Hello everybody,
my apologises if this is a dumb question,
sometimes I stumble about the term "Free Gas Area" associatied with boiler-design of a steamlocomotive.
However, I really, never found a adequate explanation about it.
May somebody enlighten me?
Thanx in advance and
Best Regards
lars
Yeah, what's the difference between Free Gas Area and Unfree Gas Area-- no idea. You'd think gas area would be the total inside cross-sectional area of the tubes and flues, minus the outside cross-sectional area of the superheater tubes, but I've never seen that confirmed.
timzYeah, what's the difference between Free Gas Area and Unfree Gas Area-- no idea. You'd think gas area would be the total inside cross-sectional area of the tubes and flues, minus the outside cross-sectional area of the superheater tubes, but I've never seen that confirmed.
Thank you Timz,
so its OK I got puzzled with this term. Really, I did not know where to start with it.
My original guess was, it could point to the Volume of Steam, produced in the boiler.
By your explanation, it does not seem really important anymore, as someone would better calculate the
outside-heating-surface of the tubes and superheater?
Lars
-BTW- Thank you for your explanation on APL 3985 run last time.
When Type E superheaters first appeared in the US, the flues were usually 3.5 inches diameter; in later years lots of engines had 3.75-inch or 4-inch flues. So fewer flues, and less heating surface, but maybe more gas area, which I assume was the point.
Wouldn't the larger diameter increase the surface area exposed to the hot gases? If there were fewer pipes, I can see the reduction in total, but if they kept the same number, I would think there might be greater heat transfer due to an increase in total surface area.
-Crandell
Say a boiler has 200 tubes, each 2 inches in diameter. If you replace all those tubes with 4-inch tubes, each one has double the heating surface-- but you can't fit anywhere near half as many tubes into the boiler. So, less total heating surface.
The gas area isn't so obvious-- I never have learned how it works. But an example would be the C&O 4-8-4: the early ones had
62 x 2 1/4 inch tubes, 220 x 3 1/2 inch flues
while the later ones had
56 x 2 1/4 inch tubes, 177 x 4 inch flues.
Both the evaporative heating surface and the superheating surface were lower in the later engines(although the superheater tubes were also enlarged, 1 3/16 to 1 3/8 inch). The later engines also had 20-ft tubes instead of 21-ft, but the reduction in heating surface was more than 5%.
Yes, a tube with larger diameter gives a larger surface (circumference = 3.14 x diameter), and it would also make it possible to use superheater tubes with a larger diameter. The usual practice was to place four superheater tubes in each superheater flue (two with steam going in and two with steam going out). The lower tubes in the boiler, with no superheater tubes in them, did not have to be as large as those with the superheater tubes.
Johnny
Thanks, Fellas. So much to understand!!
DeggestyThe usual practice was to place four superheater tubes in each superheater flue
Timz, you see what a fossil I am. I boned up on superheat in the 1922 Locomotive Up To Date. My father operated a locomotive crane in the ACL's Tampa shops, and had the book. The section on superheat mentions only the Schmidt superheater, and states that it is (was) the most common.
There is no mention of a "free gas area" in the book's index.
timzWhen Type E superheaters first appeared in the US, the flues were usually 3.5 inches diameter; in later years lots of engines had 3.75-inch or 4-inch flues. So fewer flues, and less heating surface, but maybe more gas area, which I assume was the point.
Timz and fellows,
thank you for your explanation, which makes sense to me, now.
Interestingly, quite a number of later built engines, as your example of the C&O engines shows it, were preferably built with Type A super-heaters.
I think, all the '44 built UP engines, the very last order of Challengers, FEF's and BB, used this type.
They all, indeed, lost heating surface. E.g BB:
From the first batch to the second one, (Type E 2466sq ft. / Type A 2043sq ft.), the later ones had also larger, but fewer tubes. The total evap. heating surface sunk from 5889 to 5755sq ft.
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