Author Topic: Talking Thermodynamics  (Read 139056 times)

Online steam guy willy

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Re: Talking Thermodynamics
« Reply #1350 on: August 20, 2020, 03:14:04 AM »
HI MJM, thanks for that and i also have seen the wet, and dry bulbs. one of them was circular in shape  but never examined them in detail

Willy

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1351 on: August 20, 2020, 12:57:09 PM »
Hi Admiral, an equation would certainly be easier than that ASHRAE chart, but I suspect no one does those calculations these days as we all have access to those little electronic devices that just read the humidity directly.  The sensors are even available to combine with microprocessors so you can build your own version.  The ones the met bureau use are probably more expensive though.  I must admit that I have never thought of trying to derive an equation, something to ponder on a winter evening.

Hi Willy, the main thing is the inclusion of two thermometers, one sees the straight air flow for a direct temperature measurement, the dry bulb, and the other surrounded by a cotton wick which is kept damp with water, the wet bulb.  The airflow encourages evaporation of the water which results in a lower temperature.  Much the same in the washroom with those air driers.  The air feels quite cool, but warms to become even hot as your hands dry.  (If you can stand the noise long enough).  In reality, the air reaches full temperature quite quickly, but what you feel is affected by the evaporation.  But as I am sure you know, the best way to dry your hands with one of those things is 10 seconds in the airstream the dry them on your pants.  I assume ladies use their skirts.

MJM460



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Offline Admiral_dk

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Re: Talking Thermodynamics
« Reply #1352 on: August 20, 2020, 07:13:30 PM »
Sorry that I can't help with the equation as I got rid off all the study material that didn't directly cover electronics just a few years after I left ....

The above made me Google "wet and dry bulb formula" and there are the useful stuff as first hit :
http://www.1728.org/relhum.htm   and it includes an example on how to use the (3) formulas  :ThumbsUp:
This is great, as it's quite a bit more complicated, than I remembered  :old:

Per

Online steam guy willy

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Re: Talking Thermodynamics
« Reply #1353 on: August 21, 2020, 02:34:22 AM »
Hi Admiral,...this is quite an amazing formulae and accurate to 14 decimal places !!!! or is there a typo ??


Thanks

Willy

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1354 on: August 21, 2020, 12:18:16 PM »
Hi Admiral, thanks for those links.  I am really glad to see that formula, as I had a bit of a play with a spread sheet and steam tables, and could not see how to get a linear result unless over a very limited range.  And it seemed that the parameters would have to change depending on the dry bulb you started with.

To see this formula was very encouraging, especially as I tried some curve fitting and the best fit was actually the power law. But I can understand anyone not remembering it, its quite a mouthful.

Even though the formula looks a bit daunting, with a modern scientific calculator, or better still, a spreadsheet, it is pretty easy to handle, compared with the days when we depended on a slide rule.

Hi Willy, that e factor Is the base for natural logarithms, and if I remember correctly, the number of decimal places to specify it exactly has no end, one of those irrational numbers, I think it is called.  But in the end, the answer is no more accurate than the readings on the thermometers used to read the wet and dry bulb temperatures.  But it never hurts to have a few extra significant figures on the constants, so they do not introduce extra errors.  Nevertheless, 14 places is a bit excessive in any circumstances, but if you put e into the calculator or spreadsheet, the calculator will use the number of places it can handle, but as you probably remember, with a slide rule, three significant figures was about the best that could be done.

I have been puzzling over what the other constants might be, and at this stage, I have no constructive ideas to offer.  I dont know whether they they are resulting from unit conversions, or other factors.  Perhaps Admiral or one of the other forum members can shed some light on them.  But I think I will just stick with the little electronic display in my workshop.

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 #1355 on: August 21, 2020, 12:25:45 PM »
Thank you for explaining it for Willy MJM  :ThumbsUp:

I forgot to include the only other useful result from the Google search :

https://en.wikipedia.org/wiki/Wet-bulb_temperature

Strangely not using the same formulas - but still a great and a lot more detailed explanation + nice examples from real life .... Notice still NOT Peer Reviewed yet.

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1356 on: August 23, 2020, 02:23:33 PM »
Hi Admiral, I hope I did not steal your thunder in my enthusiasm to continue the conversation.

The Wikipedia article is quite interesting.  Not sure that I totally follow it. We have plenty of areas in Australia that experience dry bulb temperatures above 40 degrees, and heat stress is a serious issue in those areas.  Fortunately the humidity is generally quite low, so wet bulb temperatures are considerably below the dry bulb temperatures.

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

Online steam guy willy

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Re: Talking Thermodynamics
« Reply #1357 on: September 07, 2020, 11:49:51 PM »
Hi MJM,  The show is now over and so much good stuff to look at and study.... I have been pressing some apple juice and bottling it in coke bottles that have this bulbous type of bottom... I have noticed that after a while the bubbles have taken up these shapes that mirror the bottom of the bottle ?? when i put the juice in a flat bottom bottle the bubbles are perfectly flat and even ... so is there some sort of thermodynamics going on here ??

Thanks

Willy

Online crueby

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Re: Talking Thermodynamics
« Reply #1358 on: September 08, 2020, 12:18:14 AM »
More like nucleation patterns in the bubble formation. Either those depressions are a rougher surface, or more likely there are particles of apple that are settling in the depressions. Either will give bubbles a friendly place to form.

