jma1009
Elder Statesman
Posts: 5,901
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Post by jma1009 on Jan 15, 2020 20:54:28 GMT
Roger visited today for a most enjoyable time, and matters relating to miniature and fullsize injectors were discussed in some detail, in line with Roger's post above.
Some of Roger's ideas I am sure I can find in fullsize injectors of years gone by, especially where patent infringement was concerned, and in the USA injectors especially. I am now sat here reviewing what we discussed and what points could have been discussed further if we had further time today.
Cheers,
Julian
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Post by Roger on Jan 15, 2020 21:13:10 GMT
Roger visited today for a most enjoyable time, and matters relating to miniature and fullsize injectors were discussed in some detail, in line with Roger's post above. Some of Roger's ideas I am sure I can find in fullsize injectors of years gone by, especially where patent infringement was concerned, and in the USA injectors especially. I am now sat here reviewing what we discussed and what points could have been discussed further if we had further time today. Cheers, Julian Hi Julian, Many thanks for a splendid lunch and as much chat as can reasonably be had in such a short time. It was really helpful to see a variety of cones and reamers and get a handle on just how fine the reamers are. That's one mighty big folder of notes, a treasure trove if you know what you're doing! It was also good to see STEPNEY, a lovely locomotive, very nicely done indeed. I'm very much looking forward to seeing some progress on that front. Hopefully we'll get some time in the summer for another get together.
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Post by Oily Rag on Jan 15, 2020 21:42:35 GMT
I think I need to see printed drawings to get my noggin around this
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Post by Roger on Jan 15, 2020 21:47:14 GMT
I think I need to see printed drawings to get my noggin around this Hi Dazza, You're right, that's what it needs. You can use all the right terminology but nothing beats a visual representation. When I get a minute, I'll make a few diagrams to insert into that description to show what each stage in the design process controls. It's far from obvious, but one dimension or geometric constraint drives the next decision until you've got it all laid out. There isn't much room for manouver, it only works in a fairly narrow range of sizes and geometry.
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Post by Deleted on Jan 15, 2020 22:05:38 GMT
I admire you guys for getting to grips with injectors and understanding them. I will make my own but alas won't try too hard to understand their inner workings...a drawing with some explanation of what's what is good enough for me. DAG Brown a d Bob's tomes will be my guide, hopefully, they are enough... Pete
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Post by Roger on Jan 15, 2020 22:33:42 GMT
I admire you guys for getting to grips with injectors and understanding them. I will make my own but alas won't try too hard to understand their inner workings...a drawing with some explanation of what's what is good enough for me. DAG Brown a d Bob's tomes will be my guide, hopefully, they are enough... Pete When the penny drops, you can see how it's sized from the graph, and the lift is set by a simple ratio of two throat sizes. The ball valve is only needed to stop air getting drawn in. The combining cone is split to get the ejector throat and allow excess steam and water out which won't make it through to the delivery cone
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Post by Deleted on Jan 15, 2020 22:37:07 GMT
I admire you guys for getting to grips with injectors and understanding them. I will make my own but alas won't try too hard to understand their inner workings...a drawing with some explanation of what's what is good enough for me. DAG Brown a d Bob's tomes will be my guide, hopefully, they are enough... Pete When the penny drops, you can see how it's sized from the graph, and the lift is set by a simple ratio of two throat sizes. The ball valve is only needed to stop air getting drawn in. The combining cone is split to get the ejector throat and allow excess steam and water out which won't make it through to the delivery cone Cheers Roger...I'm sure that things will become much clearer once I start making them, I tend to pick things up pretty quick once my mind is concentrating on a subject... Pete
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Post by Oily Rag on Jan 16, 2020 19:59:54 GMT
When one hears or reads that such and such a design that some one has made does not work well, I some times wonder if it is the details of the design or the fault lies in the execution of making the item ? I suspect injectors often fall into this question. Any how, I like the idea of making my own injectors one day soon and this thread will be most interesting for me. I am also interested in how small a working injector can be made ??
