On Thursday, October 30, 2003, at 09:05 AM, arcstarter wrote:
Comments:
--- In VacuumX@..., "nano_tronics" <nortronics@s...>
wrote:
here are my questions
1) how much ultimate vacuum i can achive with a diffusion pump
setup
bellow
a) without a cold trap ( and oil backstreaming problem that go's
with it )
b) with a cold trap
c) a cheap an easy way to replace the LN2 coolant for the cold
trap
Here's a hair-brained idea. Since once a reasonable vacuum level is
achieved (1 micron or whatever it takes to start the diffusion
pump) - the actual mass flow through the cold trap is very low.
Therefore would it be possible to use some of the thermo-electric
cooling elements (Peltier devices) to cool the baffle plates? Total
heat flow will be low (perhaps mostly radiative), and the Peltiers
can be arranged in series to add temperature drops etc.
Temperature differential is created by moving heat. The greater the
differential, the more heat you have to move. While the heat load
entering the cold trap from the inside of the vacuum system may be
low, there is lots of other heat from the room and the material of
the trap itself, and its physical connections to the rest of the
system, which are constantly trying to warm it up. That actually
represents the lion's share of the heat you need to remove.
Now then, the amount of heat transported by each Peltier array is
proportional to the current through it. Furthermore, each Peltier
array has a maximum temperature differential it will sustain at its
maximum electrical ratings. For greater differentials, you have to
put several arrays in series. This is how very cold focal plane
array temperatures are achieved on some chilled CCD cameras for low
light photography (to reduce the inherent thermal noise of the array)
So, to get to LN2 or at least dry ice temperatures, you are going
to have to have 1) a lot of Peltier arrays in a series-parallel
arrangement and 2) an insane amount of power.
I had the chance to speak face to face with Don Mattox at the Indy
coating show yesterday, and I asked him about Peltier cold traps.
As far as he knows it hasn't been tried.
They are not very efficient. Moving fluid refrigeration is more
efficient. Their sole advantage is light weight and low cost.
Note that the portable coolers which use them have either large or
cigarette lighter cords to run off of a car's power. They also have
thicker insulation than regular ice chests. The owner's manuals
recommend that you not attempt to chill down the contents of the
cooler with the cooler alone, but rather that you put already-cold
foods into it and use the cooler only to help maintain a low
temperature.
In short, it could certainly be done, but at a very high cost
compared to the alternatives.
2) the pros an cons of air cooled VS liquid cooled diffusion pump
3) on a liquid cooled VD pump can a radiator with a fan and a
circulator pump be used instead of running faucette water
I was planning on doing exactly this. Can get a condensation pump
(intended to be used with a high efficiency fgurnace to remove flue
condensation) for about $50 or less - right off the shelf at
Grainger's etc. I think this would do the job. www.meci.com used to
have heater cores out of cars - which could be used for the radiator.
I believe that the efficiency of the diff. pump is related to how well the
walls are cooled. Warm water won't work as well as cool water, if the VD
is designed to use water cooling. A heat exchanger without refrigeration
will only lower your loop temp down to slightly above ambient, at best.
Another arrangement might be to use your cold trap AC system to
assist in cooling the closed-loop water used to chill a water-cooled
diffusion pump... This might allow you to use the fan and condenser
out of the AC to not only cool the cold trap but cool the diffusion
pump.
That's an interesting idea.
Does anyone know if it is possible to over-cool a water-cooled
diffusion pump?
That's a very good question! And one we should answer before anyone goes
to all the effort to try this.
I have a line on a used evaporating station - no vac pumps but with
misc vac valves and a 18 inch diam x 30 tall bell jar. Filament
transformer is included. They want $500 which I think is a good
deal.
Sweet! It's possible to do better, but that still seems reasonable.
Also I took apart a commercial $0.50 quartz crystal the other day -
one of those HC-49 type package styles such as this one:
I used a coarse file to remove the top of the can. Inside the can
I found a quartz slab with metallized sides connected to the
leadouts. I am considering using these as film thickness monitors
in the evaporator. The price is right!
Let us know how that works for you - I'm very curious about that myself.
