Super heated Steam Throttle Lag

[Builder01]

While it is true that cleaning the superheater flues can be problematic, they are toward the top of the tube sheet and do not get very dirty. I removed my super heater tube assembly because of taking every thing apart for painting. I cleaned out the super heater flues and they were not really that dirty after no real cleaning after two years. So, every two yeas or so, the super heater should be removed so the flues can be properly cleaned. Build it so it all comes apart easily!!

David

[Fred_V]

My throttle is in the dome ahead of the S heater. 3 long tubes go it through the smokebox and screw into nipples welded to the SH so it comes apart easily.

[SilvNor401]

"A question: Since one of the main efficiency improvements with super heated steam is at any steam pressure the steam volume increases as the temperature increases while the pressure remain the same. If the throttle is down stream from the super heater how is the increased volume realize in the cylinders? If the throttle is up stream and since there is a pressure drop through the throttle I can rationalize in my mind how the increased volume of the higher temperature steam makes it way to the cylinders and reduces the amount of steam needed to be passed through the throttle."

Dave,

I think the answer too your question is that throttle placement does not matter as long as the steam is only allowed too expand in the cylinders. In full size practice, normal operating procedure is too run with the throttle wide open and use valve cutoff too limit the cylinder horsepower too suit conditions required. Only then, are the real benefits of superheating realized, even in full sized practice. On your model railroads, we generally are not able too run our locomotives in this fashion because grades change quickly, distances run with the throttle open are relatively short, and there are usually other trains/people in the way, etc. Hope this helps!

Jason

[Builder01]

I notch up all the time, although, I do not run with the throttle wide open. The reverser is only "in the corner" for starting and going up a steep grade. Otherwise, I usually run notched up two to three turns on my reverser. (I have a screw reverser). I don't know if running this way is making good use of my "superheated" steam or not, but, it does seem to make efficient use of my "dry" steam.

David

[Asteamhead]

Dave,
Looks excellent, well done! I'm using this method of welding the SH return ends, too some 40 years now with best results
But I don't recommend enlargeing the back ends into the firebox due to the risk of overheating! Please think about of firing by means of the blower at standstill.
One can achive sufficient superheating just by means of the elements and proper dimensions. About +100 °C seems to me a good choice,, which guarantees not to destroy any sealings made of PTFE . Some 30 % of the total gas cross section through the tubes will do.
Combined cross section of the SH elements should not exceed that of the throttle to achive high speed for the steam inside. This will increase heat transfer to the dry steam!
If you like, please watch my topic 'A with Timken running gear'.
SilvNor401,
The greater volume of that superheated steam will be fully used by the cylinders - more rpm by same evaporation .
Time lag (response to the throttle in the steam dome) may be neglected - no risk. IIn case of priming however, the foam / water in the superheater elements may mean an uncontrolled and unexspected running of your (cold) engine - be careful

Best regards
Asteamhead

[Berkman]

Fred,
So does your dry pipe go from the steamdome to the smokebox, as is standard practice, then through the boiler back into the superheater assembly or directly from the steamdome back to the superheat in the firebox?

[Fred_V]

Steam from the dome throttle enters the center tube of the SH at the smokebox and goes to the firebox tee fitting. Then around the other 2 legs of the SH and back through the flues to the smokebox and to each cylinder. The pipe unions come up to a tee and are the exhaust blast.
for some reason i cannot get the photo to be right side up. I rotated the image on my computer but it is still wrong.

[daves1459]

Looks great! The return bends are the only part of my boiler that I did not make myself. I had a professional welder do that for me. To keep the inside of the return bend free from weld, I think he flooded the tube with an inert gas, maybe nitrogen, I'm not sure though. My fire box is pretty short and I have run the super heater tubes almost all the way to the firebox back head. I think they actually go into the fire box a little farther that what the photo shows.
DSCN1297 - reduced 4.JPG

At the time I was welding I didn't give any thought an inert gas in the tubes. I could have used some argon from the TIG tank. Thinking further what I should done was to machine a couple of copper rods maybe 3" long to slip down inside of the tubes with .010" diameter clearance and attach them to a couple of 1/4" diameter rods the length of the supper heater tubes plus a few inches as handles to slip down through the tubes. With a 15 degree cut on the end of each copper they would nest to each other exactly where the supper heater tubes met forming a heat sink for welding and a back up to prevent any molten metal from entering the tube joint. Thinking still further, and kicking myself for not inserting the heat sinks, they would have helped greatly in preventing the tubes from heating up and allowed for continuous welds. Right now I'm considering cutting off the current welded area and re-welding after re-machining the 15 degree angle on each tube.

