7 1/4 Dyno car

[jma1009]

hi paul,

apologies for the delay but here is the info requested re the late jim ewins' tests.

the thermocouple measuring the firebed temperture was inserted through a hole in a plate covering the firehole, and appears to be sited in/on the firebed. lowest reading was 1160 degrees celsius with no load, and highest (with 30 lb tractive force) 1600 degrees C.

your firebox temperature thermocouple is sited approximately i think where jim measured the temperature of the gases as they entered the tubes, which jim measured at again lowest 510 degrees C, and highest 576 degrees C.

jim's smokebox temperature thermocouple was placed very close to where yours was located (to measure the temperature of the gases emerging from the tubes). again, lowest 182 degrees C, highest 221 degrees C.

in addition to jim's test results being in ME for march/april 1966, they are also printed in martin evans' book 'model locomotive boilers' chapter 9.

some of the most interesting results from jim's tests were that
1. the firebox temperature is as hot as fullsize (seems obvious now, but for years it was thought that a coal fire in a miniature loco never got as hot as fullsize)
2. back pressure/exhaust pressure only just above atmospheric pressure (much lower than fullsize)
3. a considerable drop in flue gas temperature after the first 1/3rd of the tube length leading jim to conclude that in miniature only the first 1/3rd of the tubes do any useful work transfering heat to the water.

the very low back pressure may be a reflection of the overall high cylinder efficiency and good design of the large exhaust passageways and large blast nozzle. it is something that don young was particularly keen on in later years. however, jim was always very dismissive of those who tried to apply the work of Sam Ell of the GWR/WR testing section to miniature loco draughting. jim argued that because of the much lower speed of the exhaust gas in miniature locos it was pointless trying to apply fullsize draughting proportions. jim considered the Greenly proportions to be quite adequate with no further refinements required.

the primary purpose of jim's tests was to find out the temperature of the steam in the radiant superheaters he had fitted to his loco, the highest temperature recorded being 854 degrees C in the return bend inside the firebox, though this had dropped quite a bit by the time it entered the cylinders.

LBSC argued that there was no need for radiant superheaters in miniature because they werent fitted in fullsize and his own locos 'went ok' with his standard superheater design. jim argued that in miniature locos the high flow and short distance travelled by the steam was quite different to fullsize leading to nothing like the same sort of heat transfer hence his use of radiant superheaters.

i hope that is a fair and not too technical summary!

cheers,
julian

[pault]

Hi Julian
Your reply answers my questions, however it brings some more which may never be answered. Since mine was a dynamic “road” trial there will be significant variation in figures and I would expect that some of the recorded values are comparatively slightly lower, as the engine would probably be eased before the temperatures had stabilised. When doing things like this I work out in my mind what I think the measurements should be and then compare them to the results. If the two differ significantly I then look for the reason.

The fire bed temperature being higher than my flue gas temperature makes perfect sense since I was measuring well above the fire bed. When you look at Jims fire bed temperature verses my flue gas temp (945 deg c) that to me would make sense i.e. the coal (up to 1600 deg c) heating the air being drawn through it (cooling the combustion gases) to give a flue gas temp of 945 deg c. For the temperature drop to be in the region of 1000 deg c as per Jims numbers seems excessive. Interestingly I recorded a temperature drop along the tube of 544 deg c, so whilst my smoke box temperature was clearly significantly higher I would suggest that the tube was not totally inefficient.

Smokebox temperatures seem to be very much on the low side to me. Assuming a boiler pressure of 80 psi this equates to a temperature of 162 degrees within the boiler. This would seem to indicate a very high heat transfer efficiency within the tube.

Do you have a figure for the steam temperature post superheater? I find the temperature at the return bend a little hard to believe unless it was the metal temperature of the element rather than the steam temperature. If it was the steam temperature this would have increased further as the element passes back over the fire towards the tube. If the “outbound leg was in the normal position i.e. on top, then most of the heat would have been given to the steam at the return bend by gases at roughly the temperature of the gases entering the flue i.e. 576 ish deg c. as the upper part of the element is shielded from radiated heat. That being the case it is difficult to see how the steam temperature could have reached 800+ degrees c. Assuming the steam was further heated as it headed back to the flue it would then have quite a temperature drop along the flue.

Whilst I can’t make any real comment on back pressure it is one thing on the list to look at. Back pressure will obviously vary from engine to engine, I would however suggest that the noise some 7 1/4” locos make at the blast pipe would indicate a fair bit of back pressure/energy being released.

