accurate measurement of steam temperature

[nigel jones 5Participant @nigeljones5]

[nigel jones 5Participant @nigeljones5]

Im currently trying to accurately determine the effect of a small superheater addition to one of my small boilers. It dawned on me that it might actually be cooling the steam instead of heating it! Any ideas as to how I can accurately and cheaply determine the exiting steam jet temperature? Thanks

[JAParticipant @ja]

Fizzy

What is the steam pressure and how much superheat do you hope to have?

I should add I do not think this is an easy measurement. At work we measured the temperatures of gas and the usual question was "what have you actually measured?" If a thermocouple had been used you could say with complete confidence "the temperature of the thermocouple bead".

JA

[J HancockParticipant @jhancock95746]

If a comparison is all you need , then perhaps you could try wrapping a tiny piece of low melting point metal

round the pipe and see which steam melts it ?

[SillyOldDufferModerator @sillyoldduffer]

You could try one of these . The hardest part (I think) is getting the thermocouple leads safely inside the boiler. The penetrator would have to be electrically insulated and capable of standing the heat and pressure. Some sort of screw fitting stuffed with heat hardening modelling clay perhaps?

Dave

[DrDaveParticipant @drdave]

Or could you make use of an infrared thermometer if you have a clear view of the pipes both before and after the superheater (and they are not lagged)?

[duncan webster 1Participant @duncanwebster1]

Weld (or silver solder) the hot junction of a thermocouple to the steam pipe after the super heater and then wrap it in insulation. Then weld the cold junction to the boiler shell, and similarly wrap with insulation. The output might have to be amplified using an instrument amp (something like an AD623), but will give you the increase in temperature from the super heater without having to mess with cold junction compensation.

I'm told you can weld thermocouple wires to the surface by discharging a capacitor across the joint, if you get it to work tell us how, I've never tried it but it sounds useful.

[nigel jones 5Participant @nigeljones5]

thanks all – i had thought of just blasting steam at a digital thermometer, would this work?

[nigel jones 5Participant @nigeljones5]

pressure is set to blow off at 60psi which from memory would be around 120 deg c. I just neet to see if the steam leaving the superheater is hotter than that going in. It is open i.e. not connected to any pistons at the moment.

[JAParticipant @ja]

One problem with a thermocouple is that the wires conduct heat as well as electricity. Therefore the thermocouple is likely to read colder than the steam. To a degree this difference can be reduced by careful design.

JA

[Neil WyattModerator @neilwyatt]

DrDave's idea of an IR thermometer is a good one, just paint a black patch on the pipes to take a reading from, they don't work on shiny surfaces.

Neil

[J HancockParticipant @jhancock95746]

Blasting 'steam' at a thermometer won't tell you a lot about the temperature of the steam down the pipe,

only how cool it is as it expands out of the pipe !

[HopperParticipant @hopper]

Posted by J Hancock on 06/02/2017 21:36:02:

Blasting 'steam' at a thermometer won't tell you a lot about the temperature of the steam down the pipe,

only how cool it is as it expands out of the pipe !

How does steam cool as it expands? (other than from hitting the cooler atmosphere).

I know refrigerant does, as it expands and changes state from liquid to gas and needs to absorb latent heat from the surrounds as a result. But steam starts as steam and stays as steam. Where does any expansion-related temperature reduction come from?

[Russell EberhardtParticipant @russelleberhardt48058]

Posted by Hopper on 07/02/2017 01:41:43:But steam starts as steam and stays as steam. Where does any expansion-related temperature reduction come from?

Charles' Law from basic physics.

Russell

[HopperParticipant @hopper]

Posted by Russell Eberhardt on 07/02/2017 07:59:52:

Posted by Hopper on 07/02/2017 01:41:43:But steam starts as steam and stays as steam. Where does any expansion-related temperature reduction come from?

Charles' Law from basic physics.

Russell

Isn't Charles's law that gas volume is directly proportional to temperature? As one increases, so does the other. So when the steam expands to a greater volume, the temperature should also be greater in direct proportion?

[Brian OldfordParticipant @brianoldford70365]

On this occasion Wobblipedia seems to be right on the money. https://en.wikipedia.org/wiki/Joule%E2%80%93Thomson_effect

[HowiParticipant @howi]

Posted by Hopper on 07/02/2017 08:21:35:

Posted by Russell Eberhardt on 07/02/2017 07:59:52:

Posted by Hopper on 07/02/2017 01:41:43:But steam starts as steam and stays as steam. Where does any expansion-related temperature reduction come from?

Charles' Law from basic physics.

Russell

Isn't Charles's law that gas volume is directly proportional to temperature? As one increases, so does the other. So when the steam expands to a greater volume, the temperature should also be greater in direct proportion?

I think you may have that the wrong way round, if you compress a gas it's temperature will increase, if a gas is allowed to expand by definition it's temperature will decrease.

Boyle's law/Charle's law

[HopperParticipant @hopper]

And yet steam that goes through a pressure reducing valve is superheated when it goes out the other side, as in a building heating system. IE, is at the same (or near same) temperature as before but is now at a lower pressure, therefore the temperature is above the boiling temp of water at that pressure.

And does not Boyle's law apply only to a gas at constant temperature? And Charles's to a gas at constant pressure? Both of which are not constant as steam comes out the end of a super heater pipe and supposedly cools as it expands?

