Here is a list of all the postings Bob Worsley has made in our forums. Click on a thread name to jump to the thread.
|Thread: Design of boilers|
What a dismal response!
More people put to sleep than commented, and as for waking up the brain cells, forget it. It is perfectly obvious that the scale boilers are not very good, been made for years, like the Ford model T, but things do move on. Simply reading books on physics and suggestions fly of the page, steam bubble size for instance. Discussion in another thread about boiler staying, well, just read books on bending of beams, side stays could be regarded as a long girder, or as an edge restrained disc. All it needs is a bit of maths.
Ok, how much would someone charge to make four 1.1/2" Allchin boilers, all to different specifications of tube count, dimensions etc? I will provide materials other than silver solder and flux.
I don't have any facilities to make them, or I would, and with current happenings not certain when I would get facilities organised.
Too long, had to split it.
The point of these is to change the heating area from the fire tubes to a radiant type heater at the top of the firebox. I can find no information about the water temperature gradient within the boiler from above the firebox to just behind the tube plate. Whilst the pressure has to be constant, the rate of boiling doesn’t. Imagine a boiler three feet long, the water temperature will quite definitely drop substantially from one end of the fire tubes to the other.
The third flue has a similar stainless tube arrangement but this is a superheater. If a superheater isn’t wanted then can either have all three flues with the secondary circulation heater, or even just two larger flues.
The secondary circulation heaters are in circuit all the time, the superheater is only used when drawing steam from the boiler. The tube plate will need moving backwards to give room for the plumbing.
Another possibility is to fix solid copper bars to the front of the firebox so they run along inside the bottom of the barrel. If of reasonable size, 1/2” or more square, then conduction will give as much heating as the flues they replace would. Similarly a combustion chamber is just a complicated nuisance, enlarge the flues inside the barrel. With the internal secondary circulation system and the much larger flues then should still heat ok. It is all to do with water circulation around the firebox.
If that wasn’t enough, consider the Stefan-Boltzmann law. This is the power lost due to radiation from surfaces at higher temperatures than the environment.
This is power loss in watts = area in sq cm x 5.735.10^-12 Stefan constant x (work temperature ^4 - ambient temperature ^4). The important detail here is that the temperatures are in K, and that they are to the fourth power. This means that at high temperatures the losses are enormous.
To give an example, at 730C, 1000K, and an environment at 27C, 300K, what size plate can just be kept at that temperature with a 10kW gas torch?
10,000 / (5,735E-12.9.919E11) = 10,000 / 5.69 = 1757 sq cm = 42cm sq
So 10kW will maintain a 16” square copper plate at 730C, assuming the coke bed it is resting on is at the same temperature so no loss on the other side.
This of course assumes that 100% of the heat from the gas torch actually heats the plate, in reality it will be somewhat less.
This is why soldering a boiler 24” long is such a struggle. I am still working on using induction heating, also using a 10kW gas radiant heater under the boiler as background heat. Possibility of resistive heating but the high amperage connection is a puzzle.
More importantly it suggests that it would be better to reduce the size of a boiler by 1 cm all round and replace the missing size by insulation. Also that making a taper boiler is a waste of time, fill the taper under the cladding with insulation.
If really keen then can work out the heat flow from an incandescent fire through the firebox walls and what is then available for the water to absorb. If the water gap around the firebox is increased to allow better circulation then the foundation ring could become a source of weakness. Might need to stop using a solid bar and flange instead, give a bit more flexibility.
A firebox for a 3.1/2” loco obviously can be built and works, what needs testing is whether that size fitted in a 5” loco is an improvement, or not, or can’t tell the difference.
Something to think about at the moment?
I quite realise that this isn’t how it has always been done, but I have never seen any of these type of calculations in the magazines and books. The radiant heat loss equation I had seen before, but the real effect of it had never become apparent, it is a killer.
Edited By JasonB on 18/01/2021 11:29:16
Been reading books again. Going through my collection of old engineering books looking to see what to keep and what to chuck. There is quite some interesting reading in the later half of the 19C in treatises, encyclopaedias and single volumes.
How they made boilers in those days, for marine, locomotive and factory type uses. Commonly all steel shell with brass fire tubes and copper firebox.
