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Upsilon
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You know, I may just try out the pressure cooker after all. I don't think it would be too difficult to layer the aluminum with some form of protection
from corrosion. It would also be much more spacious than a PVC pipe. I see a nice for-parts one on the Goodwill website, shipped to my door for $7; I
really can't refuse that. I'll see what I can do with it.
Also, I'm not sure that the vacuum you describe is a very strong vacuum - it is only caused by the contraction of the gas inside the cooker upon
cooling. Some calculations would probably reveal how strong of a vacuum it is, but I don't really feel like doing them.
EDIT: I misread, shipping is actually $15. Still, might be worth a shot.
[Edited on 1-11-2015 by Upsilon]
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careysub
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If you cool the pot to 45 C before opening you are pulling 90% vacuum, a perfect vacuum would not contribute appreciably more force.
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Upsilon
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Out of curiosity, could wood be used as an insulator in a vacuum? Since there is no oxygen for it to combust with, what happens to it when it gets
hot, and what temperature does this occur at?
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careysub
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To start off with out-gassing while trying to bring down the vacuum could be a nuisance.
Wood will pyrolyze at a high enough temperature (starting at 200 C) even in an inert atmosphere or vacuum (releasing more gases), and suppose the
inner surface of the wood gets really hot - and then your break the vacuum to end the experiment?
Instant combustion is a real possibility.
There are good non-combustible insulators readily available: fiberglass, loose perlite, cermaic suppliers have a variety of suitable products. I have
Kaowool from Seattle Pottery. for example. I see Thermwell As5 Cermic Fiber for $11 on Amazon.
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Upsilon
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Ok. I wasn't actually planning on using wood as an insulator, I was just curious about what happens to it when it gets hot without oxygen.
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Upsilon
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Alright I decided to go for the PVC pipe chamber just for better customization and workability. I have already gotten a 1 foot long section of 6 inch
diameter schedule 40 PVC pipe. Wall is 5/16 of an inch thick. Internal pressure rating is 180 PSI so I think it should work just fine for holding a
vacuum. I also got a small 1/4 inch thick polycarbonate window. Once I get end pieces for the pipe (one end will just be a cap cemented on wile the
other will have a screw cap fitting). I'll cut a hole out of the screw cap to place the polycarbonate window over.
For including a heating element, I have decided to use induction hearing - the only apparatus inside the chamber would be a steel disk. The induction
coils will be around the outside of the chamber.
After this I'll need to add some fiberglass insulation sandwiched between the inner wall of the pipe and a layer of aluminum foil. Being in a vacuum,
the pipe should remain well below its decomposition temperature with the aluminum foil deflecting radiation away from the walls. The fiberglass is
just for added protection. This will also make the heater very very efficient.
Later on I may add onto the design. Something I'll likely do in the future is have some electrode ports for conducting sensitive electrolysis
experiments, e.g. sodium from sodium hydroxide. I don't think I'll be giving it inert gas capabilities, especially since having a gas inside the
chamber risks heating the PVC too much.
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Upsilon
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So, uh, I've got an issue - I can't find any PVC caps that fit the pipe I bought. The pipe has a 6.625" (6 5/8") outer diameter and a 6" inner
diameter. I went to the local home improvement store and all of their caps were only a 6" inner diameter when I need a 6.625" inner diameter. I can't
find squat online either - any help here?
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Magpie
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What you have is 6" schedule 40 PVC pipe. A 6" pipe cap should fit that. Home Depot claims to have 6" schedule 40 PVC pipe caps: http://www.homedepot.com/s/6%2522%2520PVC%2520pipe%2520caps?...
The single most important condition for a successful synthesis is good mixing - Nicodem
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Upsilon
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Unfortunately it doesn't seem to be that simple. I was at Lowes and they had this there:
http://m.lowes.com/pd/Charlotte-Pipe-6-in-dia-PVC-Cap-Fittin...
However for some stupid reason these caps seem to be 6" inner diameter instead of fitting 6" schedule 40 pipe. Maybe they had the wrong thing in the
store - I may try Home Depot next.
[Edited on 7-11-2015 by Upsilon]
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Magpie
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That cap is for DWV (drain/waste/vent) and as such may have a smaller ID for the thinner wall pipe than for schedule 40 which is a pressure pipe size.
The single most important condition for a successful synthesis is good mixing - Nicodem
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careysub
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Quote: Originally posted by Upsilon  |
After this I'll need to add some fiberglass insulation sandwiched between the inner wall of the pipe and a layer of aluminum foil. Being in a vacuum,
the pipe should remain well below its decomposition temperature with the aluminum foil deflecting radiation away from the walls. The fiberglass is
just for added protection. This will also make the heater very very efficient. |
How hard do you expect the vacuum to be?
The reduced pressure may be less effective as insulation than you think.
People are always surprised to learn that vacuums do not reduce heat conduction through gases *unless* the pressure is so low that the mean free path
is larger than the dimensions of the space across which the heat is being transported. (I agree it is surprising - the reason for it is that although
fewer molecules are hitting a surface, they carry heat farther, so that it balances exactly.) The ratio of the MFP to the distance scale is called the
Knudsen number, when Kn > 1 the low pressure gas acts as an insulator.
