accurate measurement of steam temperature

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

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

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 ?

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

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)?

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.

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

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.

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

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

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?

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?

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

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

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

I think that means you have discovered perpetual motion!

Ian P

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

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.