OK. "We're from the government and we're here to help."

Fascinating insight into the effect of well meaning bureaucracy on reality.

Wonder if the application of an under-throat mounted burner with a brick arch might have made a difference? Maybe depending on the introduction of secondary air through the door? Or does the brick arch alone make for such a drastic difference between top and bottom firebox temperature no matter where the burner is located?

dave

Matt, if you can spare the time, could you explain this again in even simpler terms? I'm really interested in understanding, but my limited real-world experience--and all of that with coal burners-- is making it hard for me to visualize clearly the problem you're describing.

I'll start by visualizing a big empty rectangle as a simplified firebox. Now I'll add a brick arch in my imagination-- call it a plane filling the firebox from side to side and running from the bottom of the throat sheet in the front upwards toward the back, with an air gap at the rear for the gasses to flow around it and into the upper part of the firebox.

Now, help me understand in the "normal" configuration and in the CP configuration where the burners are located, what direction they are facing, how combustion air is introduced, and how it flows.

Thanks to Matt and anyone who can take the time to walk me through this so I can understand this better.

Not (ever!) speaking for Matt but those I have seen have the burner (basic Van Boden Ingles flame thrower) mounted just below the mud ring in the center under the firedoor and cab, aimed towards the bottom of the throat sheet under the front low part of the brick arch. The main primary source of combustion air is towards the forward part of the firepan, coming up from below towards the front third / quarter of the pan. This flow seems to tend to force the fire out and back along the sides of the firebox below the level of the brick arch, making them extremely hot. Figure the bottom half of the firebox sides triangularly speaking.

Some secondary combustion air comes in through the door, but apparently the greater amount of heat flows through the center of the top firebox space and doesn't directly impinge for heat transfer above the arch, thus rendering the upper triangular part - above the door, across the crown and towards the tube sheet - cold in comparison.

Fire runs in a Z pattern across the firepan to against the throat, back up along the brick arch, and across the top of the firebox to the tube openings in a side view - and we all have been conditioned to see arches in side view in all the old manuals I have. Apparently, looking from above or below, it also runs across the sides of the bottom then towards the center above the arch. Way cool to learn this.

I am becoming very interested in the cluster burners and steel arch developed by Sulzer, and the central sprinklerhead vaporizing burner used in Australia and the UK without arches at all. A less specifically directed flame might be least destructive in the long run.

dave

Both the locomotive draft inlet air will move the flame around the firebox after it leaves the burner. In SP 3420 and many other locomotives the burner sits near the front of the Mud ring pointed toward the rear of the locomotive. Air is mixed both at the burner from air passing around the burner and through drafting passages in the brick usually located near the rear of the firebox. As the flame leaves the burner it heads toward the flash wall and does a 180 degree turn before traveling the length of the firebox and entering the tubes.

Based upon the apparent firebox layout the flame coming from the Burner located under the firebox door instead of under the front of the mud ring. Remember most gas behavior is like a fluid in and it will go from high pressure areas (atmosphere) to low pressure areas (firebox and then the smokebox) thru a path of least resistance and distance. This design shortens the flame length and allows air from the damper under the rear tube sheet to cool both the sheet in the area and drive the flame up to the tubes without covering the lower sections of the boiler effectively reducing the surface area used to heat the water. To get the same steam production you have to over fire the locomotive which puts more heat stress on the metal. The temperature gradient along the sheet can be severe. The temperature flux can result in severe deformation.

When firing up a coal burning locomotive, you can sometimes find some cool areas of the boiler where the mudring is below the grates. This area usually has a pocket of cold water below the line of the grate. This localized environment is both slow to change and creates a thermal flux along the sheet. Jack Anderson and I noticed this issue on NN 93 during a fire up and were considering a circulation system before Jack passed away.

When you have a thermal stress along a side sheet you can and most likely will begin to break things. One locomotive that I fired had an oil burner that was misaligned and even with the oil valve wide open you could not get the flame to cover all of the fireman's side sheet. The result was several staybolts were replaced due to fracture. Yes, I as the fireman reported the problem. It did not take too many trips to cause the damage and the resultant maintenance work.


The importance of approximately 1000 degree F is the phase change that Iron goes thou from a BCC atomic structure to an FCC structure and the resulting strength change. Iron‘s (as with any material) tensile strength is temperature dependent and it will have a change in strength in correlation with its temperature. An Fe-C phase diagram will show the structural changes steel goes thru between its molten state and room temperature. They can be found in a material selection or physical metallurgy book. When heat treating steel you are using this atomic structure change and carbon’s influence on it to manipulate its strength. With flame temperatures near 3000 degree F, the importance of having good water contact becomes critical for well known reasons long before the sheet melts.


