Re: dumping a lot of heat

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:
>

john larkin <jl@glen--canyon.com> wrote:
>

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.
>
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.
>
https://highlandtechnology.com/Product/P945
>
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.
>
A small hair dryer can dump a kilowatt. So some sort of red-hot
nichrome coils and a vicious fan might work.
>
I'd prefer to not use water.
>
I wonder if there is some sort of runs-red-hot power resistor.

>
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).

>
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.
John

 

The solution to the IR problem is to reduce the element temperature
so's to shift the dissipation from radiation to air heat transfer. I
think the math on that is good.

 The economics aren't so good.if you have to spread the heat over a large
area by conduction alone.
 

If you need a rapidly-controllable load, valves can dissipate energy at
a much higher temperature than transistors, so they might be worth
considering.

>
Probably not  practical. Tubes are big  and full of expensive vacuum
which doesn't conduct heat well. May as well use some giant tubular
incandescent lamp which has the same glass and heater as a big tube.
Most are filled with a gas that conducts heat better than vacuum, so
would bet more air cooling from the glass.
>
Tubes radiate heat from the plate, so I'd need to step up my source to
high voltage.

  Yes, I did say 'rapidly-controllable'; if you just want a fixed
resistor, a valve would be a highly uneconomical way of doing it.