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# superheated steam as ideal gas

## superheated steam as ideal gas

(OP)
I want to know that whether superheated steam can be treated as ideal gas. If yes then why pressure of steam not increases with temperature as by ideal gas equation pressure is directly proportional to temperature. I am talking of steam in superheater of boiler. Please guide me.

Regards

### RE: superheated steam as ideal gas

No, steam is not ideal.
There are published equations of state for steam, use one of them.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

### RE: superheated steam as ideal gas

I imagine, if it never gains or loses energy, temperature, pressure, mass or velocity, and never touches a colder surface or changes volume, you could approximate superheated steam as an ideal gas for a short time. But not usefully, only theoretically.

As EdStainless pointed out above, use the tables.

### RE: superheated steam as ideal gas

I am not sure of what you mean, but as the steam moves through the superheater to the steam turbine or to the steam consumer the pressure must decrease in the direction of flow, regardless of the temperature. In order for a fluid to flow, it must flow from high pressure zone to a lower pressure zone, as per the 2nd law of thermodynamics.
If you place steam in an insulated cylinder and reduce the volume of the cylinder ( by compressing with a piston)both the pressure and temperature will increase, and you will also need to add work by compressing the piston. But this is a different situation than steam flow through a superheater and absorbing heat from the fluegases.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

### RE: superheated steam as ideal gas

A search of the periodical magazine Chemical Engineering will have the published equations that EdStainless is mentioning. On the other hand if you are willing to try the ideal gas equations for steam pressure and temperature in the super heated region you will find that you are in the ball park under certain pressures and temperatures . I have personally try values for specific volume calculations at several pressures and temperatures and compared the results to the Mollier diagram. The steam constant used was 85.76 ft-lbm/lbm-dR.

The results were as follows:
At P= 4000psia and T=1200 dF, v= 0.247 cu.ft./lbm (close to the 0.25 cuft/lbm isometric line)

At P=40psia and T=1200 dF, v=24.71 (closer to the 20 than the 40 cuft/lbm isometric lines).

At P =4000psia and T= 800dF, v= 0.188 (not good probably due to being in the vicinity of the critical point; the 0.188 value was close to the isometric line of 0.10cuft/lbm)

At P=40 psia and T=400dF, v= 12.80 (which was close to the10 cuft/lbm isometric line);

At P=1 psia and T=100 dF which is nearly on the saturated vapor line, v=333.31. The saturated vapor temp. at 1.0psia is 101.74 dF and the "v" value is 333.6 cuft/lbm.

The values compared favorably to my Mollier diagram in several but not all cases. You'll have to try other combinations of pressures, temperatures and specific volumes within the superheat region because in some section of the superheated region, there will be significant errors.

### RE: superheated steam as ideal gas

NIST publishes steam tables, and the equation versions of them.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

### RE: superheated steam as ideal gas

there are load of easy (and freely) accessible steam data. E.g. the NIST tables as mentioned. Use those (or even a HX diagram load of fun )

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