Since the MFP at STP is 66 nm this means the pressure must be 0.005 torr to increase the MFP to 1 cm. At 1 torr (5 micron MFP) fiberglass insulation
is improved in it insulation value by about 1/3 over atmospheric, which is something - but not a lot. You get the same improvement (1/3) by replacing
the air with argon or CO2. To get a really substantial improvement you need to go down to <0.1 torr.
You best bet is pack the chamber with fiberglass insulation pretty tightly - this suppresses convection, reduces the MFP in the insulation, and makes
the insulation opaque to radiation conduction (glass is opaque to thermal radiation). A final foil layer against the PVC is a good idea.
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Upsilon
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| Quote: Originally posted by careysub |
How hard do you expect the vacuum to be?
The reduced pressure may be less effective as insulation than you think.
People are always surprised to learn that vacuums do not reduce heat conduction through gases *unless* the pressure is so low that the mean free path
is larger than the dimensions of the space across which the heat is being transported. (I agree it is surprising - the reason for it is that although
fewer molecules are hitting a surface, they carry heat farther, so that it balances exactly.) The ratio of the MFP to the distance scale is called the
Knudsen number, when Kn > 1 the low pressure gas acts as an insulator.
Since the MFP at STP is 66 nm this means the pressure must be 0.005 torr to increase the MFP to 1 cm. At 1 torr (5 micron MFP) fiberglass insulation
is improved in it insulation value by about 1/3 over atmospheric, which is something - but not a lot. You get the same improvement (1/3) by replacing
the air with argon or CO2. To get a really substantial improvement you need to go down to <0.1 torr.
You best bet is pack the chamber with fiberglass insulation pretty tightly - this suppresses convection, reduces the MFP in the insulation, and makes
the insulation opaque to radiation conduction (glass is opaque to thermal radiation). A final foil layer against the PVC is a good idea.
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That'll definitely be something to test once I get it built. I have an old 5 CFM 2-stage vacuum pump (an equivalent modern unit seems to be ~$200) so
it should pull a decent vacuum, but I won't know for sure until I test it.
EDIT: These new equivalent models seem to claim ability to achieve 0.3 Pascals which is an insanely strong vacuum.
[Edited on 7-11-2015 by Upsilon]
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annaandherdad
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| Quote: Originally posted by careysub |
People are always surprised to learn that vacuums do not reduce heat conduction through gases *unless* the pressure is so low that the mean free path
is larger than the dimensions of the space across which the heat is being transported. (I agree it is surprising - the reason for it is that although
fewer molecules are hitting a surface, they carry heat farther, so that it balances exactly.) The ratio of the MFP to the distance scale is called the
Knudsen number, when Kn > 1 the low pressure gas acts as an insulator.
Since the MFP at STP is 66 nm this means the pressure must be 0.005 torr to increase the MFP to 1 cm. At 1 torr (5 micron MFP) fiberglass insulation
is improved in it insulation value by about 1/3 over atmospheric, which is something - but not a lot. You get the same improvement (1/3) by replacing
the air with argon or CO2. To get a really substantial improvement you need to go down to <0.1 torr.
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I'm impressed by your knowledge. The old thermos bottles you used to be able to buy, they were glass with vacuum and silvered to cut down on the
radiation...sounds like you're saying they must have had a really good vacuum, about .01 torr. All I ever knew was that they were easy to break.
Any other SF Bay chemists?
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careysub
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| Quote: Originally posted by annaandherdad | | Quote: Originally posted by careysub |
People are always surprised to learn that vacuums do not reduce heat conduction through gases *unless* the pressure is so low that the mean free path
is larger than the dimensions of the space across which the heat is being transported. (I agree it is surprising - the reason for it is that although
fewer molecules are hitting a surface, they carry heat farther, so that it balances exactly.) The ratio of the MFP to the distance scale is called the
Knudsen number, when Kn > 1 the low pressure gas acts as an insulator.
Since the MFP at STP is 66 nm this means the pressure must be 0.005 torr to increase the MFP to 1 cm. At 1 torr (5 micron MFP) fiberglass insulation
is improved in it insulation value by about 1/3 over atmospheric, which is something - but not a lot. You get the same improvement (1/3) by replacing
the air with argon or CO2. To get a really substantial improvement you need to go down to <0.1 torr.
|
I'm impressed by your knowledge. The old thermos bottles you used to be able to buy, they were glass with vacuum and silvered to cut down on the
radiation...sounds like you're saying they must have had a really good vacuum, about .01 torr. All I ever knew was that they were easy to break.
|
Thanks.
Yep. They had really good vacuums, about 0.001 torr in fact.
And, yes, they were easy to break, I remember getting to school in the wintertime as a child and having my heart sink when I heard my thermos of hot
cocoa going "slush-slush" when you moved it.
