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ecogeorge
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« Reply #15 on: February 08, 2010, 08:09:18 PM » |
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What gas are they using these days ? my understanding was that R134a (used in cars) boils at minus 28c.
rgds George.
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desperate
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« Reply #16 on: February 08, 2010, 08:20:05 PM » |
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On the back of our ff it says pentane dunno what the number of that is. Butane IIRC boils at -0.5 at atmospheric pressure, I would have thought Pentane would boil at a slightly higher temp than that, which sounds a bit odd for a freezer capable of -18??
Desp
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ecogeorge
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« Reply #17 on: February 08, 2010, 09:41:15 PM » |
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Try http://en.wikipedia.org/wiki/Pentaneboils at 36c !!! Can't work out how it's used as a refrigerant , I guess its run at lower than atmospheric pressures - long time since Boyles / Charles law  |
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desperate
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« Reply #18 on: February 09, 2010, 11:49:22 AM » |
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Definitely says Pentane on the back, the mechanicals all look pretty standard to me, maybe the radiator is bigger than normal? mind you it is a big ff. From that Wiki link you posted Iso and Neo pentane dont look any more suitable??
Desperate
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Ivan
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« Reply #19 on: February 11, 2010, 12:01:52 AM » |
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it's the freezing point, not the boiling point, that's critical.
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desperate
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« Reply #20 on: February 11, 2010, 06:12:36 PM » |
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Ehh? Surely as the phase changes from liquid to vapour, energy is absorbed from the inside of the fridge, if that occurs at 35deg c at atmospheric pressure it will be a high temp fridge.........wont it??
I cant see how the solid/liquid change works.
Desperate
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knighty
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« Reply #21 on: February 11, 2010, 11:34:49 PM » |
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Ehh? Surely as the phase changes from liquid to vapour, energy is absorbed from the inside of the fridge, if that occurs at 35deg c at atmospheric pressure it will be a high temp fridge.........wont it??
I cant see how the solid/liquid change works.
Desperate
the gas is under vaccume on the cold side and the liquid is under pressure on the hot side add plenty of pressure and the gas condences back to liquid where you dump all the heat then on the cold side it's under a vaccume to force it to evaporate into a gass |
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desperate
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« Reply #22 on: February 11, 2010, 11:48:31 PM » |
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wait a minute, you cant have a gas in a vacuum. As I understand it starting at the compressor the gas is compressed causing it to liquify, and heat up,then the liquid is cooled in the radiator and allowed to go through a jet, thus reducing the pressure rapidly which causes a drop in temperature, so you have a gas at a very low temp, now if that gas condenses at 36 deg c it would turn back to a liquid if it is at a low temp. on passing through the freezer it must boil again completely back to a gas or it will damage the compressor, wouldn't it? Hmm it all made so much sense in my brain before that glass of whiskey  Desperate |
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knighty
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« Reply #23 on: February 12, 2010, 12:08:29 AM » |
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you can have a gas in a vacuume..... just not in a total vacuume (where there's nothing at all)
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desperate
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« Reply #24 on: February 12, 2010, 08:02:07 PM » |
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Surely a vacuum is a..............vacuum Desp |
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KLD
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« Reply #25 on: February 12, 2010, 10:59:21 PM » |
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The term vacuum is a bit misleading here, I think. It's really that the pressure is below normal atmospheric pressure. The boiling point of 36°C is measured at normal pressure (probably 1bar absolute, or is it still the old fashioned 760mm Hg ?). When you reduce that pressure, the liquid starts boiling at a lower temperature. For instance, if you put a pan full of 40°C warm water into a pressure chamber and lower the pressure, then this 40°C water will start boiling once the pressure has dropped by about a factor 10 (so instead of 1bar the chamber is now at 0.1bar). Same holds for pentane in a refrigerator cycle: the expansion nozzle allows only so much pentane to evaporate, and the compressor keeps the amount of gas at a chosen low level, read: low pressure. For here on I can only speculate, but hey! it's Friday night. The varying room temperatures can only influence the room-side heat exchanger, i.e all the parts in the (half-)cycle from compressor through the cooling coils to the expansion nozzle. So, the compressor does its job and produces a hot gas, which then is cooled in the heat exchanger coils and condenses. Now, the final temperature of this fluid will depend on the efficiency of the HE, and thus on the room temperature. Cold room temperatures will lead to a colder fluid. At the nozzle, where the pressurised fluid can expand back into the gas phase, the rate of expansion will depend on the pressure difference across the nozzle, and the temperature of the fluid. If the rate of expansion (ie. the rate at which gas is produced) is too low, then the cycle collapses. QED Well, could all be complete bollxx , but was fun writing Klaus |
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knighty
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« Reply #26 on: February 13, 2010, 09:30:01 AM » |
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mabe it would be easier to understand if it was described as under suction instead of under a vacuum ? also... The boiling points of the pentane isomers range from about 9 to 36°C. As is the case for other alkanes, the more branched isomers tend to have lower boiling points most refrigeration gasses I know of are a mix of different gasses... (could it be http://en.wikipedia.org/wiki/PentafluoroethaneI guess the higher boiling point helps stop it being an ozone gass etc... (because it can't rffect the ozone if it's a liquid) (bit of a guess) |
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charlieb
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« Reply #27 on: February 15, 2010, 08:57:43 PM » |
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Cold room temperatures will lead to a colder fluid. At the nozzle, where the pressurised fluid can expand back into the gas phase, the rate of expansion will depend on the pressure difference across the nozzle, and the temperature of the fluid. If the rate of expansion (ie. the rate at which gas is produced) is too low, then the cycle collapses. QED
But does the cycle collapsing mean the freezer explodes? Or will it just not work so great at below freezing (which isn't a big deal, as it's freezing outside anyway). If the cycle will pick back up again when room temp gets back to normal then they can go ahead and buy a standard freezer. |
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KLD
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« Reply #28 on: February 15, 2010, 09:06:17 PM » |
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Ah, no, no explosion I'm afraid.
Whether or not the process is self-recovering I don't know, sadly. One of our small fridges has a little freezer compartment inside the main box. When that fridge sat in an unheated spare room the other winter, it did stop working when the room temperature dropped below 10°C or 8°C. I didn't have the nerve to wait for it to self recover. There was some meat in the freezer, so we had quite a feast as we didn't want to re-freeze the stuff =:^))
Klaus
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knighty
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« Reply #29 on: February 16, 2010, 02:20:04 AM » |
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it won't blow up....
it *could* damage it.... if the compressor sucks in liquid instad of gas and tries to compress it... then it can damage the compressor...
but... it's not very likely that it'll happen with a home freezer.... it's mostly only a problem for for big commercial/industrial fridges/freezers
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