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From: mangoe@umcp-cs.UUCP (Charley Wingate)
Newsgroups: net.sf-lovers,net.physics
Subject: Re: Discrepancies (Stillsuit Thermodynamics)
Message-ID: <708@umcp-cs.UUCP>
Date: Wed, 3-Jul-85 21:22:09 EDT
Article-I.D.: umcp-cs.708
Posted: Wed Jul  3 21:22:09 1985
Date-Received: Thu, 4-Jul-85 09:34:07 EDT
References: <82@rtp47.UUCP>
Distribution: na
Organization: U of Maryland, Computer Science Dept., College Park, MD
Lines: 46

In article <82@rtp47.UUCP> throopw@rtp47.UUCP (Wayne Throop) writes:

>Here is a model to work with, using convection cooling to cool the
>stillsuit, and evaporative cooling to cool the inhabitant.  Consider
>these layers, working from the wearer out:
>
>  1     wearer
>  2         layer that lets water out, but insulates heat very well,
>            and also prevents the water from getting back in.
>  3             water & humid air reservoir
>  4                 layer that prevents water from escaping, but is a
>                    thermal conductor.
>  5                     the outside environment
>
>The "human muscle power" here is used only to pump various substances
>around in the water & humid air reservoir for convenience (this is more
>like what Dune says they are used for).  Layer 1 is cooled by
>evaporation.  The heat from the evaporate is deposited in layer 3, which
>is in turn cooled by convection.  Layer 3 is the hotest, layer 5 the
>next hotest, and layer 1 the least hotest :-).
>
>The key to making it work is the "magic" properties of layer 2.  It
>allows water to pass one-way, and is a terrific thermal insulator.  It
>may be that in order to have the properties I state, some energy would
>have to be expended... I'm not sure on this.

Here's the problem: a packet of dry air picks up water from (1), and also
heat (since (1) is supposedly cooler than (3)).  We let it sit there until
it reaches some sort of equilibrium (assuming it can only pick up a
particular quantity of water).  Now we take it to layer (3), where it has to
get rid of the water.  To do this, it has to get rid of some heat, which it
must dump in layer (5).  The important question is: how much heat?  The
answer: not as much as it started out with.  Therefore the vapor pressure in
(3) has to grow, or (4) has to be refrigerated.  If we take the first
possibility, eventually this pressure must rise high enough to prevent the
flow of water from (1) to (3).  In addition, there is a net flow of heat
INWARD; when the water vapor cannot flow, heat is still being produced in
(1), and thus there is no cooling.

You can't cool a device in a hotter environment without disposing of the
heat in some manner other than radiation or convection, or without some sort
of refrigeration.  The problem with the stillsuits is that they explicitly
forbid the former, and that the energy supplied for refrigeration is
insufficient.

Charley Wingate