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.