[ausev] !12-volt A/C - scuba tanks can explode & dry ice notes

[Chris Cooper]

Perhaps compressed air in general is an answer to this. Turbine powered aircraft have been using compressed air for their environmental control systems for years. They bleed off the primary and/or secondary compressors, run it through a heat exchanger, recompress it, run it through another heat exchanger, and when it is expanded into ambient air pressure it is very cold; even approaching freezing temp. I noticed this during the Katrina rescues where the news cameras in the helicopters were facing the cockpit. I could see vapor coming out of the A/C vents above and behind the pilot and in the passenger compartment.
The first home A/C unit was based on compressing, cooling to ambient, and re-expanding air. 
Back in the 1950's there was this device you could buy that fit in the side window between the partially rolled up glass and the window frame. It used the ram air principle and looked like a funnel with the small opening facing forward. It had a tube in the center of the large end and as air was rammed into it at high speed, it expanded through to the large end. This caused the air to cool and the "tube" caught the cold air and fed it into the cabin. I have no idea if it really worked below 150MPH. 



C²

-----Original Message-----
From: ausev-bounces at austinev.org [mailto:ausev-bounces at austinev.org] On Behalf Of tomsmail
Sent: Friday, August 25, 2006 3:56 PM
To: AustinEV News Announcements and General Discussion
Subject: Re: [ausev] !12-volt A/C - scuba tanks can explode & dry ice notes

Go Tigers!

> :) Good questions.  Pardon me if I answer out of order.
>
> Fundamentally, the idea should be workable; however, there
> is a lot of energy stored when you compress gas and it has
> a nasty of habit of converting into to thermal energy when
> it is tapped quickly.  I read in last month's Pop Sci
> about a hybrid concept that was using compressed Nitrogen
> instead of the battery.
>
> The tank in question was a steel tank (not the more
> typical aluminum). Steel tanks have a higher risk
> exploding because the steel is less brittle than Aluminum
> (al).  Al tanks are more likely to crack/leak than
> explode.  Unfortunately, they are not very good with
> shock/vibration. That is why shop owners will visibly
> wince when you drop or bang the tanks.
>
> There are relief values, but they are a sort of last ditch
> back-up and (as my example shows) not always reliable.
> The show owner was right to be scared - they'd just put a
> hot pressurized can into a cold water bath.  Just think
> hot glass into ice water - steel can get brittle too.
>
> If you had an active system to monitor the max pressure
> then you could make this idea work. And you're right,
> bleed off the excess to cool the car in advance.
>
> I actually like combining the tank & dry ice idea.
> Converting solid CO2 into gas can generate pressure and
> cold!  It is also self regulating because increased
> pressure slows down the sublimation.  BUT - you'd need a
> special type of tank for it.  The ice would crack a steel
> or al tank if it regularly touched the sides.
>
> Sorry for the long reply - I've spent too long thinking on
> these issues in the past to be brief.  Feel free to send
> this up to the listserv if you want :)
>
> Rob
>
> -----Original Message-----
> From: tomsmail at wtez.net [mailto:tomsmail at wtez.net]
> Sent: Friday, August 25, 2006 12:59 PM
> To: rah at h-consulting.com
> Subject: Re: [ausev] !12-volt A/C - scuba tanks can
> explode & dry ice notes
>
> You gotta love the MEs - they're awesome!  Sometimes I
envy
> them too. (too bad for me I slept through Thermo).
>
> Question:  What about locating the tank UNDER the vehicle
> (in the shade)?  Also, aren't there pressure relief valves
> on those things? Also, what if you use half the volume in
> the morning on the way towork, and only half the volume is
> heated during the day?  Would that make a difference?
>
> What I'm suggesting is you have the thing recharged at 25
> deg C.  By the time you arrive at work the volume is 50%.
> By the time you are ready to go home the temperature is
> 50C so the pressure is back up to where it was in the
> morning with a full tank. Temp spikes above 50C are
> handled by venting (into the vehicle's cabin no less).
> Do you see any flaws with this?  Are the scuba tanks
> really that dangerous or was the scuba shop operator just
> being overly cautious?
>
> Thanks!
> Tom
>
>
>
>
> > WARNING: Scuba tanks do not do well sitting in hot cars.
> > Good old  Boyle's law (PV=nRT) means that the pressure
> > in the tank  will climb  quickly as it heats up and the
> > tank could EXPLODE.  That  is why they  fill tanks in a
> water bath at SCUBA shops.
>
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