Discussion in 'Steam Traction' started by ragl, Feb 19, 2016.
There is a short video here of AT&SF 2926's re-fitted extendable stack in action.
Could you get one of these on a King and have it retract for oncoming bridges via a GPS App...
I have found the pictures of files of the first chimney I built. The first shows how I got the point of aim wrong with it not impinging on the wall fully. The line of aim was at the half radius. The results from this was so disappointing that they where ignored and anyway at the time I only had a manometer.
Move on a year or two and I had realised what I had done wrong. Opertunities and materials where at a premium. The solution was to shorten the mixing chamber and add a cowling. The cowling was designed to add the 7.25 impingement angle on the flat cowl surfaces and then on to the mixing chamber wall. The first time I had to test it was on SMR4. At the time the locos boiler was in its last year and the boiler pressure was reduced from 200psi to 150. I struggled to make the engine to keep time and steam. Fitting what I now had which was a Kylpor radically improved the engine. It steamed sufficiently to keep time and cope with the higher demand of the lower working pressure. The power was improved back to where it was with the full boiler pressure.
This is the graph that shows the before and after. I by that time had a back pressure gauge and using the info I had obtained set the tests up to match the method used by Porta And Wardale. There. Graph had comparative systems included for reference. The basic conclusion was as Porta said that the fraught could be up to 100% more than it was originally. I look at these results with the hind sight of knowing that the glancing impingement on the mixing chamber wall or cowling makes this major gain. So from a set of drawrings of one Lempor I learnt through practical tests why boundary layer effects where so important.
Can you please explain how you measured the blast pressure? I am very intrigued by this. I would be very interested in as much detail as you can supply, please.
The back pressure in the exhaust steam pipe is measured slightly below the blast pipe you make a small fitting with a turned down entrance facing into the oncoming steam. This is abou 1/3 radius from the wall of the exhaust pipe. (See picture below.) From there and out of the smoke box you take a pipe to where your gauge will be. The gauge is ushaily up to 25 psi. A lower reading one is convenient for the bottom end of results but an engine worked hard and with crude nozzle profiles will exceed even 25 psi. In the fitting below the gauge I place one or maybe two small discs of shim with just a small puncture hole in them to act as a damper on the pulsing of the exhaust.
The vacuum is measured with a manometer. This consists of a length of plastic tubing is attached to a plank marked of in half inches. The tube is bent in a u shape and filled with water. I add one or two drops of food colouring to aid readings. The vacuum is measured by the difrence between the two water levels in the tube thus the half inch difrence becomes 1". I have made many manometers over the decades. Some have been up to 4 or five feet tall. Others are short and the pipes at an angle. The flatter angle of the pipe means a finer reading can be had on small vacuums. These are then connected to the smoke box wall or where ever you want the reading.
A word about piping. The pressure gauge I ushaily do in copper but in some instances of bad draughting smoke boxes are so hot that it will burn the copper, so I substituted stainless or steel. The manometer is ushaily plastic but the last section is copper due to the heat.
I am away from home so I don't have the full acsses to my pictures but below are the cab manometer and gauge on an engine and the exhaust pressure fitting in the exhaust pipe.
Although by today's modern technology like computers and pressure sensors this method is basic it is simple and cheep to do. It also provides a direct comparison to what our predecessors did thus giving a very good comparison to their work. If you want more info just ask.
Seeing the post about the star nozzle reminds me of 4960 on the Grand Canyon railroad. We had done a Lempor for 29 and the was time to spare so we did 4960. This had a star nozzle. We built it a Lempor and made mods to the fire pan so sufficient air could get in. The picture below shows the nozzle being fabricated. There was a lot of stream lining done internally which is not visable in the photo.
Like most Lempor improvements the vacuum was doubled for a given back pressure and the loco would steam better than before. In the end we switched fuel to vegetable oil from waste and diesel.
Many thanks for your detailed reply and pics which are very interesting.
The back pressure measurement only measures the velocity pressure. That is correct if the cross sections of the blast pipe including the orifices are of a constant area, the static pressure would be zero. However, if the orifice(s) have a smaller area there is a static pressure that should be added to the velocity pressure to get the total back pressure which is used in the graph. So question: is the orifice area smaller or equal to that of the blast pipe?
