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Re: [PSUBS-MAILIST] Pressure air drive.
I think there is a missunderstanding .. ?
The motor needs 3 cfm means : "A volume of air of 3 cf in one minute
at normal atmospheric pressure.."
That means in other words the air consuption of the engine is given as
expanded air - for example on the surface after using the motor and
collected in a bag..
The pressure figure of 90 psi ist just the differnce between the motor
inlet and the motor outlet..
If you have a 220 cf tank and a 3 cfm air consumption
the time figue is 220 cf air / 3 cfm = 73 Minutes (on surface only)
and it doesn't matter if the pressure differntial the motor
need is 9 or 90 or 900 PSI..
The Psi figure is only relevant if the boat dives deeper
because the rest amout of air not useable in the tank will be more in
greater deeps. The motor will stop running at least if the internal tank
rest pressure and the pressure outside the boat is equal.
In metric figures:
1cf is equal to 28,3 liter
Scuba Tank size 10 Liters in german means
a scuba tank of 10 liter (size is 10/28,3 = 0,3533 cf)
but with a pressure of 225 atmosp. (bar)
= 2250 Liter (80cf) expanded air on the surface pressure.
and yes that is equal to a 80 cf tank in your imperial language..
a 80 cf tank has a run time of 80/3 = about 27 minutes
Thats funny : we give allways the real size of the tank..
you can fill 10 Liters of water in - but 2250 Liter of compressed air..
But your language give the volume of the air inside if the air
is expanded to a standard atmosphere - right ?
This indicates that you use normal tanks with differnt
max.pressures ?
We here use all the time 200 bar tanks - which are today normaly
filled up to 225 bar..
regards Carsten
jbarlow@bjservices.ca schrieb:
>
> The motor needs 3 cfm @ 90 psi and expels approximately (3*90 / 14.7
> (depending on where you live ... presumably sea level if you are in a
> submarine, but if you dive in Lake Minnewanka* @ high altitude then
> presumably you could get more out of the air supply)) = 18.3 cfm
> ...therefore 220 cubic feet / 18.3 = 11.9 minutes.
>
> 220 cfm doesn't equal 2250 litres The original assumption was for a std
> scuba tank. (Note: 220 cfm is one very large scuba tank)
> 2250 litres equals 80 cubic feet which is a std scuba tank.
>
> Also the assumption was for 2 motors or 6 cfm @ 90 psi which makes the 12
> minutes actually 6 minutes. Actually less because air isn't an ideal gas.
>
> Unless you heat the air after (or before I guess) regulating it from the
> high pressure to add back in the energy lost due to the cooling effect of
> expansion.
>
>
>
>
>
>
>
>
>
>
> *Minnewanka, Lake (mi-ni-WAHNG-kuh) SW Alta., Canada, near B.C. border, in Rocky Mts., in Banff
> Natl. Park, 6 mi/10 km NE of Banff, at foot of Mts. Aylmer and Girouard; 12 mi/19 km long, 1
> mi/1.6 km wide. Elev. 4,769 ft/1,454 m.
>
>
>
>
> Jay.
>
>
> "Pierre Poulin"
> <pipo305@hotmail.com> To: personal_submersibles@psubs.org
> Sent by: cc:
> owner-personal_submersible Subject: Re: [PSUBS-MAILIST] Pressure air drive.
> s@psubs.org
>
>
> 08/01/2003 06:59 PM
> Please respond to
> personal_submersibles
>
>
>
> The math is simple:
>
> one tank of 220 cubic foot. divided by 3cubic foot/minute = 73 minutes of
> non-stop full throttle operation. So air reserve doesn't seem like an issue
>
> to me.
>
> My question is more the 25000 rpm and 0.6 HP. Would that be enough for
> moving my 1092 lbs ambient sub? And there is also the propeller design
> consideration (which I'm no expert)
>
> So, What do you think about RPM and HP?
>
> Pierre
>
> >From: Coalbunny <coalbunny@vcn.com>
> >Reply-To: personal_submersibles@psubs.org
> >To: personal_submersibles@psubs.org
> >Subject: Re: [PSUBS-MAILIST] Pressure air drive.
> >Date: Wed, 08 Jan 2003 16:52:29 -0700
> >
> >And that would explain why WW2 torps had a range of less than a mile.
