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ABfish
12-14-2010, 10:47 AM
Okay guys, here's the question: My construction company installs utility lines among other things, and we are currently "dewatering" an excavation with pumps running 24/7. Yesterday, with an air temp of 36F, some of the discharge water was freezing on tall grass adjacent to the pump discharges. How can "wind chill" cause the water temp to go below the ambient air temp?

I gotta admit, as an engineer, this is bugging the crap out of me. It also bugs me that one of my employees asked me if it was possible for the pump discharge to freeze at 36F, and I said "No!" (He set me up and got a kick out of proving me wrong, BTW).

The only explanation that I can come up with is some evaporative cooling from the wind that could cause the water temp to go below freezing.

Milo Christensen
12-14-2010, 10:58 AM
It could also be a factor that there's water vapor, expanding like the coolant in a refrigerator and losing considerable heat during expansion. Google adiabatic cooling.

CK 17
12-14-2010, 11:00 AM
Are you using a nozzle of some sort? could there be a pressure and temp drop at the nozzle causing the spray to go below freezing. . . .

ABfish
12-14-2010, 11:01 AM
It could also be a factor that there's water vapor, expanding like the coolant in a refrigerator and losing considerable heat during expansion. Google adiabatic cooling.

Just the term "adiabatic cooling" gives me flashbacks to Basic Thermodynamics, the only class in college that I enjoyed enough to take twice!

ABfish
12-14-2010, 11:03 AM
Are you using a nozzle of some sort? could there be a pressure and temp drop at the nozzle causing the spray to go below freezing. . . .

The water that froze on the grass was splashing off of some rocks at the discharge point.

Milo Christensen
12-14-2010, 11:04 AM
Sympathetic chuckle at this end. I'd a been a doctor if it weren't for coeds and organic chemistry.

Milo Christensen
12-14-2010, 11:09 AM
Technically wind chill only applies to pieces of warm meat exposed to below freezing temperatures and winds of various speeds. It's a "how fast the temp/wind combination will freeze exposed parts as if it were calm and whatever temperature the wind chill is computed to be" kind of measurement.

Iceboy
12-14-2010, 11:14 AM
Wind can take some heat away but I don't think they call it wind chill.

Iceboy
12-14-2010, 11:15 AM
What Milo said.

Phillip Allen
12-14-2010, 11:17 AM
compare ambient humidity on days when the temps are the same but no ice occurs

Brian Palmer
12-14-2010, 11:20 AM
A body of water will cool more quickly at the same temperature if there is wind than if the air is still (so "wind chill" is still relevant to any body that has heat to lose to the ambient air, not just a living organism), but it does not seem it will cool much below the ambient temperature except by evaporative cooling.

Brian

Domesticated_Mr. Know It All
12-14-2010, 04:24 PM
Wind Chill Factor will freeze bone.

Mike DeHart
12-14-2010, 04:32 PM
The pump discharge water was freezing on the grass where it was splashing against a rock. The water at that point was atomized to an extent. Very fine droplets then have an enormous surface area compared to their mass. Add a blowing wind, forced convection, in the cold air which will have very little relative humidity. The water is starting at near the freezing point. Then you maximize the surface area of that water and blow very dry air through the mist to get a quick flash of maximum evaporation. Evaporation cools the liquid rather sharply in this case, bringing some of the mist to the freezing point. If you stop the pump, I would bet that the ice already formed would melt quickly.

If you have water in a closed container in 33 degree air and you blow wind across it that makes the news anchors scare you with wind chill reports of negative degrees, that water will never freeze. It will drop to 33 degrees very quickly, but it will not freeze.

They never mention the "wind chill factor" in the summer when they are scaring you with the "heat index" either.

Flying Orca
12-14-2010, 04:45 PM
Sympathetic chuckle at this end. I'd a been a doctor if it weren't for coeds and organic chemistry.

Oooh, sympathetic chuckle here too... although my issues with organic chemistry may not have been the same as yours. :D

Keith Wilson
12-14-2010, 08:29 PM
Basic Thermodynamics, the only class in college that I enjoyed enough to take twice!LOL! :d For me it was Differential Equations. Everything I leaned in that class evaporated out of my head as I walked out the door after the final, probably causing some adiabatic cooling. (I actually liked Organic Chemistry, and was pretty good at it.)

Flying Orca
12-14-2010, 08:33 PM
Yeah, I liked and excelled at organic chemistry. It's like Lego for science geeks.

