Heat loss in buried chamber

[JunglayBM]

Hi guys.

I have a pipe chamber containing a pressure transducer that has been subject to freezing over the last year. I want to calculate the length of time for the temperature inside the chamber to drop to the atmospheric temperature above ground. Assume the chamber is 2m x 2m x 2m insulated by ground on all sides except the top and the cover for the chamber is non-insulated ductile iron with a 1mm gap around the edges where air can transfer. How would I go about calculating the heat loss and the time it takes to go from 0 degrees C to -10 degrees C. Numbers are just arbitrary so I can get an idea of the problem. Thanks for any help and apologies if this is in the wrong forum.

 

[LittleInch]

Can you draw this so we understand where this 1mm gap is please.

Also 0 degrees is freezing - why does -10 make any difference?

heat transfer in water is quite different to ice

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[JunglayBM]

Thanks for the reply.

I already have a calculation for the water going to the pressure transducer to freeze but the calculation is dependent on the temperature of the air inside the chamber. I'm just trying to get an idea of how long it would take for the air inside the chamber to reach the outside temperature. The lid to the chamber doesn't actually have a 1mm gap, I just wrote that to give an arbitrary number for calculations. In reality the lid is sat on the chamber but it is not an air tight seal or insulated.

The reason -10 makes a difference is because the temperature fluctuates during the day above and below freezing and calculations imply that at -10 the water in the pipe would freeze within 90 minutes whereas at 0 it could take up to 24 hours.

 

[LittleInch]

Aaah, I get it.

This is 2 x 2 x 2 chamber buried fully on 5 sides, full of air, which has a lid on it (6th side)with very minor air gap open to ambient air.

Doing any meaningful calculations or simulations without knowing the temperature or temperature gradient of the soil surrounding 5/6 of the volume will be difficult I feel.

This might be setting up a circulation where the cold centre of the chamber creates a falling column of air cooled by the iron lid whist the slightly warmer air on the sides rises and hence slowly circulates gradually cooling the entire air mass.

How long??

I don't think you have enough data to even guess.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[JunglayBM]

Yeah that's the right layout and I think you are right that I need more information. It might be the case that some experimentation is needed to figure it out. Thanks for your help

 

[itsmoked]

You can calculate all day long and still be way off. If this thing exists why not toss a temp logger in the hole? I'd get one with multiple channels and tape a sensor to the pipe, a wall at the bottom, wall at the top, bottom of the lid, and hang one in the air in the box center. Put one out in the ambient with a little umbrella over it so it doesn't see the night sky. A couple of day's cycles and you'd actually have a good model that could be extrapolated to various air temps.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Compositepro]

You have not even mentioned ground temperature, which will be one of the most important factors.

 

[JunglayBM]

Problem is that I don't have data for ground temperatures. The installation in question is owned by a water company over 200 miles away so throwing a logger into the installation isn't really practical. A test rig is being set up with an industrial freezer to gain experimental results so Hopefully that will yield satisfactory results.

Just for the sake of argument, if I was to assume a linear temperature gradient between 0 degrees and say, 5 degrees for the ground temperature would It be a waste of my time?

 

[Compositepro]

Yes it would be a waste of time. The temperature in the box will vary as the heat flow into the box varies with time and due to weather conditions over the previous weeks or months, and the heat flow out of the box varies with time also.

 

[itsmoked]

200 miles is going to be cheap compared to your simulated freezer chamber exercise. There is simply no way for you to simulate this. You need empirical data gathered in-place.

Keith Cress
kcress -

http://www.flaminsystems.com