Dave

[daves1459]

Just an idea but would it be possible for the dry pipe to run directly from the steam dome to the back of the firebox as per Fred's pic then have a few back and forth loops across the top of the firebox, then two pipes, again per Fred's pic back to the smokebox, then combine at the throttle valve, then to the cylinders?

Seems that would provide a good amount of superheat area above the firebed but reduce the amount of plumbing needed and also eliminate the possibility of creating flue/tube situation that would be hard to clean with coal.

It is rather late to consider your Idea. It would be a major design change and reconstruction project to implement. I get you idea though and it is a novel concept. Would there be some sort of threaded fitting for the steam inlet and to the two tubes going down the boiler tubes? If so they would be exposed to the fire heat in the fire box. I'm firing with propane so soot will not be a problem.

Dave

[daves1459]

Dave,
Looks excellent, well done! I'm using this method of welding the SH return ends, too some 40 years now with best results
But I don't recommend enlargeing the back ends into the firebox due to the risk of overheating! Please think about of firing by means of the blower at standstill.
One can achive sufficient superheating just by means of the elements and proper dimensions. About +100 °C seems to me a good choice,, which guarantees not to destroy any sealings made of PTFE . Some 30 % of the total gas cross section through the tubes will do.
Combined cross section of the SH elements should not exceed that of the throttle to achive high speed for the steam inside. This will increase heat transfer to the dry steam!
If you like, please watch my topic 'A with Timken running gear'.
SilvNor401,
The greater volume of that superheated steam will be fully used by the cylinders - more rpm by same evaporation .
Time lag (response to the throttle in the steam dome) may be neglected - no risk. IIn case of priming however, the foam / water in the superheater elements may mean an uncontrolled and unexspected running of your (cold) engine - be careful

Best regards
Asteamhead

Thanks for the compliments. I'm still at a loss as to how far to extend the ends of the super heater elements into the fire box. Everything I've read says for models to get any super heat the elements must protrude into the fire box. But, I have found nothing about how far. All of the critical dimensions within my boiler came from Martin Evans' excellent book "Model Locomotive Boilers" including the super heater tube diameter. I have built other boilers to his ratios with excellent results. But, as I recall while extending the elements into the firebox is mentioned the distance is not.

I can see how over heating the elements at stand still with a coal fire and the blower on can be a significant problem. However, in my case I'm using slotted cap propane burners that I can easily partially close the gas valve to reduce burner heat so as to reduce the safeties from popping and lower the heat to the super heater.

Laminar flow is a ever present problem with small diameter tubes. Most everyone using propane installs "turbolizers" in the boiler tubes to increase turbulence in the tubes and this improve heat transfer through the tubes. I wonder if a smaller version of the turbolizers inserted into the super heater element tubes would increase the heat transfer to the steam and thus improve the super heater overall efficiency.

Dave

[daves1459]

Steam from the dome throttle enters the center tube of the SH at the smokebox and goes to the firebox tee fitting. Then around the other 2 legs of the SH and back through the flues to the smokebox and to each cylinder. The pipe unions come up to a tee and are the exhaust blast.
for some reason i cannot get the photo to be right side up. I rotated the image on my computer but it is still wrong.

What is the best type of threaded fitting for either superheated or saturated steam when located in the smoke box? Flared, compression, unons. or what?

Dave

[southwestern737]

I have enjoyed following this thread, I just got my Heavy Mikado fired up a few months ago, it has a superheater. I am in the process of decreasing the amount of superheat because I am destroying throttle valves. Originally my superheater elements ran all the way to the back of the firebox, I am going to shorten them by 6” and see what happens. The way you have yours piped you should not have any trouble with your throttle. As has been mentioned when welding the tubes you should get the oxygen out of them, I have a T on my shielding gas and run a low flow to the tube, you will also find that this helps a little with burn through.
Good luck.
Brent