Kn > 1 is also a necessary condition for Crookes radiometers to work (those black and white spinning vanes in a glass bulb). The force that spins
them is the black vane surfaces getting warm, and the molecules bouncing off them getting "hot" (gaining kinetic energy and momentum) and depositing
that energy on the enclosing bulb. Molecules bouncing off the white sides are cooler, slower, and thus a net torque is generated. If the MFPs were
smaller then the fast molecules would collide with other molecules, and the momentum would be redistributed and end up being redeposited on the vane
surface, cancelling out the original force.
[Edited on 7-11-2015 by careysub]
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Upsilon
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Home Depot was even worse than Lowes. Looks like I'll have to be ordering the stuff online and hoping it fits.
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Magpie
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Do you have an industrial supply business in your community? There's not much need for 6" pressure piping in residential projects.
The single most important condition for a successful synthesis is good mixing - Nicodem
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careysub
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See if any companies specialize in sewer line installation in your area. They can be sources for larger sizes. Fitting should not be a problem if the
part is available since the sizes are standard.
I have ordered hard to get pipe fitting from Zoro Tools, with good success:
http://www.zoro.com/lasco-cap-6-in-slip-447060/i/G5275243/
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Upsilon
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I am actually looking at their inventory currently. So, I think the cap will fit. Now I also need a screw cap for the top end of the chamber. I
imagine something like this should fit as well?
http://www.zoro.com/genova-cleanout-body-pvc-6-in-41659/i/G6...
EDIT: Ok so these won't work after all - their standard is ASTM D3034 which is for sewer lines. The pipe I have is ASTM D1785.
[Edited on 7-11-2015 by Upsilon]
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zed
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Stainless steel pressure cookers abound at my Portland Goodwill stores. Usually, about 10 to 20 dollars, when they have 'em. The so called,
"Superstore" on SE 6th might be the best place to look.
The sides are round, and the tops are slightly domed. Making the flat bottom the weakest link. One with an Aluminum clad bottom, might be somewhat
stronger.
Should be strong enough, to not cave-in under vacuum. But, as always, such things remain to be seen. You'll find out.
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Upsilon
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| Quote: Originally posted by zed | Stainless steel pressure cookers abound at my Portland Goodwill stores. Usually, about 10 to 20 dollars, when they have 'em. The so called,
"Superstore" on SE 6th might be the best place to look.
The sides are round, and the tops are slightly domed. Making the flat bottom the weakest link. One with an Aluminum clad bottom, might be somewhat
stronger.
Should be strong enough, to not cave-in under vacuum. But, as always, such things remain to be seen. You'll find out. |
It just seemed like a hassle to try and deal with a pressure cooker, since they have 2 layers of metal with a heating element in between. Regardless
it's too late now, I've already invested into the PVC design.
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Upsilon
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Ok, so I have definitely found caps that will work. However, the screw cap isn't going to happen - it costs way too much. Therefore I need some kind
of strong ring to put over the other end of the pipe. It would need at least a 6 5/8" outer diameter and around a 5" inner diameter. This would be
cemented to the rim of the pipe. Over the hole in this ring, I would place the polycarbonate window. On the window I would use something like a
refrigerator door seal so that when the vacuum is pulled, the window will be held fast to the ring.
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Magpie
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If the open end of the pipe is perfectly flat you can likely just use a flat piece of plywood, metal, plastic or glass of sufficient thickness. Buy a
small piece of rubber (say EPDM) and silicone glue it the plate. The vacuum should hold it in place with virtually no leakage. This method is used
for bell jars that hold vacuum.
I have bought 1/16" and 1/8" EPDM cut to size at a rubber suppliers.
The single most important condition for a successful synthesis is good mixing - Nicodem
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zed
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"It just seemed like a hassle to try and deal with a pressure cooker, since they have 2 layers of metal with a heating element in between. Regardless
it's too late now, I've already invested into the PVC design."
We are envisioning different machines. Hereabouts your average pressure cooker is a simple, long handled, heavy gauge, metal pot, with a gasketed
lid. Doesn't have double walls, or anything like it. Doesn't have internal heating elements.
[Edited on 8-11-2015 by zed]
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Upsilon
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| Quote: Originally posted by zed |
"It just seemed like a hassle to try and deal with a pressure cooker, since they have 2 layers of metal with a heating element in between. Regardless
it's too late now, I've already invested into the PVC design."
We are envisioning different machines. Hereabouts your average pressure cooker is a simple, long handled, heavy gauge, metal pot, with a gasketed
lid. Doesn't have double walls, or anything like it. Doesn't have internal heating elements.
[Edited on 8-11-2015 by zed] |
Well, regardless I've already started with the PVC project so I'm bound to stick to it. I've ordered a 1/8" thick 8" steel disk that I'll need to cut
a hole out of. This relatively small thickness should be cuttable with a propane torch.
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Upsilon
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Got the cap in the mail today and it fits perfectly. I also should have gotten the steel disk today, but USPS mail people are lazy as crap. It
wouldn't fit in the mailbox so they just put in a "Sorry we missed you!" slip in the mailbox, saying that it "requires a signature" (for a $5 piece of
steel??). Definitely a load of BS since nobody ever rang the doorbell.
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