6 replies to this topic

[Celine_D]

Hi everyone,

 

I hope you are all doing well.

 

I would really appreciate some help understanding vaporizers.

 

I am working on a project that involves storage of liquid nitrogen in a cryogenic vessel. The liquid nitrogen leaves the bottom of the vessel and is then routed to liquid nitrogen vaporizers before it is sent to the supply header for distribution.

 

I was just wondering what exactly is the function of the liquid nitrogen vaporizer (other than actual vaporization of the nitrogen);

-  I have read that the vaporizers are employed to increase the flowrate of nitrogen from the cryogenic vessel but I am not sure how this would increase the flowrate?

- I have also read that the vaporizers prevent freezing of the liquid in the pipeline.

 

I have attached my schematic below.

 

I hope you can help.

 

Many thanks,

 

Celine   liquid nitrogen vaporizer.docx   43.35KB   52 downloads

[breizh]

hi ,

The purpose of the vaporizer is to convert liquified Nitrogen to gas using an exchanger .

You may find ice of the tubes of the exchanger . 

Be cautious with the pipe material because of the low temperature . Check with vendor .

The load depends on the capacity of the vaporizer (surface /number of fins).

https://www.chartind...-Air-Vaporizers

https://www.incryosy...principles.html

 

 

Consider to use your favorite search engine , key words Nitrogen vaporizer .

Good luck

Breizh

Edited by breizh, 24 September 2020 - 12:19 AM.

[Celine_D]

Thank you very much for your prompt response.

 

Do the vaporizers allow increased flowrates from the vessel and if so how?

 

I am finding it difficult to understand the attached article which states that:

"If the demand of gas needed (also known as flow rate) is higher than the output of a liquid cylinder the pressure and flow rate will drop. Adding a vaporizer to the liquid cylinder increases the flow rate of the liquid cylinder while keeping the same pressure. Meaning the flow rate and

pressure stays constant."

 

I understand as the level in the vessel drops, the static head will decrease meaning a lower DP available across the supply pipeline and hence a lower flow velocity. But how does vaporizing the liquid after the vessel help increase the flowrate? 

 

Maybe the attached set up is somewhat different to my set up (picture in 1st post) which is causing confusion.

 

Many thanks,

 

Celine 

Attached Files

•  39A-Vaporizers.pdf   7.84MB   62 downloads

[latexman]

If there is NO vaporizer, the surface area of the N2 cylinder provides the heat transfer needed to keep the liquid N2 inside at ambient temperature.

 

Assume you start with a full cylinder that has set around long enough to be at ambient temperature.  You open a valve that delivers vapor N2 (no dip tube) and leave it open.  At first, the temperature of the vapor is ambient temperature.  (Let's ignore the Joule-Thomson effect of N2 vapor, which is not so strong anyway, about 0.2 ^oC/bar at 25 ^oC.)  The vapor that leaves the cylinder lowers the pressure in the vapor space.  Lower pressure causes the liquid N2 to boil.  The latent heat of vaporization of this boiled off vapor lowers the temperature and vapor pressure of the liquid N2.  The longer the valve flows vapor, and the faster the valve flows vapor, the colder the liquid gets.  And, the liquid N2 gets colder and has less vapor pressure.  It is a dynamic situation, changing minute by minute.  So, now heat will transfer from ambient into the below ambient cylinder.  There is a limit to the rate and how long you can take vapor N2 out of the cylinder, which is determined by the surface area/heat transfer area of the cylinder.

 

By adding a vaporizer, you can send liquid N2 to the vaporizer, and vaporize it to vapor in it's much greater surface area.  The heat balance is mostly, but not totally, disconnected from the surface area of the cylinder.  This increases the rate and time one can withdraw vapors from the cylinder before temperature and vapor pressure inside the cylinder reduce to levels that will not keep up with the rate needed.

Edited by latexman, 23 September 2020 - 01:51 PM.

[Bobby Strain]

This link might help.  http://www.airproduc...ge-systems.aspx

 

Bobby

[IGC]

Celine_D, on 23 Sept 2020 - 11:33 AM, said:

Hi everyone,

 

I hope you are all doing well.

 

I would really appreciate some help understanding vaporizers.

 

I am working on a project that involves storage of liquid nitrogen in a cryogenic vessel. The liquid nitrogen leaves the bottom of the vessel and is then routed to liquid nitrogen vaporizers before it is sent to the supply header for distribution.

 

I was just wondering what exactly is the function of the liquid nitrogen vaporizer (other than actual vaporization of the nitrogen);

-  I have read that the vaporizers are employed to increase the flowrate of nitrogen from the cryogenic vessel but I am not sure how this would increase the flowrate?

- I have also read that the vaporizers prevent freezing of the liquid in the pipeline.

 

I have attached my schematic below.

 

I hope you can help.

 

Many thanks,

 

Celine  liquid nitrogen vaporizer.docx

 

 

Celine_D, on 23 Sept 2020 - 3:52 PM, said:

 

Thank you very much for your prompt response.

 

Do the vaporizers allow increased flowrates from the vessel and if so how?

 

I am finding it difficult to understand the attached article which states that:

"If the demand of gas needed (also known as flow rate) is higher than the output of a liquid cylinder the pressure and flow rate will drop. Adding a vaporizer to the liquid cylinder increases the flow rate of the liquid cylinder while keeping the same pressure. Meaning the flow rate and

pressure stays constant."

 

I understand as the level in the vessel drops, the static head will decrease meaning a lower DP available across the supply pipeline and hence a lower flow velocity. But how does vaporizing the liquid after the vessel help increase the flowrate? 

 

Maybe the attached set up is somewhat different to my set up (picture in 1st post) which is causing confusion.

 

Many thanks,

 

Celine 

 

 

 

My original comment deleted itself

 

Summary:

 

There should be nothing else in the cryogenic pipework.  It should be CLEAN.

Freezing can only occur if the pipework is not dry (dew point <-28 degC and below.  Typical for ambients is around -40 degC).

 

You can pull liquid and vaporise it, or you can directly pull vapour out of the vapour space is the flowrate is going to be small enough.  Pulling vapour out will affect the tank pressure a lot quicker and this will have an influence on the delivered flowrate.  Pulling the liquid is a lot more stable but you do still require a pressure build up vaporiser to maintain the required set pressure in the tank.

You are missing a pressure regulating station downstream of the ambient vaporisers.

 

P.s. I prefer liquid side switching on the ambients with non-return valves on the ambient outlets.  It is more expensive than gas side switching but it allows for the vaps to defrost better.  With gas side switching the liquid still pools in the vap meaning it needs to evaporate first before it can begin defrosting.

 

This is a good presentation about cryo tanks:

 

https://www.boconlin...cm410-39416.pdf

 

Most vendors for cryogenic tanks post their P&IDs online:  Chart, VRV, InoxCVA, Cryolor, Taylor Wharton/Auguste Cryogenics, Cryovat...

 

Look on the Gasworld Directories listings to find others.

Edited by IGC, 24 September 2020 - 02:39 AM.

[Celine_D]

All,

 

Thank you so very much for your detailed explanations. My understanding has been greatly improved.

 

I have only just joined and this was my first post but I know where to come when I am having difficulty understanding similar concepts :-)

 

Celine