Is there any significant difference in temperatures of a steam locomotives exhaust whether it be an oil burner or coal burner? I know that during warmer months and climates the smoke can linger for quite a while while in colder areas and times of the year, the smoke can start out black but then change over to a more whiter shade of gray in a relatively short amount of time. Is this a "cooling trend"? I have witnessed first hand steam doing the same thing as well. Can saturated (or unsaturated) steam hang out for a longer period of time in colder air rather than warmer?

The temperature of the exhaust is a function of how hard the engine is working, regardless of the fuel being burned.

Smoke is smoke, and always stays smoke colored (brown or black). What turns gray to white in the smoke plume in colder weather is the exhaust steam that goes out the stack mixed with the smoke.

You can see the steam in the air during colder temperatures for the same reason that you can see your breath during colder temperatures. The colder air is the less moisture it can absorb. Once the air is saturated, what’s left stays as steam (actually water vapor) until it finally gets absorbed into the surrounding air.

Here in the oh-so-humid east, you can see the steam in the smoke trail even during warm weather if the humidity is high, again, because the air is close enough to the saturation point that it can’t absorb what the locomotive is putting out, at least not right away.

Kelly's said most of it, although I would pedantically note that according to what I was taught, any "steam" you can see should be called 'water vapor' instead. The white color is nucleate condensation, the same effect that hard-limits expansion even in the best-insulated or -jacketed cylinders...

Historical sources indicate that oil firing produces higher gas temperatures at points on the rear tubeplate. That superheater element ends are at greater risk when a locomotive is oil-fired is almost a truism. Matt Austin is a voice we should hear from with respect to this general subject.

Regardless of firing method, there is excess heat in the exhaust gas above what the exhaust steam (at its effective back pressure saturation temperature) produces.

I too am interested in the pyrometer question: not only 'when', but where'. I have seen them used in firebox corners to gauge radiant heat release, and I have seen them used to determine superheat temperature. In steam days, a pyrometer movement would have been a comparatively delicate and expensive mechanism, and not many roads would likely have put the money there instead of on other improvements.

Why would an everyday fireman need a pyrometer? If he's building water and making steam when the locomotive is working hard, it is obvious that the fire is hot enough. Extra gauges will not tell the fireman anything he does not already know about his fire.

Pyrometers would only be used for testing a locomotive with a dynamometer car otherwise it something else to maintain.

Back to lingering steam exhaust. Our saturated oil burner, in 40 degree damp weather, and working hard leaves a heavy contrail that hangs for quite some time behind the train. Our superheated oil burner under the same conditions does not leave the same heavy contrail.

Johnathon

The exhaust of a locomotive forms a "cloud" in the same manner as regular clouds are formed. The water vapor condenses to liquid water at the local dew point of the surrounding air. A superheated locomotive will leave a smaller exhaust signature than a saturated locomotive for two reasons. First, superheating puts more energy into the steam thus requiring LESS steam for a given amount of work. and second, the exhaust is almost certainly at a higher temperature than the exhaust of the saturated engine which would allow the steam to disperse more before it reached the dew point of the surrounding air and condensed to form the cloud of water droplets that we see as steam.

Greetings:
The 1944 Locomotive Cyclopedia, page 363, Fig.D has an illustration and description of the "Elesco Superheated Steam Pyrometer".
In the same Cyclopedia, page 500, Fig.6.03 has a photo of an NYC Hudson listing a Superheater Pyrometer. Unfortunately, the devise is obscured by a cab gauge light fixture so we can't tell if it is an Elesco or another brand.
I recall that NYC 3001 (displayed at Elkhart, IN) is/was equipped with an Elesco.
J.David

OK....you have to do an amount of work that requires X number of BTUs of energy to perform. If all you gain by superheating is a hotter exhaust without having used those BTUs to do the work you have not gained anything by superheating. If you use superheated steam more expansively and reduce its exhaust temperature to that of the saturated steam also expended in an otherwise identical locomotive pulling and identical train on the same line, then you have gained efficiency. If you're not using your heat more efficiently, why pay for the maintenance of the superheaters?

dave

The other concept at play here is efficient firing and type of coal used.
If you fresh fired an engine with new coal you may spout off a lot of black smoke, but once you get the heat up and start effectively burning the coal properly you can get minimal dark smoke, the less dark smoke the more efficient you are burning the coal.

