int v\,\mathrm{d}p$, and a liquid's specific volume is thousands of times smaller

Liquids are at lower temperature, so less work is needed

The answer is: Reversible steady-flow work is $\int v\,\mathrm{d}p$, and a liquid's specific volume is thousands of times smaller

6.[2p]
A condenser receives $15.61$ kg/s of steam at $2263.5$ kJ/kg and returns it as saturated liquid at $191.81$ kJ/kg. What is the heat duty, in MW?

CorrectNot quite: 32.3

7.[3p]
For that condenser, how much cooling water is needed for a $10$ K rise, taking $c = 4.18$ kJ/(kg K), in kg/s?

CorrectNot quite: 774

8.[3p]
Which deletions are justified for a throttling valve?

Select all that apply

No shaft work, since it has no moving part

Correct

No heat transfer, since it is small and the fluid is inside briefly

Correct

Negligible velocity change, when the pipes either side are similar

Correct

No entropy generation, since no work is extracted

The answer is: No shaft work, since it has no moving part, No heat transfer, since it is small and the fluid is inside briefly, Negligible velocity change, when the pipes either side are similar

9.[2p]
Draw the control surface around a whole condenser rather than around one stream. What happens to the heat transferred between the streams?

It disappears from the balance, because it never crosses the surface that was drawn

Correct

It doubles, because both streams are now inside

The answer is: It disappears from the balance, because it never crosses the surface that was drawn

It becomes the shaft work term

The answer is: It disappears from the balance, because it never crosses the surface that was drawn

Nothing changes, since heat is a state function

The answer is: It disappears from the balance, because it never crosses the surface that was drawn

1.[3p] Match each device to the form the energy equation takes for it.

2.[3p] Air enters a nozzle at 200 degrees Celsius and 30 m/s and leaves at 100 degrees Celsius. With cp=1.005 kJ/(kg K), what is the exit velocity, in m/s?

3.[3p] Saturated liquid R-134a at 800 kPa, hf=95.47 kJ/kg, is throttled to 132.8 kPa where hf=25.49 and hfg=212.91 kJ/kg. What quality results?

4.[2p] Throttling an ideal gas leaves its temperature unchanged.

5.[2p] Why does a pump cost so much less work than a compressor over the same pressure range?

6.[2p] A condenser receives 15.61 kg/s of steam at 2263.5 kJ/kg and returns it as saturated liquid at 191.81 kJ/kg. What is the heat duty, in MW?

7.[3p] For that condenser, how much cooling water is needed for a 10 K rise, taking c=4.18 kJ/(kg K), in kg/s?

8.[3p] Which deletions are justified for a throttling valve?

9.[2p] Draw the control surface around a whole condenser rather than around one stream. What happens to the heat transferred between the streams?

1.[3p]
Match each device to the form the energy equation takes for it.

2.[3p]
Air enters a nozzle at $200$ degrees Celsius and $30$ m/s and leaves at $100$ degrees Celsius. With $c_{p} = 1.005$ kJ/(kg K), what is the exit velocity, in m/s?

3.[3p]
Saturated liquid R-134a at $800$ kPa, $h_{f} = 95.47$ kJ/kg, is throttled to $132.8$ kPa where $h_{f} = 25.49$ and $h_{f g} = 212.91$ kJ/kg. What quality results?

4.[2p]
Throttling an ideal gas leaves its temperature unchanged.

5.[2p]
Why does a pump cost so much less work than a compressor over the same pressure range?

6.[2p]
A condenser receives $15.61$ kg/s of steam at $2263.5$ kJ/kg and returns it as saturated liquid at $191.81$ kJ/kg. What is the heat duty, in MW?

7.[3p]
For that condenser, how much cooling water is needed for a $10$ K rise, taking $c = 4.18$ kJ/(kg K), in kg/s?

8.[3p]
Which deletions are justified for a throttling valve?

9.[2p]
Draw the control surface around a whole condenser rather than around one stream. What happens to the heat transferred between the streams?