7 1/4 Dyno car

[GeorgeRay]

Julian in full size I think the only time you might expect to see a pressure drop from boiler to steam chest of 220psi would be with the regulator fairly well closed. Did you mean a steam chest pressure of 220psi? If not it is unlikey that the loco would be able to move a train with a steam chest pressure of 30 psi.
With the regulator open even on a Merchant navy at 250 psi boiler pressure the steam chest pressure on starting will rise to about 160 before things start to slip. When moving it depends on the cutoff position, being lower with a long cutoff and higher with a short one. The actual values as you would expect depend on speed and regulator opening. But on a preserved line I have seen steam chest pressures up in the 180psi. On our miniature locomotives the boiler pressure is much lower so I would expect the ratio of steam chest to boiler presssure to be similar, i.e. boiler pressure 80 steam chest pressure between 40 & 60.

[jma1009]

hi george,
my apologies for my lack of clarity...
fullsize
boiler pressure 220 psi
steamchest pressure 200psi
[figures taken by Holcroft on the LORD NELSONs]

even jim ewins only managed a maximum of 41psi steamchest pressure on his most heavily loaded test (his famous 5"g 0-6-2T had 80 psi working pressure which was maintained for the tests)

cheers,
julian

[pault]

Hi All
I may have totally missed the point here, but reading the document that Reg kindly posted the link for it seems clear to me that in Marine and probably full size railway practice a significant pressure drop was DELIBRATLY DESIGNED INTO the superheaters. When you look at full size railway locos with up to say 40 superheater elements and marine boilers with significantly more elements the only way to prevent elements burning and ensure that the superheater produces its design temperature is to balance as far as possible the flow rate through each element. This would probably be next to impossible to achieve just by clever (for clever read without restriction) header design. Failure to balance the flow could result in particular elements with low flow failing prematurely due to overheating.
The big difference with our locos is that we have very much fewer elements and very little risk of causing failure due to overheating. All the failed elements I have seen have been caused by corrosion or erosion, not overheating. When you think about the number of elements we generally have, between 1 and 5, some of them balance the flow, and the others probably are unbalanced. However since we do not suffer with the overheating issue we can make superheaters with as little pressure drop as possible. In all the running I have done to date with the dynocar I have concentrated on trying to maintain a steady speed and so have not paid much attention to the relationship between boiler and steam chest pressure. I will go through the data and look at the relationship. Next time the dynocar is out I will try and see how much I can get in the steam chest relative to boiler pressure.
We have a few locos with steam chest pressure gauges at the GCR and typically with a 3 car train most of the work is done with no more than 30 to 40psi in the steam chest. This is not because we can’t get any more in but because the loco is doing the job, and most of the locos which have the gauges fitted have pole reversers and so tend to be driven on the regulator rather than the reverser.
I am learning a lot from this
Regards
Paul

[pault]

Hi All
Ok had a quick look through the data and found a couple of interesting examples, and have done a graph of one. This was a run where I tried a few times to look at the acceleration, from a standing start, of the loco with a heavy load, on an uphill section of track. This was the third attempt and in some respects was the gentlest of the three runs primarily because the others were spoiled with slips. Admittedly the boiler and steam chest pressures are only very close for about 12 seconds however had the condition been sustained for longer it may well have stayed that way. This does suggest that superheaters in our size can function with minimal pressure drop. The boiler pressure was a bit down after the previous two drag racing starts, however as slipping was an issue the fact the boiler pressure was a bit down was not considered a big issue. Would the same thing be possible if the boiler pressure was up at the red line, don’t know yet.

Interestingly when, to date when I was design superheaters I paid attention to the cross sectional areas of the inner and outer elements to get them the same and also that the combined area of the elements was greater than the area of the regulator. Possibly wrong as far as full size practice goes but fine for miniatures.
One thing this has done one is encourage me to get cracking in the spring with more tests.

Regards
Paul

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[chris vine]

Hi Julian and Paul and all others of course,

On a general note, one thing is interesting. We were asking why you needed a pressure drop across a superheater to make it work. I was wondering about heat transfer because I assumed that was what was meant by "to make it work". It sounds as though the reason is to balance the flow through the tubes by making a high pressure pressure in the wet header so that the steam finds all the small tubes (in full size). This just shows where misunderstandings creep in! What is there is usually easy enough to see. Buy "Why" is much more difficult.

