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Author Topic: Flash steam plant control.  (Read 93361 times)

Fargo

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Re: Flash steam plant control.
« Reply #75 on: November 06, 2009, 12:25:47 PM »

Gooday all :-)

I'm a newcomer here, and absolutely hooked on the concept of flashboiler systems because I've never liked or trusted conventional boilers. I'd like to develop a flashsteam sytem that could eventually push my 12' aluminium "tinnie" along at a nice sedate pace, instead of the outboard which it currently endures.

Unfortunately I've not been able to find much information on such basics as coil configurations, types of burners (petrol, propane), or suitable engines, so I guess I''ll be asking plenty of questions. Can anyone suggest a very basic type of steam engine to use as an initial testbed? Something that has no reversing or throttling, and that can be built from stock materials preferrably? Am envisioning something on the order of 2 cylinders of about 30mm bore, to be fed by up to 20 metres of 1/4" copper tubing fired by a petrol or propane burner.

It's my belief that such systems can finally be made to operate safely and reliably, given the currrent abilities of microcontrollers available today.
Or am I just a dreamer??

I'm a retired Black Arts (electrical/electronic) guru, and self-taught machinist. Having built many different kinds of engines, I'm finally hoping to tackle the big one, flashsteam.

Regards all,
George Z


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flashtwo

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Re: Flash steam plant control.
« Reply #76 on: November 06, 2009, 04:36:01 PM »

Hi George and welcome to the Mayhemers,

I've come the conclusion that whats traditionally regarded as model boat flash boilers are in fact heavily overfired causing the pipes to glow red hot and resulting in very high steam temperatures and pressures - the water really does flash off uncontrollably into steam. They use a tremendously lot of fuel for very short runs.

The system that I have developed is much more sedate and I think I should use the term "Monotube boiler", i.e. a single tube boiler, where the water is turned into steam in a controlled fashion, much more fuel economic and capable of extended running.

As I have said before, to obtain full control, the water pump flow must be independant of the boiler pressure, but settable for for different desired engine speeds. To this effect I have, for the model, used electric water pumps but, with a bit of inginuity, a purely mechanical version might be developed.

Regarding a full size version, you would have to start with the expected steam consumption of your engine and calculate the boiler copper/water surface area and from that the coil length depending on its diameter. One would also have to estimate fuel consumption and then the size of burner required.

My model boat has to look after itself out on the lake with me only transmitting the required feed flow and the onboard computer monitoring the boiler and making the necessary control changes. If you are on board it would more relaxing to have similar automatic controls giving constant attention to the boiler needs.

The boiler has a fast response to load changes and there is no throttle valve between the boiler and engine, speed control is purely done by altering the feed flow.

One of my other ongoing projects is marine propulsion with no engine at all!

There are other Mayhamers who are much more knowledgable than me regarding "full size" engines, perhaps they may wish to comment. With a controlled flash boiler, a conventional steam engine is acceptable.

In conclusion, what you are suggesting is entirely possible.

all the best

Ian G.
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Fargo

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Re: Flash steam plant control.
« Reply #77 on: November 11, 2009, 07:26:33 AM »

Thank you for your input Ian.
A superfast model is not what I'm aiming for, but as I said, a sedate but reliable propulsion system. I'm currently doing my sums and trying to source components, but having a devil of a time trying to track down suitable temperature sensors at a reasonable price. Hoping to use thermistors rather than thermocouples.
Cheers,
George Z
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flashtwo

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Re: Flash steam plant control.
« Reply #78 on: November 25, 2009, 09:39:49 AM »

Hi George,

Your choice of thermocouples (t/cs) has made me consider them as an alternative.

I last used thermistors many years ago and, although they were cheap and sensitive, were not very consistant in their accuracy. I see that their technology has now moved on and they are now better than thermocouples.

The only thing I'm concerned about is their temperature range, I've seen figures for -100 to +130C, but do they go above the higher figure?

I was really brought up using thermocouples and know their foibles. I've been using 1.5mm diameter type K t/cs, which are a good size to bend around the boiler and be fitted into the steam /water flow in the pipework.  I connect the t/cs to the AD597AH i.c. made by Analogue Devices, which linearizes the signal, applies the necessary cold junction compensation and gives a 0 to 5V linear output for a range of 0 to 350C - it can higher if the supply volts are higher.