The cab photos I have seen where an Elesco superheated steam pyrometer is used show the gauge on the engineer's side. Keeping the superheat up was more a function of proper throttle and valve setting than anything the fireman was doing. That of course is when the fireman is doing his job correctly.

The exhaust creates a vacuum effect in the smokebox forcing air through the boiler and thru the burning coal, the design is all about moving heated air effectively and heat transferring to create steam. Fireman doesnt control the exhaust. He does have a steam vent thru the stack to force steam up to create air flow when the engine is idle.

Dave,

Let me see if I can contribute on this subject. Extracting work in a thermodynamic cycle is often dependent on the quality of the energy and the physical state of the matter. If there is a small temperature difference between the energized matter and the surroundings it is technically difficult to extract the energy (think extremely large and expensive heat exchangers). Also if the available energy is "locked" in a state of matter, it may be there but not extractable.

In the specific case of superheated steam expanding in a cylinder, the steam is not only losing pressure, but is cooling down. When it crosses the saturated vapor line, the temperature remains constant, but a percentage of the steam mass which entered the cylinder begins to condense. Work cannot be extracted from the "water" thus formed. However, work can be extracted from the remaining fraction of steam which is why a saturated locomotive can still pull trains.

In steam turbine power plant design, the presence of condensed water not only represents lost work potential, but the entrained water droplets will actually pit the turbine blades. Accordingly, power plant designers use the Mollier diagram to carefully pick the boiler pressure and degree of superheat of the incoming steam so that the steam exiting the turbine is very near to the saturated vapor line and thus nearly all steam.

This is why many compound locomotive designs performed poorly. The steam exiting the first set of cylinders was already at or very close to the saturated line so there was much condensation going on in the low pressure cylinders and not much work. Superheating helps a lot as it moves the steam further away from the saturation line in a direction that is better for keeping the expanded steam away from starting to condense. Simply raising the pressure is not enough to do the job because of the shape of the "steam dome" or saturation curve. Power plant designers thus began to employ "reheat". With reheat, the steam expands through several turbine stages until it is close to the saturated vapor line. It is then diverted to another set of superheater units before being expanded to its end pressure in another set of turbine stages.

Reheat was tried on some compound locomotives, such as the infamous Santa Fe mallets of the early 1900s, but I suspect poor overall detailed design rendered the reheat ineffective.

The most successful implementation of reheat on locomotives that I am aware of was done by Chapelon, most notably on his 2-12-0 rebuild.

So, thanks to the two-phase region in the "steam dome" on the Mollier diagram where H20 can coexist as both liquid and vapor, it is actually better if the exhaust steam has some superheat in it. This is actually not wasted in that the drafting system can make use of that energy to produce smokebox vacuum (assuming the nozzle, mixer and diffuser are well designed of course).

Thank you Wolf; your explanation makes a few things clear that I hadn't understood based on the problems with compounding you mention. I do see your point in a theoretical perspective, but also agree that it wasn't often achieved in real world steam railroading, especially in the US.

I wonder of the steam jacketing of the cylinders isn't worth taking a closer look at to diminish the temperature differential between the steam and cylinder wall? Don't know if it was ever done other than on Red Devil, or how easily traditional existing cylinders could be retrofit. Hell, at what critical mass of use is it even worth trying?

I do not believe the limiting factor in steam as we know it is the ability to produce exhaust vacuum pressures more than adequately using present day evolutions of Lempor and other ejectors from the perspective of the quality of the fire. Once you start to remove unburnt fuel from the firebox, further vacuum is a loss of the heat value found in the consumption of the fuel and the waste of the energy used to remove it from a useful place in the system. We could do this even with the Master Mechanic's front end easily, but at the cost of the higher back pressure. I'm thinking about the juggling act between the force of the vacuum being held at the point of good combustion while limiting the loss of fuel out the stack, and using the efficiency of the ejector more to reduce back pressure than force products of combustion - and all this in a constantly variable state.

headache.....going away now

dave

Dave,

Chapelon introduced steam jacketing on the 2-12-0 I mentioned. It was designed to be a slow speed drag engine, a type of service where the condensation problems were particularly problematic...the other being intermittent switching service. Besides offsetting cylinder condensation, the jackets would have pre-warmed the cylinder block and in effect insulated the cylinder walls from ambient affects. If I remember correctly, Porta once did a study which indicated that it took about 20 minutes of heavy working to get the mass of metal in the cylinder block up to a steady state temperature. Your mileage will vary according to size, design, steam temps, and ambient conditions. So from that perspective, steam jacketing would be useful in preventing condensation during that initial start-up period as the cold cylinder walls would be playing a larger role.

