monotubes boiler theory

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richard orr:
Guess what I'm saying is this mental illustration points out another way to go wrong with a monotube. Of cource, this is an extreeme case in that INTENSE heat is applied to a concentrated area yet if one is designing with rocket stove technology in mind, that intense heat will be found just beyond the riser.

vtsteam:

--- Quote from: richard orr on October 01, 2013, 07:59:29 PM ---Vtsteam... O.K. .... second hand radiation by way of convection - kinda like a regenerater in an air engine? Still groping though....convective heat collecting on a tube surface...that heat conducting inward to the water...that's the main story, but are you saying that the heat collected onto the tubes by gasses of convection are also accumulating heat and radiating outward to add to the convective heat?
--- End quote ---

No. You don't add heat to a gas by radiation.

I'm saying that other masses besides the tubes can be made to absorb heat from the gasses and then in turn radiate it to the tubes. That can be encouraged by what is in the chamber near the tubes, including the casing if it is externally insulated and allowed to increase in temperature.

Imagine a heavy wall pipe, insulated on the outside, a monotube inside, gasses at 1100 F passing through. The casing is heated to nearly 1100 degrees and radiates heat inwards towards the monotube.

Ever put your hand near a metal plate that was at 1100 degrees F? There is a serious amount of radiant heat reaching the tubes from the casing -- in effect the casing is acting as part of the heat exchanger, absorbing convective heat from the gasses flowing through the system converting it to radiant heat and emitting that towards the monotube exchanger.

We no longer have a strictly convective heat exchanger, we have a system of heat exchange that is both radiant and convective. The efficiency of this system is much higher than the monotube would have been alone in convection mode surrounded by an 1100 degree gas at equivalent mass flow.

The conventional idea that one must put a monotube in the fire chamber to reap the benefit of the radiant heat generated there, or lose out by trying to absorb at greatly reduced efficiency via convective heat exchange at a later stage is an over simplification.

richard orr:
Ah , so stated that adds up.

richard orr:
Vtsteam, hello,
I am in agreement with you about the idea that there is a multitude of variables in dealing with fluid-like substances, Experience with almost anything in life has taught me that sometimes the changing of one variable can take something that isn't working at all and turn it into an all new ball game . It's called thinking out of the box.
I am cautioned ( to say the least ) by knowledgeable people, mostly of steam car experience, that a low pressure monotube is such a challange as to be shunned...something that an inexperienced person should not aproach, but thats not going to happen. I will say that having heeded the voice of their experience, I won't be building a boiler first off, but rather a set of boiler experiments starting with the firebox assembly.
One design feature fixed firmly in my mind is to make it along the lines of a rocket stove and anything I say in following posts regarding the boiler is refrenced to this approach. The two main reasons for chosing this design path are the ability to isolate the fire and the ability to attain a very clean burn using small demensioned fuel such as bamboo and switchgrass.
I have seen on youtube some rocket stove designs made to produce steam but think putting the generating tubes directly in the riser area is defeating the main principal. Otherwis, you are the first person I have come across in various posts that seems to have given searious thought to melding rocket stove technology with steam engines and so I thought I would bounce some questions your way if you are amenable and keep you posted on what I am learning along the way.
My first question to you is have you come up with firebox demensioning relating to tube surface area and horse power requirements? I myself, have thought about starting off with conventional emperical firebox demensioning of a forced draft boiler and assume a 50 to 1 ratio of tube surface to grate area.

vtsteam:
Sorry Richard, I don't have anything helpful for boiler dimensioning and configuration. That's the part that in my case needs to be tried, rather than theorized about, and I haven't done that yet.

btw. I tend to be very wary of steam design constants thrown out there, like grate area per tube length or size taken from some authority on the subject. The range of configurations, variables, unknowns, and result requirements, as well as practical considerations is too great to make that meaningful in my opinion. Unless you're following someone's specific design style and using their results.

re. the term "rocket stove" I used earlier: I actually have used some conceptual aspects of a rocket stove I observed in burner trials several years ago, but it isn't a rocket stove I ended up with. It's a new burner design. And it's going to require different physical boiler configurations than I would guess a rocket stove would. That's not meant to discourage the use of a rocket stove as a boiler heat source -- I think it can be worked out quite well. I just don't have any suggestions, other than try something out and see what the problems are. Then eliminate them! :thumbup:

ps. I think it's silly for an "authority" to warn "beginners" away from experimenting with a process because it's "difficult" or "such a challenge".

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