Seeing the photo of the firebox posted here a few days ago got me thinking. A grate area that big will draw lots of air for combustion. So how and where does all that combustion air get into the firebox?

I don't know the answer but I'd be interested in finding this out as well! I'll just keep my eye on this thread then. Cheers, Rob

Oh grate! the draft!

"Combustion air", is draft. the flow, for a steam locomotive is through the flues and tubes,and up the stack......but, before it gets to the flues and tubes, it has to be heated by the fire, in the firebox, on the grates, which sit just above the ashpan, and are shaken to get the ash out of the way so that air can pass more easily from the lower edges of the firebox( just below the mud ring), under and up through the grates, the fuel, cause combustion and get heated, then through the flues and tubes where the heat is transferred to the water in the boiler...etc.etc.etc.
When firing up a steam locomotive( cold start), a fan is placed over the stack to start the draft, or, steam from a stationary boiler can be exhausted up through the stack to help draw the air, to get the fire started, and steam up. Once the boiler has sufficient pressure, it can provide its own steam to aid the draft, as needed.
The fireman, controls the draft by using steam through the exhaust pipe, via a valve in the cab.
I think I can be reasonably sure that the grates are the one piece of a steam locomotive that rarely ever get modeled.....unless it is a live steam model.
The space between the grate bars, the sizes of flues and tubes, the stack diameter, and height, all determine the amount of draft, all of which is determined by the type of fuel, and how slowly, or quickly it burns.
The Challengers, that the Clinchfield purchased from the D&RGW, had to be modified from the original double stack, to single stack, and a number of internal boiler changes had to be made so the coal, available to the Clinchfield, could be more efficiently burned.

Sumpter250 Wrote:Oh grate! the draft!

"Combustion air", is draft. the flow, for a steam locomotive is through the flues and tubes,and up the stack......but, before it gets to the flues and tubes, it has to be heated by the fire, in the firebox, on the grates, which sit just above the ashpan, and are shaken to get the ash out of the way so that air can pass more easily from the lower edges of the firebox( just below the mud ring),

Is this combustion air reglulated by any sort of damper? Or is it smiply a gap above the ashpan?

It is, simply a gap above the ashpan. The gap is usually hidden behind the ashpan covers, that run on both sides. These covers are usually curved, a "quarter round", from the ashpan, up to, and about 4" to 6" out from the bottom of the firebox.
The ashpan covers, allow access to the top of the grates, for removal of clinker that doesn't shake down, and has to be manually removed.

There may be some additional regulation - I was recently reading an article on the design of PRR boilers which was really quite interesting. To get to the point, several references were made to air-hole size as one component of how much air is allowed into the grate area for combustion.

I'll have to dig that back up and get some more context. It discussed quite a few items that affect draft - air hole, grate area, the gap between the brick arch and the back wall of the firebox, diameter and number of tubes and flues (as noted by Sumpter), how flues packed with superheating elements affected it's ability to breathe and smoke box design (exhaust nozzle, spark arresting components, diameter of the lower end of the stack, the gap between it and the exhaust nozzle, the stack's internal shape). This is all I can remember at the moment - suffice to say that it added a lot of detail beyond my working level of knowledge about the stephenson steam locomotive.

Matt

Quote:items that affect draft - air hole, grate area, the gap between the brick arch and the back wall of the firebox, diameter and number of tubes and flues (as noted by Sumpter), how flues packed with superheating elements affected it's ability to breathe and smoke box design (exhaust nozzle, spark arresting components, diameter of the lower end of the stack, the gap between it and the exhaust nozzle, the stack's internal shape).

All of these things have to be balanced to each other, with consideration given to type of fuel, oil/coal, type of coal, array of oil feed to the fire, etc., operating speed of the loco, boiler size, and more other things than I can think of at the time.
Firebox, tubes and flues, and stack.....these are about all that's needed when the loco is standing. Everything else comes into play when the loco is working. (Exhaust nozzle, spark arresting components, diameter of the lower end of the stack, the gap between it and the exhaust nozzle, the stack's internal shape), these things are critical to working performance.
The exhaust nozzle, shapes and directs the exhaust steam from the cylinders, up the center of the stack.
Stack diameter, shape, and distance above the exhaust nozzle, determine the "venturi effect", or the ability of the exhaust steam to "draw" the hot gasses from the firebox, through the tubes and flues, and up the stack.
The idea here is, simply, the harder the loco works, the more steam is blasted up through the stack, the faster the heat is pulled through the boiler, the faster steam can be made.
Superheating elements, and spark arresting components, create resistance to the flow of "heat", and have to be compensated for.
All of this may be interesting, but where models are concerned, is never seen, and rarely ever modeled, except where it shows externally, as in air intake above the fire, to reduce smoke.

