Some time ago I made the comment on the Steam Automobile Club of America chat room or Phorum or whatever it is that I was an authority on designing and making monotube steam generators. The reason was because I had, in one of my designs, made almost every possible mistake. As we all know a person does not really know anything until they have made every possible mistake. Until then hope springs eternal and the inventor will think that the next design will work. This boiler did not steam well and then I burned a hole in one of the tubes, and this with a wood fire. I was contacted by a person who wanted to know more. This would be either a brave person or someone with a lot of extra time on his hands.
Here are the photos. This was designed to power a 2x2x2 small Vee made by Strath Steam out of Australia. For very short periods of time it more or less worked and then many things happened. What I tried to make was a water level controlled LaMont with a jet circulating pump with the power coming from the water make up pump. There was an attemperator and a buried superheater and many good features. Ultimately the design was done in by having no downcomers in the system. This was combined with it being wood fired with the heat at the bottom of everything.
To begin with we were not inside of the pipes and tubes and steam/water separating drum so we do not know for absolutely certain what was going on. We could look at the gauges and see that not much good was going on and then attempt to deduce things. Here is how it was designed.
Because the fire was at the bottom the cold makeup water was pumped into the economizer coils, which were at the top, standard counterflow. The hope was that this water would heat up and maybe get some steam bubbles in it and be at a higher pressure than the rest of the system because of the restriction of the small nozzle. It then went into the little box welded up from some square stock. Inside the box was a jet pump that was store bought with a small nozzle shooting into a venturi. A water tube went from the separator column to the body of the pump so that in theory water would be drawn from the column and circulated through the bottom hottest frusto-conical coil from which it was pumped into the column. It could be that this pipe never primed, we do not know, and if it did not prime then there was never any water drawn off the separator drum until it was completely filled up. Now that I think about it this may have been the whole problem. I did not want to drill a hole into the side of the column and that is the purpose of the design. Steam was pulled off the top of the column from which it went through the superheater coil, buried just up from the circulating coil.
The central separator drum was designed so that the bottom rounded cap was in the fire heating up the water in the drum. It was never to burn out because the water level probes made certain that water was always about half way up that pipe. In the future I was considering welding a lot of flat stock to the drum adding to the heat exchange area and giving a good swirl to the fire. All of the pipes went into the drum from the top.
One of the pipes sprayed the water with steam bubbles in as they came from the main LaMont coil. One took steam off the top. Two of the pipes were part of an upside down “U” that was the attemperator. There was a hand valve in between the two legs which, when closed, forced output steam through the loop where it was cooled off. That is, it was cooled off it the temperature was too hot. We never got that far as to test this part of the design. One pipe went to the bottom and was the blow off tube to clear scale from the bottom. Every time we thought of some other circuit for the steam generator we drilled another hole and added another pipe. When the water level probes called for water the pump pumped into the economizer coils.
I have never seen anything work so poorly in my life. Here is what I think happened. Because the fire was at the bottom this section of coils would boil dry right away. The steam bubbles formed there would pump all of the water in the coils into the drum, raising the water level there and thus shutting off the make up water pump, because that was run by the water level. Once the water pumps were stopped then the jet pump did not work and with the jet pump not working there was never any water pumped into the circulating coil and only hot water from the top of the drum went into the engine. Well, we do not know because whatever came off the top of the drum had to go through the superheater and so some heat was picked up in that coil.
Some of the problem with this boiler was because it was upside down, the heat was at the bottom and the water was pumped into the top. I did not think there would be much of a flow or leakage down because all of the water in the economizer stayed water under a higher pressure than the rest of the boiler because it all had to flow through the small nozzle in the jet pump. And then all of the water pumped through the bottom coil was discharged into the column, so there was no leakage there. I do not know if having the combustion gases routed to go from top to bottom would have made that much of a difference.
I learned many things from this design. The first thing I learned is that a person should test the jet pump under some reasonable water flow rate to see if it really does move water under 5 psi head. We just make a guess as to the size of the nozzle needed. I had hoped that as the water expanded to a lower pressure through the nozzle that some steam bubbles would form and add to the jet pumping effect. Wishful thinking did not work in this case.
The second thing I learned is that there is a reason for having downcomers in a boiler. They are very handy. The third thing I learned is that when heated boiler tubes anneal and promptly collapse against each other cutting off the expected combustion gas flow. When these coils were wound they had a lot of spring in them and it appeared that we did not need to tie them to spacers. They heated up and collapsed like soft butter. The fourth thing I learned is that a monotube boiler is very safe. When the hole burned in the tube there was ash and smoke and steam all over the place because it blew into the firebox, but there was nothing hot or under pressure anywhere. The fifth thing I learned is that wishful thinking will not make the water flow where we want it to flow.
And so then I invented the Field Tube Boiler. I was a very proud inventor for about a week and then I looked in a book from 1898 and found that someone had beat me to this excellent boiler design. The beauty of the Field Tube is that the downcomer is much larger than the upcomer. You can never have enough downcomers in my opinion anymore. Therefore we made a Field Tube boiler for the wood fired dune buggy using a vertical 6 foot length of 2 ½” schedule 40 and made sure that it had about 2 ½’ of steam separator space so that it never primed. This works great except that wood burns in three distinct stages with the last stage being a large bed of red hot coals that over heat the superheater coils to red hot making the steam temperature to go way out of sight. Now all we need is another attemperator and it will be a great success. We learned even more things once we had a boiler that made steam and did not burn out. We learned that a wood fire is impossible to modulate. If combustion air is stopped it continues to put out heat for an hour.
Therefore the next design will have provision for a large piece of ½” diamond plate to slide between the fire and the boiler. A large lever and some mechanical linkage will be needed but that is better than having to drive around the parking lot at high rates of speed in order to keep steam pressure from going past a thousand psi.