Steam pressure using thermistor

[duncan webster 1Participant @duncanwebster1]

Farnell have honeywell pressure sensor APB2 series for £15 plus vat. Rated up to 10 bar, NEAR DNOUGH 150 psi. You need to keep it cool, a length of small bore stainless pipe should work  and you probably need an instrument amplifier, but they are cheap enough.

You can also get PT100 and PT1000 sensors from China for about £8. 3mm diameter, 15mm long.

[Macolm]

On Macolm Said:

Assuming the boiler contains only water (liquid and vapour), it forms a powerful isothermal volume. As soon as the temperature at one point exceeds elsewhere, the pressure will also be higher there, and the resultant flows thus cause the temperature to equalise throughout the volume.

Hmm. Pardon me for digressing. I think you will find the pressure constant throughout (ignoring gravity). However, the density of a higher temperature region will be lower, causing either convection, especially if there is ebullition, or stratification, depending on the circumstances.

I have raised steam on the full size replica of Trevithick’s Catch Me Who Can numerous times. This has a single return-flue boiler. On a good day it starts making steam about an hour after lighting up, at which time the bottom of the boiler, below the flue, is normally still stone cold. Only when the pressure begins to rise does it quickly start to circulate.

[Robert Atkinson 2Participant @robertatkinson2]

The ABP2 sensors are a complete range with a wide choice of options including digital or analog outputs. However note that they specifically say they the sensors shoud not be used for safety critical applications.

https://prod-edam.honeywell.com/content/dam/honeywell-edam/sps/siot/en-us/products/sensors/pressure-sensors/board-mount-pressure-sensors/basic-abp2-series/documents/sps-siot-abp2-series-datasheet-32350268-en.pdf

This is of course a legal liability CMA statement. However the sensing chip is only protected by a flurosilicone gel. I would want a “proper” hermetically sealed sensor for this application.

Robert.

[duncan webster 1Participant @duncanwebster1]

Charles Lamont makes a good point. An interesting experiment for someone. The LYR built some locos with circular firebox which were reputed to take a very long time to raise steam, and Webb built some locos with water bottom firebox which were reported to have ice on even when loco working hard.

I would have thought the safety critical item was the safety valve. If a pressure sensor were safety critical I’d want to fit 2 and compare their outputs. If they differ by more than a set amount, kill the fire

[MacolmParticipant @macolm]

“Hmm. Pardon me for digressing. I think you will find the pressure constant throughout (ignoring gravity)…”

I did wonder whether to add a few paragraphs of qualifications, but decided against covering “exceptions” in what was already a (minor) lurch off topic. The usual designs of boiler have features in common with a “thermo-siphon”.

A heat pipe has a heat collector, and a cold heatsink. Vapour formed at the hot end flows to the heatsink, condenses, and flows back in a wick to the hot end. A thermo-siphon dispenses with the wick, and thus must rely on gravity to carry the condensate back. The normal boiler uses the same principle for efficient heat distribution, and therefore should be configured for the heat to be applied at the lowest part.

There are plenty of contraventions of this. In the case of the Trevithick boiler, the heat is applied half way up, and the water below is isolated from circulation for a long while. Any dead run may suffer in the same way, be it a long horizontal pipe run, a capillary running to a pressure gauge, vapour locked water tubes, or dead areas due to poor design. But any open, freely circulating volume in a boiler will act as a thermo-siphon.

[old mart]

On old mart Said:

When electronic pressure sensors are easy to get now and cheap, why advocating risking someone elses life and limb  seems such a good idea is beyond me.

Well said.
I had to check it was not April 1.

[Dave HalfordParticipant @davehalford22513]

Would this idea pass a club steam test?

[Clive India]

On Clive India Said:

On old mart Said:

When electronic pressure sensors are easy to get now and cheap, why advocating risking someone elses life and limb  seems such a good idea is beyond me.

Well said.
I had to check it was not April 1.

Keep away from most airliners built since the Airbus 320 then! And much else; fly by wire is common as muck these days.

Counter example: Bourdon Gauges are too dangerous for use in spacecraft.  Last thing an astronaut wants is to share the  cockpit with a pipe connected to a tank full of rocket fuel!

🙂

Dave

[Robert Atkinson 2Participant @robertatkinson2]

Not just spacecraft, aircraft too. Older ones and many light aircraft use them for fuel and oil pressure. I’ve designed a modification for a aircraft on the UK warbird circuit to remove this risk.
The Airbus fly by wire system is safer than rods, cables and hydraulics….