Michael D. Storey
12-14-2010, 08:36 PM
Just the term "adiabatic cooling" gives me flashbacks to Basic Thermodynamics, the only class in college that I enjoyed enough to take twice!
I first heard it in a Power Squadron Weather course in 1970

pumpkin
12-14-2010, 11:08 PM
Wind chill can freeze water.

During accident investigations it has been found that airplane carburetors can ice up at ambient temps close to 100F if the conditions are right. I have looked into the throat of a carb at 75F and seen clumps of ice. A nozzle acts like a venturi and has a cooling effect in the low pressure created at the outlet.

Call bull if you want but you better have damn good citations. I learned way more about this subject than I wanted to arguing with other mechanics.

I'm sure there are a few pilots on these boards who can verify this.

Matthew

Flying Orca
12-15-2010, 08:11 AM
But that's not wind chill, it's "pressure drop" chill. (I say when it drops, oh you gonna feel it... Know that you were doing wrong...)

Joe Dupere
12-15-2010, 08:47 AM
For me it was statistics, (those damned lies!!). :D

Joe, FFPoP

Milo Christensen
12-15-2010, 10:52 AM
. . . During accident investigations it has been found that airplane carburetors can ice up at ambient temps close to 100F if the conditions are right. I have looked into the throat of a carb at 75F and seen clumps of ice. A nozzle acts like a venturi and has a cooling effect in the low pressure created at the outlet. . . .

You would also know all about this if you had been foolish enough to buy one of the first Toyota Corolla station wagon sold in the U.S. in 1970 or so.


For me it was statistics, (those damned lies!!). :D

Joe, FFPoP

Now that stuff I turned out to be surprisingly good at, but I was older and married and living off campus. Regrets, and I have a few, that I didn't really realize that organic chemistry was one of those "door" classes, you know, like Algebra in high school. Do good in the class and all kinds of doors open, do poorly and all those doors of opportunity close with a bang.

paladin
12-15-2010, 11:09 AM
Sympathetic chuckle at this end. I'd a been a doctor if it weren't for coeds and organic chemist

By that time I had a spouse who expected me home after class, dinner, and made sure the house was quiet so that I could study, except she could also see that I was getting a bit tired and she would come over and sit in my lap.....then all bets were off.

David W Pratt
12-15-2010, 12:57 PM
I like the venturi effect.
Ski areas make snow at temps > 0 Celsius by mixing in compressed air, when it expands it sucks up some heat (PV=nrT) and freezes the water.
That was one of the first mnemonics I learned.

Chip-skiff
12-15-2010, 08:55 PM
Water under pressure, issuing from a nozzle, will be subject to adiabatic cooling at surface/normal pressure. If there's sub-freezing air blowing through it, it will freeze more quickly, since the air mixes with the water and takes away still more heat.

A still body of water (a deep lake) will circulate within a few degrees of the freezing point. At higher or lower temps, it tends to stratify, i.e. form distinct layers with more than 1C difference.

C. Ross
12-15-2010, 11:00 PM
"Wind chill" at 36 degrees?!

It isn't wind chill unless it's degrees below zero farenheit!

In the past few days I've been in Baltimore, Philadelphia, Newark, Portland OR, San Francisco, Washington DC and everyone is complaining about "bitter cold" that would be picnic weather in Minnesota.

Fair warning to Austin TX...I'll be there tomorrow and if you complain about the cold and "wind chill" I'm gonna slap you!

bluedog225
12-15-2010, 11:28 PM
Low 70's by Monday. Good Christmas weather....

Chip-skiff
12-16-2010, 02:00 AM
"Wind chill" at 36 degrees?! It isn't wind chill unless it's degrees below zero farenheit!

Wind chill can take place at 36F, as anyone who has sailed (or mountaineered or biked) at that temperature can attest. The more air that passes over an object, with a lower temperature than said object, the greater the wind chill. Above freezing, air with higher humidity has a greater chilling effect.

Doesn't anybody take applied physics these days?

SamSam
12-16-2010, 11:29 AM
Above freezing, air with higher humidity has a greater chilling effect.

That must be why it's so cold here in the summer! |;)

oznabrag
12-16-2010, 11:57 AM
"Wind chill" at 36 degrees?!

It isn't wind chill unless it's degrees below zero farenheit!

In the past few days I've been in Baltimore, Philadelphia, Newark, Portland OR, San Francisco, Washington DC and everyone is complaining about "bitter cold" that would be picnic weather in Minnesota.

Fair warning to Austin TX...I'll be there tomorrow and if you complain about the cold and "wind chill" I'm gonna slap you!

For the love of God, man! Save yourself! Right now, it's a frigid 49F with winds above 5 MPH! if it weren't for a roaring fire and a mug of cocoa, I'd be history!

oznabrag
12-16-2010, 04:39 PM
Jeezle, Cris! I was only kiddin' around!