It is a recurrent comment that the boiler has too little space around the tubes, there needs to be a gap of at least 1/2” between the tubes. The tubes can be only 1.5/8” outside diameter. This gap won’t allow the ebullition (what a wonderful word!) or boiling of the water to pass upwards. Priming also seems to be a problem, possibly due to the poor steam flow past the tubes. The flow comes in fits and starts, and carries water into the steam space.
In a model boiler since scale water is unobtainable then perhaps this minimum spacing of 1/2” should also be used? Looking in Model Locomotive Boilers by Evans and the 3.1/2” Jubilee boiler has 3/8”diameter flues on 1/2” centres for example, all the other boilers in the book are pretty similar. This leaves just 1/8” space to allow both the steam to rise and water to descend. Simple observation in a boiling kettle points out that the steam bubbles produced on the tube, or element, are much larger than 1/16”. Whether the size of the bubble reduces as the steam pressure increases I have not been able to determine, but rather doubt it. If the metal is too hot then putting water on it just makes the water turn into spheres and skate around. This could be a problem where the firebox is really hot, the sphere floats on a layer of steam?
Even worse is the space given to the circulation of steam and water on the sides of the firebox. Again from the book this gap is only 3/8” on 3.1/2” Jubilee, 1/2” or 7/16” for a 5” 2-6-4 and 3/4” on a 7.1/4” Highlander. From the old books it is reckoned that 3/4 of the steam raised is done by the firebox. And that shortening the tubes by a 1/4 actually increases the amount of steam raised.
So perhaps the firebox needs to be made quite a bit smaller to allow at least 1/2” water space, even on a 3.1/2” loco? This would also apply to traction engines since their boilers are very similar?
The problem is that the fire has to burn red hot, so that the coal starts to gassify and then burn. A black fire achieves nothing, lots of practise with coal fires and Rayburns! This means that the fire is around 700C, and the copper conducts this to the water almost immediately. It is very necessary for the eater to wet the firebox in order to accept this heat and then a very fast flow of boiling water taken upwards. A vigorous constant circulation of water down and steam up is essential.
Perhaps the cramming in of lots of tubes also needs to stop? Why not just put a very few, three, large diameter tubes at the top of the firebox and one row of smaller ones under if room? So for the 2-6-4 you would have three 1.1/4” or even 1.3/8” flues with perhaps eight 3/4” flues in a zig-zag pattern underneath? There are no flues in the bottom two rows as currently described. For the Jubilee with a much smaller barrel just three large flues at the top, none at all underneath. Also reduce the length of the flues to give room in the smokebox.
What this gives is plenty of space for both a superheater and secondary circulation heater in the flues.
Two of these large diameter flues have a type of stainless steel superheater. Can’t immediately find an example to quote but called a spear superheater? The firebox end is formed form a block of stainless shaped like a spear point. But these heaters are plumbed so that one end goes into a bush at the bottom of the tube plate, the other end a similar bush right at the top of the boiler in the steam space. If these flues are 1” diameter then can fit a 3/8” heater, if 1.1/4” diameter then 1/2” heater. These heaters are the normal there and back type with a return block brazed or welded on. These can be continued inside the firebox, possibly with the tubes arranged vertically to minimise the shadowing of the crown by the heaters.
There will be a mix of water and steam in these tubes, and I would assume that as the water is turned into steam then is would flow upwards to emerge in the steam space. It is unlikely that the steam have a choice of upwards against steam or downwards against water will always choose the easier option. This is why they need to be as large as possible, reduce the flow friction. Perhaps smaller boilers, 3.1/2”, would only need one very large flue tube to get 3/4” secondary tubes in. The superheater is different in that there is a pressure difference due to the flow of steam so the diameter isn’t so important.
|Thread: Clayton wagon valve gear|
Is this the right forum? Did search and all of them seemed to be in All Itens.
The Clayton wagon used a rather unusual valve gear which was replaced by Robin Dyer by Joy, with a follow up article by Bernard Lundberg suggesting using the original gear. I only have part one of his article, July 1988, so not certain if he actually made it.
Doing a search through my hoarded copies of old engineering magazines and came across an article on valve gears in the Practical Mechanic and Engineer's Magazine for 1846 page 271.
This describes it as Mr Dodds valve gear and used by Robert Stephenson on his locomotives. Only half a page with a coupe of woodcuts but undoubtedly the same method.
