# Steam Tables - g vs. fg



## mechgirl

I know this is very basic, but I'm feeling like I'm missing something.

Enthalpy as an example: I know that hf is enthalpy of saturated liquid, and hg is enthalpy of saturated vapor, and hfg = (hg - hf).

I was working some vapor cycle problems last week, and from the steam tables, I would select hg for saturated steam at the given temp/pressure. Instead of hg, the solution selected hfg. I wish I could remember which problem, but I don't even remember which book I was working in. I just continued working problems at the time, but now I'm thinking about it and feeling confused.

So, when would I select "fg" instead of "g" from the steam tables?

I just feel so overloaded and tired and frustrated now, and like I'll never pass.  A week ago, I was feeling pretty confident.  Ugh. Ok, must refocus...


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## Firefly

hfg can be used as the latent energy and is used when finding the quality.


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## Bman

fg is the enthalpy (entropy, etc) required for evaporation. There are certain equations that require you to use the "fg" such as the liquid-vapor equations- h = hf + (x)hfg where x is the quality. It's also used in other equations. I think you just need to be careful on what the question is asking and what you need to solve it.


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## mechgirl

Bman said:


> fg is the enthalpy (entropy, etc) required for evaporation. There are certain equations that require you to use the "fg" such as the liquid-vapor equations- h = hf + (x)hfg where x is the quality. It's also used in other equations. I think you just need to be careful on what the question is asking and what you need to solve it.


I know to use hfg for quality. I think this problem was a vapor power cycle, and I was finding the enthalpy of sat steam at a point. The solution used hfg while I used hg. They weren't finding quality, but said that hfg was the enthalpy of the steam at that point. It didn't make sense to me.

What other times, besides finding quality, would I need hfg? Why would I select hfg instead of hg at a sat steam state point?


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## Baltimore Joe

mechgirl said:


> Bman said:
> 
> 
> 
> fg is the enthalpy (entropy, etc) required for evaporation. There are certain equations that require you to use the "fg" such as the liquid-vapor equations- h = hf + (x)hfg where x is the quality. It's also used in other equations. I think you just need to be careful on what the question is asking and what you need to solve it.
> 
> 
> 
> I know to use hfg for quality. I think this problem was a vapor power cycle, and I was finding the enthalpy of sat steam at a point. The solution used hfg while I used hg. They weren't finding quality, but said that hfg was the enthalpy of the steam at that point. It didn't make sense to me.
> 
> What other times, besides finding quality, would I need hfg? Why would I select hfg instead of hg at a sat steam state point?
Click to expand...

I’d focus on phase change, specifically at the boiler or condenser. Here are locations when H fg may be applied, provided that the steam is going from a saturated liquid to a saturated gas or visa versa.


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## gaidox

Baltimore Joe said:


> mechgirl said:
> 
> 
> 
> 
> 
> Bman said:
> 
> 
> 
> fg is the enthalpy (entropy, etc) required for evaporation. There are certain equations that require you to use the "fg" such as the liquid-vapor equations- h = hf + (x)hfg where x is the quality. It's also used in other equations. I think you just need to be careful on what the question is asking and what you need to solve it.
> 
> 
> 
> I know to use hfg for quality. I think this problem was a vapor power cycle, and I was finding the enthalpy of sat steam at a point. The solution used hfg while I used hg. They weren't finding quality, but said that hfg was the enthalpy of the steam at that point. It didn't make sense to me.
> 
> What other times, besides finding quality, would I need hfg? Why would I select hfg instead of hg at a sat steam state point?
> 
> Click to expand...
> 
> I’d focus on phase change, specifically at the boiler or condenser. Here are locations when H fg may be applied, provided that the steam is going from a saturated liquid to a saturated gas or visa versa.
Click to expand...

Agree with them..

In addition to this, when things were little confusing with thermo phases,

I recommend refreshing the rules for determining phases (merm 24.2), latent heats (merm 24.16), and thermo. properties (merm 24.25~30). These are good basic stuffs for thermo.

I've noticed in samples exams that a lot of problems relies on the basics and will definitely helpful (to really master these) come exam day. I keep refreshing these when I got confused.


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