Online steam guy willy

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Re: Talking Thermodynamics
« Reply #1359 on: September 08, 2020, 01:04:20 AM »
hi Chris, Ok, Interesting hypothesis... I will have to think about this and possibly learn some new words...thanks

willy

Online crueby

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Re: Talking Thermodynamics
« Reply #1360 on: September 08, 2020, 03:12:38 AM »
Willy, I learned about such things back when I was a firmware/image-science engineer working on inkjet printers for a decade or so, and was on a team with some really good chemists and fluidics experts. Inkjet printers like you might have on your PC are amazing little machines, with tiny rows of tiny chambers filled with ink, with a tiny little heating element at the back and a tiny little hole at the front (using up a box of the word tiny, I know, but they spit out a 3 to 6 picoliter drop, which is tiny!) . When the heater is pulsed with electricity in the right waveform, it gets hot enough to vaporize some of the water in the ink, causing a little (yes, tiny) bubble to form that fills the chamber, forcing a tiny drop of ink out at very high speed towards the paper. Imagine rows of these firing at 24 to 48 khz, all spaced out at 600 or 1200 chambers per inch. We had a statistician who wanted to do a life test on a print head, and not knowing better set one up, sitting over a beaker, to fire all nozzles of all colors at full speed constantly, so the test would go quickly. It did. The head got so hot it melted the nozzle chambers, the plastic holding the nozzle chip, the plastic carrier, and part of the ink tanks! :Mad:

Anyway, in a 'machine' like that, unintended bubbles in the lines or chambers are a very bad thing, so it was a frequent topic of discussion. I learned that in a saturated solution, bubbles of air, CO2, whatever, want to come out of solution, and 'nucleation sites' are where a lot of it happens - just a pit or imperfection in a smooth surface where the surface tension of the water causes a bubble to stick, grow, release when it gets too big leaving a proto-bubble behind, which starts it all over again. You see it in glasses of anything fizzy - pop, champaign, beer, whatever, where you see a single spot at the bottom generating a never ending stream of bubbles. Now, if your apple juice is 'contaminated' with some yeast especially, it is constantly putting more CO2 into solution, and it wants to get out. Any rough surface, or particle of apple or yeast at the bottom is a perfect spot to form bubbles. In your bottle, the dimples at the bottom will collect those bits and concentrate the bubble formation.
If it gets to a nice alcohol concentration, all you can do is quietly consume the evidence!   :DrinkPint:

Offline paul gough

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Re: Talking Thermodynamics
« Reply #1361 on: September 08, 2020, 06:36:52 AM »
Hi Chris & Willy, Chris your succinct exposition on the 'micro' workings of a printer was very interesting. Half jokingly, I wondered how we could co-opt the overheating process from pulsed 'sqirters' and adapt or harness such a process to fire Willy's electric boiler.

The description of bubble propagation from imperfect surfaces made me think of the often relatively 'rough as guts' surfaces on our steam, (or pneumatic for some), circuits and the turbulence engendered. I also wondered if these surface 'pits or pores' might not attract and hold minute quantities of condensate which were 'seeding' further condensation, causing even more disturbances to perfect flow. Regards, Paul Gough.

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1362 on: September 08, 2020, 11:01:26 AM »
Hi Chris, I think you have nailed it.  I had been trying to think of subtle variations in the airflow around those shapes and the minute temperature changes they might cause, but nothing seemed convincing.

That nucleation process occurs in many areas, but that explanation of the inkjet printer is extremely interesting and certainly moves the science of nucleation to another level.  Thank you. 

Hi Willy, another great question.  Very observant of you.  Or perhaps you make large quantities of cider?

Hi Paul, good to hear from you again.  I hope you are well and that your little locomotives are still running around the track.

MJM460

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

Offline AVTUR

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Re: Talking Thermodynamics
« Reply #1363 on: September 08, 2020, 11:31:18 AM »
For a liquid to boil or freeze or a gas to come out of soliution or liquify it needs something to attach to, a nucleation point. I remember as a kid putting asprin in Cola-Cola. More seriously, the water in the atmosphere needs nucleation to produce clouds and rain. Usually dust does the job but man has been known to intervene using suitable crystals (silver nitrate commons to mind). Molten metal need nucleation points to solidify, usually the side of the mould but sometimes a seed crystal is added to grow a single crystal of metal. Further some chemical reactions will only occur on nucleation sites, the production of smoke during combustion being one.

AVTUR
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Offline paul gough

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Re: Talking Thermodynamics
« Reply #1364 on: September 08, 2020, 12:35:51 PM »
Hi MJM, Too much time with oncologists and cardiologists to have done anything over the past year, but they both lost interest in me so suppose I'll be fine. As to the little loco, I am planning on inverting the cylinders, steam chests underneath. This will allow replacing the front driver,( as an 0-4-2), and replacing it with a pony wheel, turning it into a 2-2-2. the cylinder inversion is necessary to accommodate the the lower axle height which is located between the cylinder and valve rod guides/stuffing boxes. Have to try running the cylinders upside down with steam first before I do anything much to see if the valves will seal and lubrication of surfaces are satisfactory. Also might have to put a drain on the steam chest. Very much hope to get a 'Sherline' set up in the near future as these machines seem to be appropriate for my tiny work and if worst comes to worst they'll fit in my coffin and I can take them with me anywhere. Though they'll probably melt where I'm headed. Regards, Paul Gough