Bit obscure, and not mentioned by BL so might be new, worth making a note of just in case.
|Thread: Brushless motor question|
A brushless motor is one where electronics does the current switching in the stator, not a commutator in the rotor, or armature. The electronics senses where the armature is and energizes the relevant windings. Saves lots of money.
There is a disadvantage to this that isn't often mentioned. If the controller is too energetic in putting current through the stator windings then it will noticeably demagnetise the permanent magnet rotor. With an old commutator motor any amount of current can be put through the armature and field because the only disadvantage is magnetic saturation. When you stop the saturation stops. So if your brushless controller stops the rotor instantly then by definition a lot of magnetic flux has been created to counter the rotating inertia and magnetic flux, and this will demagnetise the rotor magnet. This is probably why they say turn speed to zero before starting or stopping, they know if they don't then there will be endless complaints about the motor lost its power.
All permanent magnets have a remanence. When magnetised part of the procedure is to demagnetise it back to a known point, this is the remanence. By putting more and more reverse magnetic flux through the permanent magnet and the remanence point will slowly move down the curve closer and closer to zero.
Exactly the same occurs with hand tools. If drilling say and it snatches and stops then the motor current will peak, and demagnetise the rotor magnet. A decent make will hopefully catch this current rise in time. Similarly if running a cordless drill hard for long periods then this too will demagnetise the rotor. I don't have a brushless drill but wonder what happens if you connect one with a brushed drill and turn both on, which one wins, which one heats up?
I would have thought that all these type of controllers have a soft start and ramp up and down time? Obviously not.
|Thread: Heating copper boilers|
You are making the mistake of equating the heat from the gas torch to what is needed to heat the boiler. My calculations above, assuming zero losses, suggest a vastly lower heat requirement. The problem with gas is that 99%, or so, of the heat is basically wasted by the flame only being used to heat the boiler for a second or so. It is then blown around the brazing hearth heating everything else. This is why oxy acetylene is so useful, the flame is only 2kW or so, but it is used where it is needed.
Just been reading about induction heating, and this does seem a far more sensible route. Can buy 2kW heaters for £2k, and the heat generated is within the metal, not blowing around your workshop.
Anyone any practical experience of induction heating? People like Sealey seem to sell self contained heaters for taking oxygen sensors out of exhaust pipes.
Not too enthusiastic response, seems no one has ever tried it other than Ed, thanks.
Yes, there will be lots of amps needed, but was just thinking of kW input to see what the figures looked like. Change welding transformer to spot welding transformer. Or just strip the secondary off and replace with just a couple of turns or so. The important thing is the kVA rating of the laminations to carry the power continually. There is a low power requirement, easily managed by a domestic 100A supply, the shower takes far more. It is this low power requirement compared to a gas torch that is the attraction.
With the boiler almost totally shrouded with insulation and with the excellent heat conduction of copper I would expect the whole boiler to be at a similar temperature, possibly only 50C from one end to the other. This solves many of the temperature coefficient problems with connecting the power, but it will need multiple points with 2000A.
I have lots of high power resistors, possibly use them as a hot bed to indirectly heat the boiler but getting back to the gas torch problem. Has anyone used these gas radiant heaters as typically sold by Machine Mart to act as a hot bed? There is a 10kW one, couple of those with thermal shrouding might work.
What did surprise me when I looked in RS was the absence of any high temperature cables, insulation and similar, about 350C maximum, far too low. The idea of using the ceramic cases from fuses has been duly filched from another forum.
Charcoal was recommended as a bedding material in the fire as being clean burning?
Also one of the boiler books said dropping steel clamps in the pickle is fine. Remember being warned never to do that, meant the pickle needed to be changed?
Another 'why do we do it this way' posts to antagonise and irritate the members of this forum.
Looking at something like a 2" scale traction engine, or probably a 5" loco.
These boilers are large and heavy, and require enormous amounts of heat to reliably silver solder, so need to heat the whole thing to 700C. The whole thing ends up at 700C radiating that temperature.
The problem seems to be that the gas flame is used to heat the boiler, but once the flame has impinged on the boiler it is then redundant. This redundant flame is still at 2000C so is going on to heat up everything around other than the boiler. Talking to people and stories of setting fire to the shed roof, or anything else, are not rare.
What is needed is a heat source that is in the copper. What I suggest is resistance heating using a normal arc welder.
