GPSA DB Example 7-1...Where Did 379 Come From? 2

[dtn6770]

The 12th edition of the GPSA Data Books contains a example (7-1, page 7-12) for separator sizing and and the mass flow equation has the value 379 in the denominator. The only thing I can figure is that value represents the product of the gas standard specific volume (17.45 ft3/lb) and molecular weight (21.72). Yes/No????

The reason I ask is because I'm familiarizing myself with how my peers size the separators we manufacture. The accepted spreadsheet uses the same mass flow rate equation with 379 as a constant; a practice I believe is in error.

Before I [the new guy] bring up this issue I'd sure appreciate a sanity check from someone more in the know. I'm a little hesitant because doing as I did, the GPSA mass flow equation would cancel the mol. wt. out of the numerator and leave just the specific volume as a denominator variable. To me, that would be a better way to show the equation which means I'm probably wrong.

 

[dcasto]

379 Cubic feet / LB-Mole, by definition of PV=nRT solving for V/n = R * T / P = 10.73 *520 / 14.696 = 379.7

 

[dtn6770]

Ah, standard condition reference...it (379) is a constant for 14.696 psia and 60 deg F standard conditions. Don't I feel silly.

After my original post I found a similar equation (w/379.5) in API 11P but it at least identifed its units (scf/mol).

Thanks for the clarification; I get turned around easily by ideal gas law principles. I'm going to go crawl back under my Mechanical rock.

 

[Montemayor]

I haven't checked my latest, electronic version of the GPSA Engineering Data Book, but the old, reliable, Hard Copy, multi-ring binder version has always had the "Constants for use in Calculations" listed in the old Section 16, Physical Properties. There, you will see the first listed constant: 1 mole = 379.49 cu ft @ 14.696 psia & 60 [sup]o[/sup]F (for an ideal gas). In the old daze, we used 380 and got away with it.

 

[dtn6770]

I just searched my electronic version and "379" showed up quite a bit but in the context of a constant only in Section 1 (General Info) as a narritive in gas volume relationships. Section 23 is now that Physical Properties section and there is a Units and Conversion Factors area on page 23-8 but it has nothing about 1 mole = 379.49 cu ft @ 14.696 psia @ 60F....

It probably became so obvious (to everyone but me) that GPSA dropped it.

My ignorance aside, there's a lot to be said about showing units all the way through one's calculations. I value the GPSA DBs but that is a short coming I've noticed.

 

[dcasto]

It use to be at the bottom foot notes for GPA 2145 included in the physical properties section.