I’m hoping this will be an interesting topic. I was just looking at my NYCSHS calender for the month of February. This month the picture is of a LS&MS Class C-44 4-4-0, built by Pittsburgh Locomotive Works in 1891. The 4-4-0 is outfitted with a fairly tall smokestack, which I’m estimating at about 6’.
My question: Why did steam locomotive smokestacks start out tall then gradually get shorter and shorter (and I assume also larger in OD) as locomotives increased in size? It seems to me that if smoke became more and more of an issue on later steam (hence the need for smoke lifters), why wouldn’t you just make the smokestack taller? Would a taller smokestack effect backpressue?
For railroads like the NYC, there were height restrictions so that locomotives and rolling stock could clear tunnels and bridges. However, this wasn’t the case for all railroads.
Anyhow, thanks for the discussion and the education.
I can think of a couple of reasons but they should be considered opinion not fact. Early engines were starting points and probaby smoked more. The engines themsellves were shorter allowing taller stacks. Considerable work was done with steam exhaustt nozzles to increase draft resulting in greater efficiency.
Look at it this way. The top of the smokestack stayed at the same height and the engine came up to meet it. The smokestack didn’t get shorter, the engines got taller.
Actually yes it was. ALL railroads have height restrictions Some were a little taller than others but ALL railroads have overhead structures (tunnels, overpasses, coaling towers, through truss bridges, engine house doors, signal bridges, etc). That’s why there are various clearance “plates” .
For railroads like the NYC, there were height restrictions so that locomotives and rolling stock could clear tunnels and bridges. However, this wasn’t the case for all railroads.
Sorry, Dave. I did realize that. Poor choice of words on my part. [:$]
Thanks everyone for taking the time to answer. [:D]
Smoke may have been an issue. That 6’ above the boiler stack may have been pushing the clearance. But the real issue would be DRAFT. The taller the stack, the better the draft thru the fire box and flues. Since early steam was ‘low’ pressure (under 150#), there was less steam at lower pressure going up the stack, hence less draft. 'Modern" steam at 300# + pressure was pushing a lot higher pressure steam and a greater volume up the stack, creating much more draft, and eliminating the need for a taller stack to make the draft.
Height + draft is the reason for the TALL stacks on many industries.
Back before he was exiled to Siberia, Mark Newton made an interesting observation.
The visible part of the stack, above the top of the firebox, is the tip of the iceberg. Where 19th century locomotives had the whole length of the venturi that creates draft out in the air, modern steam has most of it inside the smokebox. The total length of the venturi isn’t much different - just how much of it can be seen.
The need for smoke lifters is driven by reduced pressure at the steam jets that create draft. Early locos, and some that were never really refined, blasted the smoke through the venturi - and thereby wasted steam pressure which would have been better used pushing the pistons back and forth. Modern locos have had their venturis and nozzles designed to get more draft with less steam pressure, at the expense of having the exhaust cling to the boiler rather than lift away. Smoke lifters solve that problem by enlisting the loco’s forward motion to propel the smoke upward with a mass of lifted air.
Chuck (Modeling Central Japan in September, 1964 - where steam locos have low-pressure exhausts and smoke lifters)
You probably remember that Santa Fe and maybe some others had extendable stacks on some of their larger engines.I always assumed the purpose was to raise the smoke a couple of extra feet, but maybe there was more to it. The discussion about draft and venturi may apply as well.
There was a lot of spark arresting gear inside the tall stacks especially the wood burners. The big switch to coal saw a lot of experimentation by the rail roads. Some roads took to Anthracite and some used Bituminous coal. Each burned and ashed differently. Matching the smoke box/stack to the fire box for the fuel they used was a big expensive experimentation until the roads found the economies and performance they were looking for.
Whole thesis’ could be written about smoke box devlopment and improvement in efficiencies. Trains had an article years ago about a Giesel smokestack that increased efficiency. Cinders in the smoke box could affect draft also.
This isn’t directly related, but if you’re interested in the history of certain words / terms…in the 19th century it was found that steamboats with taller stacks had better draft and so steamed much better. This was particularly true on the Mississippi River, which in many places was surrounded by high bluffs. Many steamboats had tall dual stacks, and folks observing the tall pair of smokestacks would make comments along the line of “she’s very well stacked”. This term over time came to have a slang usage as a positive reference to female anatomy.
I am familiar with “blowers” as a steam jet (or jets) directed up the stack to induce draft, but I’ve never seen a fan in the stack… I would assume you mean a mechanically driven fan with blades to force draft? Could you provide a link to any references? Just curious what they looked like.
On the other hand, I have seen various stacks - such as the Radley-Hunter - that used baffles in various configurations to help separate the cinders from the exhaust by centrifugal force or other means. However, such baffles tended to impede draft rather than help it. This image is from http://www.saunalahti.fi/~animato/3003/3003ah.html
The South African Railways “condenser” locos had an exhaust turbine powered stack blower. The exhaust steam was then routed to condensers in the tender to be reclaimed - there was no water on the route where they were used.
The Kiso Forest Railway used ash separators of similar design (originally industrial cyclones, IIRC) that were bigger in diameter than the boilers of the little Baldwin 0-4-2Ts. They burned cedar, and optimum draft took second place to avoiding forest fires in an area where the 762mm gauge rails were the ONLY access. The Japanese-made cyclones were a meter in diameter, the American-made boilers were only 36 inches…
Chuck (Modeling Central Japan in September, 1964 - including the Kiso Rintetsu under a psuedonym)
Chuck, once I read the post about the condensor locos it did jog my memory. So Mark, even though you may have inadvertently used another term, there is a prototype with a fan forcing the draft… and if I had a dollar for every time I used the wrong word, I would be retired by now…[swg]
