1000 F mixture of air and superheated steam 1

[kagadpencil]

My flue gas in a commercial baking oven consists of a mixture of combustion products (CO2, N2, O2) and superheated steam (due to moisture removed from the feed in the baking process). In my current analysis, I am assuming properties of pure air to hold true for flue gases, but I guess if large quantities of moisture are removed in the baking process, significant portion of ACFM will be superheated steam. How accurate will my assumption be in that case? Please enlighten me with giving some references or sections of standard publications to read. I like to read around the engineering problems that I am currently tackling -- That is perhaps the best way for a rookie engineer to become a good professional engineer !

 

[25362]

If you can get hold of J.P. Holman's Heat Transfer edited by McGraw-Hill, you'll find, at the book's end, several tables with the thermophysical properties of the gases you mentioned. Thus by "making" various gas mixtures you'll be able to compare their properties with those of air also shown in those tables.

 

[EdStainless]

If you assume that each of the gasses is ideal then you can mix the data in the same proportions and get reasonably close. The best gas property data is from NIST at

http://webbook.nist.gov/chemistry/fluid/

You will only get in trouble when you start to condense.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[siretb]

You cannot assume your flue gas, having moisture, will have the same properties as dry air. As an example, at 150°C, 1 atm
dry air 20%H2O,8%CO2,8%O2,64%N2 (molar)
viscosity 2.38e-5 2.27e-5 Ns/m2
heat capcity 1012 1140 J/kg/°K
Pr 0.701 0.796 (non dimensionsal)

As stated in a former post, you may assume that the gas is ideal and calculate the properties of the mix from the properties of each component.

 

[kagadpencil]

EdStainless (Materials)

The link that you provided does not work for steam. It is only for water. I am looking for data on physical properties of steam from 200 F to about 1000 F

 

[GenB]

Your residual added water to your flue does not make superheated steam. you will get wet gas/air, thus when condensed you get hydrogen. very corrossive.
E.R.

 

[25362]

The NIST webbook gives the details you wish for water vapor. Go to:

http://webbook.nist.gov/cgi/cbook.cgi?Name=water&Units=SI

From Holman tabulated properties for water vapor at 1 atmosphere, as functions of temperature, you can add the Prandtl number (ratio of kinematic viscosity to thermal diffusitivity):


T, K Pr

380 1.060
400 1.040
450 1.010
500 0.996
550 0.991
600 0.986
650 0.995
700 1.000
750 1.005
800 1.010
850 1.019

 

[vpl]

kagadpencil,

I would like to echo generalblr's post. What makes you think you have superheated steam? Going back to some basics, steam is water that has changed state. The word "superheated" means that the steam temperature is independent of the steam pressure and that you have no water (in liquid form). Having a mixture of other gases with some moisture in it does not equate to steam, much less superheated steam.

Ok, getting off my soapbox, if you really believe you have steam, then what you want is a steam table or a Mollier chart. Doing a google on "steam table", you will get several hits of places that offer on-line access. This one looks reasonably good as it explains how to use the tables:

http://www.engineersedge.com/thermodynamics/steam_tables.htmCan't

vouch for it, as I have hardcopy.


Patricia Lougheed

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

 

[kagadpencil]

Thanks for the explaination, although I dont think I was not understanding the case wrong. Since flue gas temperature is around 800 F, what I am saying is the moisture part of the flue gas is in a superheated state. or say 20% fraction by volume of flue is superheated steam - I dont that is wrong. Ofcourse there are other gases and air which makes it a mixture of gases and superheated steam.

 

[25362]

Water vapor, aka steam, at a temperature above its boiling point, at the prevailing pressure, is superheated. In this case, I believe, steam is even beyond its critical point, as the accompanying gases are, and should better be identified as supercritical steam.

 

[poetix99]

The water content of flue gas at ~1000°F(?!!) is superheated steam at a partial pressure corresponding to the mole fraction of H20 in the mixture.

It is not in the liquid phase.
It is not supercritical; the pressure is too low.

As "EdStainless" said: use NIST, or some similar website for pure species properties, and then calculate the mixture properties based on mole fraction or mass fraction as appropriate.
The NIST pull-down menu simply says "Water", but "water" can be liquid or gas. The site gives good property data for gaseous phase water (aka steam).

Also, as "EdStainless" said, the gas properties becomes problematic if the water begins to condense, but it is primarily because the component fractions will change - and not only the water; CO2 and other components will go into solution with the liquid water in varying proportions.

There might be a secondary issue with the accuracy of saturated steam properties, but NIST is probably as good as any source.

 

[25362]

Poetix99 is right in the definition of supercriticality. It involves the fluid being at conditions higher than both, T[sub]c[/sub] and P[sub]c[/sub].

 

[tgmcg]

At these temperatures and pressures the consitituent gases and vapors behave as ideal gases and can be treated as such. Combining rules such as Katz's law will suffice.

Supercritical, by definition, means above the critical temperature. However, it is common practice in steam engineering and in the field of supercritical water oxidation to also require pressures above the critical pressure, 221 bar.

I've been working in the area of supercritical water oxidation since 1992, and have modelled fully-reacting SCWO reactors operating at 250 bar usng CFD. I've developed equation-of-state analytical tools to model supercritcal system including mixtures of SCW and non-condesable gases.

 

[chicopee]

Here is my solution to your problem:
1)Know the mole fractions of the products of combustion that includes the vapor from baking. Flue gas components should be CO2,N2,H20,O2(from excess air).These can be determined from stoichometry of fuel gas with combustion air that includes a 10% excess air for natural gas.
2)Calculate the partial pressures of components. Assume atmospheric pressure in the stack.
3)calculated mass rate of flue gas thru stack.
4)knowing the exhaust flue gas temp and the approximate partial pressure of the water vapor in the flue gas, determine specific volume using the Mollier diagram or steam table.
5)calculate specific volume of the other flue gas components- use equation of state v= RT/p where R,p are specific to each component. T is the same for all components.
6)determine volume flow rate of flue gas= SUM(mass rate*specific volume of each component).
7)calculate pressure drop thru exhaust stack(assume isothermal condition) of flue gas.DELTA(p)=f*L/d*Vsquared/2gc. velocity V=volume flow rate/XA of stack
8)Obtain exhaust fan characteristic curve (pressure drop vs flow rate) normally design at STP and revise curve at temperature of exhaust flue gas. May have to again revise that new curve to reflect density of flue gas.
9)develop on same graph pressure drop curve thru stack of flue gas. assume any cfm to calculate a pressure drop value using above equation in 7); then series of points can be calculated to draw the curve.
10)Allowing a 10% deviation from these two curves representing as upper and lower limits, the Volume flow rate in 6) should match or be close to the points of intersection.

 

[25362]

Don't forget the water generated in the combustion process.