Steam & Excursion > A critical firebox dimension affecting drafting in coal burners

I've never seen it discussed as topic regarding drafting of coal burning locos.
Mostly, I'm discussing the later designed 2-8-4s, 4-8-2s, 4-8-4s, 4-6-4s and high speed
4-6-6-4s, etc   The more modern locos that came on the scene after about 1938..

That "area" above the top of the firebox's brick-arch and its proximity to the 
"crown sheet" that forms the flat ceiling of the firebox. Is a much smaller area than
the published 'grate areas', and directly affects the airflow through the firebox.

That area is approximately 1/3 the total grate area, that forms the floor of the firebox.
So, that reduction in draft area affects the drafting of a theoretically-flat coal fire-bed.
The intended purpose of adding the brick arch is to prevent long-flame, soft-coal's
burning gasses --- at the grate area closest to the rear tube-sheet-- from being directly 
sucked into the fire tubes.

The front wall of the 4-sided firebox has the rear of the fire tubes that convey the
burning gasses.   Without a brick arch, the burning gasses at the front area of
the grates would immediately dive into water-surrounded boiler tubes without 
having sufficient distance to completely burn the carbon gas down to CO2.
The typical flame length, with a high-velocity draft, is about 18 to 20 feet.

By adding a brick arch you can almost double the length of the flame path,
compared to a bare firebox, with no brick arch.  The theory holds that the vacuum 
produced in the smokebox ( front cylindrical volume, ahead of the front
flue sheet) draws air evenly through the entire grate sutface.

However, at high draft rates producing the hottest fires, the open area above the 
top bricks of the arch, severely restrict the air-flow pattern through the grates, because 
the burning gases cannot all fit through the restricted air flow area over the top edge 
of the brick arch :---- At the same time.

The gasses beneath the arch, about 50% of the grate area, must all make a 180-degree
change in air flow direction, through a more restricted opening.  The gasses at the back 
of the grate area have a direct-shot, over tye arch through the flues. 

That crowding causes the rate of air flow through a large area of the grates to be
severely reduced, since an area of 100 sq ft  cannot get through an area 50% smaller,
at the same velocity.  That crowding, under the arch, slows down the flow-rate of
fresh oxygen, compared the the rear grate area not under the brick arch.

That air flow crowding and disparate rates of airflow affects the volume of oxygen 
coming through the gates, according to the varying "vacuum intensity"  above
 the grates ---- air flow at the front grate areas, by having a longer, more restricted 
flow allowance. 

The rear area of the grates has no such restrictions affecting the free-flow of air,
up the rear-wall of the firebox, called the "door-sheet" --for obvious reasons-- .
That gas stream has a straight-shot to the rear tube sheet and up the stack.
The unrestricted air-flow up the door-sheet 'crowds' the gasses trying to get around 
the top of the brick-arch, obstructing the flow velocity, 

All that air flow and varying gas velocity gets to the long explanation about disparate 
coal burning rates, at different areas of grate .  Again, the strong draft velocities 
are solely dependent on the weight and drag of the cars being being pulled.
Designers had to design fireboxes that produced maximum heat at maximum speeds
and greatest loads.

Lighter loads mean less air-flow drawn into the firebox, and slower burn rates.
Varying air flow rates directly affect the burn-rates of the coal at various areas
 of the grates...  
Thus, the coal consumption-rates are greater where the air velocity is greatest.

Experiencec firemen know how to manage the varying draft rates of the airflow,
by matching the coal-bed depth, to the varying velocities of air through the different
areas of the grates.

That's all, for now. 

W.

not proofed, yet



Edited 5 time(s). Last edit at 11/27/24 09:22 by wcamp1472.

This drawing should help those of you not intimately familiar with coal fireboxes to understand Wes' description.  Not shown in this particular firebox is a combustion chamber, which would extend the righthand wall and move the firetubes (flues) a couple feet to the right, into the boiler barrel to give a longer flame path.

Practically no burning of fuel gasses takes  place in the flues - it takes place in the firebox proper.

-LD



Edited 2 time(s). Last edit at 11/27/24 08:40 by LarryDoyle.

---

I've seen similar drawings in all sorts of 'old' texts about coal firing.
Its not applicable to conditions of violent drafting, as when hauling a 
train of heavy cars.

The artist, for this illustrstion, presumed a lite, steady, low-flow of air
through the grates; also, it's hand fired, no power stoker.
Its a wrong example for texts relating to firing stoker-equipped locos.

It is useful & illustrative of representing the two paths of air-flow:
>One path is the area under the brick-arch, and you can see the
flow- reversal which is necessary to get around the top of tte brick arch.

