Calculate desuperheated steam temp

[Hippo]

Hello. I'm working on a problem right now that I hope someone here can provide some input/ideas. Here's the situation:

I have a superheated steam supply(about 200deg of superheat) that we can't directly supply to an exchanger due to fouling from the high wall temperature. The plan is to install a drum upstream of the exchanger and bubble the superheated steam through a sparger up through the condensate that returns from the exchanger, which for all practicle purposes is at saturated conditions.

I need the steam to come out approximately with only approximately 20-25 degrees of superheat (less would be better). I need to determine the liquid level required to give adequate heat transfer such that the temperature is reached. From there, I can size my drum.

This is currently done at a few other sites, but no design info is available.

Here is what I am thinking:

1) Calculate bubble diameter(assuming sphere) for a known orifice diameter.
2) From bubble diameter, I can determine a rise velocity.
3) In Mccabe Smith and Harriot, I see a Nusselt number formula for heat transfer between a flowing fluid and the surface of a single sphere.
4) Here's where I'm getting a little uneasy: Based on the ho I calculate from the nusselt number (ho*Dp/kf), can I simply solve the formula hi=mCp(Tb-Ta)/(PI*D*L*deltatLM) for my temp? Rather than L being length of a tube, use depth of liquid, and make D bubble diameter? I don't believe I can make those changes.

There must be a correlation for something like this. I have searched through my books and on the internet, but I have not run across anything yet. Any guidance would be greatly appreciated. Thanks.

Mark

 

[TD2K]

How are you going to get the condensate from the exchanger to this drum so the steam can bubble through it? Is the exchanger above where you are doing to put the drum?

There would seem to be some hydraulic issues to work out given the inlet steam pressure has to be above the condensate pressure especially with the pressure losses through your proposed steam sparger.

I would look at a desuperheater. Some designs will entrain the condensate into the steam by means of a venturi to create a localized low pressure zone.. While there are limits how much lower than steam pressure the condensate pressure can be, your system should be a good candidate for it. Plus, you can get down to within 5F of saturation easily.

 

[MJCronin]

Hippo,

TD2K is right on.... the way that power plants deal with this issue is to desuperheat the steam using a device mounted in the steam piping. Desuperheating can be done to about 15 degrees of saturation. Large turndowns are available.

You must have a source of clean water (typically treated feedwater) at a higher pressure than the steam.

See these links....

http://www.dezurik.com/MNSD_desuperheaters.htm

http://www.yarway.com/control_valves.asp

http://www.joco.com/paper/desuperheaters.html

Good luck............


Let us know what you finally decide upon


MJC

 

[Hippo]

Thanks for the replies. The exchanger will be above the return drum, and the steam pressure drop through the exchanger is minimal. There will be enough elevation difference for the hydraulics to work. I had asked about an inline desuperheater, but that's not the direction the client wants to go. Like I said earlier, they have the type of configuration I described in operation at other sites.

Looking at the equation I listed above, I don't think I can get what I mentioned above. I hadn't given it enough thought. If anything, the PI*D*L I would think would have to be replaced with bubble surface area, but I am still not certain this gets me to where I want to be.

I'll take any suggestions, and I'll keep searching.

Mark

 

[25362]

Looking for bubble columns and gas dispersions I found Perry VI tabulates the final velocities of air bubbles rising in water at 20 Celsius as function of the bubble diameter from 10 to 300 micrometers. The velocities range from 0.61 mm/s up to 49.38 mm/s. If superheated steam bubbles sparged through condensate behave similarly, it might be that table 18-23 is of help.

 

[hacksaw]

H~


Bubbling superheated steam through condensate is not a good idea.

Steam condensers(condensing heat exchangers) have been used, but it begs the question of the cost of creating superheated steam in the first place.

Modern desuperheaters are far more controllable and cost effective.