> >NOW I understand.
> >Thanks Jay!
> >Carl
> >
> >
> >jbarlow@bjservices.ca wrote:
> > >
> > > 3 cubic feet per min at 90 psi
> > >
> > > If air is ideal gas then P1 x V1 = P2 x V2
> > >
> > > 3000 psi X .353 ft3 = 90 psi x V2
> > >
> > > V2 = 11.76 ft3 @ 90 psi
> > >
> > > 11.76 / 6 = 1.96 min.
> > >
> > > The volume gets worse by the head of water (15 psi per 32 feet) and
> also
> > > the motors lose power as then can't fully expand the air back to
> > > atmospheric to regain the stored energy. You could overcome this by
> > > feeding the motor ever increasing pressure air (measured inside your
> >boat)
> > > as you dove deeper. that is regulating it down less, to regain the
> >power
> > > loss, but that again would shorten your range. 320 feet is about 150
> >psi
> > > so now you need to feed the motors 90 + 150 = 140 psi to get .6 hp and
> >the
> > > 1.96 minutes becomes 1.26 minutes. Based on the 10 litre scuba bottle.
> > > Obviously one would need bottled gas cylinders.
> > >
> > > about 1.5 min per 10 litre bottle @ 10 atm. depth.
> > >
> > > Still a sound concept. If you have to use shop air (easy and cheap
> > > compressors) then i think it is not workable. If you have access to
> > > 3000-4000 psi compressors then it should work fine for short duration
> >use.
> > > For an occasional use would be great as it has no shelf life.
> > >
> > > Jay.
> > >
> > >
> > > MerlinSub@t-online.de
> > > (Carsten Standfuss) To:
> >personal_submersibles@psubs.org
> > > Sent by: cc:
> > > owner-personal_submersible Subject: Re:
> >[PSUBS-MAILIST] Pressure air drive.
> > > s@psubs.org
> > >
> > >
> > > 08/01/2003 10:47 AM
> > > Please respond to
> > > personal_submersibles
> > >
> > >
> > >
> > > Pierre Poulin schrieb:
> > > >
> > > > Hi,
> > > >
> > > > I've check some air grinders that could be transformed into motor.
> > > >
> > > > Free speed: 25000 rpm
> > > > - Operation pressure: 90 psi
> > > > - 0.6 HP
> > > > - Air consumption: 3 cfm
> > > > - Weight: 1.5 lb
> > > >
> > > > What do you think about that?
> > > >
> > > > Pierre
> > > >
> > >
> > > Hmm.. 3 cfm means cubif -foot- minute ?
> > > than it is
> > > are equal (sorry I think in metric..)
> > > to 3 x 0,3048 x 0,3048 x 0,3048 m = 3 x 0,028 m3 or 84 Liter
> > >
> > > if we estimate that the boat need two engines of 0,6 hp we need
> > > 2 x 84 liter = 168 Liter a minute.
> > >
> > > A normal scuba bottle has 10 Liters at 225 bars = 2250 Liters
> > > air and run the drive just 2250/168 = 13 Minutes
> > >
> > > A yard size bootle for weld gas has 50 liter and is 1,5 m (4,9 feet)
> > > high with a diameter of 280 mm (11 inch) runs about
> > > 5 times longer = 65 minutes..
> > >
> > > Two tanks of 50 Liters gives you 2 hours and 10 minutes..
> > > Not so bad - Batterys has to be replace after some years -
> > > pressure bottles not - and a 50 Liter industrial one is about
> > > 300 Dollar new and maybe cost the half if you purchase a used one.
> > >
> > > Problems:
> > > - to get an engine made from seawater resistant material
> > > - the noise ? -> no fish around the boat
> > > - the air bubbles all the time ? -> no fish around the boat
> > >
> > > Most problem is maybe that high pressure scuba compressor
> > > is mcuh more expensive than a battery charger.. :-(
> > > But for peoples which are allready scuba divers - and have allready
> > > a compressor - a solution.
> > >
> > > Carsten
> >
> >--
> >"You delight not in a city's seven or seventy wonders, but in an answer
> >it gives to a question of yours, or the question it asks you, forcing
> >you to answer, like Thebes through the mouth of the Sphinx." -- Kublai
> >Khan
>
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