question on blast nozzles

[Deleted]

hey guy's

many thanks for all of the help and advice, I'm taking all of this in and a little concerned about the 7% that you have worked out John and the fact that this is seen as to low a figure. the interesting thing is what you said about firing a loco like this..
quote:
'It was a real pig to even get the fire started and difficult to keep in steam without the blower on all the time. I'm sure the lack of tube area had a lot to do with it. Unfortunately both Gwen Elms and Lassie have combustion chambers which reduce the number of tubes that you can squeeze in.'

This is describing 4470's firing characteristics very closely, very closely indeed, so would I be right in thinking that if the latest mods that I have already done and if I still struggle firing 4470 after gaining more experience that perhaps I should try a smaller blast nozzle to increase it's effect?

kind regards

Pete

[pault]

Hi
I would agree that cylinder size should not be used to calculate the size of the blast nozzle. Some years ago I rebuilt a loco which was a very poor performer. As built the loco had 2” bore cylinders and a ½” pipe from each cylinder joined together and went to the blast pipe. The blast nozzle was styled like a Lemaître nozzle, with 5 holes in it.

The rebuilt loco has 2 3/8” bore cylinders, and I made big, sort of gas flowed passages through the blocks to a 22mm exhaust system. This went to the original blast nozzle, which had a centre hole added as I thought the extra swept volume would require a bigger nozzle area. When I steamed the loco it would not steam without significant amounts of blower. I filled in the hole I had added which made the blast nozzle the same as it was originally. In this condition the loco steamed very well, however the characteristics of the exhaust had totally changed.

As originally built the loco had a fairly loud and sharp beat, and it steamed well. As rebuilt the exhaust beats were almost inaudible even when working hard full gear and full regulator. The exhaust was more of a muffled roar when working very hard. The large exhaust system and passages acted like a plenum chamber so that the steam was exiting the nozzle in a relatively steady flow rather than the traditional pulsing flow.

When you look at the traditional pulsing flow it is actually a very inefficient way of drawing the fire, a steady flow is far more efficient. When I rebuilt the loco the idea was just to create a free flowing exhaust, it was only later that I realised what was going on. I think that you have to consider the system as a whole rather than focusing on any one aspect and excluding the rest

The interesting point is that the same blast nozzle worked with both cylinder sizes. Whilst the increase in bore by 3/8” may not seem much if I remember rightly it resulted in about a 40% increase in piston area and swept volume.

When it comes to anthracite it ignites at a higher temperature than coal which can be a problem if you let the fire cool off too much or when you are lighting up. Pete I would suggest having a go using coal only just to see how you get on. Then you can decide which way to go next.
Regards
Paul

[jma1009]

pete,

i'll send you some really decent best welsh steam coal if you want! all graded and the right size for a 3.5"g loco! (ive got a shed full of the stuff!)

Heilan Lassie is a very well proven design of LBSC. it is known to be a good performer. it doesnt like tight radius curves, but otherwise is an excellent loco and you should have years of very happy running (unless you decide to sell it to buy the A3's boiler!!) i'll come over to have a drive and perhaps show you how it's done if you like! (though paul is much closer, and my eldest daughter is a far better driver than i am these days - anno domini!). i'll pop one of my locos in the boot for you to try too as a comparison. ive never run on the NL track yet. 13 different tracks is my total so far.

another possiblity apart from crap coal is that you might have had a dodgy driving trolley with sticky brakes.

paul, david wardale reckons the pulsating effect to be advantageous. i have always incorporated an inverted 'Y' branch pipe where possible.

cheers,
julian

[Deleted]

you guys are great...

Looks like I should try another coal first then.. Julian you are always welcome... not sure what the rules are in the club for visitors.. I would guess pretty laxed.. we had a visitors day last weekend, evidently it's a yearly event run by a member who invites some friends from other clubs over( one guy bought over his 5" Tornado) so I guess any member can have visitors? If you or anyone here wanted to visit NLSME I will ask those above me to check first as I'm still relatively new having recently started my 3rd year of membership.
Thanks for the words of encouragement for lassie Julian, curves aren't really a problem at our track, or at least 4470 has no problems in negotiating them, think the smallest is 40'.Driving trolley was ok...4470 gets up to a very fast speed down the back straight and even with the regulator fully closed I still needed to use the trolley brakes to reduce speed before hitting the bend...

