Author Topic: Talking Thermodynamics  (Read 136822 times)

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1380 on: September 11, 2020, 01:52:24 PM »
Hi Willy, I really do not know much about fermentation -another teetotaller.  I know Ginger beer is usually fermented in a sealed bottle, and the process involves pressure buildup due to the CO2, but I don’t think pressure tight lids are used for all fermentation processes.

I shall watch the continuing experiment with interest.

Hi Paul, it looks like that rubberised material I’ll might be ok for the outside of a boiler where flames are not expected.  I am not sure of the idea behind the water soaking series.  Was this to see if soaking made it easier to bend around the boiler, or just to see what happens if the insulation did accidentally get wet.

 It certainly looks promising for boiler insulation, as I assume 150 degrees would normally be enough.

I bought some insulation intended for engine exhaust manifolds, hoping it would be good for a boiler casing.  I also tried the propane torch test and found that it caught fire without too much persuasion.  It is still working over a tin plate fire box so no direct flame impingement, but I am not sure about its use on an exhaust manifold.  I had previously thought that reports of race car engine fires might have been due to that insulation becoming oil soaked over time, but a simple test showed that the oil was not a necessary component.

MJM460

« Last Edit: September 11, 2020, 11:26:28 PM by MJM460 »
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Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1381 on: September 11, 2020, 02:33:49 PM »
Hi Paul, I was also doing an experiment using cork matting around a kettle ...a bit different to yours though as it was just checking up on the thermal characteristics !! this was on the post  No 1067...Page 71. 72 Also on my Moriss Minor the rocker box gasket is made of a compost cork that withstands the high temp ?!!

Willy
« Last Edit: September 11, 2020, 02:46:19 PM by steam guy willy »

Offline paul gough

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Re: Talking Thermodynamics
« Reply #1382 on: September 11, 2020, 06:23:15 PM »
Hi Willy, Thanks for your reference. The kitchen is certainly a useful laboratory, if the supervisor grants workshop staff access.

MJM, Boiling the material in a cylindrical form was to test the integrity of the material in a wet condition and at a temperature that would reasonably approximate the conditions under metal boiler cladding if things got wet. I also wanted to see if the material retained its flexibility or retained the tubular shape afterwards and whether it would be suitable gasket material in a hot well. The sheet seems to be flexible enough to use around small pipe. I'm afraid I do not understand what you mean in your question, ".....,or just to see what happens if the insulation did accidentally get red."

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1383 on: September 11, 2020, 11:25:41 PM »
Hi Paul, apologies, that was a typo, though I suspect that Apple Intelligence (AI) also played a part.  I cannot see how a typo gets from “wet” to “red”.  I read it through three times before posting, and it still got through.  I will go back and correct it.

As a hot well gasket, I wonder if time will be your enemy with the rubber component.  Will it stick and make the surfaces difficult to part after sitting for some time, especially if left tight?

MJM460
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Offline paul gough

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Re: Talking Thermodynamics
« Reply #1384 on: September 12, 2020, 12:59:01 AM »
Hi MJM, AI = Automatic Idiocy in these cases.

As you said in your previous post on Sept 10, sticking might be an issue, but  the tests seem to indicate otherwise. In fact there is a very slight 'drying' effect to the surface of the sheet which I put down to 'gassing' of the bonding medium. However, 15 minutes in a pot or oven is not the same as many hours of operation and years tightly pressed against a boiler barrel under metal cladding or secured tightly as a gasket. Time being the issue. Paul Gough.

Offline paul gough

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Re: Talking Thermodynamics
« Reply #1385 on: September 16, 2020, 11:33:16 AM »
Hi MJM, I am having trouble finding our discussion on small boiler insulation that might guide me for future applications. I did find this article as as something of a guide which is from; radiationbarrier.com/resources:

"Although two objects may be identical, if the surface of one were covered with a material of 90% emissivity, and the surface of the other with a material of 5% emissivity, the result would be a drastic difference in the rate of radiation flow from these two objects. This is demonstrated by comparison of four identical, equally heated iron radiators covered with different materials. Paint one with aluminum paint and another with ordinary enamel. Cover the third with asbestos and the fourth with aluminum foil. Although all have the same temperature, the one covered with aluminum foil would radiate the least (lowest [5%] emissivity). The radiators covered with ordinary paint or asbestos would radiate most because they have the highest emissivity (even higher than the original iron). Painting over the aluminum paint or foil with ordinary paint changes the surface to 90% emissivity."