Fascinating stuff - it makes me appreciate how much design effort went into these machines to create the power and efficiency that late steam had. And all with slide rules (or simpler)!

Matt

in essence the draft was controlled by the amount of steam exhausting through the stack , that is why a heavy handed engineer could rip holes in the fire on a coal fired locomotive. too much draft caused by too much throttle too fast.
Jim

Holes in the fire. These are places where, for whatever reason, there's little or no combustible material.
Hand firing's most critical concern is getting the coal evenly distributed across the grate, and avoiding "holes" in the fire.
These "cold spots" reduce the loco's fuel/steam efficiency.

Don't knock the slide rule, they can do things a lot of modern calculators don't always have the programs to do. What amazes me even more is with the limited diagnostic equipment of the day, how they figured out what was going on, and why, before they could begin to figure out what to do about it.

Another fascinating subject, is exactly how a steam locomotive's equalized suspension works!, and just how precise all the parts have to be, to do the job correctly, while still supporting the tons of weight! Side rod binding in a model is inconvenient......in the prototype it was disastrous!

I think some also had blowers to bring in air but could be mistaken. As far as interesting facts are concerned , Steam locos were very hard on the rails. The side rods thrashing back and forth and cylinders moving them caused a right left motion which was detrimental to the track.

Lester Perry Wrote:I think some also had blowers to bring in air but could be mistaken. As far as interesting facts are concerned , Steam locos were very hard on the rails. The side rods thrashing back and forth and cylinders moving them caused a right left motion which was detrimental to the track.

[Bold words are mine for emphasis]

You are actually quite correct. All steamers had what was called a blower. It was controlled, among other things, by the fireman. When the engine is first getting heated up to operating temperature, the flues are cooler than when the engine is the hottest and working hard with exhaust steam venting up the blast pipes, through the petitcoat pipe, and up the stack. Cooler flues, like cool chimneys, don't draught well.

With a decent fire going and gew pounds of pressure in the boiler, the hostler turns a valve and lets steam out through a pipe just under the stack, facing up. That is the blower. When an engine is stationary and not chuffing, the blower is almost essential, even if only just barely going. It helps to keep the fire healthy. The blower can be turned off when the engine is underway, and certainly when it is straining under a load. As stated by another gentleman, a fire that is worked too hard, and at the same time not carefully groomed for depth by the fireman (if coal fired), would result in areas of the coal bed literally ripped up and sucked through the flues. What would be left are holes which would allow cool incoming air an easier path into the flues. Cool air cools the flues causing rapid differential strain against the welds in the bulkhead. This could break welds and cause high temperature water and steam to enter the firebox.

Not good.

Lester Perry Wrote:As far as interesting facts are concerned , Steam locos were very hard on the rails. The side rods thrashing back and forth and cylinders moving them caused a right left motion which was detrimental to the track.

This is true. The majority of the weight in the counterweights was there to offset this rocking couple. It was always a compromise though - completely offsetting the piston thrust with counterweight would leave the wheel so unbalanced that it could begin hopping on the rail at higher speeds. This hammering was very destructive - C&O's T1 2-10-4, N&W's K3 4-8-2 and B&O's "Big Six" 2-10-2's spring to mind as locomotives that had a reputation of breaking rail.

The rocking couple is why higher speed locomotives generally had four-wheel lead trucks - more wheels to damp the oscillation. The mechanism for doing so is interesting, but that's another story.

That and other shortcomings aside, steam, as wel all know, is still > diesel....

Matt

The C&O T1 was so hard on the rails that at one place in Ohio ( I don't remember where now ) They had a track crew 24-7. Every time a T1 came through they went out to repair the track. I believe it was in a turn at the bottom of a hill. Makes one wonder, why?It must have been one he** of a valuable locomotive.
Les

Quote:Makes one wonder, why?It must have been one he** of a valuable locomotive.

Or, too tight a curve, that could not be eased due to "right of way", considering that it was "the only place" where there was a problem. It could also have been a roadbed problem, being at the bottom of a hill.