I hope this helps.
Robby

In reading these posts I think I may have an answer to why the oil burners I have seen with either syphons or arch tubes have had no brick arch. It seems when the fire is forced, and probably when it is not, the brick acts as a seperation between portions of the sidesheets allowing that portion between the arch and the rear tubesheet to be cooler than the rest of the sheet. I would think this would be the same in a coal burner but since oil fires are generally hotter than coal the brick is needed more to help reduce fuel useage and make the engine easier to fire. When firing coal burners you generally fire the sheets knowing more heat is absorbed through them than the rest of the firebox but in the case of an oil burner being fired from the rear mudring and drafted at the front third of the pan this heat on the sidesheets can be excessive enough to cause maintaince problems. In the case of the 3716 the foaming caused the water to be held off the sidesheets and created a dry sheet situation. The forced fire resulted in extreme temperatures on dry sheets. Without the arch brick the fire would not have been deflected to the sidesheet as much and more importantly the entire sidesheet would have been closer to the same temperature. Is this basically correct?

In the case of the Canadian engines with the rear mounted burner would it help to have the primary air inlet under the burner rather than in the front of the firepan? From reading all this it seems one of the big advantages of a front mounted burner is that the air is entering the firebox mainly below the flame causing the flame to be dispersed more evenly. This is similar to the way a coal burner drafts from below the grates. It seems logical a back mounted burner would do the same if the draft port were below the flame instead of in front of it.

One of the objectives of firing up a boiler slowly is to evenly expand the boiler through more even heating. Several times I have seen coal burners fired up to fast that were poping off when the mudring was still cool to the touch. I can see how this could easily happen with an oil fire. Arch tubes and syphons relieve this situation considerably. If I am reading Robby's post correctly more relief would be gained by installing circulators in the bottom of the sidesheets. Any improvements in fuel efficiency will reduce the amount of fuel required to produce the same results which will also reduce another problem we are likly to have more of as time goes on, smoke. Is the improvement enough to justify the cost of installing such tubes?

Feel free to correct anything I have written. Like Erik, I want to be sure I understand what is being said in this thread.

Thanks.

John Bohon

I think what you are missing with the brick arch is the fact that the brick arch is designed to increase the flame length of the coal under the arch before it reaches the tubes. Once in the tubes the fuel air mixture gets upset and the flame dies out in a very short distance. If there was no arch the draft would pull the flame to the tubes rather quickly and the combustion process may not be complete for the gases coming off of the coal. By having the arch you increase the air flow distance from the fire to the tube sheet giving you more time for both combustion and heat exchange. This should give you the reason for the combustion chambers on locomotives.

That, combined with holding the volatiles given off by coal in combustion hard down by the firebed to allow for more complete combustion does add efficiency, certainly. Overfire jets and the Gas Producer design also. If there's ever going to be a new generation of steam designed to burn coal, it will be interesting to see what features in combination will provide for best efficiency which pretty much guarantees least pollution as well.

dave

but all my picture (& cartoon) books are in Hawaii. And I am not.
So far this topic has touched:

1. Fluid flow, both compressible gas and liquid
2. Metal expansion due to temperature differentials
3. Plastic deformation due to temperature level and molecular phase change
4. Heat transfer by radiation and convection.

Each of these topics is a Jr or Sr level engineering class all by itself.

This can be transposed to an easierly visualized format but not without pictures. There are also a few fundamentals not mentioned that are crucial to understanding what really goes on. Please allow me a few weeks to get home and coagulate a presentation.

Until then, realize that the fundamental process that drives a locomotive is combustion.
One simple exercise to understand combustion is to look at a candle flame. Now understand that the flame is hollow. What you see is called the "flame front" where the combustion reaction occurs and is only about 20 molecules thick. The flame front is a living being and moves at a speed of about one meter per second for most gaseous hydrocarbon combustion. Just watch a movie where a guy drops a cigarette onto a trail of gasoline, watch how fast the flame moves and it is easy to grasp the meter/second flame speed.
Everything that happens in a locomotive firebox, coal or oil, is to manipulate the flame front such that all the hydrocarbon molecules pass through a flame front and are converted to H2O, CO2 and heat.

I'd certainly welcome an article about that!

dave

Actually I do understand the principals of lengthing the flame of the coal firebed around the arch brick though certainly not on the level of an engineer. I also understand teh advantages of a combustion chamber. I guess what I was thinking was that with the fire being blown toward the rear of the firebox you already sort of have the same effect without the brick. It seems the arch tubes or syphons would perform the same improtant function of increasing water circulation and heating surface regardless of the fuel being used.

Matt's description of a flame and the speed at which it travels is very interesting. I will certainly be looking forward to seeing his pictures when they get posted. Come to think of it I would rather be able to go to Hawaii and look at the photos. I suspect I am not alone in that thought.

Keep the information comming. I am enjoying this thread.

John Bohon