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Post by Roger on Jan 16, 2020 20:25:01 GMT
Ok, this is my interpretation of how to design an injector using Bob Bramson's book The Blue cone is an imaginary construction guideline to help explain why the throats are where they are. You look up the Combining, Delivery and later the Steam cone throat diameters from the graph he shows on Page 19 Fig 20 for the delivery and pressure you want. Immediately things are confusing in the book because the Combining cone is actually in two halves on our typical injectors, with a gap between them. The large diameter end, not shown in this diagram, is called the Condensing cone, and the one shown here is called the Mixing cone. If would have been less confusing if the graph used the term Mixing cone rather than Combining cone. The key thing to take from this diagram is the fact that the 9 degree blue cone controls how far to the left or right the throat diameters sit. It's like putting two different sized rings on a traffic cone. So just by drawing that 9 degree cone and putting those two diameters from the table as shown, you've already got the start of the design. The Delivery cone will need a lead in Bell mouth to the left of the throat, and this will reduce the gap, but not to less than the figure show. The exact distance between the two cones shown isn't critical. I believe the figures in the book refer to the distance between the throats. The only way to get those closer together is to have a slightly shallower angle than the 9 degrees shown. The length of the Delivery cone is between 10-12 times the diameter of the throat. The length of the Mixing cone isn't defined yet. We can't figure that out until we get the information from Step 2 Anyway, this is the business end where we've got a fully formed flow of water and condensed steam shooting out of the Mixing cone and providing enough pressure in the Delivery cone to overcome the boiler pressure. Obviously the velocity of the water slows down inside the Delivery cone as the diameter increases. Note:- ALL of the Steam needs to be condensed, there should be no Steam and no water coming out of a perfectly working injector. Steam is elastic, so any bubbles take up space and reduce the momentum of the water jet. This is what happens when you have hot feed water. The feed water is then not cold enough to fully condense the Steam and the injector stops working. A small amount of air is drawn into the gap between these two cones and is carried into the boiler. This is why some injectors have a non-return valve on the overflow to prevent this from happening. The gap shown here goes to the overflow. Step 1 by The train Man, on Flickr
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Post by John Baguley on Jan 16, 2020 21:17:14 GMT
I am also interested in how small a working injector can be made ?? When I was messing about playing with injectors (some years ago now!) I made a 4oz one that worked quite well. I had it on my Helen Long for a few years but it was too small really. It took ages to put any amount of water in the boiler. I made a few 8oz ones as well, one of which is still on the loco. That's about the right size for a 2½" gauge loco but the small ones are very susceptable to any dirt etc. in the feed water and you have to keep them spotlessly clean.
I've often thought if there is any benefit in using a harder material for the cones such as bronze or even stainless steel. Brass is easier to work with but brass steam cones seem to get eroded away after a time. Some of the ones I have looked at for people have been really bad. Mind you, that could be partially down to over enthusiastic cleaning with acid and people poking them out with bits of wire!
John
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Post by Roger on Jan 16, 2020 22:25:09 GMT
I am also interested in how small a working injector can be made ?? When I was messing about playing with injectors (some years ago now!) I made a 4oz one that worked quite well. I had it on my Helen Long for a few years but it was too small really. It took ages to put any amount of water in the boiler. I made a few 8oz ones as well, one of which is still on the loco. That's about the right size for a 2½" gauge loco but the small ones are very susceptable to any dirt etc. in the feed water and you have to keep them spotlessly clean.
I've often thought if there is any benefit in using a harder material for the cones such as bronze or even stainless steel. Brass is easier to work with but brass steam cones seem to get eroded away after a time. Some of the ones I have looked at for people have been really bad. Mind you, that could be partially down to over enthusiastic cleaning with acid and people poking them out with bits of wire!
John
Hi John, D.A.G Brown's book shows a 4oz one, but Bob Bramson says pretty much the same as you about them. I saw a Steam cone at Julian's yesterday that was dezincified and like foil on the edge. This is another reason to use End Regulation or at least something other than the usual Annular Regulation. I guess there's no reason why you can't make them from Leaded Bronze SAE660 which you can get down to about 1/2" diameter easily enough. Ok, that's on the big side, but I'm sure it's available in slightly smaller sizes if you want to hunt some down. I'm very curious to know if you can make them out of PEEK which can handle a continuous service temperature of 260C and is cheap in small diameters. A plastic such as this may prevent limescale from adhering to its surface and it ought to be dead easy to machine. I'm probably going to make mine out of that to see if it works.
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jma1009
Elder Statesman
Posts: 5,901
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Post by jma1009 on Jan 17, 2020 0:08:36 GMT
It is a very difficult subject and with so many interpretations...
People over pickling in citric acid injectors to clean them and destroying the innards...
Roger was shown a startling extreme example of this sent to me that was a 'bin job'. On the other hand, I did stress to Roger that I had an excellent Linden injector that was some 70 years old and still works perfectly albeit with a new steam cone of Bob Bramson's design.
The problem with over pickling injector innards to clean them, and applying a pair of pliers to remove the delivery cone and steam cone is just way off my radar for all this!
I think the first stage in all this is to make accurate reamers that cut keenly and are dead sharp and with a dead smooth finish... I've seen Roger's Jones Shipman grinding machine, and if anyone can make dead sharp accurate reamers of the required tapers accurately then it would be Roger!