The advantages are that there are no flames, and the whole boiler can be closely insulated to keep the heat in the metal.
Will it work? Copper has a specific heat capacity of 0.38J/g K. So to raise a 1kg boiler by 600C takes 0.38x1000gx600C = 228kJ. 1J = 1W/s so this is 3.8kW for 60s, or one minute.
A real boiler would be 20kg and 700C = 5.32MJ or 5kW for 18 min. This of course assumes no heat loss.
You would need a decent sized, or more than one, welder and because it is slow then need to run them at their 100% rating. Looking through Machine Mart the concept of continuous duty rating seems to have gone. You would also need to use one of the old transformer based welders, no silly electronics saying to turn off.
How to use them? On the barrel to firebox ring joint would need a clamp around the barrel for one lead and two clamps, one either side of the wrapper for the other lead. The current flow will then be all through the ring joint and down the throatplate. Boiler will need moving to do the top and bottom, but need to do that with gas anyway. Just also need to shift the insulating cover first.
The power leads will get hot, 700C, so not your average rubber coating, needs to be glass cloth or something similar. Because the welder is only 30V or so there is no electrical hazard, so bare wire might be ok.
It is also feasible to use some high power resistance wire or resistors under the boiler as background heat. This could be used with a gas torch as well.
Drawbacks? Will be a couple of large cables, taking 200A at 5kW, so quite stiff. Will need some heavy duty clamps for connecting, with AC looking at about 10-15A sq mm connecting area. Clamps possibly need to be connecting to a sheet rather than around the boiler with thermal expansion.
An alternative is induction heating, would be even better but a 5kW induction heater is probably rather expensive. The old transformer based welders are two a penny at sales.
The normal welder used would be AC, so to reduce losses the power cables need to be tightly twisted together, also minimum length. If DC then these worried don't apply. Also modify the welder to fit a number of old computer fans to cool the transformer.
|Thread: DRO errors, or are they??|
Zero DRO, grab hold of the toolpost and then as hard as possible push it in, record reading, pull it out, record reading.
Seems to me that the screw or nut are loose somewhere?
|Thread: Is Plastow 44T 6D.P bevel gear - 3" Fowler|
This is a general query about bevel gears.
I ave been looking at these for use in traction engines, but wonder why they have to be bevel gears, won't the contrate (as in Meccano gears) type work fine? Ok, not correct to scale, but much easier to cut, and not as if 5rpm is going to worry them. Hidden away inside the differential so not visible anyway.
|Thread: Advice required|
The end of the tube belled out?
Easiest is to just use a centre punch and tap it with a hammer. The secret of success lies in making a clamp for the tube by clamping two pieces of steel together and drilling through the join with the outside diameter of the tube. I have only done larger diameters but the hole the same size as the od will grip enough. If necessary put a tiny bell mouth on the hole so the tube splays out.
Holding steel doesn't need to be more than 1/2" wide, I used 1.1/2" by 1/2" but was forming tube up to 1/2" diameter.
For a closing in forming use the same tube clamp but hit it with a Vee shaped hole in the end of a bar, I used a centre drill but you will need a busted one reground.
|Thread: capacitance in long cables|
Why not use chips specifically designed to drive and cope with long distance cables?
The obvious choice are the RS232 1488 and 1489 devices. Feed power along two wires and signal on the third.
|Thread: Is there any tips for sawing aluminium|
If cutting with something toothed then must use a lubricant to stop it welding to the tooth tip, WD40 is fine.
Any stone angle grinder disc will happily cut aluminium, steel will not work. I know because I cut a CVRT aluminium tank in half to shift it to the scrappy.
The Evolution metal cutting saws are brilliant, the Clarke ones aren't.
|Thread: Copper boiler plate flanging, or not?|
Spoke to Reeves, I will try to get some drawings from Haining Steam Models to see what he says.
I have got the Alec Farmer book, but he swears by oxy-acetylene heating, I am not so sure. For soldering the stays OA seems ideal, tight flame and do them one at a time, but still have a 3300 deg C flame.
Don't see that it matters if the rivets bend in the middle, boiler pressure never going to straighten them so they will continue to stay the plates. Besides only 3/8"" or so between the plates so pretty stiff, not much more than the diameter.
Can't find my gas torch, still not got a hearth, progress is slow.