[Nigel Graham 2Participant @nigelgraham2]

Seems a lot of over-complification and wheel-re-inventing going-on here.

If I wanted to measure boiler-pressure electronically I would use a pressure-sensor meant for… sensing pressure. It may be a resistive strain-gauge or capacitative. It may need protecting from the heat, but so does a conventional Bourdon gauge so we interpose a U-tube or loop of pipe in the open air, giving a cool water barrier between the steam and gauge.

One post worries about “clunky” pipework… that puzzles me. Especially since an electronic gauge would create a much bulkier installation overall. A boiler for any practical purpose is liberally sprinkled with pipes and fittings anyway.

And about the soundness of the interpenetration for a sensor within the water: errr, yes? Don’t we already do something like that with all the other fittings? Putting electrical leads through a pressure-shell is hardly novel though it may be very unusual for a boiler, where the design and materials need withstand the temperature as well as pressure.

.

Pressure and temperature of water: Their relationship has been part of boiler and steam-engine design for well over a hundred years! For much of that time engineers cited the Callendar’s Steam Tables.  You might want ones in ISO units, you might want ºF and p.s.i; but don’t fret! For all practical purposes water boils at x degrees at y pressure-units; all enshrined in references used industrially in, even before, the days of Percival Marshall. Whether they may be less accurate in a miniature boiler I cannot say, but I would be surprised if such inaccuracy is significant. So…..

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….. Would a club boiler-examiner worry about an electronic gauge? Perhaps, through unfamiliarity; but there is no intrinsic reason to do so. What matters is not how the pressure-indicator works, but that it does work, and that its 0 and maximum working-pressure points tally with the boiler’s specification and safety-valve settings. The indicator can be calibrated using the test-set gauge with its own calibrated gauge; but I would want to ensure it does so in working conditions as well as by cold hydraulic test.  This might be done by temporarily attaching a conventional gauge of known accuracy to some point such as a whistle-valve connection.

[Nigel Graham 2]

On Nigel Graham 2 Said:

…One post worries about “clunky” pipework… that puzzles me. Especially since an electronic gauge would create a much bulkier installation overall. A boiler for any practical purpose is liberally sprinkled with pipes and fittings anyway.

 

…

Let me explain.  Steam engines are heat engines.  They convert energy into motion by doing work (scientific definition).  They depend on heat, not pressure, it’s all about energy.

Follows that efficiency depends on maximising the amount of heat available to do work inside the cylinder.  Anything else in the engine that wastes heat is bad news.  Steam locos are famously inefficient because the firebox structure radiates heat, heat travels through the tubes and up the chimney without warming the water, the boiler shell radiates heat, as does the pipework, cylinder, and piston.  Even when the loco is idling at a station.   Yet more heat is wasted by the safety valves and whistle!

So anything that can be done to reduce wasted energy should be.     Doesn’t help that lack of space makes it difficult to fit steam locomotives with condensers, compounding, water pre-heating systems, or thick insulation.  In full size, this resulted in steam locos being replaced outright by electric and diesel locomotives.

Though Model Engineers don’t have to worry about efficiency because it’s a hobby, there’s value and interest in tackling it.    Not “reinventing the wheel”, what needs looking at dates back to Watt and Smeaton, and there is much room for improvement!

The education system may be to blame.   Thermodynamics was commonly taught in Sixth Forms up until about 1950 because the science had direct job value in a hands-on world powered by steam.   Most of those jobs disappeared as the Steam Age came to an end, and Thermodynamics became a University subject.  The theory is vital when designing new engines, but far fewer practitioners are needed.  Model engineers who just want to build established designs needn’t concern themselves at all.  Those who like to innovate and understand how things work should dip into the theory!

Dave

[MacolmParticipant @macolm]

Surely the traditional steam locomotive has very low efficiency as a result of the very poor max to min temperature range, perhaps 420degrees K to 330degrees K, leading to at best 20% practicable efficiency. Then reduce this for poor heat transfer, heat losses and mechanical losses and you are down to a few percent.

[SillyOldDuffer]

On SillyOldDuffer Said:

…Thermodynamics became a University subject.

I am sure we did some basic thermodynamics at school in the early to mid 1970s. Although reading electronics at university I did a thermodynamics course in my first year. The properties of steam were included; I still have, and use, my steam tables. I got 98% in my first year thermodynamics exam. I know that because, to my embarrassment, the lecturer announced it in the lecture theatre in front of the whole year. I can’t remember what I got wrong that lost me the 2%.

(It might have been the ‘missing quantity’; that’s a pun on the effect well known in practical steam engineering.)

