When are work and heat calculated by differences in enthelpy or internal energy

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SUMMARY

The discussion centers on the application of the first law of thermodynamics in different contexts, specifically the use of internal energy versus enthalpy in calculating heat and work. The equation Q + W = U2 - U1 applies to closed systems with fixed mass, while for open systems, such as steam cycles, the equation transitions to Q + W = H2 - H1, where H represents enthalpy. The distinction arises because enthalpy accounts for both internal energy and the work associated with pressure and volume changes, making it essential for mass flow scenarios. The participant references "Applied Thermodynamics for Engineering Technologists" by Eastop & McConkey to support their understanding of these concepts.

PREREQUISITES
  • Understanding of the first law of thermodynamics
  • Familiarity with internal energy and enthalpy concepts
  • Knowledge of steam cycles and their thermodynamic processes
  • Basic principles of mass flow in thermodynamic systems
NEXT STEPS
  • Study the differences between internal energy and enthalpy in thermodynamics
  • Learn about the application of the first law of thermodynamics in open systems
  • Explore the concept of shaft work and its relevance in turbine operations
  • Investigate steam cycle efficiency and the role of enthalpy in energy calculations
USEFUL FOR

Engineering students, thermodynamics practitioners, and professionals involved in energy systems design and analysis will benefit from this discussion, particularly those focusing on steam cycles and energy balance calculations.

Yellow Fang
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Hi,

This is not exactly homework, but I'm not sure it belongs in any other section either. I've been reading through Applied Thermodynamics for Engineering Technologists by Eastop & McConkey. In general it's pretty good I think. However, in the first few chapters, it explained that Q+W = U2-U1, i.e. Heat plus work equals the difference in internal energy (in steam). Then I got to the chapter on steam cycles where it started calculating heat and work in differences in enthalpy. I haven't really got to the bottom of why this is. I know that enthalpy is internal energy plus the product of pressure and volume (h = u + pv). I looked up one book in the library which referred to 'shaft work' when work is done with no change in pressure, but I can't see how this could apply with a turbine because there in change in pressure during the compression and expansion processes.

It seems that enthalpies are used for calculations where mass flow is involved, and that the equation in question is

m(u1 + v12/2 + gh1 + p1v1) + Q + W = m(u2 + v22/2 + gh2 + p2v2)

Am I along the right lines? Would Q+W=U2-U1 still apply for a reciprocating steam engine?
 
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Q+W=U2-U1 is applicable when you have a system (fixed mass). For a control volume (or open system) the correct energy balance is Q+W=H2-H1 .
 

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