>The other path is more direct, up the door sheet and over tte arch,
   then into the boiler flues.... it's the path of greatest air flow, and 
    the highest burn-rate of the fuel on the grates.

Once the carbon has been burned-away, you're left with cold, dead 
ash, high wind, & cold air flowing into your firebox..  adding more coal 
to the burning area, adds cold fuel ( which must still be heated to be burnable)
and the result is both lowered fire temps, and black smoke.

When hauling a train, ( stoker-fired engine) the air velocity is immense and
the high rate of oxygen comes the coal at a much faster rate.
You cannot have a flat firebed, when you have differnt flow velocities.

The two paths have 2 different flow rates, so the faster air-flow rate burns the
fuel down to ashes about 3-times the burn rate of the coal, down under the brick arch.  
The greater volume at the rear of the grate area crowds-out the air flow trying
to make the 180-degree flow path from under brick arch.

So, if heavily drafted, any coal at the back area of the grates will burn down to
cold ashes, virtually immediately.... and it becomes a wide-open, free air-path 
through the firebox and into the boiler flues.

W.

 

And the arch spreads the heat so that it a) doesn't blow right out the tubes and
b) isn't just partially absorbed by the crown sheet due to the flame direction and
velocity.

Out 

Now, if you wanted to change a "modern" large coal buring locomotive to burn oil what changes would be needed to do the job "right"? 

I would expect that the UP conversions/arrangements are a good place to
start ---  seeking clues as to what works best...

As a coal-guy, with no "oil experiences", I'll let others 
supply their own comments.

​W.


 



Edited 1 time(s). Last edit at 11/27/24 21:29 by wcamp1472.

How did syphons fit in?  and what was their effect? 

Syphons was trademark name for firebox water circulators.

Think of a funnel.
Now, think of a funnel with the sides pushed-in, forming a funnel with, long, flat sides.
There are staybolts throughout to support the flat sides of the Syphons.
Syphons were constructed of 3/8" thick, firebox steel.... which must be stayed to withstand the 
boiler pressures, and not distort under pressure.

They were typically used in larger fireboxes and spaced with an arch-tube near each side-sheet of the firebox.
So, you had 4 sloping pipes for supporting  the refractory bricks forming the 'brick arch".

The simple, 4" diameter arch tubes'  upper ends terminated at the upper 
region of the rear firebox sheet, called the 'door sheet'.

The Syphon's flattened sides would fit between the rows of laterally
arranged crown bolts supporting the roof, or Crown Sheet.
So, water was heated directly in the path of the flames,
and fed with water taken from the coldest water in the boiler,
down and the lowest part of the firebox, the throat sheet.
They did a terrific job of increasing water circulation, promoting 
faster boiling times, from the 'cold' water state.

After several years of service, stresses would build up between
the crownsheet and  the lower ends of the Syphons, at the 'throat sheets'
of the firebox.  At "Classified repairs" schedules, boilermakers would carefully 
cut the welds that attached the Syphon tubes at the throat sheet.

The lower ends of the Syphons were extended pipes, and the throat 
sheets were formed with curved adaptor regions of the throat sheet,
with some of the lower Syphon tubing extending into the water space.

When boilermakers were 'relieving' the Syphons, they would cut the weld-bead
that attached the curved adapter portions of the throat sheet to the tubular
extensions of the lower ends of the Syphons...

Typicallly, ... as they were carefully cutting the circumferential weld attaching the Syphon
 at the  throat sheet, there was such terrific tension between the crown sheet and where the lower
throat sheets were formed, that the accumulated, strong tension would break, tear-apart,
the last 1 or 2 inches of original weldment with a loud BOOOM... heard throughout the backshop.

After 'releasing the Syphons' ,  the Syphons' lower supply pipes would be welded &
attached back to the throat-sheet. with a fresh, circumferential, weld-bead and about
3" to 4 inches of the 'new', exposed, lower pipe-portion of the Syphon, now visible.

 Its about time for NKP 765 to have it's 'Syphons released'.....
That would make great "audio" of the last inch, or so, of the Syphon-welds being
ripped apart, in one sudden yank!

The simpler arch tubes were longer and curved; so, unless there
were other age or 'defect' reasons, arch tubes were not 'released',
the way that Syphons were, in order to be continued in service.

​W.

( Now that I’m thinking about it,  I think that the boilermakers’ term for the
   stress-relieving process for Syphons was called: “Releasing” the Syphons,
   and not “relieving”. My badd!)

not proofed, yet...



Edited 8 time(s). Last edit at 11/28/24 22:56 by wcamp1472.