Author Topic: Talking Thermodynamics  (Read 143409 times)

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1335 on: July 07, 2020, 03:01:35 AM »
Hi MJM, just a quick question ...wondering what the central bar of the thermostat is made of ?? I usually saw these apart to use the brass tube ??!!  and  don't worry Per  about the toaster as I count to 100 and then switch it off. I am sure one could do a calculation here ...Colour = time ,,volts. area.. waste heat.. bread density .. etc etc    :lolb: :lolb: :lolb:  ok I know we are in lockdown but I'm sure we have better things to do !!!  also carboot find ...a part Boley staking tool set !!

thanks

Willy.

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1336 on: July 07, 2020, 10:35:32 AM »
Hi Willy, great to have you back, I was starting to think that you were missing in action.

I don’t really know the answer to your question.  It depends on just what is in that junction box at the end of the metal tube.

Some have a thermocouple in that outer sheath, so the junction box is simply for convenient connection.

Others have a micro switch and operate by a difference in thermal expansion coefficient between the metal of the outer sheath and the centre rod which would be welded to the sheath at the end dearest from the junction box.  Just what metal is used would depend on the required switch actuation temperature.

I found a table with a few values. 

Brass    18.7
Stainless steel. 17.3
Aluminium   23.9
Carbon steel 10.8
Tungsten 4.3

To calculate the expansion for a change in temperature, divide those values by a million, and multiply by the number of degrees C change.  Then each metre expands or contracts by that rather small number of meters.  If the outer tube is brass then carbon steel or, better still tungsten would give you some differential expansion to operate a switch.  But these are just a few standard materials, I believe there are other alloys which might have a much bigger difference that would be used in that application, but not for building bridges.

It is also possible to amplify that rather small difference using levers to increase the movement.  You can’t stop thermal expansion with any reasonable force, so there is plenty of force available to operate levers and a switch.


MJM460

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Offline Admiral_dk

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Re: Talking Thermodynamics
« Reply #1337 on: July 07, 2020, 11:56:11 AM »
Willy - the safety is there for when you forget or are suddenly otherwise called away.

Weller used another solution in their old soldering irons. The tip had a magnet on the backside, and it lost it's magnatism when it reached a certain temperature. An iron rod 'connected' the magnet to a magnetic switch inside the handle, that switched the heating element On and Off.

Offline derekwarner

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Re: Talking Thermodynamics
« Reply #1338 on: July 07, 2020, 01:15:01 PM »
mmm... Chromium is put to good use in such thermocouple type devices with it's curious characteristic...... :happyreader:

'Chromium has unique magnetic properties in the sense that chromium is the only elemental solid which shows antiferromagnetic ordering at room temperature (and below). Above 38 °C, its magnetic ordering changes to paramagnetic'

[I stumbled on this obscure fact when trying to understand the apparent difference in the reported thickness of the electrolytic Nickle deposit [under Chromium] on 380mm diameter hydraulic cylinder piston rods as scientifically measured by a NATA accredited body in Abou Daubi , then the same cylinder rods checked for Nickle depth as measured in Denmark by a DNV registered facility....

Abou Daubi certificates confirmed 42 degrees C, Danish certificates confirmed 3 degrees C  :hammerbash:]

Derek

« Last Edit: July 07, 2020, 01:34:39 PM by derekwarner »
Derek L Warner - Honorary Secretary [Retired]
Illawarra Live Steamers Co-op - Australia
www.ils.org.au

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1339 on: July 08, 2020, 02:29:40 AM »
Hi MJM, et al ,   I have taken the thermostat apart and the central rod is magnetic there is a makers plate with it...I have not done a spark test yet , or attempted to turn it ..yet ?
 Derek.. interesting about Nickel plating..between steel and chrome  this is the correct way to do it chemically !!! Nowadays on bikes the chrome goes strait on the steel. so in a few years it starts to peel off...after the guarantee ,of course..

willy

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1340 on: July 08, 2020, 12:52:47 PM »
Hi Willy, there you two more possible explanations of the mechanism.  From the picture, I am not sure that it can be identified, but with it in your hand and turning it around, you might be able to work it out.

As for the rod material, your original question, your spark test and a test cut should tell you something useful, but I am no expert on metallurgy.  And worth testing it for corrosion before putting a lot of work into it.

MJM460
The more I learn, the more I find that I still have to learn!

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1341 on: July 10, 2020, 02:31:57 AM »
Hi MJM, ok...the spark test is a deep red dusty looking a bit like the tipped tool steel but more red than orange   and it turns nicely with curly swarfe it also files well and docent want to go rusty ???? also a pic of the thermo contact breaker...

Willy

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1342 on: July 10, 2020, 10:50:53 AM »
Hi Willy,

It sounds like that rod will be useful for something.  Can’t beat material that is good to machine and also resists corrosion. You will have to tell me what material the spark test colour is indicating.  You might also rescue some points for an engine, though the spring material might not be good for a high number of cycles.  Perhaps more suitable as a boiler cut off switch.

As to the switch, is there a chance that you have removed a part that allows the movement of that part with the green tape to open and close the contact as it moves?  It certainly looks like a simple device, that would have been quite reliable.

MJM460
The more I learn, the more I find that I still have to learn!

Offline Admiral_dk

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Re: Talking Thermodynamics
« Reply #1343 on: July 10, 2020, 12:12:15 PM »
The picture of the switch imidiately made me think Bi-Metal when I first saw it and these can be adjusted simply with a screw pressing on the Bi-Metal part ....
If this is the case, there should be a "Heat transport system" - so is the yellow rod by any chance copper ?... Certain brass types are ok (not great) in this aplication too ....