I connect the 0-5V signal to my PIC based control system, but it could drive an analogue or digital voltmeter.


Ian G.
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flashtwo

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Re: Flash steam plant control.
« Reply #79 on: November 25, 2009, 10:12:27 AM »

Hi Mayhemers,

Vital Byte has been out on the lake a couple of times recently in colder weather with the ambient at 11C.

As expected the gas cylinder has started to get cold and its pressure drops under hard running conditions. Despite this, the boat is still performaing well with the gas valve opening automatically to compensate for the lower available gas pressure.

There is a limit though and I can end up pumping too much water into the coil and end up with very wet steam. This occurs when the gas valve is fully open, due to the lower gas pressure, and no more heat is available. This can be solved by limiting the feed flow if the gas valve is approaching it maximum open position. I can either do this automatically in software or use  VB's "bridge" indicator to show gas valve position so I can check visually from the shore before applying more feed.

Last Sunday, I accidently left the computer display unit at home and had to start and run VB "blind". It worked ok (as designed), but I wasn't able to collect any running data as I would normally do. It does mean that,  perhaps on a smaller model, the display unit is not necessary to be permanently on board.

The forward/reversing on the engine is very reliable now, since remaking the valve rods and re-timing the engine.

At the Warwick International Boat Show, I visited the Prop-shop stand and discovered that they make a 5inch prop specifically for the Stuart D10 engine (I wish they had a proper web-site). I've ordered one and will be able to return the one lent to me  by Malcolm Beak.

Vital Byte will be on the Blackheath Stand at the London Model Engineers Exhibtion at Alexandra Palace in January. She should have, by then, her new smaller control AE-35 computer fitted. The development of the AE-35 is going well with the RPM and display code working. The serial communication between the chips is proved and it just leaves the control code to be installed, modified and tested.

Ian G.
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Fargo

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Re: Flash steam plant control.
« Reply #80 on: November 25, 2009, 01:55:00 PM »

Hi Ian,

I've given the thermistor idea away for the very reason you mentioned ie. too low a max. temp., and bought myself a K thermocouple with a ring tongue which Ive bolted to a copper flag which will eventually be brazed to the boiler tubing. I'm hoping the thermal response will not be too slow doing it this way, but it seems to me to be the most convenient way of mounting it.

I'm currently using a 741 op amp as an amplifier, but getting the correct signal to the PIC will no doubt be messy. I'll probably end up copying your idea of the AD597AH.

Managed to get an absolutely beautiful little pressure transducer from a bloke in Norfolk, and "Experimental Flash Steam" is on its way from the US, so the project is gradually gaining momentum!

From your photos it seems that you feed your gas unregulated to the burner. I've tried this on some hot air engines years ago, and had so much trouble that I gave the idea away. The combination of liquid gas and ultrafine jets was just too unmanageable.

I'm hoping to fire my boiler using propane regulated to 5 to 20 PSI and a burner of my own design similar to the one I use in my casting furnace. It fires horizontally, and produces a tangential swirl of flame which is great for the furnace, but I have doubts about how efficient such a burner configuration would be firing directly into a tube coil horizontally. Obviously it works, as evidenced by VB, but why is your boiler horizontal? Wouldn't the burner be more effective pointing straight up into a totally vertical tube coil? Daresay I'll get more answers when the book arrives ...

At least I shouldn't have the same gas problems as you fellas have up there at the North Pole, it's quite often 37 Deg C here downunder! But even in the dead of winter, my casting furnace burner puts out about 45,000 BTU/hr without freezing the bottle valve. It pushes the gas through a 0.9mm dia. orifice at 18 PSI.

Just finished the pattern for the cylinder of my engine today, and will probably pour it next week. More news then I hope.
Cheers,
George Z

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Circlip

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Re: Flash steam plant control.
« Reply #81 on: November 25, 2009, 02:19:53 PM »

Cos of the various inputs to the control system Ian G is it not possible to put a Temp sensor on the gas tank and allow a system of bleed heat to the tank for cold weather ops??

  Regards  Ian.
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flashtwo

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Re: Flash steam plant control.
« Reply #82 on: November 25, 2009, 04:47:27 PM »

Hi George,

Thanks for the info regarding thermistors - they would certainly be useful at the lower temperatures.