Regards

Pete

[pault]

Hi Julian,
Interestingly I have just started reading “Red Devil” by David Wardale, but have not got very far yet. My understanding is that the relatively sharp pulsing exhaust is useful to reduce back pressure, by the pressure wave through the exhaust creating a reduced pressure in other branches of the exhaust system. This has been developed with car engines and especially race engines to a fine degree. This effect will by created by your inverted Y. Measurement of the “back pressure” in the system is not a straight forward thing since depending on the measurement location and the response time/sampling rate of your measuring/recording equipment you will see different results. The pressures in the exhaust fluctuate to a large degree as each pulse travels through the system.

When you consider that the flow of gases through the ashpan, grate, fire, tubes etc is created by friction between the exhaust and the combustion gases there are losses involved in this. Allowing the velocity of the combustion gasses to ebb and flow with the exhaust pulses creates the repeated need to accelerate the gas stream which takes more doing that maintaining a continuous flow.
Obviously these effects become less as the speed increases and the flow becomes more stable. The effects would be most felt when working a slow heavy freight, when you can see the pulsation of the fire. The trouble is that as a machine the steam loco has a very large number of variables, far more than an internal combustion engine. When you look at the sort of running most of us do, a steady state is quite a rare thing.
The above are just my thoughts and maybe wrong, I am happy to debate them with anyone sitting by a bonfire, drinking beer in the early hours of the morning.

Regards
Paul

[Deleted]

Aug 21, 2013 8:22:42 GMT pault said:

The above are just my thoughts and maybe wrong, I am happy to debate them with anyone sitting by a bonfire, drinking beer in the early hours of the morning.

Regards
Paul

Now I for one would like to be sitting around that fire, I'm sure I would learn an awful lot about the black arts behind our fascinating hobby..

Pete

[ejparrott]

Aug 20, 2013 8:18:31 GMT jma1009 said:

Keiller proportions for length and diameter.

?? Google's failed me on this one!

[jma1009]

hi ed,

Kt = L / d squared

L = length of tube

d = internal diameter of tube

C.M Keiller suggested that for satisfactory operation Kt should be a value between 65 and 70

jim ewins was of the view that a higher figure of 80 should be used.

Clifton Macnee Keiller (1888-1968) spent some time at Swindon following his apprenticeship there. he was a very famous model engineer. he also worked out the miniature injector proportions, and built some exceptionally fine and ground breaking 2.5"g locos notable for their high pressure boilers and compounding. he wrote numerous articles and letters published in ME and the SMEE journal over many years.

cheers,
julian

[jma1009]

i should add that the above figure of 80 as per jim ewins' views actually can have a detrimental effect on free gas flow - the higher value indicating smaller diameter tubes for a given length etc. i would be inclined to go the other way and go up a size on the tubes which is what don young generally did. his RAILMOTOR N0.1 boiler was a very short barrel with 7/16" dia tubes, and as indicated previously (and others can vouch for this) steams on a candle! according to jim ewins' boiler formula the RAILMOTOR boiler would be a bad steamer but the contrary was certainly the case. i also enlarged the chimney and petticoat internal diameters, and went up a size on the exhaust pipes from 1/4" dia pipes to 5/16" dia. in the days before i had my dremel engraver the passageways between cylinder and port face were laboriously filed out into slots.

that was all many years ago but the lessons learned were applied to subsequent locos.

cheers,
julian

[Deleted]

Julian, I've reached the conclusion that the Keiller ratio is not particularly important. So long as the tubes are not excessively long (as you suggest) you can afford to have them shorter giving less friction to the gas flow with no detrimental effect on the heat transfer. In our sizes only the first few inches of the tubes contribute anything to steam production anyway so the length of the tubes with regard to heat transfer doesn't matter.

I realised some time ago that Jim's 'Boiler factor' doesn't hold for a lot of boiler designs. It does match some of them reasonably well but others are way out. Often, the only way you can get near Jim's 'magic figure' is to drastically reduce the number of tubes which seriously lowers the free gas area with a detrimental effect on the steaming.

I've reached the conclusion that so long as the grate area matches the cylinders/wheel diameters and the gas area through the tubes matches the grate area, then everything should be ok. On that basis, I would expect both no.1 and no.2 Rail-motors to be good steamers as they score highly on both counts.

