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Old 05-28-2009, 05:50 AM   #1
Mar 2008
Eugene, OR
Posts: 7

Some questions to tease out a better intuition about the form and function of flash boilers.

kladue in particular:
Why go tall and skinny instead of short and chubby on your second generation flash boiler? It seems like you'll transfer more heat that way, but is it really that much more? Just considering options in terms of portability (short and chubby seems more portable).

What about eight coils instead of four? Perhaps three forming a sort of triangle in the middle, with five surrounding the inner three?

What about baffles? Would installing baffles on the coil help to disrupt the chimney effect and provide better heat transfer? Perhaps an idiotic question!

Also, how did you arrive upon your particular design. I've seen a few other designs now, from different industries. Here are a couple:

Boiler Basics: Waste Oil Generator System, Ken Rieli, Phoenix Turbine Builders Club, Tesla boundary layer disk turbines
Clayton Thermal Products UK - Steam Generators Principle of Operation
Boiler Efficiency

(Forget about the pressurized component; not interested in dying at the moment.)

For example, why did you go vertical (i.e. chimney) instead of horizontal (like a real BBQ)? (Perhaps that would require more extensive insulation? Thicker chimney material?)

Why coils instead of parallel tubing? (see phoenixnavigation link above)

I think that's it for my questions for now!

Can't wait to build one!

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Old 05-28-2009, 01:52 PM   #2
Senior Member
Dec 2006
Turner, Oregon, Oregon most of the time now
Posts: 2,305
Liked 54 Times on 47 Posts

The flash boiler design evolved around the tubing cross fitting from swagelok ( 4 coils) and a 6 " diameter tube to match the 6" cast iron burner. The tube chimney design was used to get natural draft circulation of the hot gasses through the boiler coils. The vertical upflow circulation of the water allows the air that seperates when the water is heated to flow up and not block water flow in tubes. Down flow circulation is more efficient but does not work at low flow rates used for sparge and steam generation, air bubbles block flow and boiler pops and snaps as water flashes on the hot spots. Increasing the number of coils will help heat transfer at higher flow rates for water heating, but increases the chances of boiling the liquid in the tubing at low flow rates. With both of the boilers built from 1/4" OD stainless tubing the efficiency is not what one could get if they were built using copper tubing but the ability to withstand the thermal abuse and availability of materials for experimenting were the reason for using stainless coils.

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