"Flash Freezing" of Water in a Metal Pipe

[ericskiba]

Hello,

I have a simple question that I hope some of you may be able to help me with.

In an hypothetical situation you have a body of water inside a home at room temperature. The outdoor temperature is sub zero (~ -25oF). If all the piping was say for example 3/4" copper could you freeze the water inside the empty pipe if the flow rate was about 1 gallon per minute.

I am thinking that since the thermal mass of the copper is much lower than that of the water there is no way that you can freeze all the way through. You may get some quick freezing on the pipe wall which will disappear as the warm water flows over it.

What do you guys think?

 

[racookpe1978]

Be more accurate to assume the exposed pipe is in a basement/crawl space or garage rather than an abandoned house. Or a house being left not used over the winter?

If there is flow, the problem becomes a conflict between the "warm" water from the street system (assume 40-45 deg F if the water main is buried 2-3 feet (1 meter maybe) underground?) continuous flowing in the pipe and the "room's" ability to remove heat from the outside of the pipe.

Given enough flow, there never will be ice formed because the water doesn't get cold enough to freeze before it leaves the room. Given too little flow, the ice will have a chance to form - which will either reduce flow further and lead to icing completely; or to a restriction in flow and increase in velocity which will remove the ice. Depends on what is moving your water.

If no flow, then the whole problem changes and (eventually) ALL of the water in the static parts of the system will freeze when the pipes and water pass through freezing to reach thermal equilibrium, breaking the pipes. Notice that even -1 or 2 degrees C is enough to feeze everything eventually, but the lower the temperature is below zero C, the faster heat transfer is out of the pipe to the room, and the greater the flow is needed.

Common practice in the south US is to trickle water in the outside hoses to prevent water lines going outside from freezing: Further north, pipes are better insulated in the walls of the house, and are isolated and drained completely when freezing weather starts in the fall.

Through-wall delta T of the copper pipe will be small, and the change in thickness of the ice at the wall as the water freezes will be of greater interest. Through-wall delta will at least be constant: the through-ice delta T will change as the ice gets thicker with time. Delta t of the room to the pipe wall will be the biggest unknown. Pipe assumed horizontal? Sloping downhill slightly? A mix of verticals and horizontals?

 

[IRstuff]

3/4 in diameter corresponds to 1 gal vol in 13.3 m of length. Assume double that, so 26.6m of length. At a gal/min, the water is exposed to the cooling environment for 2 minutes.

1 gal of water frozen from 1°C over 2 minutes will require 10.7 kW of power

10.7 kW/area of pipe/deltaT(assume 5°C) = 4.2kW/m^2-K would be the required heat transfer coefficient.

That's not sustainable by anything to do with heat transfer via air.

Working backwards, you'd need a flow rate on the order of 1 gal/day to get a heat transfer coefficient low enough to be supported by air.

TTFN

FAQ731-376

 

[ericskiba]

In this scenario the water would be room temperature not cold city temperature. Assume the pipe is insulated with 1" insulation and is exposed outside for a length of say 20 feet.

I think with those above calculations it seems like it is not possible to freeze.

 

[vpl]

Is this homework?

Patricia Lougheed

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[ericskiba]

not homework what so ever. when dealing with solar hot water systems in a drain back format it is possible to have a situation where on a super cold day the sun comes out and turns on the circulation pump. while the collector may be hot it is possible for the pipes to and from the collector to be at ambient temperature.

 

[vpl]

ericskiba

If the pipes were empty (i.e., the drain back system worked properly) and then the collector starts, the one gallon per minute flow rate is probably sufficient to prevent freezing (as IRStuff pointed out) I'm guessing from your description that the water starts inside the house at some ambient (70ºF?). It then exits to the roof for the 20 feet of piping until it enters the solar collectors. Then another 20 feet of return piping. The water wouldn't have time to drop 38º during the time it's exposed before entering the collection system. Afterwards, it's at a higher temperature so freezing is even less likely.

The only problem would come if the drain back wasn't sufficient such you started building up ice inside the pipes whenever the system wasn't running (such as night time). I've read that some systems use anti-freeze to prevent that sort of problem.

But I don't normally deal with solar heaters, so I don't claim any particular expertise in this area.



Patricia Lougheed

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