I rebuilt a 7 ¼” loco some years ago with new cylinders and valve gear. Having read all the teachings about streamlined steam and exhaust passages I made the exhaust passages as large as possible with “slightly gas flowed “ passages. Just below the blast nozzle the exhaust plumbing was 22 mm dia. The blast nozzle was a Lemaitre style affair with 5 holes. As I had increased the cylinder volume by about 35% IIRC I added a 6th central hole to the blast pipe, reasoning that the greater volume of exhaust from the cylinders would require more blast nozzle area. When I steamed it for the first time it would not steam at all so I blocked the central hole that I had added. The loco steamed fine like that but even full gear full regulator the blast was very quiet and muffled. The exhaust system was acting as a plenum chamber and smoothed out the pulsations in the exhaust to give a more sustained draw rather than the normal pulsed draw. Previously the loco was had a small and restricted exhaust and whilst it was very much less powerful it was a lot noisier. I have often thought that a continuous steady exhaust would be much more efficient than the conventional pulsed exhaust. Another one for the list of things to investigate.

Regards
Paul

[Deleted]

Hi Paul

Just to say that I'm enjoying this thread, I love learning and this thread is teaching me a lot... Keep going...

Pete

[RGR 60130]

Jan 4, 2013 21:21:25 GMT pault said:

The exhaust system was acting as a plenum chamber and smoothed out the pulsations in the exhaust to give a more sustained draw rather than the normal pulsed draw.

This is exactly the same as turbochargers on ic engines. Marine diesels often used pulse turbocharging where individual exhausts went straight into the turbo nozzle ring. The smaller volume of pipework made the turbo more responsive during start-up and manoeuvring. The trend shifted towards constant pressure systems where all exhausts feed into a big manifold and that in turn feeds the turbos with the result of smoother running.
The attached picture shows the latter system on my present vessel. The manifold runs right across the picture and one of the turbos is visible to the left.
In our locomotives a constant pressure system makes some sense as it should still draw plenty of air through the fire but perhaps without the strong pulses encouraging ash to lift from the fire and deposit in the tubes. (Well, that's my theory anyway!).

Reg

Attachments:

[pault]

Hi Reg,
I would agree with your theory regarding the lifting of ash and would also add one of my own.
When you are running at moderate speed you can see the pulsations, caused by the exhaust pulses, in the fire so the flow of gases is speeding up and slowing down, and the fire temperature is probably fluctuating a small amount in the same manner. The speeding up and slowing down of the gases takes more energy than a constant flow. We get the extra energy required by restricting the blast nozzle to increase the velocity of the exhaust. The down side of this is the increase in back pressure. In reality it is probably quite a small increase.
In my mind (normal people may not get this) I liken the gas flow through the fire, tubes etc to two cars travelling from A to B both start at the same time and must arrive at the same time. One travels at a constant speed on a motorway the other travels on a parallel A road with loads of roundabouts which the car almost stops at. The fuel consumption and hence the energy expended of the A road car will be considerably higher than the motorway car (unless it is on the M25)
The results that Jim got regarding back pressure were unique to his loco so I think it is a bit misleading of Jim to make sweeping statements regarding back pressure and drafting based on one loco. We have all seen to different locos hauling very similar loads over the same track. One mumbles round without making any fuss the other shouts its way round in spectacular style. Both are doing about the same amount of work but are doing it with very different draughting arrangements. Both do the job asked of them.
The one of the problems with back pressure is that it is not a constant pressure so measuring it is generally done one of two ways depending on what you want to look at. You can connect a plenum chamber to the exhaust and measure the pressure in the plenum chamber. This will allow you to use a relatively low sampling rate, however the results are effectively an average value of the highs and lows of the pulses. Alternatively you can connect to the exhaust with a minimum volume and a high sampling rate so you can look at the actual pressure trace which shows the pulses. When you throw in the high gas flow rate quite a lot of care needs to be taken when measuring exhaust pressures and it is not difficult to get misleading results, particularly if you are using gauges rather than transducers or pick the wrong sampling rate. The location of the tapping can also change things significantly. Since the whole thing is a pressure gradient, which ends at atmospheric pressure three different locations in the system can give three different results. This makes comparisons between different locos tricky
Overall does any of this matter? No I just find it interesting.
Regards
Paul

[pault]

Hi Reg
Any chance of a picture of you main engine with a person to give it scale, it looks quite large. Is the thing at the bottom (looking at the picture) of the turbo the air filter?
Regards
Paul

[chris vine]

Hi Paul,

You are exactly correct with your "more energy for pulsating flow" comment.

For a given mass flow, the steady flow will require less energy or power to move it. Because the energy in the flow is proportional to the square of the velocity (power proportional to the cube of velocity), you use a huge amount of extra energy in the faster parts of the cycle and save only a small amount of energy in the slower parts.

