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[Chris Haslego]

--------------Original Post-------------------
What are the factors that decides the Height and Diameter of Cylinderical flash tank. Can any body give the correlations and thumb rules in this regard.


--------------Responses----------------------
Much of the design can very well be dependent on what you are flashing. You are not specific as to what you are flashing.

The first response came from me and it is as follows:
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I would strongly suggest you get a copy of the follwing articles.

1. "Design Two-Phase Separators Within the Right Limits", W.Y. Svrcek and W. D. Monnery, Chemical Engineering Progress, October 1993. There was a letters to the editor for this article but I do not have the date (I could kick myself for this!). It refers to a correction to an equation in Table 6 of the article. However, see my comment for reference 2 below.

2. "Successfully Specify Three-Phase Separators", same authors, Chemical Engineering Progress, September, 1994. (The beauty of getting this article is that it uses the same basic equations as in the first article and the equation that was referenced in the "letters to the editor" that was wrong in the first article was corrected here).

The second response came from a great person, Art Montemayor and it is as follows:
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For flash chambers of the type you have described, I have used the classical Souders-Brown relationship that has been offered by Milton Beychok on a previous problem:

V = Ks [(dl - dg)/dg]^0.5

Where,
V = allowable superficial gas velocity in the flash chamber;
dl = liquid phase density;
dg = gas phase density;
Ks = An empirical constant; its value depends on all factors that affect separation other than density.

You will have to calculate the resulting flash vapor flowrate by using a mass and heat balance around your flash chamber. Or, you can use a Temperature-Entropy diagram or Mollier diagram of your system and employ the lever rule to find the flash vapor flowrate.

In a Vertical separator, Ks can vary between 0.06 to 0.35.
In a Horizontal separator, it can vary between 0.4 to 0.5.
These values pertain to density units of lb/ft3 and the units of Ks are ft/sec.

I have used 0.25 with stainless steel mesh as internals on vertical vessels. In your application, I would recommend 0.15 for Ks, with no steel mesh or internals.

By calculating the allowable superficial velocity and using the vapor flowrate, you calculate the cross-sectional area of the vessel. Be sure to allow generous vapor disengagement space and a liquid inventory level in the vessel. I usually us a height to diameter ratio of 2:1 on vertical vessels. In your assumption of a Ks consider that the lower Ks will yield a larger and more "conservative" design.

As Art included in his closing, "I hope this helps".
Phil Leckner
First Content Manager
The Chemical Engineers' Resource Page