9 replies to this topic

[A mukherjee]

Hello all...

I want to design a steam bath vaporizer.

Here we 'll vaporize Liquid nitrogen (LIN). Steam is distributed through the sparger present at the bottom of a trough, that contains water at around 45 degC. LIN is passed through the coils submerged in the bath. Can anyone help me in designing this vaporizer? Any suggestion regarding the sparger design?

Thnxx in advance...

Edited by Art Montemayor, 15 August 2010 - 11:59 AM.
Spelling, grammar, sentence spacing

[Zauberberg]

LIN vaporization is normally accomplished by heat transfer with ambient air, at least on an industrial scale basis, in real plants. I haven't seen water bath LIN vaporizers so far. And why would you use water for that? It isn't practical at all in my opinion.

[Padmakar Katre]

Dear,

Yes I absolutely agree with 'Zauberberg's' opinion.

I will suggest to explore the possibility of recovering the cold energy of the Liquid nitrogen stream which is a common practice in Air Separation Unit. Again, I need further clarity on the basis of the Liq N2 heating system, like a regeneration gas at a specified temperature. In that case, I can understand you concept but still am not clear as the bath temperature you are going to maintain is just 40 Deg C and I am not aware of regeneration (temperature swing adsorption) system at such a low temperature.

Waiting for your further comments.

[A mukherjee]

Actually, we use both an ambient vaporizer system and a steam bath vaporizer system for vaporizing LIN.

Here the client has specified a steam bath vaporizer. So we have to go for it.

In the steam bath vaporizer, we supply steam via a sparger present at the bottom of a trough, which contains water at around 40 degC. The LIN is passed through coils submerged in the trough and it gets vaporized.

I hope that I'm now clear.

Edited by Art Montemayor, 16 August 2010 - 11:25 AM.
Grammar, spelling, sentence spacing

[Art Montemayor]

All:

I’ve taken the liberty of editing and correcting the OP’s posts because I believe some of the corrected elements in the posts have given the wrong impression or communication.

I believe this is what has provoked a veteran and experienced field engineer like Zauberberg to make the comment(s) in his post. One cannot expose steam to a cryogenic wall temperature (-195.75 ^oC at atm. Pressure) without having the droplets of instantly condensed steam immediately freeze on the tube surface. This water ice formation continues to form and forms a barrier for further heat transfer and converts itself eventually into an “iceberg”. I believe the erroneously applied term, “steam bath”, has caused confusion.

This, in my opinion, is NOT a steam bath heater. Since I started out in the compressed gas industry, I am familiar with and have designed and used this type of vaporizer out in the field before. This is nothing more that a hot water, submerged coil heater/vaporizer. The steam is sparged into the water to heat it (the water) – and NOT the LIN. The LIN only comes into contact with hot water.

This type of vaporizer is very unstable and is not recommended for use with cryogenic liquids because it requires an inordinate amount of heat energy and requires constant, steady heating and agitation (which causes water convection). It is subject to a high degree of empirical factors and experienced design. An inexperienced or bad design usually results in a frozen block of water ice and little or no vaporization.

This type of vaporizer CAN be made to work – but at the expense of manual supervision and constant monitoring. This design relies basically on supplying sufficient water “flow” (in the form of a constant, moving film coefficient) such that it will immediately carry away cooled water and replace it with a “warm” water film. To do this in a static water “tub” that contains a submerged coil – and NO mechanical agitator – demands that one supply agitation and convection currents in the static water bath through other means. This is done by designing a steam sparger that essentially supplies both required heat transfer ingredients: a hot thermal fluid supply source and an agitation source (pressure differential causing expansion, turbulence, mixing, and steam condensation. This has all the potential for causing some negative effects as well: mechanical movement, noise, violent mixing, and potential stresses in what is a non-pressure vessel (a coil box or “tub”).

The process design is rather straight-forward. This type of vaporizer produces a saturated vapor – hardly never a superheated vapor. That means that all latent heat of vaporization is furnished by the sparged steam. This is easy. However, the coil box (or “tub”) must be kept “open”, full of water, and allowed to drain the subsequently condensed steam. This is usually into an open drain system and all condensate is lost to the boiler plant.

The mechanical design is the real “secret” of making this operation work. The steam must be sparged and distributed underwater in such a manner that it affects the entire coil in a positive, heat transfer mode. To do this, it must cause sufficient convection currents within the static water bath such that the entire coil is kept in continuous, positive heat transfer mode without suffering from localized freezing of the surrounding water. Once the surface of the coil freezes the surrounding water, any external convection currents are terminated and heat transfer, for all practical purposes, comes to a halt. Bear in mind that any localized surface freezing of the water affects the induced convection currents produced by the sparger. When this happens, more and more water surface starts to freeze and the result is a “Domino Effect”. The vaporizer starts to drop in efficiency and the effect can be a collection of LIN within the coil that may be entrained out with the little vapor produced – all depending on the mechanical and instrumentation design employed. Characteristically, this design relies on much empirical mechanical design know-how and only experienced and knowledgeable persons or firms should undertake the design and installation of such a device.

[Zauberberg]

Art,

Thank you for editing and clarifying Mukherjee's original post. My initial thoughts were exactly like you have described, and this particular design appeared to me as immanently wrong in the very concept itself.

I don't have any experience in design and operation of hot water bath facilities, particularly those handling cryogenic fluids. And it doesn't make sense to me - using a liquid bath that can easily experience freezing and the subsequent "snowballing effect" once when the first pieces of ice start to form around the outer coil surface, as you described. Why would anyone want to do that? Even if having the entire know-how in his hands, such design poses many operational and safety concerns. People have been using ambient air for vaporizing cryogenic liquids for many years, and it is proven, safe, and reliable design. Keep it as simple as possible - that's the principle which ensures steady and trouble-free operation.

[A mukherjee]

Thnxx a lot ART 4 making my post crystal clear to all..
Can you provide me name of any book or any source from where I can approach this design?
Also any information regarding the sparger design will be highly appreciated.

[Art Montemayor]

I am sorry to say that I don't know of any book, journal, or article that describes or deals with the design and workings of a steam-hot water vaporizer - especially one for cryogenic fluids.

As I have stated, much of this design (in my opinion) is based on acquired experience and empirical knowledge.

[A mukherjee]

Ok...then...

[Zauberberg]

I don't know the reasons why your client insists on the steam/water bath vaporizer for Liquid Nitrogen, but what you certainly can do is to prepare a brief summary and present all the arguments - as indicated by Art in his reply - as to why steam/water bath should not be considered as a reliable design solution. As process engineer, you are definitely entitled to do that.