And in the example of compressing a gas and its temperature increases, is not the temperature increase due to the work (energy) added to the system by compressing it, forcing the molecules closer together and so on?

 

Edited By Hopper on 07/02/2017 09:43:15

[Ian PParticipant @ianp]

Posted by Hopper on 07/02/2017 01:41:43:cool as it expands? (other than from hitting the cooler atmosphere).

. But steam starts as steam and stays as steam.

I think that means you have discovered perpetual motion!

Ian P

[Ian S CParticipant @iansc]

There will be a chart some where, then all you need is a pressure gauge, as the pressure goes up, so does the temperature.

Ian S C

Edited By Ian S C on 07/02/2017 10:12:52

[Russell EberhardtParticipant @russelleberhardt48058]

Posted by Hopper on 07/02/2017 08:21:35:Isn't Charles's law that gas volume is directly proportional to temperature? As one increases, so does the other. So when the steam expands to a greater volume, the temperature should also be greater in direct proportion?

Sorry, Ishould have said it's a combination of Charles' Law and Boyle's Law. If you "blast steam at a thermometer" it will expand as a result of the pressure drop and thus the temperature will fall. When it falls sufficiently condensation will occur and the gas laws no longer apply as it is no longer only steam but a mixture of steam and water droplets.

Russell.

[Anonymous]

Posted by Ian S C on 07/02/2017 10:12:14:

……… all you need is a pressure gauge, as the pressure goes up, so does the temperature.

That's true for saturated steam, but not for superheated steam, which is what the OP is trying to measure.

Andrew

[HopperParticipant @hopper]

Posted by Russell Eberhardt on 07/02/2017 11:08:35:

Posted by Hopper on 07/02/2017 08:21:35:Isn't Charles's law that gas volume is directly proportional to temperature? As one increases, so does the other. So when the steam expands to a greater volume, the temperature should also be greater in direct proportion?

Sorry, Ishould have said it's a combination of Charles' Law and Boyle's Law. If you "blast steam at a thermometer" it will expand as a result of the pressure drop and thus the temperature will fall. When it falls sufficiently condensation will occur and the gas laws no longer apply as it is no longer only steam but a mixture of steam and water droplets.

Russell.

Dusting off some ancient grey cells now. And then there is Gay-Lussac's law. And some Googling around shows if you combine all three, you get the combined gas law, PV/T = k. Still not clear on exactly how it applies though.

Then looking at how a steam pressure reducing valve works, it turns out that yes, there is some reduction in temp but relatively small, so the steam coming out of the PRV is lower temp than the input but still much higher than saturated steam at that temp, therefore it technically superheated. That's why if you are having trouble with water carrying over into your model steam engine, you can throttle back the steam valve on the boiler and the steam in the line to the engine is then superheated, ie its pressure drops but its temp does not drop as much, relatively. Has to do with enthalpy, steam tables and someting called isenthalpic expansion. Interesting.

[SillyOldDufferModerator @sillyoldduffer]

Having read the various replies, I think I was on the right track suggesting a thermometer of the type sold by Amazon. That particular model can read two temperatures at the same time and is supplied with a pair of thermocouples. (If more thermocouples are needed they cost about £5)

Using a thermocouple to take truly accurate measurements is quite complicated involving hot and cold junctions and other fuss. The device sold by Amazon is relatively crude (±1.5%), but all you need to do is position the end of the thermocouple wire at the point the temperature is taken. I guess ±1.5% accurate enough for fizzy's needs.

I'd suggest fizzy position one thermocouple in the steam dome and the other close to the output of the super-heater. As others have pointed out, the measurement needs to be taken under realistic conditions. I suppose that means with the pistons connected so that the pressures at the super-heater output are 'normal'.

It would be very dangerous for me to mess with a boiler to get the thermocouple wires inside but fizzy clearly knows about boilers and fittings. What's needed are boiler fittings that allow a pair of wires to pass safely into the steam space. For me, that's the hard part. Once the wires are inside the boiler, you just plug in the meter and take the measurements.

The other methods discussed are all more or less indirect. Using an IR meter seems the best, but the other options seem to involve corrections requiring a rather deep understanding of steam, for example you can't just take the pressure and easily infer the temperature from that. (Though the technique would be useful in other circumstances).

I wonder if anyone has ever comprehensively fitted a model loco with thermocouples? Comparing steam temperature in the boiler, super-heater, cylinder inputs, and cylinder outputs might suggest improvements.

Dave

Edited By SillyOldDuffer on 07/02/2017 12:37:38

[John BaguleyParticipant @johnbaguley78655]

Posted by SillyOldDuffer on 07/02/2017 12:36:29

I wonder if anyone has ever comprehensively fitted a model loco with thermocouples? Comparing steam temperature in the boiler, super-heater, cylinder inputs, and cylinder outputs might suggest improvements.

Dave

Yes, the late Jim Ewins did it on his O-6-2T loco and measured the temperatures at many points in the boiler, flues, etc. under various load conditions. His results were written up in ME for 18th March, 1st April, and 20th May 1966.

It is interesting to note that the maximum superheat temperature measured was 378°F (192°C) giving a final steam temperature of 620°F (327°C). The maximum superheater temperature recorded was 1570°F (854°C!)

John

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