While I think steam jackets could certainly be implemented by welding on some steel and using saturated boiler steam [a reasonable effect could be achieved by steam insulating the ends of the cylinder (not to be confused with the cylinder heads) so that stays would not be needed], I suspect a reasonable level of improvement could be achieved by doing an intelligent job on heavily insulating the cylinder walls and heads. An approach much more readily accomplished by a typical preservation operation.

Keep in mind that with a Lempor applied to a GPCS firebox, the much smaller percentage of air entering from under the fire means that fire lifting is minimized.

As applied to a conventional firebox, one would proportion the Lempor to achieve whatever smokebox vacuum was appropriate for good fire keeping and enjoy lower back pressure. Another option would be to increase the percentage of exhaust steam going to a feedwater heating system. Yet another would be to use roughened or finned tubes (with higher flow resistance and thus a higher draft requirement) to make the boiler more efficient...careful attention to design would assure that the available vacuum in the firebox would again be appropriate to not lift the fire.

The Stephensonian steam locomotive is such an integrated system that the designer really needs to pay attention to the impacts of a change on one part as it can impact others...this is why it is so important for a steam locomotive designer to really understand the theoretical as well as the practical...

Chapelon's approach on the 160 A1 was a little more interesting than described: the full steam flow going to the steam chests passed through the jacketing volume. This gave the cylinder block a higher temperature than would have been the case with exhaust-steam jacketing.

Wardale rather conclusively dismissed exhaust-steam jackets in favor of good cylinder insulation, for a number of reasons. The supposed advantages of the steam jackets are ougweighed by maintenance requirements, and of course they do not relieve the need for cylinder-cock opening at starting, outsize relief discs to prevent cylinder-head damage, etc.

My own suggestion for this (on high-performance locomotives) involves circulating boiler water (saturated at whatever the available boiler pressure determines) through tracer lines with heat-pipe sinks around the cylinder block, with modern insulation around as much of the block as possible. This involves much less bulk than effective steam jacketing, provides much higher heat transfer, and warms the cylinders up proportional with boiler water-temperature rise during fire-up (and can maintain cylinder temperature during stops and idling regardless of 'steam demand').

There's a difference between nucleate condensation (which occurs in the volume of the steam) and wall condensation. The heating of the cylinder block does not address the whole physics of wall condensation -- note that at even moderate cyclic rpm, only the surface .007" of the cylinder wall metal cycles between intake and exhaust steam-temperature extremes, so additional heat transfer from the bulk of the metal won't help the situation. What does help is the absolute temperature of the metal on the "block side" of that .007", and having this at full boiler-pressure saturation is significant compared to the temperature achievable from steam at sensible exhaust pressure (and remember that this temperature will drop with improved back-pressure from better front-end design).

The nucleate condensation is, in my opinion, the principal reason for the importance of higher superheat; the longer the superheat can avoid phase change in the mass of expanding steam, the longer the steam will act in expansion as a gas, and not start losing pressure as 'heat' is abstracted in the process of doing work on the piston. (As a peripheral note, this also controls how short a 'practical' cutoff a steam engine can use, regardless of the technical precision of its valve gear.)

Remember also the characteristics of exhaust steam in a high-speed engine. Nucleate condensation is quickly reversible with changing pressure, accounting for the large relative volume increase (useless for any effective locomotive purpose!) upon exhaust release. At low pressure but significant required mass flow, the exhaust plenum must be made propoortionally larger, and exhaust passages need to be carefully sized or flow speeds may easily rise into the transonic range, which as David Wardale notes is Not Good from a flow-shock perspective. In my opinion there are all sorts of unexpected shock fronts in a front end, and sensible CFD models might identify them... and I think careful attention to this needs to be part of sensible design and modeling.

The frustration of being an English major with a substantial practical background trying to understand the theoretical aspects, which only seem to be understood in a static but not dynamic state........

I appreciate all the good technical commentary, and am working on digesting it. Not that I'll ever design a new locomotive from the back of the envelope out, but it helps me think about making our existing locomotives more sustainable, which is going to become more and more important.

dave