Julie

 

[duncan webster 1Participant @duncanwebster1]

Boiler efficiency, both full size and model, is quite high, about 70% unless you push the grate load too high, in which case it can drop down to 50% or so. The heat loss from the shell and pipework is fairly small compared with the heat generated in the firebox. There is a maximum efficiency with a loco type boiler as the exhaust gas cannot be lower than the temperature of the water in the boiler. Crosti boiler was tried in UK and was more trouble than it was worth, similarly feed water heaters.exhaust injectors do much the same job and we’re successful. Superheat temperature in full size is typically about 650F, 616K, but most of the heat transfer is at a lower temperature, boiling the water, so traditional Carnot efficiency isn’t really applicable

[Macolm]

On Macolm Said:

Surely the traditional steam locomotive has very low efficiency as a result of the very poor max to min temperature range, perhaps 420degrees K to 330degrees K, leading to at best 20% practicable efficiency. Then reduce this for poor heat transfer, heat losses and mechanical losses and you are down to a few percent.

Yes. that’s part of of it.   The figures don’t consider the whole system though, I guess they are steam temperatures at the cylinder?

Coal in a firebox burns at about 2800°K, so the in-out temperature difference is about 2500°K between firebox and chimney.    Misleading though because the bigger difference doesn’t mean steam engines are efficient!  More a hint that heat is being wasted.

One way of looking at a boiler is that it’s a device for converting heat at high temperature to something manageable.  Early superheat experiments failed because superheated steam is hot enough to destroy animal fat lubricants causing severe frictional losses and rapid wear in the cylinder.  So superheat had to wait for Mineral Oil, and not just any oil!  Still a problem today, most lubricants fail at high temperature.

Macolm’s temperature range is about the best that can be done in practice with steam in the cylinder of a reciprocating engine, and he’s right – the in-out temperature difference at that point fundamentally limits efficiency.

So please add superheated steam to my list of potential improvements!

What’s “quite interesting” I think, is that science has been ahead of practice for 201 years.  Carnot published his theory in 1824,  when the materials and engineering methods of the day were far too primitive to implement much of it.  Sixty years later Parsons had significant bother developing his Steam Turbine even though Victorian technology was much improved.   Likewise, 30 years later again, Whittle and others developing gas turbines all struggled to build reliable engines, most of the problems being materials related.  Special steels needed etc.   In 2025, the makers of modern jet engines are struggling to find materials that can cope with Macolm’s “bigger the better” temperature gap.

As home workshops are limited compared with Industry and R&D establishments, Model Engineers can’t expect to achieve  much in the way of efficiency improvements.  But modern materials and CAD/CAM mean a determined Model Engineer isn’t restricted to copying designs dating from before Greenly.   Not easy to innovate though:  LBSC’s 1922 Ayesha already has all the easy to make efficiencies, and improving on Ayesha today would require multiple well designed small changes, careful testing, and accurate measurements.

Same is true of full size, when it became obvious in the 1940s that steam was pretty much as efficient as it could be, the world moved to electric and diesel.  Diesel wasn’t an easy ride either:  struggled to get 20% efficiency before WW2, gradually rising up to 40% in the decades after 1950, and the best maritime diesel made today gets close to 60%, but that’s exceptional.  The theory hasn’t changed, rather small improvements due to better methods, materials and optimisations gradually applied over decades.

Dave

 

[noel shelleyParticipant @noelshelley55608]

Dave, I was aware that diesels were good for about 40%, but 60%, which and how ? Noel.

[noel shelley]

On noel shelley Said:

Dave, I was aware that diesels were good for about 40%, but 60%, which and how ? Noel.

The Wartsila-Sulzer RTA96-C is over 50% efficient apparently:

https://web.archive.org/web/20100716202400/http://people.bath.ac.uk/ccsshb/12cyl/index.html.o

https://newatlas.com/go/3263/

[Martin KyteParticipant @martinkyte99762]

Going back to pressure sensing. Maybe a strain gauge on the boiler shell would provide a more direct form of electronic sensing. After all the strain in the shell is the very thing you are trying to limit.

 

[duncan webster 1Participant @duncanwebster1]

<p style=”text-align: left;”>Now that’s a neat idea. You can get epoxy that will take the temperature. I’d put 2 on in a Wheatstone bridge arrangement with 2 fixed resistors. One question we always asked in safety reviews was ‘what if if falls off’? Usual answer was to fit 2 and compare, if wildly different assume a fault. You might need to calibrate with steam pressure rather than hydraulic to allow for temperature effects</p>

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