Madinventions
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| posted on 19/1/11 at 10:12 AM |
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More maths/physics help please! (Liquid Nitrogen...)
I've been doing some calculations at work, and would be grateful of any insight that the Locostbuilders collective may have please?
I'm working on developing a small cryogenic cooler to cool a sample to -100°C (maybe -150°C if possible), and we are going to do this by boiling
a given amount of LN2 into gaseous form using a heater immersed in the LN2, and using the resulting cold gas to cool our sample. From experiment and
experience, we need a cold gas flow of around 30 litres/minute, and I'm trying to work out how much heater power is required to boil off a
sufficient quantity of LN2.
Here's the formula I'm using:
R = (IV+h)/(pL)
R = flow rate in litres/minute at standard temperature and pressure
IV = heater power
h = extra ambient heat input from environment
p = density of gaseous Nitrogen at stp
L = latent heat of vaporisation
R = 30lpm / expansion ratio of LN2
= 30 / 694:1
= 0.0432 lpm of LN2 (about 2.6 litres per hour)
p = 1.185 (or 1.165 depending on the text book you read?)
L = 198.38
So, I rearranged the formula to:
IV+h = RpL
= 0.0432 x 1.185 x 198.38
= 10.16 Watts
I think that the split between heater power (IV) and losses (h) would need to be determined empirically, but does 10W sound about right to those with
more experience than me?
Thanks in advance!
Ed.
Mojo build diary: http://www.madinventions.co.uk
Solo music project: Syrrenfor http://www.reverbnation.com/syrrenfor
View my band website:
http://www.shadowlight.org.uk
http://www.eastangliankitcars.co.uk/
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Daddylonglegs
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| posted on 19/1/11 at 10:36 AM |
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I need to go fo a lie down 
It looks like the Midget is winning at the moment......
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SPYDER
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| posted on 19/1/11 at 10:44 AM |
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42
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matt_gsxr
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| posted on 19/1/11 at 10:44 AM |
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I did all the calc in units of mass. I also assumed the 30litre/min was at STP.
30 litre per min required flow rate
/60
0.5 litre per second flow rate
1.185 kg/cubic meter N2 density at STP
/1000
0.001185 kg/litre air density at STP
*
0.5 litre per second flow rate
0.0005925 kg/second flow of nitrogen per second
x
200 kJ/kg latent heat of vaporisation of LN
=
0.1185 kJ/sec kW
=
118W
I can't claim to be any good at these calculations though. Different between 1.185 and 1.165 is probably STP versus RTP ( 273K and 293K)
Where do you convert from minutes to seconds in your calcs? (Watt is Joule/sec)
You do need to be careful you don't let air into the LN container, as the oxygen will liquefy in there which is very dangerous.
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britishtrident
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| posted on 19/1/11 at 10:52 AM |
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(1) Convert volume flow rate to litres/second
(2) Convert volume flow rate to Mass Flow rate in Kg/s
(3) Multiply mass flow rate (in litres/second) by latent heat evapouration of nitrogen (199kJ/Kg) this will an answer in KW.
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Madinventions
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| posted on 19/1/11 at 11:00 AM |
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Thanks guys, once again, Locostbuilders has proved it can answer any question! Thankyou ever so much for your help, it makes much more sense the way
you've both done it, and yes - it appears that I hadn't converted correctly between minutes and seconds when handling the units...
Ed.
Mojo build diary: http://www.madinventions.co.uk
Solo music project: Syrrenfor http://www.reverbnation.com/syrrenfor
View my band website:
http://www.shadowlight.org.uk
http://www.eastangliankitcars.co.uk/
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daviep
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| posted on 19/1/11 at 11:58 AM |
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I was doing a similar thing yesterday and today. So far I've converted 28,000 litres of LN2 to GN2, I'm using a 300hp diesel engine to
supply the heat and pumping it at 1000scf/min and 3200psi 
“A truly great library contains something in it to offend everyone.”
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Peteff
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| posted on 19/1/11 at 04:13 PM |
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I've been a walk and stroked some donkeys. I want my head cryogenically preserving if you get the thing working so I can have it put on a monkey
body when they find a cure for whatever killed me.
yours, Pete
I went into the RSPCA office the other day. It was so small you could hardly swing a cat in there.
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Madinventions
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| posted on 20/1/11 at 12:48 AM |
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quote:
I've been a walk and stroked some donkeys. I want my head cryogenically preserving if you get the thing working so I can have it put on a monkey
body when they find a cure for whatever killed me.
Ok.... maths and physics I can just about handle, but that's really confused me!
Mojo build diary: http://www.madinventions.co.uk
Solo music project: Syrrenfor http://www.reverbnation.com/syrrenfor
View my band website:
http://www.shadowlight.org.uk
http://www.eastangliankitcars.co.uk/
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