Re: The Refrigerator

[Tom Droege <droege@wwa.com>]

Chris,

It is more like when your wife complains.  She wants sympathy, not
solutions.  I am after neat tidy permanent solutions.  I know how to
compute most of these things, I was just disappointed that the refrigerator
did not have more cooling power.

I spent part of the afternoon designing various TEC solutions.  None are
very good.  You end up with two cooling loops.   Sigh!  The best way is
with a multi-stage TEC.  But the head is all designed, and there is really
no room without a lot of redesign.   I just need about 10 C more.  At the
moment I just have a plastic window on the head.  I could put in one of the
filters and pump it down with a vacuum pump.  I know this will gain 5 C or
so, even with the simple hand piston pump I have.  I could also back fill
with Nitrogen, or Argon.  I have a tank of Argon.  

So the ice is no solution unless you want to run up to Mt. Wilson every few
hours through the summer.  

I like to run the camera TEC at 10 volts and 1.5 amps.  So that is 15 watts
of DC power in.  On top of that is 5 or so watts that is pumped out.  So a
guess is 20 watts per camera head.  This would require 4 large TECs to
lower the chilled water 10 degrees or so, depending on how well you can get
rid of the heat on the other side.  It is just not a great way to do
things.  Don't show me curves that say you can do better.  There are lots
of things that happen that keep you from getting what the curves say.

Still, it is interesting to see that $40 worth of TECs will do as well as a
$200 refrigerator.  It is really a question of acceptance, mass production,
and marketing.  Else it would appear that we would be using TEC cooled
refrigerators.  Or possibly I have a bad unit.  Sure solves the Freon
problem. 

Yep, heat transfer is a real pain.  When you are trying to conduct heat
into that tank in the frige, you have to get by a thermal resistance for
just the wall of the tank of 1 degree F per square foot per Btu.   It is
hard to get many square ft in a tank that fits in a small frige so the
thermal coupling between the air in the frig and the water tank is not so
good.  Sigh!  The air should get real cold in the frig and take care of
this, but who knows how well the refrigerant does this.  Lots of problems I
don't want to solve today.   It is true that a fan would help a lot.  But
things just get more and more complicated.  Now you have the load of the
fan motor heat. 

Just griping that my "easy" solution will not work very well.

Tom Droege 
  

At 03:57 PM 10/23/98 -0700, you wrote:
>A few ideas:
>
>1) Why not just dump a 20 pound bag of ice into the water and not 
>   have to wait so long?
>   
>2) How many watts will the Mk IV's TEC put into the water.  I had
>   thought a TEC chiller would be a good way to cool the water.
>
>3) In darkroom work I sometimes adjust the temperature of a 500ml
>   bottle of chemicals by placing it in a temperature controlled
>   water bath and just letting it sit.  It is surprizing how long
>   it takes to cool or heat only 500ml this way.  It could take
>   45 minutes.  If I am in a hurry I stir the bottle with a glass
>   thermometer and drop a small (1.5 inch cube) pump into the water
>   bath to stir the bath.  Takes only a few minutes by this method.
>
>   So maybe try putting a small box fan in the 'fridge to keep the
>   air moving and a small pump in the wast basket.  The pump came
>   from an aquarium.  They are called "power heads" and run on only
>   a few watts
>
>   Maybe you have a big fan you could direct on the cooling coils of
>   your refrigerator.   
>
>Tom Droege wrote:
>> 
>> I have been running the refrigerator since noon with a waste basket in it
>> full of water.  With 57.2# of water it is cooling 2.6 degrees per hour.  I
>> figure that this is 148 Btu per hour or 43 watts.  What a bummer.  I had to
>> guess which sized refrigerator to buy.  All I could do was to estimate what
><SNIP>
>
>> I note that I could match this $200 (plus shipping) refrigerator with 4
>> small thermoelectric (@ $10ea) devices.  I am thinking of just plastering
>> some TEC on the frame of the telescope.  There is lots of area, and warming
>> the whole telescope body would reduce problems with dew.  Serious
>> astronomers will be horrified that I am thinking of heating the telescope.
>> But we need only 7 second seeing.  Should be hard to do this much damage by
>> heating things up a few degrees.
>
>No, don't do that.  Use a standard heat sink with fins and put a big muffin
>fan in there.
>If nothing else you will have a cooling unit that can be easyly replaced
>or upgraded.  You will have to fix and test it who wants to bring the
>whole telescope indoors to do that.
>Coolling will be hardest just when it is most needed too,
>on warm summer nights.  A fan cooled TEC system may be cheaper too as you
>can use a smaller TEC and power suppply. 
>
>
>> 
>> Just waiting for the screams of horror.
>> 
>> Tom Droege
>
>-- 
>--Chris Albertson
>
>  chris@topdog.logicon.com                Voice:  626-351-0089  X127
>  Logicon RDA, Pasadena California          Fax:  626-351-0699
>
>
>