Hi Byro, You have "crossed the river..". When you understand that steam is NOT just boiled water, but H2O as a gas, you are getting there. (Not sure how you did, but this is correct: "the steam is more like air now so I’ll just be adding energy into it ").
Water is boiler in the boiler and in the large space above the water you have wet steam: a mix of water spray, droplets, aerosol, whatever, and steam at EA+XACTLY the same temperature of the water. It is called WET because ANY loss of pressure or energy - after the steam has left the boiler - will result in MORE droplets of water condensing as the latent heat is used to compensate for the loss of pressure: Even a 1psi drop down the feed pipe is enough! - and more pressure drop through the throttle valve.... The the steam enters the valve chest - with further expansion and loss of pressure, then whistles past the valve into the cylinder - Wow! MORE loss of pressure - so the steam entering the cylinder has lost latent heat all the way getting to where we want it to do work. So now, all that hot water and steam is expanded in the cylinder (as the work is extracted to move metal) and the ensuing water droplets and remaining steam (Hopefully only a small fraction of what we stuffed into the cylinder) are discharged out of the exhaust.
- Now if the steam has had MORE Heat given to it after the spray and droplets from the boiler have been left behind - in the boiler where we want them to stay, courtesy of the steam dome - then this "Superheat" - meaning the correct use of the adjective Super to be "Above, beyond" - becomes the energy stored in the steam "Above" the boiling point of latent heat conversion of the change of state. So the external heater you plan for this is correct - except you DO NOT RETURN this to the boiler, but take the superheated steam to the engine.
Actually, many model superheaters do not release pure steam into the engine, because of "transmission losses getting the steam to the engine. Most static models I see at shows, etc, have bright shiny steam pipe in COLD air, without any lagging, so the steam is drastically cooled before it gets to the engine. I CRINGE! But MOST locos have superheaters that are long tubes from smoke box to firebox and returning in parallel to the smoke box before entering the engine. GOOD. This is how they do it on full sized locos, so these are correct models. But Thermodynamically, the firebox may be 400C or more... and the smoke box much cooler, say 200 degrees C or whatever, so the returning loop of the superheater tube (with steam at 400C) cools in the cooling smoke so it gets to the engine not much above the 200degrees of the smoke box! Yet still with some superheat when it gets to the engine. Long boilers have smoke that is cooled almost to the temperature of the "wet" steam by the time they get to the smoke box, and adiabatic expansion as they leave the tubes and enter the firebox - with the vacuum in the firebox from the forced draught - cause the firebox to be much cooler than desirable for the superheated steam pipes!
However, in the model boiler we cheat - the way full-sized static boilers do - by taking superheated steam from the hottest point of the superheat path and then directly take that to the engine.
For my vertical boiler, this means taking the steam out of the dome, down the flue to the firebox, then sideways straight out to the engine. On my horizontal boiler, and on simple commercial tank boilers for the cheapest models (e.g. Mamod), with a fire beneath, these take the steam from the dome and down to the firebox, and have a simple loop of pipe inside the fire zone. This superheated (DRY) steam then goes to the engine. My only problem, is that the superheater pipe exiting the firebox is usually TOO HOT and chars the cotton string I use to insulate the steam pipe. I have to find something else that can stand over 200C! (or let that bit of pipe cool the steam a bit so the rest of the lagging doesn't char! - I need to find glass-fibre string!). Therefore I have superheat on everything I make, so the engines run better. (More power in the steam that can be converted to moving metal).
Superheat also raises the pressure of the steam (NO NEED for a pressure relief valve on superheater pipework. Just use something that can take the higher temperature and pressure!), and when in the engine it allows expansion in the passages so only steam enters the cylinder. This steam expands as the piston motion extracts heat, until it has cooled to the condensing temperature, when the latent heat starts to be used and water droplets (vapour) appear. Then when expelled from the exhaust the WET steam expands more showing the plume of water vapour after the expansion has lost all the latent heat. It is nice to see a little gap of clear steam expelled before the water vapour plume starts - just like on a good kettle! This shows that you have more than enough superheat to keep the engine "dry" inside. So you can give the engine more overlap to use more steam energy and be more efficient!
Just a word on caution. Hot water (
over 60C) and vapour can scald "a little" ("Scald" means "to
burn the flesh with
boiling liquid or
steam: "). But real (Clear, invisible) STEAM will take flesh off your arm quicker than you can yell as loud as you can! (I have seen the scars on others!). This won't grow back! so NEVER stick a finger (or any other living flesh) in the CLEAR part of water vapour. I NEVER PUT ANYTHING IN ANY PLUME, as it is often hotter than expected and hurts.
Cheers!thank you for the treatises. You have confirmed my studies. I initially thought of returning the super heated steam to the boiler but I finally got it through my thick head this would at best lead to increased pressures have a self imposed limit to boiler pressure . Also you confirmed my chase to find an easy way to get hotter steam and contain it . My latest idea is to have smaller heavier boiler to contain super heated steam. I’ll have a port to use this directly through an oiler and throttle. So I’ll have steam oiled “gas” to run the engines. This tank or smaller boiler will have check valves set up to gather vaporous “ steam” from the main boiler it can then be super heated thus replenishing the volume also I can have a separate valve to pressurize the water fill tank more or less filling the main boiler as needed. Yes it gets complicated but the valves can be simple ballbearing valves getting relief pressure adjusted by altering or optimizing pressures . The secondary storage boiler can have higher relief valve pressure to insure against over pressurizing I may have to use manual valves to get it stabilized during operation. My concern is how well the induction heater works. Your note also raises a question . If I build a smaller storage tank would it be possible to I use this as a super heater for air from the air compressor when running on air compressor. Again exit air could be steam oiled rather than light oiled . It’s contents would essentially be heated dried air. Again it would take some optimizing I’ve come up with a way to force the air flow over considerable heated surface I gues incision a labrnith type device increasing air contact with heated surface provided by the induction heater. This will be a try it an see. Thing. I have almost al the materials needed. I’m going to give my TIG welder another test to see if I can be visually steady enough to weld whe here Bessarabia . I have a good friend who can inspect my stuff so it’s correctly done . I’m going to work on some sketches an a process to create this. So far I have not seen much in the elevated temp pressure range beyond the RR guys so I’m kinda on my own I think. Depending on you guys to critique my thoughts I have some stainless steel seamless tubing and all the welding stuff to do this . I’m liking stainless as it does not transfer heat very fast so once the induction heater gets it hot it will stay hot giving heat off to the incoming air or vapor. Maybe a lubricator could be used as a collector for moisture driven out of incoming air or boiler vapor . Have to think about that one .
Dinner time
Byron
K2
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