This is equally applicable to driving a car on a long journey. a constant 50 mph will use much less fuel than part of the journey at 40 and part at 70.

This is the most fascinating thread still!!

Chris.

[RGR 60130]

Hi Paul,

That is indeed the turbocharger intake screen you can see in the bottom of the picture. The second one is further Aft out of sight.
This picture shows the guys starting to change out an exhaust valve. These use hydraulics to open and air to close them. The silver things down to the right of each set of steps between the units are the fuel pumps, 1 per cylinder.
This is a B&W 6S80MC 2 stroke engine. Air enters through ports in the bottom of the liner in the usual 2 stroke way and exhausts through the exhaust valves on top. Bore 800mm. Stroke 3056mm. 22110BHP. 16257KW. Dry Weight 825 tons. Speed 67.7RPM.
Getting back to steam and exhausts, if you take indicator cards off a multi cylinder engine you can sometimes see a little blip of increased pressure towards the end of the expansion stroke. This is caused by the exhaust on an adjacent cylinder starting to open and increasing the back pressure on the first cylinder. A bigger exhaust manifold (or larger blast pipe orifice) would help reduce this. Apparently this can be seen in 'our' sizes using the latest electronic indicating equipment.

Reg

Attachments:

[pault]

Hi All,
I’ve been going through the data to find the next run to post and came across this that I thought might be interesting. Whilst the run is not particularly interesting as it is very similar to the last one I posted the early stage of the run does have an interesting event. The graphs below show the start of the run. Starting from a stand the track starts off flat, dips slightly down, levels off and then starts to climb part way into the climb the gradient suddenly increases.
At about 90 seconds both the post superheat temperatures drop fairly suddenly down to just above the pre superheat temperatures. This coincides with a fairly dramatic increase in steam chest pressure, normally the increase in steam chest pressure would result in higher steam temperatures.
The increase in steam chest pressure was me being a bit heavy handed with the regulator. The sudden crash in superheater temps was caused by the loco priming when I suddenly opened the regulator more. The priming event only appeared, based on evidence at the chimney, to last for a few seconds. As can be seen the steam chest pressure remains relatively stable at about 3/3.5 bar for about 1 min 30 after in initial priming event. When you look at the superheat temperatures they have still not returned to their pre priming levels even though the engine has been working quite hard.
Interestingly the priming seemed to have no effect on the exhaust temperature or cylinder block temperature, which would seem to indicate that significant amounts of water did not reach the cylinders. This would indicate that the superheaters were boiling the water so the cylinders were getting wet saturated steam rather than water.
The second smaller drop at about 200 seconds is caused by the drop in steam chest pressure as the loco is eased.
Enough for now
Regards
Paul

Attachments:

[pault]

the same event different parameters

Attachments:

[jma1009]

re exhaust i think one should aim for as low back pressure as possible. our miniature locos have a much lower working pressure than fullsize and the steam chest pressure in many miniature locos is considerably lower than one might expect, so proportionately high back pressure is a problem. it never ceases to amaze me how some miniature locos will only run for a few hours before they have to come off the track because the fire is full of clinker and half the tubes blocked. this may be due to inadequate firebar spacing and insufficient ashpan depth and poor coal selection and bad firing and driving, but is more often the result of the fire being over forced by too strong a draught IMHO. a badly made (and badly designed) loco (particularly if the valvegear cant be notched up) will use more steam (and coal and water) so a strong draught isnt necessarily evident by excessive 'blowing off' from the safety valves.

the Maunsell LORD NELSON's had 8 beats per revolution, partly to provide a more even turning movement. one was fitted with axles giving 4 beats per revolution and its steaming was the same as the remainder of the class. it was never altered to 8 beats per revolution. in marine steam engines the exhaust was condensed and the fire had a constant draught.

paul, the temperature of the steam on jim's tests as it entered the steam chest was lowest 260 degrees C (without a load), and 312 degrees C with a tractive force of 30 lb.

jim took the view that gas velocity of the flue tube gases was roughly scale ie 1/12 in a 1" scale miniature loco.

over the years in ME much has been written about superheaters in miniature and what they do to the steam. the steam is greatly expanded and heated increasing its quality and flow characteristics. heat is added as extra energy to the steam from the superheater flues. what i have never quite understood is the often stated principle that there must also be a pressure drop in the superheater elements... if anyone can give me an idiot's explanation as to why this is so i would be very grateful!

cheers,
julian

[chris vine]

Hi Julian,

Re the pressure drop in superheater tubes: I think this is just a funny way of saying things as it is more of a result than a reason.

