monotubes boiler theory

[richard orr]

 Hello all.
Earlier posts by  Vtsteam's  on monotube theory is what drew me to this forum in the first place. It was an interesting read and obviously much thought went into it.  Most of his design priorities are right in line with my own with safety being at the top of the list and efficiency bowing down before steady steaming tendencys.  An off-the-shelf quality is my second priority right along side with using sched.  40 galvanised pipe and sched. 80 screw-together fittings. I should say at this point that the boiler I am designing for will operate within lower pressures of  10 to 40  p.s.i.g.
  Many experts on boiler theory claim that monotube design is a very tricky and exacting process and there are legions of ways to go wrong.  And that is why I am following a hair pin configuration, screw-together approach --- especially for a first boiler attempt. It  makes experimentation and re routing of tubes very convienient. It also makes repair of burnt tubes a matter solveable by a pipe wrench.
  If impingement of gases upon pipe threads is a concern, then solutions of wrapping the fittings with insulation are easily doable. Contriving some kind metal type walled barrier is a design complication but also do-able.
   I would indeed like to come upon a list describing the reasons for given design failures, but have not come across such yet.  If any reader knows of a source please shoot it my way.  I imagine the  predominent issues are water sluggung, water hammer, and inability to  estabilish or maintain  a stable transition zone. I do believe that most of the bugaboo about monotubes comes from those using them in the highly variable load demands of steam cars.
 As mentioned in Vtsteam's post, it would seem diameter of tube for a given horsepower rating  needs to be nailed down pretty tight.  Surely turbulant flow must be obtained for good steambubble scrubbing,  while at the same time trying to minimise pressure drop along the tube or tubes if multi-path is being used.
  For me, the pipe diamiter has been predetermined by oppertunism as I was able to obtain bucket loads of  1/2" sched. 80 threaded pipe fittings at scrapyard prices....not exactly the scientific, technicalogical approach but "rather just going with the flow when the flow seems to be going in my general direction.
   Vtsteam in his earlier Feburary post, pondered the use of  flow vs counterflow  to hopefully stabalize the transition zone. His suggestion of parrallel flow seems logical to me  as the cooler tubes with their greater temp differential to the gases will more rapidly absorb heat and the hotter tubes would  have a much lower differential resulting in less effecient heat absorbtion and higher  flu gas temperatures. These higher exiting temps could simply be recovered with a larger economiser which would be a small price to pay for transition zone stablization.
  Besides parrallel flow and counter flow there is also variations of the two. For example, devide the entire generation tube assembly into thirds and play around with different stacking combinations. Another possibility is to use both parrallel flow and counter flow together in different combinations.
  One concern, however that may have to be concidered is  shocking  the tubes by pulsing water that is too cool through pipes located in the zone of   hottest gasses.  Again, a larger economiser could remedy that. This is one of the issues that I will have to be on my toes about as  I will be using a " rocket stove" type set up to burn solid fuel . Such a firebox design achieves  very high temperatures and a clean burn which will possibly eliiminate sooting problems altogether.
  Another bennifit of a "rocket stove" technology is that it allows for the isolation of the firebox from the generating tubes --- safetywise a great advantage...but at a high price. I have read that radiant heat can be as high as 90% more effecient then convective heat.  Just on a gut level, that seems high. What ever the effeciency trade off though, some of the loss can be made up with the higher heat of the gasses typically produced by rocket stoves, and maxamising the angle of impingement...and of cource more tube surface.
 

[vtsteam]

Richard, I have been working on these ideas for a very log time -- including the rocket stove combustion side of it. It is difficult to talk about.... let me rephrase that..... it is easy to talk about, more difficult to do. Doing is the only important thing. It's easy to get lost in generalized concepts, which seem to work together, but very difficult to work out specifics in a real object, by comparison. Le me rephrase that... it is easy to work out specifics in a real object, if you are working out specifics in a real object, much harder to work out specifics in a real object by talking about it.

So I try to stop myself talking about it too much until I get to the stage where I'm working with something real and solid. The wood combustion part I have done in the past, and am satisfied with. The tube and pump and control side is what needs doing, not talking about.

I can talk about combustion because I've done it. We all generalize our ideas and so tend to miss important facts. I do. Everybody does. Here's one. The generalized idea that radiant heat is more effective in transferring heat to a monotube boiler than convective heat.

That's okay, but the conclusion isn't. A set of tubes removed up the exhaust beyond the firebox receives convective heat, while one in the firebox receives radiant heat.

Actually, it can receive radiant heat if heat is transferred from the convective medium to a radiating medium at that location. Which occurs whenever hot gasses impinge on solid objects. It may not be as intense a level of radiant heat as in the firebox. But that doesn't negate the fact that it is radiant heat. It will require more area and mass to transfer the same amount of heat, but it may not be an impossible or impractical amount of area and mass for some specific purposes. Do you understand what I'm saying?

[richard orr]

And howdy there Vtsteam!
 I agree, talking the talk is never quite as interesting as observations from those  further down the road. Hope I don't come off  as some  voice of authority....just musing along on a fishing trip to see what others have learned.
  I do not follow your last paragraph. To my knowledge radiant energy is line of sight from the source and so after a few banks of tubes it becomes convectional. Your feed back welcome.
   

[vtsteam]

Richard you sound fine. It's me who is backing off until I actually have something in front of me.

Yes radiant heat travels in a straight line until absorbed or reflected.

But radiant heat is given off by any hot solid. If you heat a solid by convection, it will start to radiate. So when hot gas impinges on a solid, no matter where it's located, the object will tend to radiate the heat it collects. Radiant heat can be moved in a sense by transferring from radiant to convective and back to radiant. It won't be as intense and concentrated as it was in the firebox. But it's a mistake to assume that a heat exchanger located in the exhaust stream cannot receive any radiant heat. As an example, a mesh surrounding copper coils can collect convective heat and re-radiate in that vicinity. A heavy walled externally insulated exhaust chamber containing a coil exchanger can do that too.

I think people over simplify mechanisms of heat exchange. Everything is black and white. One place is radiant, the other convective. When we simplify like that we overlook other possibilities which may have value.

[richard orr]

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?
  Now here's the latest thought through my knoggen concerning flow /counterflow. It's simple but worth thinking about: Take a given length of pipe...let's say 4 feet....put a cap on one end and fill with water. Lay the pipe  horizontal, angled just high enough that the water doesn't run out. Apply entense heat on the lower caped end and observe. Are you seeing a water slug? I am.

[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]

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?

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! 

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".

[richard orr]

  Concept philosophys parrallels. Because I am determined to us hairpin piping, conventional rocket stove  configuration must be modified. The tubular riser must be  both elongated and narrowed  and  incorporated as part of the horrizontal throat  (nomenclature?) with the tubes running to either side of the riser and located just outside.
  In real world terms it would be useful to set up a rocket stove configuration which demensions are conveniently adjustable. In principal imagine before you a piece of paper. Write two letter L's upon it. (L.1 and L.2) Flip L.2 upside down and join it to L.1 to form a rectangle. By sliding L.1 back and forth from side to side  or L.2  up and down, we now have adjustment  of two demensions. By giving both L's a third demention of depth, one can now slide one L in and out  in relation to the other and facilitate convenient adjustment of depth. 

[vtsteam]

Build it.   
Try it. 
Make it work. 
Enjoy! 

[JohnHaine]

I don't know if it helps but there was a very interesting articles (articles?) on monotubes in Funnel the SBA magazine a few years back.  You probably know all about them (even wrote them?) but if not I could lay my hands on them but it would involve turning a bookcase upside down!