Question from a customer

As an aside, re vacuum brakes on British railways.

The Great Western ran at 25" Hg. This caused problems when coupling stock from another railway. It meant that the strings had to be pulled on every vehicle, destroying the 21" Hg vacuum, so that the GWR loco could then "make a brake". And the same procedure had to be carried out when coupling GWR stock to any of the other railway companies in Britain.

Howard

..Higher and reduced pressure vessels require different approaches. Long ago a bulk powder lorry tank ( filled by gravity from a silo, unloaded by aeration and blower into a silo) was converted to operate as a large vacuum cleaner to pick up spillages. It was not a simple job, as I recall, due to the risk of implosion.

The classic school lab experiment was that of reducing the pressure in an aerosol can. When evacuated, if it did not collapse as air was removed, it would implode with a slight tap with a suitable edge (a ruler, usually).

Submarines are under increased pressure of one Bar for about every 10m they are submerged.

The submarine has increasing external pressure due to depth, but the internal pressure won't increase very much, as it depends on the internal volume, which will reduce slightly. The original poster's question regarded a reduction in internal pressure, caused by drawing in water and the condensing of steam - the principle of atmospheric engines, such as Newcomen's.

Agreed, the pressure inside has to remain close to atmospheric as a submarine dives - or submariners escaping from a submerged vessel would all suffer from the bends after escaping, if only a few atmospheres delta!.

The analogy was to demonstate that the external pressure can only be increased, above atmospheric pressure, by those two scenarios described - there is no internal forces acting outwards when there is a perfect vacuum within.

The time when the internal volume (of a sub) decreases rapidly is when the forces on the hull crush the sub. Collapse is all about differential pressures inside and out, along with the design of the vessel (shape, thickness and material, plus any internal bracing).

When you are talking about ultra high vacuum (UHV) such as is used in semiconductor manufacturing and electron microscopes, pressure is no longer measured as a force. You are talking about how many molecules of gas are present. Measurement is usually electronic as in how much current will flow. The pressures are measured in Pascal and UHV is below 1x10^-7 Pascal or 0.0000001 Pa. One atmosphere is about 100,000 Pa (101325 standard sea level 1013.25 millibar) so that is 1x10^-12 Bar or 1.47^-11 PSI 0.00000000000147 PSI.

The pumps used look like multistage axial turbines running at very high speed but are actually sweeping individual molecules out of the UHV volume rather than pumping. They have to be "backed" by a high vacuum system of less than 100 Pascals (0.015 PSIA).

Robert G8RPI.

. . . and oil diffusion pumps and ion getter pumps and the further reduction in pressure by cryo cooling.

regards Martin

Fizzy's thoughts follow early steps in the path made by Newcomen, Watt, Carnot, Stirling, Joule, Clausius, Otto, Rankine, Diesel and other geniuses!

For many years I imagined steam and internal combustion engines as simply driven by pressure. It's a useful model but turns out to be only a small part of the story. Looking deeper it's found the prime mover is heat, not pressure.

When heat is applied to water it warms up linearly until it reaches boiling point. This is called 'Sensible Heat'. At boiling point something strange happens: a hefty blob of extra heat is needed to convert water from a liquid into a gas. This is called Latent Heat. Then sensible heat takes over again and raises the temperature and pressure of confined steam linearly.

Water requires rather a lot of extra energy to convert from water to steam - about 2300kJ to convert 1 kilogram of water into steam. The effect can be seen in the kitchen where it takes a long time to boil a saucepan of water dry. Almost all the energy coming from the cooker ring is used to convert water to steam. The temperature of the pan doesn't rise above 100°C, because it's kept cool by the water. An empty aluminium saucepan soon melts.

One way of visualising steam being converted to work is to imagine it applying pressure to a piston, more accurately the piston converts energy released by expansion into mechanical work by cooling steam. The maths of the latter explanation is much more informative and complete.

