From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.space.tech
Subject: Re: Venting Helium Gas
Date: Thu, 2 Mar 2000 16:20:33 GMT

In article <l74rbs0umjafat1us7t3e4hfr0lnq3gsvv@4ax.com>,
Ephraim Gadsby  <Ephraim.Gadsby@The.Drones> wrote:
>I understand that some satellites have liquid helium cooled
>instrumentation, with reservoirs that are well enough insulated so
>that the helium lasts for the course of the mission.

Correct.  The mission lifetime is typically limited to a year or two, but
within that, this works reasonably well.  It's mostly been done for
infrared astronomy, where detectors work much better if chilled to nearly
absolute zero.

>In micro-gravity how do they vent the boil-off, without ejecting
>liquid?

The orthodox way of doing this sort of thing is a "phase-control heat
exchanger".  You run the fluid leaving the tank through a pressure drop,
reducing its boiling point.  Then you run it through a heat exchanger
connected to the rest of the tank (simply bonding a run of tubing to the
wall of the tank works fairly well).  The departing fluid, being at a
lower pressure, boils, and this cools the tank.  This kills two birds with
one stone:  it ensures gas in the output, while also refrigerating the
remaining liquid.

I'm not sure this is *exactly* what the liquid-helium guys do, though.
Liquid helium has some strange properties, and I have a dim recollection
that at least some of the helium-cooled systems exploit those to help.
--
Computer disaster in February?  Oh, you |  Henry Spencer   henry@spsystems.net
must mean the release of Windows 2000.  |      (aka henry@zoo.toronto.edu)

From: Doug Jones <random@eau.net>
Subject: Re: Spacesuit technology - a question...
Date: 06 Aug 1997
Newsgroups: sci.space.tech

Henry Spencer wrote:

> I see some practical problems with helium.  For one thing, I expect that
> its much lower density would interfere with air-cooling of electronics
> (which is one major reason for wanting to run at a total pressure not far
> below 14.7psi).

Actually, helium is a far better thermal conductor than air- I had to
release a large amount of helium from a balloon in a launch abort, and
the helium felt cooler than the air at the same temperature.  At the
same pressure, He will transport much more heat than air- about 8x,
IIRC.

Helium's thermal conductivity is a problem for saturation divers,
requiring higher temperatures in their habitats for comfort.

--
Doug Jones	random@eau.net
If they call it tourist season, why can't we shoot them?