Steam Turbine: Solve for Velocity

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Discussion Overview

The discussion revolves around a homework problem involving a steam turbine, specifically focusing on calculating the exit velocity of steam given certain inlet conditions and enthalpy values. The scope includes theoretical reasoning and mathematical problem-solving related to thermodynamics and energy processes in turbines.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant suggests that the steam flow should be in lb/min rather than Btu/min, indicating a potential error in the problem statement.
  • Another participant questions the validity of the exit enthalpy being given in Btu/min, arguing that it should be in Btu/lb to match the units of heat per pound.
  • A participant proposes recalculating the exit velocity using corrected units for enthalpy and steam flow, implying that the original answer of 50 ft/s may be incorrect.
  • One participant claims to have resolved the issue by changing the exit enthalpy to Btu/lb and confirming the steam flow as 200 lb/min, suggesting a successful recalculation.

Areas of Agreement / Disagreement

Participants express disagreement regarding the units used for steam flow and exit enthalpy, with no consensus reached on the original problem's correctness or the validity of the provided answer.

Contextual Notes

Participants highlight potential limitations in the problem statement, including unit inconsistencies and assumptions about the definitions of enthalpy and steam flow rates. These factors may affect the calculations and interpretations of the problem.

Paul Lasdivan
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Homework Statement


Homework Statement [/B]
A steam turbine developed 2372.20 Hp when its inlet condition is 1300 Btu/lb enthalpy and 400 ft/s velocity and steam flow of 200 Btu/min :The exit enthalpy is 800 Btu/min. Find the exit velocity.

That is the original problem statement but i think the steam flow should be in lb/min.
The answer to the problem is 50 feet per second but no solution was provided.

Homework Equations


Energy process in turbine is mh1 + KEi = mh2 + KEf + W + Q , Heat is neglected so Q = 0.

The Attempt at a Solution


I assumed that steam flow isn't in Btu/min but in lb/min so I converted this to lb/sec to fit with the given velocity.
mh1 = (1300Btu/lb)(3.33lb/sec) = 4329 Btu/sec
KEi = 1/2(3.33lb/sec)(4002)(ft2/s2) = 266400(ft2lb/sec3)
Also since the exit enthalpy is given in Btu/min, I assumed this to be mh2
mh2 = 800Btu/min(1min/60sec) = 13.33Btu/sec
W = 2372.20Hp ((0.707Btu/sec)/Hp) = 1677.14 Btu/sec
1lbf = 32.2lbm-ft/sec so KE1 = 8273.29ft-lbf/sec = 10.63Btu/sec
Solving for KEf, KEf = 4329+10.63 - 13.33 -1677.14 = 2649.16btu/sec
I solved Vf by converting btu again to ft-lbf and then to ft-lbm from this but the answer is much greater than 50fps.
 
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This problem is solvable but it seems that the given problem answer is incorrect
 
Since no one has answered you just yet, I will do my best to help you. I'm thinking that the 800 Btu/min is wrong because enthalpy is a total amount of heat, and Btu/lb is heat per pound, it's a mass unit. But 800 Btu/min is a measure of the speed of heat loss. One is a total and one is a speed, it doesn't match up.

I would recalculate it with 800 Btu/lb and see if you get the right answer.
 
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Also, the steam flow should be 200 lb/ min .
 
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Got it by changing exit enthalpy to Btu/lb and of course steam flow to 200 lb/min :D Thank you for helping.
 
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