The realisation that heat, rather than pressure, drives engines is important. For example, it becomes possible to calculate how big an engine will be needed to do a known quantity of work, and how long it will take to do it, and how much fuel will be consumed in the process. In the 19th century, heat theory (Thermodynamics) predicted correctly that internal combustion engines could be made with a much better power to weight ratio whilst being far more energy efficient than any reciprocating steam engine. It predicts when it will be impossible to improve a heat engine beyond a certain limit. It also anticipated that steam turbines could be made more efficient than any internal combustion engine, which is handy to know when building power stations. The same theory explains refrigeration.

Fizzy said: 'Let it cool down and the pressure comes down to exactly where it was when you started. Nothing can have changed. The only difference when using it for real is that there will be less water in it when it cools down, assuming no more has been added.' This would be correct, except the amount of energy in the boiler changed massively and might have altered the balance.

'Might'! But to create a vacuum inside the boiler, it's necessary for it to have done some external work. However, even without driving a piston or heating radiators, leaks or stretching the metal of the boiler does 'work'. So a boiler might collapse.

Only a completely sealed and rigid container would behave as Fizzy suggests, which is 'not wrong' - there are plenty of real world examples. For instance, I don't think boiling a Carbon Dioxide soda syphon Sparklet would cause it to collapse on cooling. (As Sparklets are about 900psi at room temperature, the thought of one going pop inside a pan of boiling water puts me off cooking one!) One reason pressure vessels need to be operated and maintained carefully is the rules change depending on circumstances. A faulty valve can cause an otherwise sound boiler to collapse or explode. In full-size there have been many nasty accidents.

Ought to be emphasised that the size of the boiler is all important. It limits their capacity to store energy. The boilers used in model engineering are tiddlers, meaning not much energy is involved even if one goes badly wrong. In addition they are usually made of copper which tends to split rather than explode when a boiler is over-pressed. Slowing down the energy release makes small copper boilers even less likely to hurt anyone. Likewise the surface area of a model boiler collapsed by an accidental vacuum is tiny compared with a CRT, ie the boiler is safer than an old-TV set. It's misleading to describe model boilers as 'bombs'. Yes they're capable of causing injury, but they're not in the same league as a hand-grenade...

Dave

Great Post SOD

Re "Ought to be emphasised that the size of the boiler is all important. It limits their capacity to store energy. "

So true, this is where the 250 Bar/Litre "rule comes from. If the product of pressure and volume is less than 250 many of the regulations don't apply. 250l tank at 1 bar is same energy as a 1l at 250 bar.

Robert G8RPI

Those could put Maxwell's Demon out of a job.

Ah but, something has changed. When you initially filled the boiler, the space above the water was full of air. When the boiler is supplying steam to an engine, this air passes out with the steam, so the space above the water is full of steam (water vapour). Stop the engine and let it all cool down to say 20C, if no air leaks in, and water can't get in via the clacks, this water vapour has a pressure of 2337 Pascals, 0.34 psi absolute (not full vacuum), so the shell is subject to an external pressure of 14.36 psi. Any stays supporting the ends will be in compression, and so could easily buckle, but I doubt this is a problem in real life. Tubes subject to external pressure can fail due to elastic instability. As others have pointed out this is not a gradual process, one minute they are fine next minute implosion. However the ends will tend to inhibit this process by stiffening the tube. Superheater flues have exactly the same situation, they have a much greater length/diameter ratio, a smaller thickness/diameter ratio and a much higher external pressure, so they are in a much worse situation, but they seems to last.  I'm not going to lose any sleep over this. If you have the normal design of whistle valve air will readily leak in by pushing the ball off its seat.

Submarines have internal stiffening hoops for exactly the same reason, but of course they are subject to a much higher external pressure, approximately 1 psi for every 2 ft depth

Edited By duncan webster on 07/10/2019 19:57:41

It's not Bar/litre if it's the product, also said disapplication specifically says it cannot be used if steam is the relevant fluid.