I am really not interested in getting it above 1000 RPM, being as it is not easily balanced, and it will start beating itself to a pulp above those speeds (I put 120PSI to it once... . I will never run this dude in public (I'm willing to stake my own life .... so public certification will probably never be an issue.
Ill also weld me up a 3/16 in thick firebox just in the case it does decide to go. I don't know how to calculate strength of a blast shield, but I have a hard time believing that would come apart given sufficient venting.
As for the boiler design itself, I recently found a couple steel compressor tanks, about 4 inches diameter and 12 inches long, from what you are saying, and adjusting for less RPM and being single action, It sounds like one of these could work. Please correct me if I am wrong.
I don't currently have any plans for powering anything with it, but I am definitely open to ideas for novelty pieces.
I am also very interested in what you called a "super heater" I have thought about running the output of the boiler back through the firebox (steel line of course) to make sure there were no droplets left, is this what you are talking about?
Hi TIG, I guess that pseudonym means you are a TIG welder? Certified or something? - Or just a hobby?
Just to feedback on your post:
I guess you read my comments about running on air... as you said you whacked
120psi into your poor little engine... when it threw a real Wobbler!
As to
certification, I guess you have house insurance, Life insurance, Health insurance, etc. In the event of something unexpected going wrong, you may want these to pay out. An uncertified boiler "accident" will prevent any payment, whereas a certified boiler just may help get some money from those insurance merchants?
Welding the firebox... if the boiler goes "bang in a big way, the firebox won't be an issue. It is the steam that does the damage. I'll explain below.
I have asked a few more questions about these
air cylinders below.
Super heating the steam - I have said some more below on this subject.
I hope this explanation helps you understand a bit more about Boilers and steam?
Most hobby boilers are made of Copper in small sizes, and silver soldered, and the regulations cover these. Making steel boilers is usually in larger sizes, such as 6" diameter and above on traction engines - where welded boilers are more practical to make than silver soldered copper. But even full sized locomotives had copper boilers, as a hundred years ago it was cheaper to make a copper boiler that would last without corroding, than to make 3 steel boilers for the life of a locomotive. But by the 1950s most full sized boilers were steel. With corrosion inhibitors they can be very practical.
Are you able to make a welded air receiver? A boiler is a lot like that, but with a lot more fittings - such as water level gauge, clack-valve for the feedwater (a non-return valve), etc.
But the key to most steam boilers is the large surface area for heat transfer from "flames" to Steam. Just putting an air receiver in a blow-torch flame does not make a good boiler.
The other thing you need to grasp is that an air receiver failing will split the metal, but then only make a loud bang as the air powers a shock wave ... But if a steam boiler fails... :- if you are lucky there is only a large hiss of steam, that can cook flesh like an oxy-acetylene cutting torch (except that Steam is invisible, unlike most flames!), or if if goes "Bang" the stored energy in the hot steam cooks everything with 3rd degree burns about as effectively as the flash burns from a gas explosion, or dynamite. Oh, and just like the Bang from compressed air, the shock wave will deafen you (burst ear-drums), so you will be the only one who cannot hear your screams. So we are careful to design and make steam boilers to the regulations or approved designs, so they DO NOT FAIL. It is common sense, in my book.
Safety sermon apart, to run at only 1000rpm, needs only 1/3rd of the size of boiler I suggested, so a 3" diameter boiler 7 or 8" long with a few kW of gas flame beneath would power the engine - and possibly drive a small dynamo to light an LED or 2. (Maybe 1000th of the input heat? These engines are not powerful, despite all the" power" we stuff into them!).
So a design converting a steel compressed air cylinder, with suitable connections for Safety valve, gauges, feed and steam take off is feasible, but I have not seen any modellers doing that. Here's a couple of images of popular commercial boilers, for powering engines similar in size to yours. As you can see, there are more fittings than on an air receiver, so the whole thing is just more complex. I have included a view of the array of 3 to 6 tubes filled with water beneath the main copper drum, in the space below where the gas fire lives, and where most of the steam is generated.
The only thing I don't like is the lack of insulation over the top and ends of the boiler, so 1/2 the gas heat escapes in warming the room, instead of making steam!
These boilers usually have a single or double run of steam pipe through the fire-box, to dry the steam before it gets to the engine, Not exactly superheated, but just enough to help considerably.
Here's a vertical boiler (Much smaller fire, so less power per lb of copper!):
This is a boiler kit supplied by "PM Research" - here's a picture showing what is inside the boiler as flue tubes where the steam is generated from the hot gas from the fire:
Perhaps you can give some more details of the air cylinder you think may suit? What is the primary use (air, at what pressure? what cylinder dimensions? Wall thickness, etc?). Have you examined it closely and carefully for any rusting on the inside and outside? If rusted, please do not use as a boiler unless you have had it shot-blasted, ultra-sonically tested, and crack checked. Rust (corrosion) attacks weak spots, creates stress raisers and generally erodes the safety margin almost to nil, before most people even consider it to be something to look at. (Safety margin at "Nil" is when it leaks air!). Most "cheap" air cylinders are without an internal zinc coating for corrosion resistance, and all compressed air is WET, with distilled water (condensate) that is more corrosive than water we drink. So I would expect the air cylinder you have to be rusted = No good.
Hope this gives you a better idea of what is involved in making a decent model boiler?
Here's some of mine... (rotate to view as required).
3" Bench horizontal boiler: NWP = 40psi.
Yarrow: 1 1/2" drum, NWP = 25psi
Small vertical boiler 2" diameter NWP = 67psi.
And here is a large stainless steel boiler for a 5" gauge loco - NWP 100psi - for the truly ambitious modeler!
Cheers!
K2
P.S. Maybe your TIG skills can TIG-weld a 1/8"~ 3/16"mm thick copper boiler? - There are professionally made commercial boilers that are TIG welded copper, so with the skills and certification these are OK as well. Or can you silver solder thick copper with your TIG set?).
