Re: Boiler Discussion Thread/Boilers only-
Posted by:
Peter Brow (IP Logged)
Date: July 26, 2002 08:08AM
<HTML>Hi George,
Hardly anyone in the world of steam experimentation is as thorough as you when it comes to mathematically analyzing original boiler designs. I certainly am not. I am still trying to digest the info in the B&W book, so that I can fill notebooks with heat transfer calculations as you do.
The general layout & characteristics of my current boiler concept, despite the odd tube size and some other differences, are very similar to the White steam generators, which made about 10 lbs/hr of steam per square foot of heat-transfer surface, if memory serves. These used vaporizing burners with lower fuel pressure than Stanleys (and lower than I plan), and had secondary air inlets in the burner grate too if I recall correctly, which also cuts gas pressure. On the basis of this & other simplified reasoning, I think that I should be able to duplicate the White lbs/sq ft evaporation rate.
Unfortunately, I have not been able to find info on the gas flowpath area of White tube stacks (I use a square inches/gph ratio). That is why I used the Stanley figure. If anything I may need to open up the flowpath. Due to the peculiar shape of the tube elements (not coils, strictly speaking) in the multipath zone, changing gas flowpath area should not be difficult, though it would require changing the case diameter and some other sheetmetal, insulation, etc rework. This steam generator's gas velocity should be lower than in a Stanley, but higher than in a White.
Gas velocity and aerodynamics, of course, are not the only factors in heat transfer; radiant heat, steam/water velocity (negligible in Stanley boilers), etc are also factors. My steam/water velocity should be about the same as White, and gas velocity (or at least pressure) higher than White. Radiant heating area about the same as a White of same output. Same modified counterflow regime. Smaller tubes in upper section should give more gas turbulence, but gas flow should be about the same due to higher pressure hopefully compensating for turbulence. I can always change fuel/gas pressure (and/or tube spacing) as needed to achieve design goal.
My current approach is to get in the ballpark with simple calculations, and then cut & try as needed to fine tune results. Hopefully I will not have to go thru 8 different tube stacks as SES did! The advantage of your calculation-intensive approach is that you end up far closer to the final working boiler on your first try, with a great reduction in trial & error work. I'm working toward that goal! A few successful monotube builders have told me that they used simplified-analysis "starting design" approaches very similar to mine, and that their subsequent modifications were not too daunting, which I find reassuring.
Does anybody know, offhand, the spacing between the tubes in a White tube stack? Often this is expressed as a ratio of tube diameter to distance between adjacent turns of tubing. In some monotubes with powerful fan burners, the space is reportedly 1/3 the tube diameter or so, but it varies according to a number of factors. I have never been able to locate the White gas-flowpath area information in print. Size of White exhaust flue would also be helpful in estimates; total gas flowpath in tube stack would have roughly 4x the exhaust flue cross-section area, I estimate. I have some of Prof. Carpenter's famous test result tables and other info to extrapolate from.
Getting to the first-time building stage with your own car-sized system is pricey and difficult; sounds like you found a great shortcut! I may yet convince one of my local friends to build my design. Currently, they seem convinced that I am a kook who will never build anything. Too many years of designing, learning more, then redesigning, I guess. Fortunately, the desire to demonstrate sanity can be a strong motivator. :)
Peter</HTML>