Interesting tests on boilers. Soft solder doesn't seem a good option, but silver solder seems to take almost anything.
Any more? Seems an expensive hobby even at £4 kg scrap value!
Yes, as Jason says, poke rivet through to the outside, cut a cross in the end, heat to red to anneal, hit with suitably shaped punch to flatted down the four quadrants, silver solder.
Many boilers use rivets but just solder them on the outside, thought that splaying them would provide a similar mechanical hold a a nut.
Just an idea. One day might be able to get along to a club, but not at the moment.
That drawing is exactly what I was expecting from Reeves, but no, just telling me to return it in an undamaged condition. Snag is they use thick paper from a shredder and it isn't easy to disinter the parts from this paper. Was missing the firehole ring, found it in the wheely bin.
No other progress either, trying to contact Tyler Steam Models to ask about drawings, buy a new set if necessary, but no response to email or phone call.
That photo of the BB boiler is impressive, built and safe! Would probably pass all the safety tests, simply because the copper and solder do make a good joint.
As to the rivet stays, they are 3/16" diameter, what I wondered was cutting a cross in the end, using a four way chisel to splay the ends out flat and then soldering. Different to threading and nuts, but the volume of copper mechanically holding the stay is the same. End of rivet will need to be annealed prior to splaying. Any thoughts?
JB, don't be worried, I look, read and think. I missed that throatplate overlap probably because at that moment I didn't see it as being important which way round it was made, you corrected that, thanks. But I will say that continuing reading the ME and issue 3781 has the Waverly boiler with this joint the other way round, Martin Evans.
I mentioned I had contacted Reeves about the modified BB boiler. Not much of a reply. But the boiler as they supplied needs a joint ring inside the boiler to join the barrel to the wrapper. This ring is flat material 16" long by 3/4" by 3mm. So will go all the way around the boiler. Being 3/4" gives 3/8" on barrel and wrapper. The water gap between firebox front and throatplate is 3/8". So the ring completely blocks this? I have asked what I am supposed to do. Similarly the side stays are provided as copper rivets, what am I supposed to do with them, different to drawing.
Yes, propane can melt copper, but it can't boil/vapourise it like an OA torch can.
SoD makes some important points about old, out of date, builders or designs? I like copper for boilers though, its cost is only part of the cost of the model, but still less than a miniscule holiday.
Thanks to all again.
|Thread: Is a hand scraper pulled or pushed?|
To answer Paul, no, I didn't do the scraping.
At the time, near 35 years ago, I had a friend who rebuilt and mended machine tools, he did it. It was on a Hassison L5A, which I still have and use, bought from Rotagrip. It wouldn't turn anything. Undid the few capscrews to remove the top of the saddle, bit of marking blue, front left, under the chuck, side of the saddle Vee ways no blue for 2-3 inches. Scaped the centre of the Vee ways, only contacts at the ends for 2" or so. I think he used a power scraper but only took minutes to get it done, blued and checked. End result was amazing, could turn to a size, repeatedly.
The difference I see between push and pull scraping is that pushing is digging into the metal, pulling isn't. With a 90 degree end, held at 45 degrees then the angle of the scraper to work is the same. Just seems more controllable pulling.
|Thread: 1.0mm 2 flute cutter HSS or Carbide.|
Slitting saw or a couple of hacksaw blades side by side, power hacksaw blade?
|Thread: Is a hand scraper pulled or pushed?|
As is usual, after posting this, sat down with an ME off the pile to read and an article about scraping, 15 Jan 1988 issue 3817.
Their scraper, made from an old flat file, the end has the teeth removed and then slightly hollow ground. Use the periphery of the grindstone to put a slight hollow right at the very cutting end of the file. Grind the end with a slight curve and finally stone on an oilstone.
In use the description is "The tool is drawn steadily across the work with slight pressure applied to enable the cutting edge to make the cut.". My immediate reaction was 'ah, it is pulled'.
In use the scraper seems to be held at about 45 degrees to the work, so the 90 degree end face is at the same angle to the work if pulled or pushed.
Must say that pulling makes more sense if you have a handle the length of a chair leg, 20" or so, plus the length of the old file, 12" or so. Internal scrapers can only be pushed but they are hollow ground and the trailing edge touching sets the cutting angle.
Another problem solved! Thanks to everyone who read and contributed.
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