From this I take it that wrapping our little boilers with aluminium foil is, perhaps, a good idea. But how far does one take this. Should we have just one foil layer and where, eg against the barrel or around the outside of the cork or Kaowool layer. Or, should we have two foil layers one against the barrel and one around the insulation. Finally is there any practical value in repeating this foil/insulation layering. I am thinking here of kitchen aluminium foil. Would you care to comment on this please. Regards, Paul Gough.

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1386 on: September 16, 2020, 02:02:03 PM »
Hi Paul,

I suggest that the key to understanding this issue is in understanding the different modes of heat transfer that are occurring.

One of the early lessons on heat transfer is that there are three modes of heat transfer, conduction, convection and radiation.  In effect only two modes, conduction and radiation, as convection is basically conduction involving a fluid.  The effect of the heat on the density of the fluid starts movement due to buoyancy in natural convection or due the the fluid velocity in forced convection, making a huge difference to the transfer rates by changing the temperature gradient near the heat transfer surface, hence it is usually analysed as a separate mode.  Usually, one mode is more important than the others, and the analysis is limited to one mode.  In reality, usually, all three modes are involved, and combined mode problems are quite important in some problems.

The article you have quoted is talking about radiation.  The difficulty in applying this directly to a boiler insulation comes in determining whether most of the boiler heat loss is by radiation, or whether conduction and convection are significant.  In addition, the surface temperature of the material is an important factor.

The figures quoted all assume the surfaces are at the same temperature, and all heat transfer is by radiation.  However, in the boiler case, we are actually interested in loss from the boiler internal contents (steam and water), so the relevant starting temperature is the steam temperature.

Now look at the heat path and the temperature profile from the steam, through the copper shell, somewhat imperfect contact between the shell and the insulating layer, convection from the insulation surface and finally to the bulk ambient temperature of the room and the wall temperature of the building walls for an inside layout, or the sky and surrounding objects if out doors.

Each interface involves a resistance to heat transfer and hence a temperature difference.  So for our boiler, the asbestos - these days we should use ceramic fibre or cork - is heated on the inside surface by that imperfect contact with the shell, and will introduce a resistance to heat transfer through the solid material to the outside surface, so the outside surface will be at a lower temperature than the steam.  The article talks about radiation, so we need to understand that radiation heat transfer is proportional to the fourth power of the absolute temperature.  The absolute temperature bit means we are talking about large numbers, and the fourth power emphasises even small difference.  And the insulating properties of the asbestos means that we are starting at a temperature well below the steam temperature.

On the other hand, aluminium is a good conductor, and even with that imperfect contact with the shell, will be quite close to steam temperature, so the difference in heat loss between aluminium and asbestos will not be as great as first appears by a pure radiation calculation made assuming identical surface temperatures.  And the asbestos surface temperature will be highly dependent on the thickness of the material.  I don’t really know what a detailed comparison would show.

My marine boiler is insulated with cork and timber strips, and the surface temperature is about 50 degrees measured with an infrared temperature monitor, while a foil of aluminium would be much close to steam temperature.  The steam temperature is usually about 120 degreesC.

A layer of foil is a very useful radiation shield.  However it needs to be spaced from the shell to minimise contact, so relying mostly on radiation from the shell combined with convection to heat the foil.  Then the foil will be reach a temperature lower than the boiler shell or the steam.  It is quite reasonable to then support a second layer, again spaced, perhaps by rings or strips cork or timber, so the radiation becomes a two step process and the temperature drops in two steps, each governed by that difference in T^4.  I vaguely remember doing the calculations many years ago, and more layers are useful, but economics becomes a limit, as does the point where convection becomes more significant.  Or resulting outside dimensions for a small model locomotive.