John

[jma1009]

hi john,
one of the other things that jim advocated which i dont think he published during his lifetime was a higher than average firebox crown. i have a summary of his unpublished 'thoughts'. it is quite revealing how many of say martin evans' early designs have quite low firebox crowns. his 'model locomotive boiler' book gives ridiculously low proportions for firebox crowns. ok, it might aid the tyro keeping the water level between the top and bottom nut compared to a loco where the effective water gauge reading is within small parameters, but it doesnt do much for good boiler proportions. for example, my GWR 3.5"g KING has a water gauge that shows only 5/8" of glass, but ive never had a problem keeping the right level in the boiler. as you are used to 2.5"g locos im sure you would agree!
cheers,
julian

[Deleted]

Ok I've been taking a closer look at 4470's blast nozzle arrangements, let's just say they are a long way from the drawings which could explain or at least be another pointer towards my steaming issues. I have little idea of what the discrepancy's mean but I'm sure you guys will enlighten me as to what needs doing as they are not big jobs to put right, very simple in fact but I just wanted to check first.

first picture shows the blast nozzle's drawing as LBSC designed it.



next picture shows the blast nozzles as they have been made, seems the OB has added a hex section perhaps to make it easier to tighten up but in doing so he's made a nozzle that sits a lot higher than the drawing states.



As you can see the base pipe should be an inch long or 25.4mm from base to bottom of nozzle but in fact it's closer to 35mm long, the nozzle top itself seems to be to drawing except for the actual hole through it which should be 4.04mm but is in fact 4.36mm


for clarity I'll show the drawing for the smokebox and also a rough drawing giving actual sizes... these are approx figures as some parts are difficult to measure.

drawing




actual sizes



I forgot to write down the distance between centres on the sketch which is 30mm so not to far away from what it's supposed to be

So guys over to you... what suggestions for nozzle height if indeed it needs to be different to the drawing... I'm assuming that it will have to change as it's no where close to drawing but considering the nozzle holes are a little larger than they should be what height should I aim for between petticoat bell mouth and top of nozzle?

looking forward to your everyone's input...

Pete

[jma1009]

hi pete,
there's a vital dimension missing from your sketch - the point at which the 'bell' at the bottom of the petticoat starts inside the petticoat pipe (the 'choke')to the distance to the bottom of the petticoat pipe. or in other words how deep is the 'bell' on the petticoat pipe? if you shove a rule vertically down the petticoat pipe and scrape it around keeping it parallel to the upper part of the petticoat pipe it will leave a mark to show where the choke is.
cheers,
julian

[jma1009]

subject to the above, the blast nozzle should be 45mm below the choke for a choke diameter of 20mm. unless the depth of the 'bell' is 15mm, your blastpipes are too high.

what is the shape of the inside of the blast nozzle? it should have a converging taper then a short parallel section before the exit point on the top.

cheers,
julian

[Deleted]

hi Julian

here's a picture showing the petticoat... not easy to measure the choke... outside is probably 3-4mm but it looks like it goes a little higher internally, perhaps 5-6mm in total... don't think it's any deeper than that



inner nozzle is cone shaped so yes similar to what you describe...good news is I now understand what you guys mean when talking about the 'choke'....

Regards

Pete

[digger]

As an aside, I upset the steaming of my royal scot by opening out the blast nozzle caps too much, I have rectified this by threading out the blast nozzle caps and turning up a variety of different sized nozzles, changable in a few minitues.
As an experiement I chocked the locomotive up, and connected my air compressor to the boiler, full boiler pressure was attained, I lit a smoke pellet threw it in the fire box and gently opened the regulator, the smoke was quite visible being white, it exited the double chimney in a continious column the exhaust beats being visable as well as audible.
It proved little but it was good fun, these smoke pellets can be obtained from B&Q and are used by gas fitters to test flues etc.

Digger

[robmort]

Julian,

The subject of draughting and blast nozzle sizing is very interesting. But the sources you mentioned:

Brian Hughes of the birmingham club and Bob Sanderson's article in ME 15th June 1984

are no doubt very interesting but obscure. Would there be any possibility of getting a copy of these?

Rob

[jma1009]

hi rob,
the bob sanderson articles are to hand in my book case but the brian hughes article is missing from my bookcase and is in one of my loco files - i will have to hunt for it. i can take pics of same for you and email them to you.
incidentally a lot was written about applying the Sam Ell Swindon formula to miniature locos in ME over the years, particularly as applied by the late Bert Perryman and Lionel Woodhead. one must treat the Swindon formula with a bit of caution in the smaller miniature sizes due to the much lower speed of the exhaust.
cheers,
julian