9 replies to this topic

[Steve Hall]

From an outdoor liquid nitrogen tank, nitrogen is vaporized as it flows at 80 kg/hr to a pilot plant. Please tell me a typical temperature for the vaporized gas. Do the vaporizers typically warm the vaporized nitrogen near ambient temperature, or is it usually much colder, like minus 30 or so? 

 

Thanks,

Steve

[latexman]

Steve,

 

I just checked ours.  We don't measure the flow, only level for ordering.  It was essentially ambient.  The pipe from the tank to the vaporizer is ice covered.  The ice ends right at the inlet to vaporizer.  It is wet for a short while past that, then it is dry.  After the vaporizer it feels essentially ambient.  It depends on the load, but that's how it is now.

Edited by latexman, 14 April 2015 - 12:09 PM.

[Steve Hall]

Thanks. 

[Bodhisatya]

It warms up to the ambient temperature as end users be it refinery or steel plants utilize N2 at ambient temperatures.

 

Bodhisatya

[Zauberberg]

I wouldn't say it will be at exactly at the ambient temperature but normally 10-15 degC lower (theoretically an infinite surface would be required). You also want to be at a temperature measurably above the Carbon Steel minimum design temperature (-29 degC) since distribution piping is mostly made of CS.

[Art Montemayor]

I've designed and installed a lot of ambient vaporizers - including nitrogen, oxygen, and argon.  All these involved liquids in the cryogenic zone (less than -150 ^oF).  All cryogenic vaporizers will do exactly that: vaporize a saturated liquid at the saturated temperature related to that fluid.  However, the excess heat transfer area given to the ambient vaporizer coil allows for post sensible heat transfer as well as the basic latent heat transfer and this causes the final end product to exit the coil at close to the existing ambient temperature used to effect the basic vaporization.  In humid climates - such as here in Houston, Texas - it is not unusual to have the ambient vaporizer coils coated with water ice (subcooled to cryogenic temperatures) and the deposition of this hard ice forms an efficient insulation around the coils, lessening the effective latent heat transfer area and, subsequently, decreasing the total heat transfer available - both latent and sensible.  This would apply to both natural or forced circulation ambient vaporizers.  Most cryogenic vaporizers I've found today are of the natural circulation type, with a generous excess heat transfer area.  This is considered much more practical than furnishing and maintaining an air ventilation system.

 

Therefore, what ultimately happens, if the coil does not have sufficient excess surface area, is that the exit gas temperature starts to decrease.  Experienced cryogenic vaporizer designers such as Air Products, Praxair, etc., know this and provide the empirical coil area necessary for the process and the ambient environment involved.  The coldest cryo fluid I've vaporized is liquid hydrogen and the exit temperature from my vaporizer varied - but hardly ever got lower than 20 ^oF below that of the ambient air - as Zauberberg has noted.

[Bodhisatya]

Excellent summing up by Art sir.

 

In India we face this problem of hard ice coating forming around the vaporizer fins quite often,thus reducing the effective heat transfer,which becomes evident with temp dropping of Gaseous Nitrogen.

 

So the general philosophy is one working one standby,the standby one generally remains in defrosting.

 

If you have a close look at the interlock description,one can find closing of GAN distribution valve at Vaporizer exit  if the recorded temp is critically low (We maintain it at 5 DegC),the reason being the transition from SS pipeline to CS pipeline as explained by Mr. Zauberberg

 

Bodhisatya

[curious_cat]

Since, this ( e.g. frosting, too low N2 temp.) seems a common problem, I'm wondering aren't there any alternative solutions? What about a steam traced / hot water / condensate return exchanger? Would the cost be excessive? Or is the danger of freezing those lines? Or the delta T across the walls would be too high?

 

Why are the Liq. N2 vaporizers mostly air heat exchangers?  Just curious

[Zauberberg]

The temperature driving force for vaporizing liquid Nitrogen is huge, if one uses ambient air for heating. As Art has pointed out, the excess surface area covers for ambient moisture freezing effects while still keeping the system safe and simple.

[Bodhisatya]

Hot water bath vaporizers with helical coils passing Liquid nitrogen and steam bath vaporizers are quite common and are in use.But they are at the expense of shelling out bit more money.

 

Steam bath Vaporizers also require huge floor space,so attractive option is Vertical water bath vaporizers.But they do add a contingency factor.

 

For instance if there is power outrage (mostly when Vaporizers come on line) and hot water or steam is not available ,one can't use those vaporizers,while for Ambient vaporizers dependence is less and hence they are more widely used

 

Bodhisatya