Offline MJM460

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Re: Talking Thermodynamics
« Reply #1344 on: July 10, 2020, 12:38:12 PM »
Hi Admiral, it’s certainly a puzzle.

In Willy’s first picture, and implied by the original question, the operation seems to depend on a rod within a brass tube, and perhaps differential expansion, but how that connects to the switch is still mysterious.  I am not so familiar with these things.  The oil industry changed to electronic controllers long ago.  I did not have much to do with the early switches that were used in some applications for lower cost, but did not have a good reputation for reliability.  Hence the change to controllers

MJM460

The more I learn, the more I find that I still have to learn!

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1345 on: August 18, 2020, 02:26:50 AM »
Hi MJM,  we have just hade quite a few days with the mercury going up to about 37 degrees !! this has set me wondering about cooling fans.......if I am in a sealed room and switch on a fan it will cool me down ? however the fan is just moving the ambient temperature about. there is no heating or cooling attachment so why do if feel colder ?? also if the fan is directed at a piece of metal will this also get colder ??... also I saw this comment in a 1917  Model Engineer magazine about a blackened kettle. I think we have talked about this in a previous post , and as this is true are there any tables to support the science ??

Willy

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1346 on: August 18, 2020, 12:53:21 PM »
Hi Willy, you won’t be surprised to find that there is not a simple answer to your question about the fan.  It is necessary to think about what is going on.

Your house is gaining energy through the walls and roof by radiation from the sun as well as conduction from the air.  In addition, depending on how the house is ventilated you may be bringing in some of that 37 degree air.  The ground is a little more complicated.  I suspect that you loose a little heat to the ground during the day, but that is an even more complex question. 

And you being in the room contribute a little more heat, at least until the room is up to 37.  The temperature in the room results from the heat balance between the gains and losses.  How hot you feel depends on the difference between your body temperature and the room temperature.  You might guess that as the room approaches 37 degrees you will get quite uncomfortable (and will probably fail the basic COVID screening test!). You do need to be able to loose enough heat to stop your temperature rising further.

When you switch on the fan, you are adding more energy to the room, so it will become a little warmer.  It’s a very small amount in the grand scheme of things, but directionally, you are heating the room.

So why does the fan feel like it makes you cooler?  Well, it’s because of evaporation.  Under those conditions, I am sure that you will be sweating which will make your skin damp.  The air flow from the fan will evaporate some of that sweat, so taking away the latent heat.  That will cool the remaining moisture on your skin, and that does really help you stay a little cooler.  Part of the secret is of course the humidity.  The humidity has to be low, or there is no evaporation, and it is even more uncomfortable.

If the room is fully sealed, the humidity will rise as you sit there in front of the fan, so the fan becomes less effective.  You need low humidity air coming in from outside to maintain the low humidity inside and expel the excess moisture with the exhaust air.

Because the cooling you feel is dependent on moisture evaporation, the block of metal will not be cooled by the fan unless it has a damp surface.  Providing you measure it with a thermometer, just using your finger to test the temperature will fool you.

The kettle question is similarly complex, and involves heat transfer by conduction, convection and radiation, along with issues of emissivity and absorptivity of the surface, so that will have to wait for some other time.

But I hope that I have helped your understanding of the fan question.

 MJM460


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Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1347 on: August 19, 2020, 02:17:07 AM »
Hi MJM, thanks for this..so yes that makes sense !!  and there are so many variables in these calculations ?!!.  I could do more experiments with the electric boiler as well as the heat input is fixed ?..when I have time  etc etc

Willy

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1348 on: August 19, 2020, 12:59:34 PM »
Hi Willy, I was thinking about yesterday’s topic, if you have a little spare time in these unusual days.  I know that I just don’t seem to have any spare time, despite all the complaints in the media about having nothing to do. 

The Met bureau used to use a device called a sling psychometer to determine humidity.  It had two thermometers, mounted in a frame so air flowed past the bulbs when the device was swung around.  One had a little pot of water and some cotton cloth wrapped around the bulb, with a tail in the water pot, so that the cotton was kept moist by the wicking action.  The temperatures of the wet bulb and the dry bulb were used with a chart like the one attached, to determine the humidity.  I seem to remember that the device had to be swung at arms length and the number of circles per minute was specified, along with the duration.  The chart is quite difficult to work out how to read, these days it is easier to use an electronic device.

Of course most appartments these days don’t have room to swing a psych, (or was that supposed to be a cat?), even if you had one.  But if you read the room temperature with your thermocouple just in still air, then wrap a strip of rag around the end of the thermocouple, with the end of the rag in a cup of water, then once the rag was nicely wet, I would expect that you would see a difference in temperature depending on whether the fan was running or not, so long as you give the reading time to settle.

 You might find the readings a bit hard to interpret if you read the temperature with the cloth wet, but no fan, as the water might take some time to reach equilibrium with the room temperature. 

MJM460
« Last Edit: August 19, 2020, 01:04:18 PM by MJM460 »
The more I learn, the more I find that I still have to learn!

Offline Admiral_dk

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Re: Talking Thermodynamics
« Reply #1349 on: August 19, 2020, 05:14:43 PM »
I learned to do these calculations in my youth - a linear equation if memory serves) - but I never used it for anything => completely forgotten til you mention it, or more correctly - I remember the wet and dry bulb bits and the fact that you could use it to calculate the 'missing parameter' in indoor and outdoor climate.