Home made liquid nitrogen generator

[carbon-rod]

Hello all!

This is my first post on the forum, I have been reading some threads here lately and have decided that it was time to sign up and post a question. It is apparent that this forum contains a plethora of information from a lot of very experienced engineers and I would be very grateful if you could indulge me with some of your knowledge.

So a while back I saw a build of a home made liquid nitrogen generator by Ben Krasnow, Ben used a stirling cycle cryocooler which he extracted from a superconductor filter to produce liquid nitrogen at home. I thought it was an excellent build and very inspiring, although when I looked on ebay I found that the cheaper superconductor filters had already been sold and I wasn't really looking at spending $1k+ in order to obtain one for primarily hobby purposes

so I started doing some research into the JT effect and adiabatic expansion refrigeration etc and looked at some ways of converting a regular window air conditioner into a liquid nitrogen generator of my own. I do not have any refrigeration experience and do not fully comprehend the limitations of conventional refrigeration however I do know that it will be somewhat difficult to obtain that temperature and provide and substantial quantity of cooling capacity.

I have been toying of the idea of using Helium as a refrigerant, in a window air conditioner, leaving the condensor in situe, installing an oil separator, a manually operated TX valve and a heat exchanger in order to cool the high pressure line using the low pressure line.

After theorising this technique I found that it had already been discovered a long time ago and was patented as a hampson-linde air liquefier, the primary difference is that I would be looking at producing a closed system with a 'cold finger' which I would flood with nitrogen which will condensate and yield liquid nitrogen. Obviously in order for this to work I will either need a very large cooling capacity of a vaccuum flask dewar for storing the liquid nitrogen as Ben Krasnow has previously done.

I realise this system would not be very efficient and the uplift burden at these temperatures will be quite low, if I could achieve 10-20W at liquid nitrogen temperatures (from a 1.5kW electrical compressor) I would be pretty happy! I believe that the stirling cryocooler that Ben used is capable of 6W uplift at 40K utilising only a few hundred watts of electrical power.

The problem I have recently discovered is that the Joule-Thompson inversion temperature for Helium is very low, therefore under 'free-expansion' with constant enthalpy this would cause a temperature increase. Commercially this is overcome by using turboexpanders in order to extract work from the helium instead of using a TX valve, only once the helium has cooled to below its inversion temperature do they cool it further using the JT effect.

I will likely be trying to operate the system at as low suction pressure as possible and highest discharge pressure as possible in order to create a sufficient differential pressure to obtain low enough temperatures, also the continous cooling of the high pressure line using the low pressure line will gradually bring the temperature down as well.

The questions I have are,

Will the cooling of the nitrogen gas around the cold finger cause enough work to be done by the helium expanding through a TX valve to become cold?

Will I likely need a method of extracting mechanical work from the expansion of the helium in order to produce a cooling effect? if yes any ideas on how to produce a simple device that I would be capable of producing at home? (I can't think of much!)

Do any of you think this system is somewhat feasible?

as a backup plan I would probably use nitrogen as a refrigerant instead of helium as the JT inversion temperature is well above room temperature and therefore I could provide cooling at least to the point of liquification of it inside the system which I think would be a decent achievment.

Any way I appreciate any comments and constructive criticism.

Cheers

Murray.

[edit] Ben Krasnow article for cross reference
http://benkrasnow.blogspot.co.uk/200...generator.html