How to calculate the thickness of a wall of a dewar?

[Perilun]

TL;DR

Wall thickness cryostat dewar

Hello,

i am trying to design a dewar for experiments at very low temperature. Unfortunately i do not know how to calculate the wall thickness. I can calculate the heat flow through the wall, but i do not know what to do further. Do someone know how to calculate the thickness of the wall for a dewar?

 

[berkeman]

Welcome to the PF.

A dewar does not have a single wall. Which wall are you asking about? What are the different requirements on the walls?

What material and what thicknesses are you asking about? What welding technology are you going to use?

Is this just a schoolwork/homework question, or are you actually going to build some of these dewars that will be used in labs?

 

[Perilun]

For some experiments a dewar was bought. To make some experiments i need to make a cover at the top of the dewar, for putting some bulks in it. The construction should not be mounted to the rest of the dewar, but some other devices should be mounted with screws on it. The requirements for the top cover are it should be leight and could isolate the innerspace. The maximum thickness of it should be max. 6 cm, but as possible not over 3 cm.

 

[berkeman]

For some experiments a dewar was bought. To make some experiments i need to make a cover at the top of the dewar, for putting some bulks in it. The construction should not be mounted to the rest of the dewar, but some other devices should be mounted with screws on it. The requirements for the top cover are it should be leight and could isolate the innerspace. The maximum thickness of it should be max. 6 cm, but as possible not over 3 cm.

Is this a schoolwork assignment?

I'm having a hard time picturing what you are describing. This is the kind of dewar I work with. Is it what you are talking about when you say "dewar"? What is your application?

https://media.glassdoor.com/l/74/29/2e/fe/liquid-nitrogen-dewar-shiny-and-new.jpg

 

[Perilun]

No its not this dewar, its more a freezer. Something like this: https://www.alibaba.com/product-detail/SY-LAB-ICE-CUBE-14S-freezer_166562265.html. The problem is i have to put some mechanics on the top of the dewar/freezer and that is why the temperature on the top of the freezer should be over zero and inside the freezer between -150°C and -180°C.

I am a student at the university. I study mechanical engineering and that's why i do not have access to such equipment (dewars and so on) and do not know how to make isolation between such a freezer from the picture and the mechanical part, which should be on it (with motor and grease).

I thought about making the end plate at the top with perplex for the isolation and stainless steel for the stability, but i do not know if it works and how i can calculate the thickness of each layer. I read at the moment the book from ekin, but i do not understand how to make calculation.

I hope my little description answered your question.

 

[bobob]

The reason that dewars are made of stainless is that stainless is a terrible heat conductor. The way a dewar is made is that there is an inner bottle made of thin walled stainless steel that only contacts the outer bottle at the top, i.e., the inner bottle hangs from a flange welded to it and that flange mates with the flange on the outer bottle. Then, you pull a vacuum in the space between the inner and outer bottles. When you do that, the primary heat leak is through the mating flanges, not the dewar itself. Without a vacuum, your dewar won't work very well at all. You would be better off buying a generic thermos.

 

[Perilun]

Thank you for the answer. Its a little bit complicated, but i try to describe it. I do not want to make a dewar, because i do not have the possibility to make the vacuum. I have the freezer from the picture the link i posted to. it has a top, which is made from glass. I have to made a construction for this, one part of which will be put inside and the motor, which should drive the mechanism should be outside. To avoid of destruction of the parts, which are outside of the freezer, i want to make a top between the inner and the outside parts, which will be placed between the inner parts and outside mechanism. The freezer as itself exists, but i have unfortunately no access to the freezer (please do not ask why.) Because on the picture it seems that as the top is used glass, i think a vacuum isn't needed, but i do not know how good it isolates. That is why i am asking, is it possible to make the top from periflex to isolate (to keep cold inside or get the outside parts at least to -10°C) and what thickness the layer should have?

I would also apprechiate other suggestions. Unfortunately i do not have the possibilities to make very complicated constructions, like vacuum. I don't think liquid nitrogen is need for the isolation, because it is used as the cryogen.

Summary: Freezer/Dewar exists, Question how could be the top be constructed (which material, calculation thickness, vacuum not possible)

I can not give more details.

 

[Nik_2213]

Hi ! We used cryo-rated stainless-steel 'thermos' bottles for inhaler propellant sampling. Partly because they were literally unbreakable during manual handling, partly because they'd shrug off the thermal shock of going from ambient to whatever-below in one splash...

They had thick, plastic, screw-in lids with a burst-disk safety valve. Seems a centimetre of cryo-rated plastic makes a fairly good insulator !

 

[Perilun]

Thank you for the information.

Do you know which plastic it is, g-10 or something else and is it possible to calculate the thickness of the layer?

 

[Nik_2213]

I'm sorry, it was an 'aerospace' material with a really, really low 'plastic transition temperature' so it would not shatter.

Hunt around for "Plastics for Extreme Temperature Applications" and cryo-rated plastics.

Care: following link has much useful info, but produced by manufacturer.
https://indico.fnal.gov/event/13139/material/slides/1?contribId=0

 

[Perilun]

My problem in first place is i do not understand how i can calculate the thickness of the layer i need. I know that the heat transfer should be calculated, but i do not understand what values should be. For example i get 200 W as the result of the heat transfer. But is it good or not? I do not understand it.

 

[Tom.G]

For strength use whatever material is sufficient and easy to work with.

Put a few cm. of plastic foam on the bottom of it for thermal insulation. Others here can likely pick a low temperature plastic, but foamed polyethylene, polyurethane, and poly styrene come to mind. Even a few layers of a wool blanket would be a better insulator than a glass cover.