It is for this reason that I have enjoyed reading books by the old chief engineers. ES Cox, R Bond, Bowen Cooke, H Holcroft. They often tell you why they did things because they knew! A lot of books written by observers only tell you what was done.

Chapelon is interesting about draughting. His work was largely to reduce the back pressure on the pistons by clever design of the smoke box and pipework. He found that this had a much larger effect on power output and efficiency than almost anything else. his reasons were that the exhaust back pressure is acting on the whole of the piston's stroke whereas small improvements in getting steam into the cylinder at the start of the stroke only have a big effect for the first part of the stroke. But more importantly: The back pressure on the piston has to be paid for by the piston and this work is only got from the steam on the other side. And (it is a big and) if you work out the efficiency of the cylinder in converting steam to work using an indicator) you will be very lucky to see 10%. So 100 horsepower wasted in back pressure is 1000 horsepower of coal. At least!

In one example of a locomotive he rebuilt from a previous designer, he almost doubled the power output and, at the same time, nearly halved the fuel consumption.... I will try to look that bit up when I have a bit of spare time...

As for Bongo: I was aware of trying to make it (her) efficient. The following might help: Superheated, carefully lapped bores to cylinders and valves. Clupet piston rings on the valve heads. Also it is the edges of the rings which do the timing (the edges of the valve heads are machined and chamfered away). The steam pipes are large and the regulator has a maximum opening of 13mm diameter hole. The exhaust pipes are fairly generous.

One extra item fitted is a stainless steel "brick arch". I think this would have quite a large effect on efficiency because the bottom tubes (without the arch) receive flames which have not had time to burn all the gases. I understand that once the flames enter the relatively cool tubes, they are extinguished very quickly. In fact it was the discovery of the brick arch which allowed the early loco builders to use coal because it meant that the loco would now consume it's own smoke.

Another thought, but I have never done anything with it: I would love to make an engine with cylinder walls made of an insulating material, on the inside working surfaces. The problem with steam engines is that the steam comes in hot and then touches all the cooler bits of metal on the surface of the cylinders, piston ends, cylinder ends etc. All these surfaces fluctuate in temperature between that of the exhaust steam and inlet steam. The outside of the cylinder, if insulated, will be an average of this temperature.

however insulating the cylinder does very little to reduce the loss. Every time a hot bit of new steam enters the cylinder, it is cooled down by the walls. Then at the end of the stroke when it has cooled (by giving up work) it is being heated as it is being exhausted. In the old days this used to be called the missing quantity of steam as I don't think it shows up on an indicator diagram.

The situation is far worse with saturated steam than it would be with air as the working fluid. This is because as the steam cools on the walls etc, it condenses and reduces its volume and ability to do work drastically. Then, even worse, because it has contracted, it brings more steam into contact with the cool cylinder wall. It is a sort of super heat transfer situation and I think explains why steam burns to skin are so unpleasant/dangerous.

I was told that this is the reason that superheating in steam engines is so important. It is less to do with the simple thermodynamic efficiency being better with a bigger temperature difference, but largely to do with reducing condensation in the cylinder. It is much hotter so less likely to cool down to a condensing situation.

The other reason why superheating is so successful is that there is a gigantic quantity of heat required to raise water from ambient temperature to boiling and then about 3 or 4 times as much again to turn it into steam at 100 degrees C. However carefully the engine is made or designed, all this energy is completely wasted because it goes up the chimney as exhaust steam! So anything you can do to add energy to the steam (in a superheater) which enables you to get more out of the cylinder without boiling any more water is a big improvement.

I am sure I have ranted on enough now!!

Chris.

[pault]

Hi All
Chris someone, I think his name was Jack Pickup, built a pair of North Eastern 4-6-0’s one was conventional and the other was a Stumpf uniflow which IIRC has a steam jacket round the cylinders, I also think it was compound although that may incorrect. In 7 ¼ there did not seem to be any benefit, if anything the conventional one seemed the better of the two.
Julian, did Jims test loco have a brick arch?
Regards
Paul

[jma1009]

hi chris and paul,

some more very interesting posts!

chris, jim ewins was also concerned about the steam turning into wet steam or, even worse, condensate in miniature locos. paul's research and that of jim, and my own rather crude empirical way of assessing these things all go to show that this isnt a problem with superheaters of various types, so long as the flues are clear and with a fire that is hot (but at the same time isnt overloaded).