The only way to avoid having a pressure drop would be to have a really large diameter tube and then the steam would be a long way from the hot tube surface. Of course it would go slowly and have a long time to heat up but there would be no room for this.

The other way of looking at it is to say that you need the steam (in this case) to have turbulent flow so that all the bits of the steam come into contact (or get close to) with the hot wall of the tube. This turbulent flow creates a pressure drop simply because it is turbulent and wasting kinetic energy.

If the steam flow was laminar (opposite of turbulent) there would be a much smaller pressure drop but a lot of the steam would just go down the middle of the tube and not get heated much.

I don't know if I have got all the reasons, but maybe part of it?!!

Chris.

[pault]

Hi All
I think there may be a simpler reason behind the pressure loss in superheaters statements. The superheater is one of those very rare things where you get more out than you put in due to significant increase in volume. Throw in that superheated steam flows better than saturated steam, and I think it quite probable that these things cancel out to a large degree the pressure drop created by wall friction, bends, turbulent flow etc.
So let us assume just for a moment that there is very little or no pressure drop in superheaters, why do a lot of people believe there is. The most likely thing is that people are looking at boiler and steam chest pressure gauges that have different readings when the regulator is wide open and saying ah the S/C pressure is X psi lower than the boiler pressure. The superheater must be to blame as its lots of little tubes.
Well to prove that you would need to measure the pressure at the inlet and outlet of the superheater. It is not just a simple flow/pressure calculation due to the changes that take place in the steam as it is heated, and probably cooled in the superheater.
I think the main cause is the fact that the steam chest pressure fluctuates every time a valve opens or shuts. As a result even if the S/C pressure gauge is stable enough to give a reading it is only a figure somewhere between the high and low of the fluctuations. The highs may well be boiler pressure but the steam chest pressure the gauge will show something lower.
Then again the pressure drop could be caused by the regulator. And as our locos are all different what is true for one may be different for another.
Chris I have sent you a PM
Regards
Paul

[chris vine]

PS to pressure drop in superheaters:

I was lucky to have had a physics master at school who was fantastically pedantic. It was supremely helpful when trying to get one's head round a tricky problem. Paddy Whelan wrote the school text books on physics and was famous at school for lots of reasons. My favourite was when a boy told him that his kettle was boiling. "Oh no," came the reply. "The water in the kettle is boiling!"....

I am sure he would be pleased to note an obvious point about the superheater/pressure drop question. There must be a pressure drop across the superheater, otherwise steam would not flow through it!

Chris.

[pault]

Hi All
Good point well made, Chris, maybe the term excessive pressure drop would be more appropriate . Your physics master sounds like someone I used to know. If you packed a gland he would say “is it going to leak”? If you said it “no it shouldn’t leak” his reply would be “I know it shouldn’t leak but is it going to”? If you said “no I don’t think it will leak” he would just say “good”
Regards
Paul

[jma1009]

thanks paul and chris!

i may be missing something here, but as i understand it steam 'flows' into the cylinders by way of the cylinders effectively being at the end of a 'stopped' pipe which is attached to the boiler, the 'stopped' end of the pipe having an end that moves outwards (the piston). the steam is cut off before release to exhaust by the valve. apart from friction and wiredrawing in the regulator and steam pipes to the cylinders in theory it shouldnt matter whether there is a pressure drop, irrespective of whether the steam is superheated or saturated. the only difference between a saturated and superheated loco is the cylinder's ability to use steam expansively and use less coal and water. the pressure drop along the pipes caused by the moving piston is the same in each case (though an allowance can be made for the increased length of the 'stopped' pipe with superheater elements causing friction - partly or completely cancelled out by the improved flow characteristics of superheated steam).

my knowledge of physics is pretty basic, but if we apply a variant of 'Boyle's Law' and temperature isnt a constant but is increased, then volume AND pressure can increase in a superheater? (or at least the pressure drop should be minimised?). i am quite sure that in fullsize effective superheaters are stated to rely upon a pressure drop (which i have never understood), which no doubt was familiar to steam engineers consulting tables that have been out of print for decades! i will have a look at some of my dusty old books re fullsize and see what i can find!

cheers,
julian

[RGR 60130]

There is an interesting article on superheaters which some may find interesting here:

http://v_ganapathy.tripod.com/superhtr.pdf

This article explains the requirement for a pressure drop across superheaters to give even flow through all the elements. Unless you have this even flow of steam to provide cooling for the superheater it will only last a matter of minutes before it melts. Retractable soot blowers melt just the same if they get stuck and someone shuts the steam off.