Cheers,
Julain
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Post by John Baguley on Jan 17, 2020 0:20:32 GMT
I'm very curious to know if you can make them out of PEEK which can handle a continuous service temperature of 260C and is cheap in small diameters. A plastic such as this may prevent limescale from adhering to its surface and it ought to be dead easy to machine. I'm probably going to make mine out of that to see if it works. That's an interesting idea Roger and definitely worth looking at.
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Post by John Baguley on Jan 17, 2020 0:26:16 GMT
I think the first stage in all this is to make accurate reamers that cut keenly and are dead sharp and with a dead smooth finish... I've seen Roger's Jones Shipman grinding machine, and if anyone can make dead sharp accurate reamers of the required tapers accurately then it would be Roger! Cheers, Julain Quite agree Julian. My reamers were far from perfect. I've now got a Quorn T&C grinder so should be able to do better next time. That's assuming that I can work out how to use it properly. I tried grinding an endmill on it and eventually gave up as I couldn't get the right angle on the cutting edge! Definitely need more practice!
I'm surprised that injector reamers have not been available commercially (maybe they were?). There's definitely a market there for somebody.
John
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Post by ilvaporista on Jan 17, 2020 4:12:01 GMT
Whilst I am always in favour of experimentation and pushing the boundaries of our knowledge I would add from personal experience to take care with creep in plastic components. PEEK is better than some but still will exhibit creep over time. This accelerates at high temperatures and with high humidity, both of which we find inside an injector. Creep is what led to the end of the mass production plastic throttle body used on the K series engine. And partially lead to the closure of SU of carbs. fame after many years of operation. I am now super sensitive to the use of plastics where dimensional accuracy over time is critical to successful operation.
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Post by Roger on Jan 17, 2020 20:07:11 GMT
Whilst I am always in favour of experimentation and pushing the boundaries of our knowledge I would add from personal experience to take care with creep in plastic components. PEEK is better than some but still will exhibit creep over time. This accelerates at high temperatures and with high humidity, both of which we find inside an injector. Creep is what led to the end of the mass production plastic throttle body used on the K series engine. And partially lead to the closure of SU of carbs. fame after many years of operation. I am now super sensitive to the use of plastics where dimensional accuracy over time is critical to successful operation. Duly noted, it's certainly something to be carefully considered. My gut feeling is that PEEK would probably not work in the conventional design with separate cones. My concern is mainly that they might not stay where they were put in the body, and that would be hopeless. However, all of the cones with the exception of the Steam cone could be made in one piece, then I don't think creep or small changes in dimensions would be enough to stop it from working. We know from the published material that there's a bit of leeway in the dimensions, so as long as we start somewhere in the middle of the acceptable dimensions and geometry, I think it's likely to be ok. In the end, all of this is speculation until someone bites the bullet and tries it.
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Post by Roger on Jan 17, 2020 21:01:20 GMT
Step 1 defined most of the delivery end but we couldn't define the length of the Mixing cone at that stage. We still won't be able to do that here, but we are working towards that goal. So here we've looked up the Steam cone throat diameter from Bob Bramson's Fig 20 on Page 19 from the same delivery figure of course. There are a few things to note here. The entry of Steam to the throat is not critical, but it's advisable to have a small parallel section. You don't need the long tapered lead in shown in D.A.G Brown's book. The second thing to notice is the fact that the Steam nozzle is Divergent, not Convergent. This is a similar setup we're familiar with seeing on Rocket nozzles, and although it seems counter intuitive, the velocity actually increases as the nozzle diverges! That's because we're dealing with a compressible fluid. As I understand it, the flow before the throat increases to reach Mach 1 (the speed of sound) at the throat and then continues to increase after the throat. A thermodynamicist might like to comment and put me straight on this, but the fact remains that the velocity increases after the throat has been passed. For the moment, forget about the gap and the length of the nozzles and just consider the ratio of the Steam and Condensing throat diameters. Bob Bramson's book tells us that we need to keep the ratio between 1.38 and 1.45 The reason for this is that we want to create enough suction between the two cones shown here to draw in the water. Just in case it's not clear, the water will surround the outside of the Steam cone and is drawn into the gap between the two cones. Water can't get past the outside of the Condensing cone because it's in a housing. For convenience, Bob Bramson usually makes the diameter of the Steam cone outlet the same as the Condensing cone throat. This now defines the length of the Steam cone. I've drawn in the Condensing cone at an approximate length, we'll return to that later. Obviously, the longer we make the Condensing cone, the larger its inlet end will be. We can now set an approximate gap of say 0.15mm between the two cones. The exact length of this gap is usually set by trial and error, the purpose being to regulate the flow ie to prevent more water passing into the cones than can exit the other end. That's because any excess is just going to go out of the overflow and be wasted. It may well also prevent the injector from picking up in the