I wonder if you could enlarge a little on your highest back pressures and where your lowest smokebox vacuums were.
What might have been the highest pressures in the exhaust if there ever were a number over 25 psi:
quite exceptionally high ones would be interesting but what I would really ask is if there might be any number at about 30 psi or a bit more?
Do the low smoke box vacuums seem to be associated with any particular type of boilers that do not need much draught?
This might of course be the grate or the fuel or the ash pan rather than the boiler itself.
I'd be much obliged for any pointers, particularly if any boilers steamed well but did not need much vacuum at the front end.
Some of the high back pressures are tests to find the limits, not nesicarily within the normal working range. An engine working over 12 inches of vacuum is unusual. What is significant is that at 12psi the terminal speed of sound is reached for steam. To exceed this the delavel converging - diverging nozzles are needed to prevent the terminal conditions thus allowing a continuous rise in the vacuum. If this is not done you will get a decreasing vacuum with an increase in back pressure. Additionally with this increase in back pressure the ability to do work is reduced. An engine which has a restrictive nozzle which dose not steam will not nesicarily steam more if the nozzle size is reduced as the back pressure will limit the expansion posible in the cylinder before release. To gain the work wanted it would thus require more steam to be produced and the ever decreasing cycle of limited return continues. The rules of extracting power out of a steam engine apply just the same as that of the internal combustion engine. This is why such things as the design of the exhaust pipes should follow these rules also. It has not been touched here but the Abt engines where we replaced the exhaust pipes made a significant gain to the whole picture. On another engine such issues of bad design of the exhaust system rendered the Lempor ineffective because what was happening at the cylinders was not reflected at the blast pipe base.
The gauge shown is what was made avalibe and its upper limit is not relivent in this context.
Although the test gear above has limitations like the position of the back pressure fitting which is at a compromised position at 1/3 radius from the wall of the exhaust pipe. It is the standard point that has been used by many elite steam engineers in the past, it gives a resnable comparison between the difrent systems. There is no doupt that you could use much more modern and much more expensive equipment and in difrent position in the system, but they where not avalibe when their work was done, a consequence of evolution. Money is always an issue and weather you want to get your engine working better or just obtain an endless stream of data for what ever. Given the variations which could affect the results like the streamlining of ether pipe work, the relative bore of the pipe work, the form of the nozzles, the cutoff used and so on the use of the above method gives a resnable comparison between systems. The best is to do back to back tests of the old systems from single nozzle systems to the new system. Even for that the results gains or losses can be clearly seen and then the absolute detail of testing becomes less relivent. What is significant with my work is that the back to back results show on average a 100% increase in draughting for a given back pressure when fitted with a chimney of the Kylchap or Lempor family.
In designing a draughting system you can play around with the proportions to allow for the fuel, boiler proportions and the relative steam demand of the cylinders. The two extremes are a large blast pipe size with a large mixing chamber. Hence a low back pressure and a high gas flow but low efecentcy and very free steaming. The second alternative is if the mixing chamber is proportional smaller then you will have a higher back pressure to force the gasses through the smaller mixing chamber. Highly efcient but you can reach a draughting limit before the grate or boiler heating surface limits. The ideal middle compromise is the best to aim for. As an aside oil firing has a major effect on the draughting required to over come the steam worked burner ushaily used so a highly efcient draughting system like a Lempor becomes vital. Again with the allowance having to be made even for the evaporation of the atomising steam.
If to clarify a few points. I can't say that I have ever seen a back pressure of 30 psi or over. The working vacuum is proportional to the evaporative rate and the boiler proportions. I can't say that one boiler requires a very low vacuum compered to another. Curtainly the grate area to the gas flow area through the tubes is important. The larger the grate the lower the combustion rate is on the fire but that dose not directly effect the design of the draughting which is more determined but the heating area of the boiler and gas flow area of the tube bundle for a given evaporative rate.
I've just been reading up about the recent work on the Island of Sodor. They've been trying some interesting modifications but no results yet ...
Here is my latest Lempor. A smaller than average one. Note the new exhaust pipes which are stream lined to give higher velocities and lower back pressures.
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