Reg

[jma1009]

thanks reg for that interesting link which i have read and tried to digest.

i hope i am not labouring a point, but there is a significant drop in pressure between boiler and steam chest on a superheated miniature loco, made more pronounced by the low working pressures in use. i am quite sure that this pressure drop isnt simply the result of friction/resistance caused by the superheater elements, which in any event compared to fullsize are considerably shorter. paul has quite correctly referrred to the temperature of saturated steam at 80 psi. once steam is superheated i am quite sure something odd happens to the 'normal' connection between pressure and temperature... ie the 2 become independant (though ive never understood why!). it seems that the steam increases in volume and temperature at the expense of pressure despite the added energy added via the superheaters. this is one reason why the best superheated locos in fullsize (to be efficient) also had much higher boiler pressures than a saturated loco.

in fullsize you might expect a drop of say 220 psi to 200 psi between boiler and steamchests on a superheated loco. in our miniature locos most designs have a drop of 80 psi to 40 psi. given the vastly increased flow characteristics of superheated steam in miniature, something very odd is happening somewhere!

cheers,
julian

[chris vine]

Hi Julian,

I understand your frustration with trying to understand steam!

I think it is seriously difficult trying to make comparisons between large and small engines when everything is so different.

As to steam and Boyles law, the only thing to remember about steam is that it doesn't obey any laws except that it will do what it says in the steam tables! The amount of heat you add to create pressure or temperature is not calculable from a simple formula, you just have to look it up to find out what it does.

The pressure drop you refer to will occur for a number of reasons and you can ignore the piston if you think of the flow of steam into the cylinder, through the port. If the piston moves, then the pressure in the cylinder drops so it is now less than is in the bit of pipe just upstream, so steam now flows into the cylinder.

The pressure drops occur because of friction with the sidewalls of the pipes etc. This will depend on the flow rate of the steam, size of pipes etc.

Another pressure drop occurs because of change of section of the pipe/regulator etc. IE restrictions. If the steam (any fluid) moving through a pipe, meets a restriction, eg the regulator, then to go through the smaller hole, it has to be accelerated. To accelerate the steam, there must be a pressure upstream of the restriction from Newton's laws of motion. (really basic stuff!!). However downstream of the restriction it would be nice to think that the pressure builds up again as the fast steam hits the slow steam, but this does not happen because of turbulence. So you get a pressure difference.

If I keep thinking about it, it will all become clear as mud!

Chris.

[jma1009]

hi chris,

thank you very much for the trouble you have taken in replying to my previous post.

i have a very keen interest in paul's research, having taken careful note of the only other similar research by jim ewins many years ago, and having entered my 5"g GWR Armstrong Goods 0-6-0 loco in IMLEC in 1995. dynomometer tests = efficiency tests in my brain! i have tried to incorporated as many 'efficiency' (and reliability) details in my own locos as possible, so paul's research deserves to be considered very highly, and should rank along side that of jim ewins.

chris, i would be very interested to know whether you incorporated any of your specialist knowledge in the building of your superb loco BONGO? as is probably apparent my own knowledge of physics is singularly lacking!

i have always tried to enlarge the steam and exhaust circuits as much as possible in my locos, and maximise valvegear efficiency. i also design my own draughting and smokebox arrangements.

so far as superheaters are concerned, paul's research and testing i gather is primarily aimed at investigations of superheater efficiency. i have only carried out empirical evaluations of my own locos and others i have driven. i was privileged to have met jim ewins and bill carter, and knew don young very well. at some stage i would very much like to enter IMLEC again so have a certain proprietary interest in the results of paul's research!

if one can minimise the pressure drop in 1 1/16" scale between boiler and steamchest, and at the same time obtain a free steaming boiler with sufficient superheat to prevent condensate in the cylinders with expansive working then a very useful loco can be produced, with the added benefit of a bit more adhesive weight to take advantage of the increased efficiency without overloading the grate.

my own locos have my 'standard' 1/4" dia double return bend superheater elements of non-radiant type in generous superheater flues of 1" or 7/8" OD... so that a small 3/8" dia flue brush can ensure they are kept clean and free of any obstruction. i wear glasses, so know instantly if the exhaust is wet (which it isnt!). so empirically i know that the steam when the valvegear is notched up isnt causing the steam to be wet and that the superheaters are working. at the same time the fire is hot without being overloaded, and i never have any clinker after a day's running.

my own locos so far have a higher steamchest pressure than i have quoted. there is a balance to be obtained as a result in adhesion versus tractive effort and i tend to err on the side of smaller cylinder diameter as well as adding a bit of extra adhesive weight where possible.

cheers,
julian