first place. I'm sure you've all had to back off the water feed at some point to get it to pick up, and that's what this is designed to avoid. I'll return to this topic later with more discussion about regulation in general. So let's now imagine what's going to happen if we fire up this injector. Firstly, the water is turned on and it will dribble through the gap and out through the Condensing cone and onwards to eventually end up at the overflow, so long as the tank is above the injector. Either way, when the steam is turned on, this part of the injector will work as an Ejector, causing water to be drawn into the gap between the cones. The water and steam now mix and ALL of the steam should condense to water. However, the initial flow will be chaotic and there will be incomplete condensing of the steam. So a chaotic mixture of water and some uncondensed steam will exit the Condensing cone, and that's a problem. The chaotic mixture of water and steam exceeds the volume that can pass through the Mixing cone throat, which is downstream from this. However, as it stands, the Mixing cone isn't shown here yet, and that's because we're going to leave a gap for all this excess volume to escape. Step 2 by The train Man, on Flickr
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Post by Oily Rag on Jan 17, 2020 23:33:58 GMT
Second best thread ever
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Post by Roger on Jan 18, 2020 10:08:14 GMT
In the first two steps we've set some things that cannot be changed, such as the position of the two throats shown here in Red. The length of the smaller Mixing cone is decided by the size of the gap between the two cones. This needs to be large enough to allow the excess steam and water to escape during starting, but also has to leave the Mixing cone long enough to control the flow through to the throat. Clearly if the gap was huge, the water would have to jump an enormous gap, and then enter a very short Mixing cone. It's not hard to see why that's not going to work. So the idea is to make the gap big enough to do the job of letting the excess steam and water escape but no more. It turns out from practical experiment that in small injectors the cones need to be pretty much the same length. Step 3 by The train Man, on Flickr
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Post by Roger on Jan 18, 2020 11:13:20 GMT
Finally we can put the pieces together and tidy up. The lead in on the Delivery cone extends the length left of the throat, we need to leave that where it is. You can't go too far with this because we don't want to compromise the gap which needs to be within the parameters set earlier. Let's talk a bit about regulation, because I think this area has been starved of development. First of all, what is regulation and why do we need it? The design of the injector starts with deciding what delivery flow rate we want. We'd like to only draw as much water into the injector from the supply as we deliver, without wasting it out of the overflow. Remember that we fully condense the steam into water, so there's a little bit more volume added by that. In an unregulated injector, the ejector stage at the beginning is capable of delivering more than this, and the excess will go to waste through the gap between the Condensing and Mixing cones. The idea is just to restrict the flow of water somehow so that this doesn't happen. Too much water might also prevent the injector from picking up, presumably because there's so much water and steam trying to get through the gap that it's impossible to establish a nice cone of water. Almost all the injectors in miniature appear to follow the designs in D.A.G Brown's book. This is Annular regulation, and consists of making the end of the Steam cone enter inside the Condensing cone. By moving the Steam cone in or out, the Annular gap which the water has to pass through is made larger or smaller. One advantage of this arrangement is that a large Axial adjustment has a smaller effect on the gap due to the taper. However, it does mean the the cone has a wafer thin edge, and the gap may be uneven if the cones are not very accurately made. The alternative arrangement used by Bob Bramson, and shown in my diagrams, uses a flat nozed Steam cone where the regulation gap is set by the fine adjustment of that gap. The advantage is that you lose the wafer thin nose of the Steam cone, but at the expense of more sensitive adjustment. However, I'm of the opinion that a third option ought to be explored. My suggestion is that the End regulation style of Steam cone is used, but the gap be set deliberatly on the large side, say 0.15-0.2mm. The idea is that this is enough to create a working ejector stage but not to work as a regulation gap because it's too big to do that job. I suggest that restriction orifice in the water inlet to the injector be used to regulate the flow instead. This can be something as simple as a Stainless Steel grub screw with a hole drilled through it, and a chamfer or curved lead in to break the sharp edge. Changing the regulation then becomes the simple matter of fitting a grub screw with a different hole. We've all seen the Ball valve on top of the injector, so why is that there? If you look at the diagram below, you can see I've drawn in the possible entry points for Air into the system. The valve usually fitted is to prevent the air entering on the left of the two places shown. The injector will still work without it to some degree, but it will draw air into the boiler and reduce the momentum of the water column. Less momentum will prevent the injector from overcoming the boiler pressure as easily so it may drop out. Air entering on the right of the two positions is ignored on most designs, presumably because its effect is less problematical, although stopping air getting into Steel boiler is a good reason to add a second non-return valve to that position too. Indeed, this is what's done on some designs, and it appears to also allow the injector to work when the feed water is at a slightly higher temperature. Step 4 by The train Man, on Flickr
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