Finding h2 Using Steam Tables and Enthalpy Equations

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In summary, the conversation is about finding h1 and h2 values to calculate work and heat transfer for water using enthalpy equations. The speaker has obtained h1 using steam tables and is asking for help in finding h2, which is necessary for the heat transfer calculation. The expert suggests using the equation h2=hf2+(x2hfg) or w=h1-h2 and also recommends drawing diagrams and providing the complete problem statement for further assistance.
  • #1
cookie85
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Hello again...i was wondering if you can help?

I am using enthalpy equations to try and find h1 and h2 to work out 'work and heat transfers' for water, for a piston type question.

I have calculated work done using: [w = -PDV]

To find the heat transfer i need to use: [Q=m.(h2-h1)]

i have obtained h1 using: h1 = hf + x(hg-hf) (using steam tables)

This has been calculated at 1826 kg/m3, from a revision guide all it states to calculate h2 is:

''From the steam table using h1 find h2?''

This is the only piece of information i need to complete the heat transfer calculation (to find h2) can you help at all? I don't now how to find h2.

I know m = 0.15, and h1 is 1826kg/m3 and Q is 223 kj

But h2?!

Guidance on how to look this up would be appreciated.
 
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  • #2
It would help if you could post the complete problem statement. For these types of problems, it usually helps to draw out the P-V and T-S diagrams. I suspect the equation relating enthalpy and work will help here, it should be somewhere in your text
 
  • #3
Posting the whole question would have got the job done but anyway have you tried these formulas

h2=hf2+(x2hfg)
since hf2 can be found on the tables and hfg

or w=h1-h2
 

1. What are steam tables and how are they used in thermodynamics?

Steam tables are a set of tables that present data on the thermodynamic properties of water and steam at different temperatures and pressures, such as enthalpy, entropy, and specific volume. They are used to determine the properties of steam and water in various states and to perform calculations in thermodynamics.

2. What is enthalpy and how is it related to steam tables?

Enthalpy is a measure of the total energy contained in a system, including the internal energy and the work done by or on the system. In steam tables, enthalpy is listed as one of the properties of steam and water at different temperatures and pressures, making it a useful resource for thermodynamic calculations.

3. How do you read and interpret steam tables?

Steam tables are typically organized in rows and columns, with different properties listed for different temperatures and pressures. To read and interpret steam tables, one must first identify the desired temperature and pressure, then locate the corresponding values for the properties of interest, such as enthalpy or entropy.

4. What are the limitations of steam tables?

Steam tables are based on ideal gas assumptions and do not account for real-world conditions, such as the effects of impurities or non-ideal behavior. Therefore, they may not be accurate for all situations and should be used with caution.

5. Can steam tables be used for substances other than water?

While steam tables are commonly used for water and steam calculations, they can also be used for other substances that behave similarly to water, such as refrigerants and other fluids. However, they may not be accurate for substances with significantly different properties or behaviors.

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