5C-7 : | Heat Losses From a Steam Compressor | 6 pts |
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A 25 kW compressor is used to increase the pressure of saturated steam at 140oC to 1.2 MPa.
The compressor effluent
is at 280oC. If the steam flow rate
is 3.7 kg/min… |
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a.) Calculate the rate of heat loss from the compressor b.) Assume the steam behaves as an ideal gas and calculate the % error in the heat loss that results from the ideal gas assumption. |
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Read : | This is a straightforward application of the steady-state form of the 1st Law. | |||||||||||||||||
In part (a), we can lookup properties in the Steam Tables. | ||||||||||||||||||
In part (b), we must use the Ideal Gas Heat Capacity from the NIST Webbook to evaluate DH. | ||||||||||||||||||
Given: | m | 3.7 | kg/min | P2 | 1200 | kPa | ||||||||||||
0.0617 | kg/s | T2 | 280 | oC | ||||||||||||||
x1 | 1.00 | kg vap/kg total | W | -25 | kW | |||||||||||||
T1 | 140 | oC | ||||||||||||||||
Find: | Q | ??? | kW | |||||||||||||||
Diagram: |
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Assumptions: | 1 - | Changes in kinetic and potential energy are negliqible. | ||||||||||||||||
Equations / Data / Solve: | ||||||||||||||||||
Part a.) | Begin by writing the steady-state form of the 1st Law for open systems in which changes in kinetic and potential energy are negligible. | |||||||||||||||||
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Eqn 1 | |||||||||||||||||
We can solve Eqn 1 for Q : |
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Eqn 2 | ||||||||||||||||
Now, we need to determine H1 and H2. We can lookup H1 in the NIST Webbook. | ||||||||||||||||||
H1 | 2733.4 | kJ/kg | ||||||||||||||||
For state 2, we must first determine the phase. | ||||||||||||||||||
Psat(T2) | 6416.6 | kPa | P2 < Psat, therefore we must consult the Superheated Steam Tables. | |||||||||||||||
From the NIST Webbook, we can obtain : | H2 | 3002.6 | kJ/kg | |||||||||||||||
Now, we can plug values into Eqn 2 : | Q | -8.404 | kW | |||||||||||||||
Part b.) | Eqn 2 still applies if the steam is treated as an ideal gas. | |||||||||||||||||
The difference from part (a) lies in how we evaluate the change in the specific enthalpy of the steam. | ||||||||||||||||||
In part (b) we evaluate the change in the enthalpy using : | ||||||||||||||||||
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Eqn 3 | |||||||||||||||||
The Shomate Equation for the ideal gas heat capacity is : |
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Eqn 4 | ||||||||||||||||
NIST Webbook : | where : |
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Eqn 5 | |||||||||||||||
Temp (K) | 500. - 1700. | and : |
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Eqn 6 | ||||||||||||||
A | 30.092 | |||||||||||||||||
B | 6.832514 | Let's use these Shomate constants even though 50oC is outside of the recommended temperature range. These are the best values available to us. | ||||||||||||||||
C | 6.793435 | |||||||||||||||||
D | -2.53448 | |||||||||||||||||
E | 0.082139 | |||||||||||||||||
Combining Eqns 1, 2 and 3 and integrating yields : | ||||||||||||||||||
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Eqn 7 | |||||||||||||||||
Plug in values for the temperatures and the constants to get : | DH | 4908 | J/mol | |||||||||||||||
MWH2O | 18.016 | g/mole | DH | 272.4 | kJ/kg | |||||||||||||
Now, plug this value into Eqn 2 to evaluate QIG : | QIG | -8.200 | kW | |||||||||||||||
We can calculate the %error due to assuming that the steam is an ideal gas using : | ||||||||||||||||||
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Eqn 8 | |||||||||||||||||
%error | 2.42% | |||||||||||||||||
Verify: | The assumption made in this problem solution cannot be verified. | |||||||||||||||||
Answers : | Q | -8.40 | kW | Note that the negative sign indicates that heat transfer is from the compressor to the surroundings. | ||||||||||||||
QIG | -8.20 | kW | ||||||||||||||||
%error | 2.4% |