C. Ross
12-16-2010, 10:25 PM
Whew. Somehow I survived Austin's sunshine, warm weather, hospitality, smart people, AND a nice lunch and dinner. I'll give it another go tomorrow morning, and then that's it...I'm headed back to more snowfall and temperatures at least 60 degrees lower!

Chip-skiff
12-16-2010, 10:55 PM
This thread got me so fired up (or maybe iced up) that I went down to the river and watched frazil ice forming at the tail of a riffle. That's the free-floating pebbly sort of ice.

I'm thinking that the adiabatic lapse rate (a decrease in temperature corresponding to a decrease in pressure) applies to fluids in general, not just the atmosphere. Where there's turbulence and low pressure in a river, i.e. downstream from a boulder, anchor ice forms. The zones of low pressure that develop in riffles and rapids must have a similar effect: the water temp drops quickly. Surface turbulence also allows mixing with colder air.

So I'm thinking the combination of lapse-rate cooling and air mixing is responsible for the appearance of frazil ice. Eureka!

You want wind-chill? Come visit. -12F and a brisk 20 knots this morning. Makes your face ache.

oznabrag
12-17-2010, 08:49 AM
Whew. Somehow I survived Austin's sunshine, warm weather, hospitality, smart people, AND a nice lunch and dinner. I'll give it another go tomorrow morning, and then that's it...I'm headed back to more snowfall and temperatures at least 60 degrees lower!

I'm glad you're enjoying yourself, Cris!

If you'll give me a little more warning, next time you're in town, I'll be delighted to stand you to a pint.

oznabrag
03-25-2014, 09:23 PM
For me it was statistics, (those damned lies!!). :D

Joe, FFPoP

Sayonara, senor.

Phillip Allen
03-25-2014, 09:50 PM
since I was pretty young, I've known that frost can happen below 39 degrees... others have already said why

Ian McColgin
03-25-2014, 09:50 PM
The lost calories are accounted for by the negative of the heat of transformation. Water to vapor takes heat even though both remain at the same temperature. Same with ice to water. Visa versa is just the negative.

David W Pratt
03-26-2014, 10:55 AM
Ski areas can make snow above freezing. The aerated, pressurized water is sprayed out, as it expands, the air absorbs heat, voila.
I always considered Biology the Queen of the sciences

Gerarddm
03-26-2014, 11:00 AM
Per Phillip's #37, the frost alarm starts blinking in my Jetta Sportwagen when it gets to 39 degrees F outside.

BrianY
03-26-2014, 11:13 AM
"frost alarm" in a car? What's it for?

ahp
03-26-2014, 11:22 AM
No adiabatic cooling. To have adiabatic cooling there must be expansion. Air expands but water does not, at least too little to make any difference. Evaporative cooling is very effective, especially if the humidity is low.

ccmanuals
03-26-2014, 12:10 PM
Seeing a post from Milo I wonder how he is doing. He was having some health issues when he left us abruptly.

Phillip Allen
03-26-2014, 12:20 PM
I will tell you guys a funny... I may have told it before so bear with me

carpooling with a bunch of masons to a remote location (Galistaio, NM as I recall) about 40 miles east of Albuquerque... I was driving a Park Avenue with all the bells and whistles including the dash thermometer (not many others had seen such a thing yet, though the car was several years old). A Texas mason nosticed the screen asd asked what it was. I told him it indicated the outside temp... how can it do that, he said? The chill factor would give you a false reading!

I spent a couple of miles explaining that if that were true, one could blow on a glass of cold water and freeze it. I don't think I convinced him.

Chip-skiff
03-26-2014, 03:32 PM
No adiabatic cooling. To have adiabatic cooling there must be expansion. Air expands but water does not, at least too little to make any difference. Evaporative cooling is very effective, especially if the humidity is low.

The water being pumped probably has dissolved gases that would bubble out when the pressure is released, so you'd have an adiabatic effect that would cool the water, as it is mixing with cold outside air. At 36F, the relative humidity of the air would be quite high, so evaporation (and evaporative cooling) would be negligible. The surface area of water spraying out of an outlet would also increase rapidly, increasing the radiation of heat. But the characteristic of water known as the latent heat of fusion (80 calories must be lost by a gram of water to allow it to freeze) makes water resist freezing.

Interesting, anyhow.

Flying Orca
03-26-2014, 03:56 PM
The obvious explanation that never arose in response to the OP was that the ground temp was lower than the air temp. Simple explanations are, surprisingly, sometimes correct.