in fact jim's rather 'overboard' radiant return bend superheaters showed on test that the steam was above the saturation temperature by quite a bit when the exhaust steam temperature was measured in the blastpipe.

none of this is of any real importance if all you do is leave a loco in full gear and run on the regulator. if on the other hand you have a miniature loco with a good valvegear and want to obtain the benefits of driving with the gear 'notched up' (ie worked expansively with a short cut off) then adequate superheaters are a pre-requisite in miniature particularly in the smaller scales. unfortunately jim never mentioned the cut off or regulator opening used in his tests, and although the highest loaded test was with a tractive force of 30 lb the speed the loco would have been travelling had it not been on a static test bed was very low. paul's tests are therefore of particular interest.

in 30 years i can think of only a handful of miniature locos i have driven which can be driven with the gear notched up! those locos have always been a real pleasure to drive! not because of the fun of driving them in that way but because one hardly has to add any coal and the injectors only need using every now and then, and everything does what it is supposed to with a quiet purr from the chimney and with the fire adjusted for the softer exhaust and kept hot without being pulled to pieces. the passengers also appreciated the absence of hot coal and cinders falling on top of them in the days before we all had to fit spark arrestors!

some miniature locos with good valvegear will not respond too well to being 'notched up'... perhaps due to the reduced steam consumption leading to a 'lazy fire', or wrong draughting, or inadequate exhaust circuit. i have always felt that SIMPLEX and SUPER SIMPLEX fall into this category, or at least the ones ive driven over the years.

superheaters are therefore only one part of the equation of arriving at an efficient loco that is a pleasure to drive.

in paul's table above, boiler pressure is static. therefore there is 'no' link in the table between boiler pressure and steamchest pressure in the above test. speed in the table will be a combination i presume of regulator opening, reverser position, and gradient. can you remember paul if the gear was notched up once on the move and what adjustments would have been made to the regulator opening? i would expect that once the loco was 'on its way' the regulator would be opened further, and the gear notched up, leading to the loco being able to run freer and use much less steam, therefore causing the steamchest pressure to rise as shown in your above table?

chris, i agree about the writings of E S Cox, Roland Bond, and Harold Holcroft. my favourite is Holcroft, and my volumes of 'Locomotive Adventure' are much thumbed and covered with my own pencilled notes! that superheaters are only one part of the equation is amply demonstrated by Holcroft again and again. (he was also a friend and regular visitor of LBSC and knew all about miniature locos). it could even be argued that in fullsize superheaters were less important than in minature...after all CITY OF TRURO went over 100mph with a non-superheated boiler and slide valves! (in miniature jim ewins was of the view that heat loss and condensation in the cylinders was potentially far more of a problem than in fullsize).

cheers,
julian

[jma1009]

hi paul,

no, jim's loco didnt have a brick arch.

chris, in the smaller scales (1" and below) virtually all the heat is transfered in the firebox by radiation. miniature locos are aided by the 'cube laws' in this respect. combustion should be complete in the firebox, whether at the front or back, and the proportionately increased radiation in minature assists this process.

however, i recall the late laurie joyce's 3.5"g GWR KING winning IMLEC (i think the only time a 3.5"g loco has won) and laurie's superb loco had a stainless brick arch. whether this made any difference i dont know...it was a cracking loco in every respect!

on the subject of cylinder temperature once the cylinders have got hot (ie after starting from cold) this doesnt seem to me to be problem. the problem is preventing steam going 'wet' if it is expanded in the cylinders if the gear is notched up. this can happen even in 'hot' cylinders if the steam isnt superheated first.

cheers,
julian

[RGR 60130]

In the course of this extremely interesting thread, I'm seeing the terms Wet Steam, Dry steam and Superheated steam being used. These issues can confuse many people and I found a very simple little article which helps explain such things here:

mes2005.tripod.com/Steam_and_its_properties.pdf

Unless steam is Superheated it is going to be Wet. The big question is how wet is it? This is usually expressed as a dryness fraction, i.e. the dryer the steam, the closer to 1 the dryness fraction will be. This occurs along the line from S to T in the attached link.

