Steam generator

[michael howarth 1Participant @michaelhowarth1]

Compliments of the season to everyone.

I am sure that I read somewhere of a model engineer using a kitchen steam cleaner to test one of his creations. Seems like a good way to get a quick test done ……if it works. Anyone tried it? What sort of pressure would you get?

Mick

[Bo’sunParticipant @bosun58570]

On a similar note.  Any suggestions for a small (ideally inexpensive) compressor for testing model steam engines?  I mention “inexpensive” because it wouldn’t be used for anything else, just sit on the shelf until next time.

[peter1972Participant @peter1972]

I have successfully used a car tyre inflator running off my car’s 12V socket. However it was very tricky getting the engine to run but that may have been due to my poor machining tolerances. I am thinking of making a tee connector so I can use an old wheelbarrow wheel as an air storage tank!

[duncan webster 1Participant @duncanwebster1]

I think early VW beetle cars used the spare tyre to drive the screen washer. You had to remember to over inflate it regularly. There was a clever valve in the system which stopped you running the pressure down too far.

[Harry WilkesParticipant @harrywilkes58467]

Ade Swash posted this video of him using a pressure cooker to generate steam to run his many engines link

‘Pressure Cooker Boiler for Steam Engines’

H

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[BazyleParticipant @bazyle]

If you make a little oscillator engine you can ‘run it in reverse’ as a compressor. Just need a suitable reservoir.
A slightly odd option is to make an adaptor to fit your hosepipe. Drain the hose then connect to engine and mains. You have quite a bit of volume of air to push out before the water gets to the other end. I did this with my Mamod as a child, using all sorts of bits of plastic tube and sellotape to adapt the size to the boiler filler plug.

[Bo’sunParticipant @bosun58570]

Thanks all,

I’m going to give the tyre inflator a try.  Now, where did I put the ear defenders?

[JasonBModerator @jasonb]

As Bazyle says an oscillating engine can also be turned over by hand and then it will pump air. No reason that you could not run the pumping engine off a small electric motor, the 775 dynamo in my current thread would also make a nice period looking motor.

Or there is this low cost option.

A lot will depend on the displacement of your engines and the speed they are run at as to whether a compressor or steamer will keep up. Also just running slowly for display will have a far lesser demand than an engine that is working.

[peter1972Participant @peter1972]

For using a tyre inflator, I made a two-part Schrader adaptor for a Stuart union cock:

The Schrader thread is 32tpi, which extremely close to 0.8mm pitch.

[Paul LousickParticipant @paullousick59116]

This is a link to my previous post on ME showing my engine running on a steam cleaner. Not sure how much pressure they generate but according to specs on Google. 3-5 bar (40-70 psi) but not much volume of steam.

https://www.model-engineer.co.uk/forums/topic/reversible-marine-engine/

[michael howarth 1Participant @michaelhowarth1]

That one of Paul Lousik’s will be ideal for what I need, a quick and easy source of steam to check joints etc. Thanks Paul.

Mick

[noel shelleyParticipant @noelshelley55608]

The Prestige HI DOME pressure cooker will power a steam engine ! It will have a max PSI of 15. I have run a Stuart No 1 on the pressure cooker on the stove, though at 2″ bore and 2″ stroke it needed a lot of steam, still ran ! Noel.

[JasonB]

On JasonB Said:

…
A lot will depend on the displacement of your engines and the speed they are run at as to whether a compressor or steamer will keep up. …

Displacement is key to identifying a suitable pump or boiler.  The basic factors needed to do a guesstimate are:

• Volume of gas needed to move the piston from start to exhaust, one stroke.  This allows the volume of gas needed per minute to be calculated from an engine’s measured RPM.
• Pressure of gas needed to overcome start-up inertia and running friction.   (Tick-over only! More pressure and volume will be needed if the engine is put to work driving a dynamo or whatever.)
• Time – how long the engine is required to run.

When choosing an air pump it’s necessary consider pressure, volume and time.

Here’s displacement data for a Stewart Hart PottyMill:

By experiment mine runs on between 3 and 5psi.

Each stroke takes 0.00784 litres of compressed air or steam, and at 100rpm that’s about 0.8 litres per minute.  Actually, though I haven’t measured it, I guess my PottyMill idles faster, perhaps 200rpm, and the piston is powered both forward and back. That means a pump providing about 0.8×4=3.2 litres per minute at 5psi.

This turns out to be an awkward combination in that most off-the-shelf pumps are either high-pressure/low-volume or low-pressure/high-volume.   A tyre inflator delivers tiny dollops of air at far too high a pressure, which is why Peter1972 had trouble.  Conversely, air-bed inflators deliver too much volume at too low a pressure to drive an engine.  We need something in the middle, and they aren’t common.  We have to extemporise.

Bazyle’s hosepipe suggestion works, but not for long.   Imagine the hose is the same diameter as the pottymill cylinder (⌀16mm)  and is 20 metres long.  One quarter filling the hose with water will pressurise the trapped air to 0.5bar (about 7psi), in 15 metres of hose.  Pressure good, volume low.  As each potty piston stroke consumes 39mm worth of air from the hose, we have 15000/(39*2) ≈ 200 revolutions, or a 1 minute run.  Roughly.

Duncan’s inflated tyre provides a longer run time from a more convenient reservoir because a car wheel contains more air than a typical garden hose.   But tyre air would probably be pressured at about 2.5bar (36psi) which is on the high side, though it can be reduced by bleeding the air out through a carefully adjusted valve.  A proper pressure regulator would do a better job, and every Model Engineer should know how to make one!  (Ha ha).   Again roughly, a car tyre sized torus can hold about 0.05919 cubic metres of air at standard temperature and pressure, about 60 litres, or 60×2.5 ≈ 150l when pressurised, which won’t run an engine for long.  (Boyle’s Law: Pressure is inversely proportional to Volume if anyone wants to convert 60litres at 2.5bar to x litres at 0.5bar.)  Again, the tyre suggestion works, and is probably more convenient than filling a hose with water, but still…

I tested my pottymill with a car foot-pump connected via a 2litre lemonade bottle.  A couple of vigorous pumps started the engine, which I kept going with slower footwork.  The lemonade bottle provides a small reservoir.   Proves the engine works, otherwise not good!

Never tried a steam cleaner. Not found one with a specification providing the necessary information.   Adverts often quote pressure in the 2 or 3 bar range, but we also have to know the volume of steam produced, which I suspect is not much, and how long a tankful of water lasts.  Much less steam than a proper boiler, and I’m a terrible cynic, but there may be very little dry steam produced by a steam cleaner  – what I see in the pics looks like smallish clouds of water vapour. My guess is they will drive a small engine, but not well.

Testing small engines on air is easy now I own a baby workshop compressor, except the pump is ear-defender noisy.   It has a 25litre tank at about 8bar.   Comes with a regulator that can be dialled down to less than 0.5bar on output. Can’t remember how long a PottyMill runs on a full tank with the power off – several minutes, even though my test pipework leaks!

A DIY pump outputting a steady 10 litres of air at 5 to 10 psi without deafening the neighbours would be a good ME project.

Final note, my terrible maths assumes air, not steam, is much simplified and I’ve probably made mistakes.  Intended to give show order of magnitude rather than give accurate results.    Grateful if anyone can improve the sums!

Dave

 

 

 

 

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