Regarding using the old 741 op amp - you would be able to amplify the feeble thermocouple signal but it would be difficult to compensate for the ambient temperature.

With your ambient at 37C, you would be developing a signal roughly equivalent to 63C, whereas with my ambient at 11C it would be equivlent to 89C. So you can see that a stable reference temperature is needed to make sense of the thermocouple signal.

The AD597AH most likely uses an internal thermistor to measure its own temperature, which it then adds to that received from the t/c and thus compensate for changes in ambient (to be pedandic, it should measure the internal temperature, convert it to the equivalent mV from the t/c, add it to the mV from the t/c and then convert it into temperature according to the t/c tables and referenced to 0degC - it is surprising that even some professional outfits don't do that).

The 0 to 5v output from the AD597AH connects very nicely with the PIC input and is represented internally 0 to 1023 (two bytes required).

Regarding the boiler attitude, I chose horizontal because of the lower centre of gravity. With the coil being bendable in any direction, you could have it horizontal and vertical in the same case or even as a toroidal shape.

I do regulate the gas flow to the burner. If the steam temperature goes up the PIC controller gradually closes the servo controlled gas valve and vice-versa. A new cylinder starts off at about 2.4bar (excuse the language) and I normally bring the boat in when it drops to 1bar, although I have run down to 0.6bar under these cold conditions knowing that is plenty of gas in the cylinder.

Alan Rayman, one of the "Experimental Flash Steam" authors, has seen VB going through its paces - it was an honour for him to be there.

I thought I was going back to basics, but to make your own patterns and castings thats well out of my league.

All the best
Ian G.
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flashtwo

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Re: Flash steam plant control.
« Reply #83 on: November 25, 2009, 05:23:36 PM »

Hi Circlip,

In the early days (two years ago), I did  have a steam loop in a water bath containing the gas cylinder with a manually controlled steam valve. It worked fine, but in considering a automatic system, it started to get complicated in designing a "fail-safe" system that could cope with a stuck open steam valve.

I now follow the MPBA rules in not having any direct heating of the gas cylinder - I would rather put up with a reduction in performance in the winter than have a potential bomb on the water.

I was thinking of having some heating (as opposed to cooling) fins on the gas cylinder. This would be a passive system that would scavenge any ambient heat that was above the cylinder temperature and warm up the cylinder - a bit like a heat-pump. I've done some rough calculations regarding surface area etc.  With 20 fins of 50mm width and 105mm length (= cylinder height), I would get a five-fold increase in the cylinder external surface area. It will take up more room on the boat, but I think it will be worth trying.

But, saying that,  any ideas would be most welcome.

Ian G.
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Fargo

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Re: Flash steam plant control.
« Reply #84 on: November 26, 2009, 01:36:25 AM »

Hi Ian,

thanks for the advice regarding coil config. and AD chip. I'm not an expert on thermocouples but apparently I should be able to achieve ambient compensation by using alumel termination lugs at the circuit board (for type K), which is what I've done. This is suposed to subtract the ambient mV from the measured mV. But more on this later.

I'm also in the throes of learning to program in MPASM, and obviously facing several large learning curves altogether. But it sure is interesting and a hell of a lot of fun.

Congratulations on your successes with VB, and like the other blokes, I'll be following with interest your next project.
Cheers, Georg Z
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benjaml1

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Re: Flash steam plant control.
« Reply #85 on: November 29, 2009, 10:31:45 PM »

Thermocouples:
As long as the cold junctions are isothermal ( + and - are at the same temperature) it doesn't matter what connection material is used as long as the final measured cold junction is compensated. However, for optimum design accuracy they should be of the same material as the conductor.

Check out the www.omegaengineering.com for some excellent technical assistance....
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Fargo

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Re: Flash steam plant control.
« Reply #86 on: December 04, 2009, 08:53:47 AM »

Thanks  for the reference benjaml1,
but I've opted for using an IC, which will take care of things like cold junction compensation and linearity, and simply give me an analog or digital output to feed straight into my microcontroller chip. Meanwhile back to the drawing board and machineshop.
Cheers, George Z
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benjaml1

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Re: Flash steam plant control.
« Reply #87 on: December 04, 2009, 12:09:25 PM »

This maybe of interest......

