air properties over 400F

[AO1958]

Hi, can anybody help me ?

I am facing to following problem:

I'd need to calculate air properties over 400F and 5bar

May I understand that traditional formulas that I can find in ashrae handbook cannot be utilized ?
May I understand taht at these conditions there is no more water in the air, because of this temperature, therefore it is a nonsense to consider relative humidity ?

can anybody give me a link where to find formulas for calculating these properties ?

Many thanks

 

[IRstuff]

Why would there be no more water? Where would it go?

TTFN

FAQ731-376

 

[rmw]

There still is water in that air if there was water there before it was heated and pressurized. Assuming that you may cool it some day, you will find it then. What you don't have is visible water vapor; it is superheated at those conditions.

rmw

 

[AO1958]

many thanks Mr IRstuff and Mr. rmw !

But please, can anybody tell me where to find the formulas to calculate the air properties in these conditions ?

many thanks

 

[TGS4]

Try ASTM C-680.

 

[danberry]

There can be water in the air AND relative humidity IS a nonsense.
The fact is there can be an infinite quantity of water per pound of dry air becaust the temperature is above the saturation temperature of water at 5 bar. You need to determine the water content one way or another but you cant assume "saturated conditions"

 

[AO1958]

Many thanks to all.
And please, apologizes for not being so clear.

Anyhow my current problem is:
how to determine this properties.
In usual air conditioning and refrigeration conditions, humid air can be considered as an ideal gas.
My question is:
at the conditions of 400F and 5bar can humid air still be considered an ideal gas or shall we take into consideration different factors ?

Many thanks

 

[rmw]

You can consider the air as an ideal gas but the water you have to consider as steam at that temperature. Remember that the air and water don't mix, that is to day the water is not dissolved in the air (or vice versa); the air just carrys the water vapor. Use the steam tables for your water vapor conditions of state. The engineeringtoolbox website has some information that will help you with this.

rmw

 

[UtilityLouie]

I have an old book:

Industrial Research Service's
Psychrometric Tables and Charts
by
Zimmerman and Lavine
Second Edition

This has high temp air tables and charts.

Something else that also works is an excel add-in called "@air". It is found on at:

http://www.techwareeng.com/prodAA.htm

It works great on superheated air. The only suggestions I could give you for formula to develop your own application is maybe something from a flue gas book or a boiler vendor.

That's the best help I can give.

 

[FOURe]

Here is a URL for an online calculator dealing with air physical properties when air behaves as an ideal gas:

http://www.engineering-4e.com/calc1.htm

http://www.engineering-4e.com

 

[AO1958]

Many thanks to everybody.


As a matter of fact I am going deeper into this problem, and new questions are presenting.
Indeed, if I consider the water saturation pressure at air temperature, this is higher than ambient pressure.
Does it make sense to define a relative humidity in this conditions ?

To mr. UtilityLouie
in this condition the suggested software doesn't work

to Mr.FOURe
unfortunately I have to consider moist air !

Many thanks for your help

 

[danberry]

"Indeed, if I consider the water saturation pressure at air temperature, this is higher than ambient pressure.
Does it make sense to define a relative humidity in this conditions ?"

NO.

There can be an infinite quantity of water per pound of dry air because the temperature is above the saturation temperature of water at 5 bar. Relative humidity is (actual water / maximum water) = (anything / infinity) = 0 independantly of actual water content. Not a very usefull concept is it?

 

[Compositepro]

Relative humidity is defined as actual partial pressure of water divided by the vapor pressure of water at a given temperature. Above 100C and at ambient pressure it simply isn't possible to have 100% relative humidity. The concept still applies but it may not be as useful.

 

[KiwiMace]

Try this paper -

Formulation of High-Temperature Properties for Moist Air. Nelson and Sauer. HVAC&R Research Vol 8, No. 3. July 2002.

 

[AO1958]

Many thanks to all.

Mr KiwiMace, I bought that article.

Just to integrate my question with two new - sorry for delay -.

Since at described condition do not exist saturation conditions,
does it make sense to define a dew point ?

Would air at this condition dehumidify if getting in contact with a colder surface ?



many thanks