water velocity required to prevent freezing 1

[hrdrok]

Simple,

I need to move ~10,000 usgpm of water over 3000 feet, in subartic conditions, 24/7 run conditions.

Negligible head (10m +/-) plus line loss.

Heat trace and insulated pipe not an option due to cost.

Mining application

Anybody have a minimum flow velocity to prevent line freezing,

Probably sclair DR17, water at low single digits (C). Ambient air temp -20C and warmer.

ANy ideas?

I'll put this in civil as well.

h_r

 

[CarlB]

I think you may get more help posting it in the Mechanical Engineers/Heat Transer forum.

From your question it sounds like you are counting on heating due to frictional resistance. I think that heating will be negligible, unless you go to a quite small pipe/high velocity, so that pumping energy is providing heating. You didn't mention initial temperature, that's important.

You might try some simulation at

http://www.pipeinsulation.org/

but that may not account for friction heating.

 

[PEDARRIN2]

Look at ASHRAE Fundamentals - Chapter 26 in the 2005 edition.

It gives an equation for time it takes for water to freeze in a pipe.

It mentions that if the flow of the water is sufficient to offset the heat loss - freezing will not occur.

 

[PEDARRIN2]

Thinking about how I would approach it - using the ASHRAE information - I would calculate the time it took for a 1 foot section of water in the pipe to freeze in the given conditions.

Then see what velocity the water in that 1 foot section would have to travel before it froze.

If say the calculation came out the water would freeze in 60 minutes, then the velocity required would have to be 50 feet per minute (3000 ft / 60 minutes).

Convert the gpm to velocity given your pipe size and that will tell you if 10,000 gpm is enough.

 

[TBP]

I actually have a book (Handbook Of Data Sheets For Solution Of Mechanical System Problems - edited by Roose & Roose) that has - among many other things - "Chart Gives Bleed Rates To Protect Bare Water Pipe". It's for bare steel pipe at - 20*F. with a water temp of 40*F.

All I need is your line size. Note that the chart goes up to 18" pipe, and is for 100 feet of pipe.

The example used in the book is an 8" line, 250 feet long. The chart says 5 GPM, so it's 5 X 2.5 = 12.5 GPM

For wind velocities over 15 mph, it states that the flow rates taken from the chart should be doubled.

 

[bimr]

The book that is linked below states that a minimum flow of 2 ft/sec would probably keep the fluid from freezing. There is a formula in the book to make the calculation. You should plan on some type of contingency in case the fluid stops flowing and you get a freeze up.

http://books.google.com/books?id=pt...low velocity to prevent line freezing&f=false

 

[CarlB]

The 2 ft/sec velocity cited in that reference is primarily to maintain sufficient stagnation pressure at the pitorifice to cause relatively short service lines to circulate. That section is also primarily for lines buried in permafrost, where temperatures only drop a few degrees below freezing. In addition, lines in arctic conditions are insulated, with rare exceptions.

Even with considerable velocity, given enough time (distance) and heat loss water lines can freeze solid. Line blockage can be due to both annular ice formation and frazil ice which can clump together at obstructions.

 

[bimr]

CarlB,

The important point about the book is that it contains a formula so that hrdrok has a reference to use when he makes his calculation. A reference is better than an anonymous post.

"Introduction to cold regions engineering" also shows on page 435 that water mains with 2 ft/sec velocity have been successfully used. On page 434, the book states that insulation to keep the freezing isotherm within the insulation is too expensive.

If you feel the book is in error, perhaps you can put a review on amazon.

 

[rconner]

While I’m sure all can appreciate constraints of ”cost” and likewise benefit of “24/7” run conditions in many operations, it is just kind of hard (for at least me) to imagine a dependable reliance for long-term operation in subarctic conditions on heat energy from conveyed fluid alone to prevent “freezing”. In at least other applications I think there are alas possibilities for many potential reasons of eventually at least temporary “shut-downs” of operation, line occlusions, damages, breaks, failures, etc.
While we obviously have very little information and this is indeed an interesting exercise for any thermodynamically inclined amongst, from the outside looking in (I guess there could be much we don’t know/uniqueness about the specific application) this seems like somewhat risky business, and there would be at least some possibility of inconvenience and maybe even some unexpected “cost” if/when? the line freezes.