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Fargo

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Re: Flash steam plant control.
« Reply #88 on: December 05, 2009, 08:40:41 AM »

The engine I'm building for my test rig also uses an aluminium cylinder and a brass piston. It's a unaflow design with a spool valve. It'll be interesting to see how this combination of metals works out in practice.
The cylinder is already cast and partly machined,but the brass bits still need to be cast, and that's another story again ...........
Fargo
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benjaml1

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Re: Flash steam plant control.
« Reply #89 on: December 05, 2009, 11:27:00 AM »

Upon reading, numbers like 3000 psi & 18000 rpm stick out, engines don't last long under these conditions. Building an engine with survivability would not be a problem in this day of modern metallurgy. Pay special attention to the spool valve, stainless steel would be a minimum requirement unless "unobtainiums" are available. Stellite comes to mind....
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flashtwo

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Re: Flash steam plant control.
« Reply #90 on: December 05, 2009, 02:56:29 PM »

Hi,

The whole point about the flash steam control system that I have developed is that is avoids the very high pressures and temperature that the traditional flash steam vessels operate under.

The Vital Byte runs very well at 30psi and normal saturation steam temperatures (plus a little bit of superheat), it is controllable, fuel efficient and uses normal engines.

Ian G.
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Fargo

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Re: Flash steam plant control.
« Reply #91 on: December 06, 2009, 03:57:52 AM »

The engine I'm planning on using for my monotube test rig is a scaled up version of  #48 "Elmers Engines" on jon-toms.com website. Its actually a horizontal "mill" type engine which is relatively quick and easy to build and by no means an exotic, meant for high performance.

Up till now I've been fairly isolated regarding the modelling scene (25 years out in the bush), and consequently have very little experience with steam whatsoever, apart from building one small 2 cylinder vertical open column type with rotary valving, which runs beautifully. But having caught the "microcontroller" bug, I needed a simple, appropriately sized load to apply to my proposed boiler system, and the above engine seemed ideally suited.

I'm attempting to emulate Ian's project complete with low-stress parameters, with the ultimate goal of working it up into a
'lifesize" system suitable for a one-man canoe or fishing dinghy. Eventually I'd like to investigate the possibility of coupling a monotube system to a Tesla turbine ..........

Cheers,
George  Z
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kiwimodeller

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Re: Flash steam plant control.
« Reply #92 on: December 06, 2009, 09:13:33 AM »

This maybe of interest......


Hi Benjaml1, can you please take a look at an earlier posting of mine and see if the engine in the picture you have posted looks anything like the engine in my posting? The thread was http://www.modelboatmayhem.co.uk/forum/index.php?topic=18582.msg196432#msg196432
I also spotted a similar engine on Ebay except that it had different valve gear but I missed out on it. If the article you have is about a similar engine I would appreciate a copy of the article either by email or personal message. Thanks, Ian V.
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benjaml1

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Re: Flash steam plant control.
« Reply #93 on: December 06, 2009, 12:01:33 PM »

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kiwimodeller

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Re: Flash steam plant control.
« Reply #94 on: December 07, 2009, 10:47:19 AM »

Great, thanks for the links. The motor on Ebay looks just like the one I have. I have ordered a copy of the Ebook and bookmarked the On the Wire pages. Thanks for your help. Regards, Ian.
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flashtwo

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Re: Flash steam plant control.
« Reply #95 on: January 20, 2010, 01:49:22 PM »

Hi,

Just a quick note to say that "Vital Byte" will be on the Blackheath Model Power Boat Club stand (with many thanks to Steamboat Phil) at the "London Model Engineering Exhibition" this weekend.

Vital Byte will have the new AE-35 control computer installed.

The exhibition is at Alexandra Palace from Friday 22nd to Sunday 24th January - opens at 10am.

Looking forward to seeing a good number of "Mayhemers" there.

Ian.

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flashtwo

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Re: Flash steam plant control.
« Reply #96 on: June 27, 2010, 01:27:16 PM »

Hi,

Well, as you can see from the "Steam Jet Engine" thread, "Vital Byte" has been on the back burner for a few months, but it is now being put through its paces in the test tank to flush out the expected software bugs.

