Re: dumping a lot of heat

On a sunny day (Fri, 06 Dec 2024 06:23:19 -0800) it happened john larkin
<JL@gct.com> wrote in <sg16lj5atqibsg0b51pdgph71u2ha4ck6i@4ax.com>:

On Fri, 6 Dec 2024 08:29:33 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
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John R Walliker <jrwalliker@gmail.com> wrote:
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On 05/12/2024 22:03, Liz Tuddenham wrote:

john larkin <JL@gct.com> wrote:
 

On Thu, 5 Dec 2024 08:55:32 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
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john larkin <jl@glen--canyon.com> wrote:
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I'm thinking about building a biggish rackmount dummy load box. It
would simulate series resistance and inductance. Part of the problem
is that it will need to dump a lot of heat.
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We are using copper CPU coolers on PC boards, which are great up to a
couple of hundred watts, but I'd like to do a kilowatt or two.
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https://highlandtechnology.com/Product/P945
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It would take a heap of expensive extruded heat sinks and fans to get
rid of a kilowatt. At 1 K/W, a pretty good heat sink, that's 1000 degC
temp rise.
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A small hair dryer can dump a kilowatt. So some sort of red-hot
nichrome coils and a vicious fan might work.
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I'd prefer to not use water.
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I wonder if there is some sort of runs-red-hot power resistor.

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If you are using elements at near red heat, remember you need to keep
the radiant heat away from the outer walls of the cabinet.  Reflectors
just throw the problem elsewhere and eventually will tarnish, the best
system is several spaced blackened steel baffle plates with vertical air
passages between them (visual black is not always IR black).

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Seems to me that black baffles will absorb IR and get hot, so devolve
to air-cooled heat sinks.

 
Yes, all the energy is eventually going to finish up heating the air,
the only question is the pathway it takes.  One way to avoid that would
be to construct a massive infra-red searchlight beaming the energy away
from the earth - or a broadcast transmitter beaming upwards.
 
To dump heat into the air, you either have to have something very
conductive with a large surface area or you need another way of
spreading the energy across a suface, such as heat radiation.  A big
sheet of thin, blackened steel plate for heating by radiation is a lot
cheaper than a thick die-cast aluminium lump with fins for heating by
conduction.
 
The economics of mechanically-forced air cooling are better than
convection unless you are able to use a tall 'chimney', so that the
energy of the waste heat is used to generate the draught.

 
A full sized rack cabinet could be remarkably similar to a chimney
if it had baffles in the right places.

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Yes, as long as the ceiling of the room isn't lined with polystyrene
tiles.  It might not be very good for the air conditioning system in the
building where this is installed, regardless of which dissipation method
is used.
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In the days of germainium transistors, one firm used to make cabinets
with a 'clerestory' roof, like an upturned tray suppoted on spacing
pillars above the ventilation holes in the roof proper.  This allowed
the heated air to flow out under the lip in case some idiot put the
instruction manual on the top of the cabinet.
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Another possibility, especially if there aren't going to be many of
these on sale and the installation will be done by the firm that makes
them, is to make a hole in the wall and stick some stainless-steel
boiler flues up the outside of the building.  Even better, in an old
building. use a redundant fireplace and put the contol box in a
decorative housing on the mantlepiece.

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We have in mind some commercial rackmount products, 1U to maybe 5U
dummy loads that people would buy and bolt into their 19" racks like
any other instrument. They would blow hot air out the back, like most
other gear.
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My users typically have cold forced air enter a rack from below and
expect it to exit near the top. I have one customer that controls the
air temp to milliKelvins, in what may be the world's biggest clean
room.
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https://lasers.llnl.gov/multimedia/photo-gallery?tid%5B%5D=401&tid%5B%5D=402&
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One rack that I know of is 2/3 full with kilowatts of Tek scopes and
ARBs. Things you need youngsters to help lift. I insisted that my
boxes be mounted BELOW the hot Tek stuff.

In the TV studios, plus all the technical control rooms,
many kW say from spotlights, lighting and all technical equiment
was dealt with by a huge airco system,