Measuring the dryness fraction of the steam that is exhausted is nigh on impossible. In locomotives it will also receive a little bit of re-heat after leaving the blast pipe when it mixes with the flue gases. Looking at what is coming out of the chimney tells you more about the weather than the steam quality.

However, we don't have to worry much because by the time the steam is exhausted it won't be too far above atmospheric pressure and the temperature is going to be well above 100c so we aren't likely to start pumping water out of the stack!

Reg

[jma1009]

hi reg,

i apologise for using imprecise descriptions of types of steam... i was trying not to get too technical.

i dont agree that a driver cant tell if the steam is 'wet' from the exhaust. a great many things can be judged from making a careful note of the exhaust, regardless of the weather. anyway, that is my experience for what it's worth!

cheers,
julian

[ejparrott]

I have to agree with Julian. I've been driving so long that I tend now to drive by the emissions from the chimney, knowing what each and every state requires me to do.

Unfamiliar engines of course I have to resort to watching gauges, but regular engines I know how to read.

[chris vine]

On Bongo, I find that when working in full gear, the exhaust steam is still steamy. (dryness fraction unknown!!) However when linked up (almost to mid gear) I get quite wet.

I am assuming that this is because the steam has been expanded more and cooled.

Maybe if the run was longer and the engine could work harder, the fire would get really hot so the superheater put more heat into the steam and then I wouldn't get wet.

Perhaps we need a seriously long test track!

Chris.

[jma1009]

hi chris,

im surprised to hear that your lovely loco BONGO steams as you describe.

there could be many reasons why, and without taking the regulator myself i hesitate to speculate.

some on here will remember the late Mike Killick of the Sussex Miniature Locomotive Society (Beech Hurst, Haywards Heath). when i was 19 i finished re-building a 3.5"g MAISEE into a H2 Brighton Atlantic loco (same as currently being built at the Bluebell Railway). i offered Mike a drive, and he took the loco over from me with my usual thick fire and yellow flames with black coal with orange lumps underneath. for those who know the design it has a very shallow firebox. mine didnt have the combustion chamber. after quite awhile Mike handed the loco back to me saying it was a lovely well behaved loco etc. i resumed my position on the driving trolley and looked at the fire and lo and behold encountered the most perfect fire i had ever seen up to then or since. Mike had reduced the fire level to a thin fire and everything was incandescent with not a yellow flame in sight... in fact the flames looked purple. perfect combustion! Mike had taken some trouble to get the fire how he wanted it. i was very impressed and also noted how Mike had driven the loco in a very gentle manner and 'notched back', and nothing could be seen from the chimney. the fire wasnt 'burnt through' when handed back to me. we had a bit of a chat as he jumped on behind me as a passanger, and i learnt a great deal. the fire wasnt in that condition due to the blower being left on or the loco being worked hard... it was just plain and simple perfect firing. in fact what Mike didnt know about driving and firing miniature locos wasnt worth knowing!

we were burning soft anthracite beans of uniform size, and no dust or slack was allowed in the tender.

incidentally, Mike had a store of very funny stories about jim ewins.

cheers,
julian

[chris vine]

Hi Julian,

I had not seen it as a problem, only showing that the steam was cooler at the exhaust when well linked up. The track was an up and down line and so the fire never really gets hot, especially at the beginning of the run!

The best continuous run I ever had was at the Bristol track some years ago. We had 4 of their heavy bogie carriages with 4 men on each. It was fantastic, linking up and opening the regulator as speed picked up. Then on the back straight you could open the regulator fully and set almost full gear.

When I shut the regulator at the end of their straight section, there was a tremendous sense of momentum. I think there was about 2 or 2.5 tons travelling at quite a good speed. later they told me there was a speed limit of 10 mph!! (it was the only engine on the line).

The fire certainly got going properly then and even when linked up, I didn't seem to get wet. Or if I did, I didn't notice!!

Chris.

[pault]

Hi All
It would seem that the statement “you need pressure drop to make a superheater work” should read “you need pressure drop to make a superheater work reliably and efficiently in full or large size” I have found on a number of occasions that people take principles theories methods from the big railway, especially if they have worked on the big railway, and try to apply them directly to our locos. Whilst this can provide a good starting point, consideration should be given to treating it as an engineering problem in its own right.

