Is there dust free PC case to buy?
Not reasonably, PCs create heat and must then be cooled by
airflow. There are two remaining options, neither deemed
reasonable by "most" people.
1) Filter as much as you can before the increased noise
becomes unacceptible.
2) Very expensive chassis that 'sinks all heat to it's
external skin, and ideally, doesn't ignore parts that age
from heat, rather than only focusing on short-term
stability. For example, one can just eliminate fans which
will quite obviously decrease dust buildup to the level seen
in any passive device in the same environment (room), but to
get there, many only do it crudely, only putting waterblock
heatsinks on 2 or 3 of the hottest parts, ignoring the other
components on parts which where engineered within the
assumption that the system would have active airflow.
i mean something hermeticly closed, and cooled by inside wather
cooling.
You seem to be wanting to do what I just mentioned, ignoring
that many parts need airflow, even if not wearing a large
heatsink that could have a water block on it.
The extreme measures you propose are best avoided unless you
have deliberately engineered the system to be very low heat.
This is quite possible, but seldom done as it is
substantially lower performance per $. NASA might consider
it, but someone building a PC would tend to prefer modern
performance levels and if they have to clean the heatsink
every now and then, so be it.
You could put filter panels in the system, this is a popular
alternative and you might not have to clean or replace them
very often, but of course it depends a lot on how dirty
(dusty) the environment. If a very dusty environment, a
room air-cleaner is kinder to the user, and benefits the
system as well.
Otherwise, the key to least (approaching or achieving
passive chassis) airflow is that each component have very
*good* heatsinks. This is a large variable, we'd need to
know exactly what your intended system is. 10W is not too
hard to 'sink in closed chassis if you're creative (but in a
typical chassis, 10W is enough you may not be able to just
throw a heatsink on and forget about it without "some"
airflow from whatever source) but it becomes a matter of how
much effort and ability you have to source parts, engineer
this, and machine the solution out of a *bunch* of metal.
If you are talking about a modern component like a CPU or
most video cards, don't bother trying to use a closed
chassis and typical means. You will need pumps (defeating
the purpose, since pumps are as loud and as (moderately low)
reliability as a fan at low RPM for chassis airflow) or
quite custom heatpipes 'sunk to the chassis skin. This is
quite possible if you have the means to do it, but a custom
heatpipe and well (soldered) connected passive radiator is
drifting into the realm of very expensive custom case, not a
typical cost effective one-case-fits-all mass produced
product.
On the other hand, if you don't care how long it runs, if it
dying in a year or two is ok, it opens up more
possibilities. It just seems an awful waste though, to add
to landfills without reason (since you didn't express any
reason, for all we know all you would need for an
appropriate enough result is to put some filter panel(s)
over your case intake and leave the rest as-is, except that
having filter means that to keep a similar component(s)
temp, the fans have to have higher RPM to offset the
impedance of the filter material (which will also vary based
on how effective it is at stopping dust/dirt/etc intake).