V.B. now has a new controller (the AE-35 unit), a professional 5inch prop (Propshop) and  a 200 sq.inch monotube coil.

Having sorted out a few software bugs, I had trouble with some erratic behaviour akin to that caused by steam induced static electricity that I had in the very early days. Well, all effort was made to earth everything down, but the problems persisted. I finally traced it down to the servos that, when making large movements, were causing the supply volts to drop on the controller. The offending servos were moved to an existing servo power board that is supplied directly from the battery, thus stopping the interference on the AE-35.

Having finally got the controller sorted out, I then concentrated on the overall performance of the boiler and engine. I had noticed a lot of soot on the boiler coil indicative of poor combustion, also, when running with the gas valve open above 50%, there was the sound of an unstable burner.

With the experience gained on steam nozzles with the steam jet engine, I've just made and tested ( the sort of thing one does on the hottest day of the year) a Induced Draught nozzle that tees-off some steam from the coil just before the boiler exit. The steam is directed up the stack and greatly improves the draught resulting in a great increase  in boiler pressure and engine RPM. The engine speed has now increased from about 480RPM to higher than 700RPM (ahead and astern!), I say higher because the RPM code is currently limited to 700, but will be changed in the near future.

The substantial increase in RPM has been achieved inspite of using some steam for the Induced Draught nozzle and the beauty of the system is the higher the steam pressure the more draught is created.

As you can imagine 700RPM with a 5inch prop there is a lot of water sloshing around.

I suppose this method of induced draught can also be applied to those Mayhemers with conventional boilers.

Ian.
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steamboatmodel

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Re: Flash steam plant control.
« Reply #97 on: June 27, 2010, 10:43:02 PM »

Hi Ian,
"I suppose this method of induced draught can also be applied to those Mayhemers with conventional boilers."
I think the train guys refer to this as a blast pipe, Blower, Petticoat or something like that.
Regards,
Gerald.
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gondolier88

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Re: Flash steam plant control.
« Reply #98 on: June 28, 2010, 12:12:17 AM »

Hi Flashtwo,

Yay, engineering I can relate to again!!! :-))

The way you have increased the draft is an elegant but inefficient way of doing it- you will be using excess steam, water and gas by using steam to induce a forced draft.

A better way to do it, especially at the speeds you have on your shaft, is to run a short layshaft next to the propshaft, just some 25mm conduit or similar, this would have the layshaft through it centrally, the layshaft would end in a small diameter (22mm-ish) three bladed coarse prop, this would then have a slight venturi down to 16-18mm pipe which would then 'T' into the funnel.

This is the way full size boats that ran on the edge of what reciprocating steam did, it uses far less energy than using steam does, and it's speed controlled, not pressure controlled.

The sooting could also be from when you first fire up and the pipe is cold- chilling a flame can also cause incomplete combustion, even if the fuel/air mix is correct.

Greg
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flashtwo

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Re: Flash steam plant control.
« Reply #99 on: June 28, 2010, 09:49:08 AM »

Gerald,
I did consider using the engine exhaust steam with a blast pipe in the traditional way, until I witnessed a model steam tug at our local club.

As the tug chugged across the pond it left a trail of oil spots on the water and, as you know, they spread out to a size almost unrelated to the quantity.

It is not the sort of thing to explain to the local water bailif / enviromental officer ( he who walks around with a noise meter ensuring the i.c. boats are within the limits!).

Obviously it was the engine lub oil being chucked out the stack, so I decided to use the steam directly from the boiler - oil free.


Greg,
Thanks for the compliment. Yes, it was simple solution and was relatively easy to make and install especially for a monotube boiler where you just tee-in to the line. For a conventional boiler you would have to tee-in to the external steam line or put yet another tapping point on the boiler shell.

To keep the steam consumption low, I used a 0.7mm nozzle, which is far less than the steam ports on the engine itself. I made the nozzle inter-changable just in case the small one didn't work - it worked, so I didn't bother making a larger nozzle.

An intriging bit about the mechanically driven fan - was there any provision for when the engine was in reverse or didn't it matter for short periods and, also, are there any examples that one can visit?

 That's a good point about the sooting. I shall have to give the coil a good clean and see how things progress with the increased draught.

Ian
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