I would have to disagree to some extent with Julian’s statement that “none of this is of any real importance if all you do is leave a loco in full gear and run on the regulator.” Whilst if you are looking for the best efficiency you can get from a loco you will need to notch it back. However if the loco has a pole reverser the chances are that the best you can do is a combination of reverser and regulator. Very often with pole reversers the difference between notches is too great for the regulator not to be touched when notching up or down. As a result on a hilly railway very often it is easier just to run on the regulator. Whilst the engine is not been driven at its most economical, the savings created by superheating are still equally applicable. We have a number of locos which if you don’t actually get stopped by signals can complete a whole trip with the regulator wide open, controlling the loco totally on the reverser. These trips are generally quiet, very smooth and relaxing to drive. By the same token sometimes it is fun to put the engineer in you to one side, get on a loud unrefined loco, put a heavy freight train (no passengers) behind it and blast round making lots of noise.

some miniature locos with good valvegear will not respond too well to being 'notched up'... perhaps due to the reduced steam consumption leading to a 'lazy fire', or wrong draughting, or inadequate exhaust circuit. i have always felt that SIMPLEX and SUPER SIMPLEX fall into this category, or at least the ones ive driven over the years.

Julian raises an interesting point with the above paragraph. Quite often when new locos have come to the GCR we have had problems with the way they steam, this can be either not well or too well. We then “tune” the blast nozzle to achieve the balance we need for the job we want the engine to do. In days gone by we have also altered blast nozzles when we had deliveries of coal which had significantly different characteristics to the previous load. The nozzle sizes shown on drawings should be regarded as a starting point rather than a definitive size. If the fuel used is different to the fuel the loco was designed for a different nozzle size may be needed. Likewise a short steep up and down railway, a flat circular railway and a long undulating railway may require different nozzle sizes to make a loco steam properly.

During the 3 acceleration runs there was no finesse involved. I was curious see how quickly a heavy train could be accelerated. On the first 2 runs the loco slipped which obviously put a dent in the performance. As a result on the third one (the one shown above) I was gentler with the regulator to make sure that there was no slipping. Since performance not economy was the objective the loco was left in full gear throughout with the regulator being used for control. The step changes in steam chest pressure are as a result of the regulator being opened more when I was sure the loco would not slip. The reason the speed had dropped and did not increase with full regulator (when the steam chest and boiler pressure were the same) was that the train was on a curve where the train binds.

I would also agree that the writings of the old and in some cases not so old steam engineers are informative and often a good starting point for mods to our little engines. I met Roland Bond at the GCR and A. B. Macleod who was a member there, was known to the kids at the GCR as Uncle Mac. Unfortunately I was about 14 at the time and did not really know who they were, oh to be able to wind back the clock.

I would agree that in some ways superheating could be said to be less important in full size, but then again it depends on how you look at it. In full size superheating was primarily about economy not increasing performance. I would agree with Jim Ewins was of the view that heat loss and condensation in the cylinders was potentially far more of a problem than in full size which would have an effect on performance.

With regard to Bongo, Chris you do not have a problem with the loco. Driving at the GCR on the first trip of the day you open the regulator to get rolling from the start. After about 60/70 feet you can shut the regulator and roll down the first bank. At the bottom of the bank you open the regulator for the first climb and wet fluffy clouds come out of the chimney. As the climb progresses the exhaust clears as the superheat increases. As can be seen on some of the previous graphs on the loco used superheat increased over about 3 minutes, whilst the loco was working. When you notch back more of the heat energy in the steam is given up to the cylinders, resulting in the exhaust temperature decreasing and some of the steam being condensed. This is especially true if the superheat temperature is low.

You don’t say if Bongo has radiant elements, however if she does they may well not be giving as much superheat as they could. Most of the increase in temperature created by radiant elements is as the name implies created by heat radiated by the fire. The brick arch would shield the elements from a significant amount of the radiated heat so reducing the superheat temperature.

Whilst brick arches were important in obtaining complete combustion in full size they never had to my knowledge (on railway locos) had radiant superheaters. It could be that the brick arch is improving things by promoting better combustion, having its effect cancelled out by reduced superheat, or reducing overall efficiency due to lower superheat. I think your right you need a longer run

Food for thought

Regards
Paul

[Deleted]

Looks like Mike at Station Road Steam is treading a slightly parallel path with this gadget.
JB

www.stationroadsteam.co.uk/indicator/index.htm