Was there ever a study done to find out what the maximum length of a boiler could be before it looses it's heat to steam conversion effectiveness.

I know there were some steam traction engines that made "wet bottom" design boilers, in which there was a water jacket all the way around the firebox so as to maximize the heating of the water / steam generation. I have actually felt around the bottoms of the "firebox" area on some of these water bottom style boilers, and the water is still cold / luke warm, even after the engine has been fired up for a while and / or has been moving or working. Are the flue gas temps on these articulated loco's really hot enough to convert the water into steam at the smokebox end?

Mark

Removing superheaters from the equation, the key thing you would hope to achieve as best as possible is for the flue gases to leave the stack the same temperature of the steam inside of the boiler. The steam inside of the boiler is a set temperature based off the saturation point of the water at the operating pressure (ie. the point where water and vapor exist together). The temperature in the flues can't drop below that temperature or else the boiler would start heating the flue gas. If you let the smoke head up the stack any warmer then the water in the boiler you are wasting energy. Now this is with in reason, since heat flows from hot to cold, you will have your best heat absorption with the greatest temperature difference.

At least this is what I would think. As for the length and volume of the flow through the tubes, all of these should be rather standard straight forward calculation from any college heat transfer text book, for a "tube in shell heat exchanger" as far as I know. I guess the only variable that could make things interesting is if any additional combustion takes place in the tubes themselves.

I could be wrong.

Adam

There have been numerous studies done on the effects of tube length vs diameter and the associated heat absorbtion rates.
Two of the more interesting are those published by Cole in 1914, referred to as "The Cole Ratios", and research done by a German engineer, Wagner.
There is a definite relationship between the length to diameter ratio, commonly referred to as the A/S ratio. The main problem becomes the length that can be self supporting without infringing on the holding power of the tube to tube sheet joint. Some of the longest are recorded as 24 feet, however, in most cases they seldom went beyond 22 feet.
The whole point was centered on how much steam would be generated per square foot of tube surface, which becomes fairly clear to be less as the gases give up their heat on the way out of the boiler.
Some of the more interesting information on this is found in reviewing the locomotive test reports from the Altoona Plant----some high temperatures in the smokebox when all the"corks" are pulled out on all out testing.
R.P Johnson's book brings most of the knowledge(up to that time) together in one place in his remarks on evaporation (1942).
A very interesting area of research.
Respectfully'
Dave Griner
Wasatch Railroad Contractors

Besides the thermal considerations, you must also take into account friction in the tubes. The longer the tube, the more friction you have to fight to get the gases/cinders to the smokebox. This requires a stronger draft and so more back pressure in the cylinders.

This was one of the advantages of Garratt type locomotives over conventional articulateds. Garratt's have short fat boilers and so lots of short tubes. For the same surface area of tubes as a conventional articulated you get better heat transfer with less friction.

Roger

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David,

Would it have been possible for them to put in maybe a second tube sheet that has holes unoccupied by tubes to allow for water flow and even possibly behave as a set of baffles to cut down on boiler surge effects also or would this have screwed things up too much for the fireman watching the water level?

Adam

Another point worth mentioning here is that if you "extract" too much heat from the flue gases, the moisture in them begins to condense which leads to severe corrosion problems. The sulfur in the exhaust gases forms sulfuric acid which can eat your front flue sheet and smokebox components. This can be a problem with industrial and heating boilers.

IIRC this was a problem with the Italian Franco-Crosti locomotive boilers. These boilers had a long firetube heat exchanger mounted under or on each side of the boiler barrel that pre-heated the feedwater before it entered the main boiler. The exhaust gases exited the smokebox of an otherwise conventional locomotive boiler, then made a 180 degree turn and traveled back towards the firebox through the supplementary heat exchanger(s). The exhaust stack(s) were right in front of the cab. While these engines were more efficient, they ran into significant problems due to condensation of the moisture in the flue gases. I believe some of them used "enameled" tubes (like old time cooking utensils) to try to combat this.

Hello Adam,
The problem with internal supports is that the tube "bundle" spacing between tubes is usually not more than 1 inch. Hence, any support is going to act as a baffel and impede circulation. There is a definite flow pattern of the water and it wants it's freedom, especially in locomotive boilers. This type of boiler provides some of the highest steaming rates vs unit volume, again requiring unimpeded circulation.
Other types of pressure vessels use baffels, albeit applying different flow characteristics.
Respectfully,
Dave Griner
Wasatch Railroad Contractors

WOW! Thank you all! Very insightfull and very helpfull!

Mark

The answer to this is very complex.

Small boilers use A/S ratios of 1/350 where a large boiler can be as much as 1/500. What is critical is the overal gas flow areas through the boiler relative to the heating serface. Mentioned in the thread is both extrems. neither are ideal but can be balanced with care in draughting the boiler. What is realy significant is keeping within the prescribed limits for gas flow areas. If a locomotive falls out side these then its all but imposible to correct or make a balance. What realy throws the whole situation out is if the boiler is scaled up due to lack of proper boiler water treatment then its all but irelivent what you do for any proportions.

I noticed after exploring C&O 1601's vacuous firebox that the rear tubesheet is not located where the firebox ends, there is almost a course without tubes. Therefore the length of the tubes are not as long as you would imagine on an Allegheny.

This is called a combustion chamber. A big open space where hot gases have a chance to keep burning before being quenched in the tubes.