Time required to cool the Tube metal with Liquid N2

[DK44]

6000 Lit of Liquid N2 is required to passed through Tubes of Heat Exchanger to Cool the Tube Metal to -160 C from 40 C. The inside wetted surface of the Tubes is about 700 m^2. Out side of Tubes is ambient air at 40 C. How much time is required to cool it to the temperature. How to assess the time. Request guidance.

 

[IRstuff]

Start by figuring out the thermal mass

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[itsmoked]

That's a tough one.

Why?

Because part of the heating from the surrounding ambient is radiative and the rest is convective. The radiative will start as omni-directional and quickly convert to strictly just those directions that can't see other parts of the heat exchanger. The convective will start being from all surfaces as the warmer air moves by but will rapidly slow as the surrounding air becomes chilled. Then, to really make matters worse it will all slow considerably further as insulating frost appears on the entire surface.

All of these are time changing aspects that vary over the local wind-speed, local humidity, air temperature, and solar gain, and make it very tough to do.

So tough, you're only going to do it with empirical data. That data will come from a similar heat exchanger and it's published thermodynamics.

Keith Cress
kcress -

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[LittleInch]

Also in the first instance the liquid nitrogen will boil, creating a cold gas or two phase flow.

flow rate is also important.

normally you want to cool these sorts of things very slowly to prevent excess stress from one part being colder than another part.

Not to mention what exactly is on the other sides of the tubes? Air, liquid of some sort?

So no idea how to start doing this?

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[racookpe1978]

Homework assignment?
It does not sound like a real world problem because of the rapid cooling of liquid N2 in many adjacent very thin wall heat exchanger tubes Vs the thickness of the tubesheet itself? The splashing, the boiling, the cooling of the tubes downstream of the tubesheet.

Bad idea in practice.

 

[Caloooomi]

Are you looking to cool down the complete tube length / exchanger or just a portion of it?

You would need to work out the energy required to cool down the metal from ambient to -160 degC of not just the tubing, but all the parts of the unit in contact with the LIN.

What time scale are you considering for the 6000 ltr? Seems like a very small amount compared to the internal surface area of the tubes, well, unless it's enhanced.

Is it ambient air on the outside of the tube bundle? If so, remember that'll ice up too (which will help to extend the boiling section if flow boiling).