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| vtsteam:
Thanks 1hand! Too much "knowledge" on my part and too little doing on this subject -- talk is cheap! Still don't have my engine running so I can move on to the boiler. But as soon as that baby turns over on its own, I'll be onto the monotube trials here. And gradually work through plans A, B, C, etc. Wonder how many letters I'll go through this time? :lol: |
| tom osselton:
Probably a to h that will give you the " AH " factor |
| Tony:
I'm new to this so this is my first post! I have been working on various designs of monotube boilers over the last few years and find the views on this forum most interesting. My original challenge was to design and build a 3HP double acting steam engine and boiler from scratch to fit in a dinghy. It needed to be light so much of the engine is ally with sealed ballraces throughout. The boiler had to be monotube for safety. The engine is fine and chugs away quite happily at about 800 RPM. It has some unusual features. The flywheel is driven by a timing belt at 3:1 above crankshaft speed, this enables the use of a flywheel that is only 1/3 of the weight and also the engine can be mounted lower in the hull reducing shaft angle and lowering the C of G. The weight of the engine assembly including condenser and scavenge pump (but not the boiler) is only 34 lbs. The boiler on the other hand is a different matter! I needed about 80 PSI of moderately superheated steam (about 200 C). It was important that it was limited to <220 to avoid damage to the various O rings in the engine and valve gear. I started off with a monotube boiler and economiser coil in the flue controlled by a PIC and R/C servos and using gas burners from a domestic boiler. After much plying around with the control system it was moderately successful but I was not really satisfied with it so went for a Lamont design with separate superheater coil and burner. The pump was the problem here and I ended up with a much modified central heating pump. Again moderately successful but it was getting very heavy and I was worried about the separator tank which was, after all, a small pressure vessel. So now I am looking at going back to a single coil again. There is much information on small units for hydroplanes producing amazing quantities of steam from 3/16 dia tube and a very noisy burner, but this is not suitable for a steam boat. From what I understand, it seems necessary to have a feed flow of at least 1m/sec for removal of vaporisation bubbles forming on the inside of the tube. This leads to very small Dia tubing and a small surface area. I have noticed in the past that if you tap the coils lightly with a small hammer, the pressure rises quite dramatically for a few seconds (until the bubbles reform again?). I am now experimenting with an ultrasonic transducer fitted near the cold end of the coil in an attempt to shake these bubbles loose. Another idea would be to use a very oversized piston type feed pump which could shuffle the water back and forth in the tubing at about 1m/sec velocity. What do you think? |
| dsquire:
--- Quote from: Tony on May 11, 2013, 02:18:00 PM ---I'm new to this so this is my first post! I have been working on various designs of monotube boilers over the last few years and find the views on this forum most interesting. My original challenge was to design and build a 3HP double acting steam engine and boiler from scratch to fit in a dinghy. It needed to be light so much of the engine is ally with sealed ballraces throughout. The boiler had to be monotube for safety. . . . . use a very oversized piston type feed pump which could shuffle the water back and forth in the tubing at about 1m/sec velocity. What do you think? --- End quote --- Tony I think that you have made a great start on your engine and boiler. I am sure that there will be much interest by the members here. If you would be so kind as to go here http://madmodder.net/index.php/board,3.0.html and post an introduction telling a bit about yourself, where you are from, etc. It would help us all get to know you a bit better. I hope to see more posts on your projects in the future. Cheers :beer: |
| vtsteam:
Hello Tony! Great info! Thanks so much for your post. It's a funny coincidence that I'm just about to start work on monotube boiler experiments since I'm just about completed with my 4 cycle to steam conversion. Okay, so now for the opinion piece..... I think that the 1 meter/sec quoted is just a theory, not necessarily written in stone. Few people if any have ever watched bubbles form inside a monotube. There are many explanations, theories, and statements of "common knowledge" made on the internet and repeated, but very little proof of these conjectures. Basically, anecdotal experience when somebody changes something and sees either an improvement or performance decrease. Here are just a few reasons why I have doubts that we are anywhere near a state of certainty on monotube function. It seems obvious that steam bubbles can form at different internal skin temperatures, different pressures, different material heat transfer rates, different relative surface to volume ratios of the conduit, etc. There are just lots of diverse factors besides flow rate. One example? If pressure is high enough, and maintained superheated water obviously can be conveyed all the way to an injector and engine -- as in true "flash steam". Where along the length of the monotube do the bubbles form there? Nowhere. What speed is critical to attachment? No speed, as long as the pressure is maintained. And re "bubbles" -- what do we mean by bubbles? Is the insulating value people have observed a result of actual "bubbles", or a sheets of steam, and if bubbles, how large are they -- do they fill the conduit, are they a small percentage of the conduit? There are just millions of questions as I see it, in a subject of much greater complexity than some arbitrary rate is going to answer. Not saying ignore the theory completely -- might be a good rule of thumb for low pressure of a specific size boiler and material tube. But I think there's room for a lot more knowledge and development. Which is a very good thing! |
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