I know what you mean about finding a topic in this thread, as it has a huge number of pages, but I have found that first opening the thread, then using the forum search function, is quite good at turning up relevant posts.  I tried “boiler insulation” just now, and the first two hits were posts from you on this subject, about mid September 2019, so that might help you find the rest of our previous discussion.  There was quite a lot there, as I did find that the discussion refreshed my memory and understanding as I trawled my textbooks for the correct formulae, and data, and applied the various concepts to the topic at hand.  Part of the fun of this thread was the remembering those topics from study and work days, and finding how it could be applied to our models.  I was often quite surprised at how much could be achieved by just applying those first principles as the topics developed.  It would take me quite a while to get back to that level again, as my memory doesn’t hold that level of detail these days, in fact, I don’t think it ever did.  But all that aside, I am quite happy to continue the conversation if it will help.

I hope that helps, or at least prompts some more questions,

MJM460

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

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1387 on: September 16, 2020, 02:09:31 PM »
Whoops, Paul, I should have read those first few posts that turned up more carefully.  However, if you go down the search results to about 24, we were talking about thickness on your boilers. 

More careful selection of the search term might find them more selectively.  Perhaps search for radiation?

MJM460
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Offline paul gough

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Re: Talking Thermodynamics
« Reply #1388 on: September 16, 2020, 08:59:02 PM »
Hi MJM, Thank you for your note regarding doing a search after opening the thread, I had failed to do this, I guess I'll never understand computer operation protocols, even at the most basic level.

I'm sorry for not starting my last paragraph with the context setting phrase, "With respect to radiant losses", this would have saved you a lot of typing. I could not remember or find any conclusions as to whether foil might be advantageous to our small boilers in combination with insulation in a single or multiple sandwich layer(s). Once we insulate our boilers it seems there is no more to do regarding convective or conduction losses, they are what they are, so I wondered if there might be anything to be gained by trying to claw back some radiation losses, even if small. As there is practically no dimensional or economic penalty in adding single or multiple layers of foil, I thought it worth investigating. Thank you for you efforts in explaining things. Regards, Paul Gough.

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1389 on: September 17, 2020, 01:50:28 PM »
Hi Paul, the theoretical calculations are quite helpful in letting us know what is going on and what are the critical variables.

Unfortunately, the calculations all depend on general typical data for the various parameters, so in order to get a really definitive answer, we need to do some practical experiments and tests to improve the accuracy of the data to be used in a particular case.

In addition to the difference in T^4 and emissivity that we have already discussed, there are also view factors between the areas being considered. 

Then the convection coefficient also depends on several factors, so after a theoretical exploration to discover the critical factors the best way (and possibly only) way to definitively answer your question is a series of experiments. 

It depends on how accessible your boiler is, to try a couple of different schemes, but in the end, it is the best way.  The different experiments can be compared by plotting cooling curves as the boiler cools. This method is sensitive enough to determine not only which is best, but even the relative difference between different insulation systems.  Sometime differences are small and convenience can outweigh very small differences unless you are trying to win a race.

I find a practical method is to make a replacement filler plug with an internal extension, drilled with a blind hole to accept a thermocouple, as supplied with most digital multimeters these days.  In industry, it is called a thermowell, and provides a way to insert the thermocouple deep into the high temperature zone without breaching pressure containment.  Then a watch to measure the time intervals, a digital kitchen scale to measure the water mass in the boiler and a quiet afternoon activity.  From these curves, the actual heat loss can be calculated for as many variations of the insulation scheme as you have the patience to carry out.  I can help you with the maths if you decide to give it a try.

One thing I meant to mention about radiation is regarding radiation heat transfer through a gas such as air.   Air in mostly transparent to heat, but some heat is absorbed by air and even some reflected back.  And importantly, the degree of absorption is highly dependent on the wavelength of the radiant heat, and is different for different gases..  This is the basis of the often quoted “greenhouse effect”.  So even with good data, a complex calculation by wavelength for each component of the gas is required to provide a thorough calculation.  That is a more specialised area than my general overview of thermodynamics.

And I am totally with you on the computing protocols.  I am sure there are ways to search only for articles which contain all the search terms instead of just any of the terms, but I’m blowed if I can remember the syntax.  I tend to avoid all but simple searches and as few links as possible.

MJM460

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