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| vtsteam:
--- Quote from: dvbydt on February 25, 2013, 04:44:35 PM ---Another web site I have bookmarked from a few years ago :- http://www.flysteam.co.uk/index.htm Might still be of use, he mentions the same names as you did. I want to make a small version (eventually), to run my models. Ian --- End quote --- dvbyvdt thanks. I have seen that site, and it's very good. In fact it had an earlier incarnation a number of years ago, and I wrote Geoff back then asking a bunch of questions about monotubes. He said he'd been working on a mechanical control mechanism and a book on monotubes. I think he even sent me a draft chapter. Besides all of the interesting stuff he has now on the website (it is much larger now than it was) one of the most important to me is the discussion of tubing diameters and the relationship of water velocity inside the monotube. There seemed to be a magic number for diameter in both his readings of tests by Edgar Westbury in Model Engineer and his experience with a small steam launch. And that was 3/16" diameter tube. His theory was that the velocity of the water in the tube (for the sizes of engine in the small range) was critical to good performance of the monotube. As a result of this I bought back then a roll of 3/16" stainless steel tube, which I still have. I'm very curious about this point. And we'll probably find out here whether it holds for an engine of increased displacement, like the one I'm adapting. It seems to me that 3/16" may not be a magic number -- but the velocity theory may very well be true. Seems reasonable to me that the steam needs of a small engine might enforce an optimal velocity with 3/16" tube, and that the steam needs of a larger engine might enforce the same velocity in a larger tube. The reason that 3/16 tube might work well for a pretty good range of small engines is because the next common (imperial measure) increment in tubing size is 1/4", and while that seems only a 33% increase in nominal size, it's a cross-sectional area increase of 77%. To achieve the same velocity the pumping capacity would need to increase that amount. So the steam volume (and possibly the engine displacement volume) would increase 77% to match. The tube surface area has only increased 33% however, unless we lengthen it -- so that also might need to increase 33% to bring the heating surface in line with the quantity of water being pumped per minute. So, basically, going up only one size in tubing has made a big difference in what size engine it would be appropriate for. And so it might seem that for a fairly wide range of small engines 1/4" is too large. Beyond some point, however, I bet that a next size tubing increment is appropriate. And that would logically continue up through the engine sizes. |
| John Hill:
Thanks for the interesting read and all those links. I have incorporated some of what I have learned from them in my latest daft idea which is a "semi-internal steam generator" engine. |
| Raggle:
I hope the following is of interest, I do tend to ramble ... About 40 years ago I made my living as a chemical descaling operator, largely on conventional steam boilers of multiple firetube type. Other work included water jackets on large compressors, cooling towers, etc., in fact anywhere a heat exchange had taken place and deposited calcium carbonate. I'd already developed an interest in the Doble steam car and I like to think my mention of it at interview helped me to get the job. One day I was called to see to a Stone's steam (or vapour) generator installed at a knitwear factory in Leicester. I believe it was to provide steam for one or more Hoffman presses, though it may have had other jobs. I arrived with my full kit of pumps, 70 gallon plastic bath and hoses, and 2 or 3 carbuoys of hydrochloric acid. On being shown the item I confessed to the owner that I had never seen one before and asked if he had a manual. This he presented and left me to it. I turned straight to the descaling section, piped it up as per instructions and got going. Most of the time is spent waiting between measuring acid strength, adding more as and when required. This gave me ample time to read the rest of the manual. The Stone's and the Doble are very similar, possibly this later one contained some Doble patents. The fire, kerosene in the case of the Doble, maybe diesel in the Stone's, came in at the top and is electrically blown. The key to steam production is continuous pumping through the coil, which I think was 3 concentric diameters. At a high point above the coils is a steam drum/water reservoir. Steam bubbles are centrifugally seperated in this vessel, inside of which there are 2 water level sensors, high and low. The pumps are interesting in being of diaphragm type. The diaphragm is at the end of a long pipe and its valves are at the other where the main flow occurs. Thus the diaphragm is not subjected to the full working temperature, being at some distance and merely generating pulses. The pumps are cam driven and the diaphragm can be held stationary by a solenoid. The two pumps looked identical but one is for continuous circulation, the other is the feed pump, triggered by the demands of the 2 sensors in the upper vessel (which I now recall was called the accumulator). Feed water was preheated by a coil, probably by-pass or parallel connection to the main coil, (or maybe somewhere late down the flue, I can't remember) I believe that for some models there is a superheater coil somewhere in the steam side, but details escape me. When acid strength ceased to fall my job was done, other than adding lime to my acid bath to neutralise it before dumping down the drain and a number of clean water flushes and I packed up my truck without having to do the usual cleanup of my work area. The owner reassembled the burner system and pressed the start button. 100 psi arrived in a few minutes which he pronounced good. The unit by the way was about 3 ft high and about 3 or 4 ft diameter, not including ancillaries. Ray |
| vtsteam:
That is very interesting stuff, Raggle. Thank you very much for posting it. :bow: Completely new to me at least are the cam driven diaphragm pumps, the method of de-scaling with hydrochloric acid and watching the pH drop. Some of the other details are more widely reported but the pumps are especially interesting. In fact I'm going WOW, that is just brilliant! Almost every feed pump I've seen in, well, mostly model engineering sources for monotube steam are plunger type. Well D.H. Chaddock experimented briefly with an oscillating cylinder type -- like an oscillating steam engine in reverse. But thank you so much! I am definitely going to think about this more. :mmr: |
| vtsteam:
Reading a second time through I'm struck by the circulation pump and accumulator, as well. And the longevity of this monotube steam generator, its industrial setting, its size. and the 100 lbs working pressure. Thank you again for this very rare and useful information! |
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