Replies

1. 
   Chris_T | Dec 23, 1998 06:50pm | #1

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   I cannot help you with the freezing temperature of the water. But if you want to determine whether your pool will freeze with the water that's presently in it then take a cup of it and put it into your freezer overnight. Your freezer's temperature is probably about zero degrees fahrenheit +/ - 5 degrees. You can get freezing temperature by measuring it every hour or so until it freezes.

2. 
   Bruce_ | Dec 23, 1998 10:00pm | #2

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   I live in Idaho and what I have seen folks do is to place a large piece of styrofoam (or a few) into the pool water so that the expansion of ice compresses the styrofoam therefore not displacing the sides of the pool. Utilizing this method along with covering the pool seems to work in all but the very worst, prolonged conditions.

1. 
   Kent | Dec 23, 1998 11:14pm | #3

   *Adam,I have my brine table before me but I don't know how much water your swimming pool holds and to what temperature you want to suppress the freezing point. For a reference point, 1 lb. salt per one gallon of water will drop the freezing point to 19.3 dF at sea level (NOTICE: to, not by). At that concentration, watch out for corrosion on all your other components of the system. That's the science but for the care and feeding of your pool seek the help of others with pools. Minus 20d F where I'm at,Kent

3. 
   Guest_ | Dec 26, 1998 10:15pm | #4

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   Adam,
   Your post has raised my curiosity. I have had swimming pools for fifteen years and have encountered every problem imaginable. Your type of automatic chlorinator is "new" to me. It seems like alot of work. Why do you need to prevent your water from freezing? I assume that you would have to drain your pool water in the spring with all the salt that you add to prevent the freezing?? The liner problems that this would cause are unthinkable to me!!
   Waiting for spring, Beatrix

1. 
   Guest_ | Dec 27, 1998 08:40am | #5

   *Wow.Quite a response. Thanks to all. (Fred, I would have guessed more than 1.8C/ 3.2F...Thanks)I'm in Northern California (Sacramento Foothils) where it "never snows", except this year when we had 2 inches (first time in 15+ years).We usually don't need to drain and "winterize" pools, so I don't. The low temps with this cold wave are therefore a problem. My pumps are below water level, so I can't drain the external piping, but I can just keep it running. If I know the Freezing point depression, I know when I need to turn on the pumps!I'm not adding salt to stop it from freezing, the salt is already there for the Clorinator. I just want to know when to turn the pumps on. (actually, a thermistor and a radioshack X10 controller can do it automatically.)PS The clorinator is a "Chlormatic" Nice unit so far, 3 years old. Did have one service call but it was under warranty.Weather here has been real cold... 50-60s in the day....what wimps, eh?

1. 
   Guest_ | Dec 27, 1998 01:24pm | #6

   *The reference I used is Perry's Chemical Engineering Handbook - if your library doesn't have it, they should. Maximum FPD you can get with NaCl is to -9.4C/15F but that would be at its solubility limit of 35%. With heat gains from the surrounding ground 15F ought to more than do it - that gives you 17F below fresh water. But it ain't going to be a normal swimming pool anymore. That's more salinity than the Great Salt Lake and you would float with half your chest out of the water - which could be kinda cool. Also, all that salt would be very drying to your skin, incredibly irritating to your eyes, and corrosive to almost all metals - you got titaninum pipes? Any dissimilar metals that touch will corrode very fast. Other brines can give you even lower FPD (CaCl to -37.2C/-35F) but have the same problems.Let's not even get started with ethanol, methanol, or ethylene glycol due to their extreme toxicities.Fred's right. Seawater (about 3%NaCL) freezes at 29F.It's -18F in Kasilof, Alaska right now. We waited until the afternoon warmed up to -8F to go skijoring behind our retriever today.-David, Chemical Engineer, Owner-builder, and Chocolate Chef

4. 
   Scott_ | Dec 29, 1998 02:34am | #7

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   Depending on your piping layout, you may not need to drain the entire pool to get the pump and external lines drained. Some pools are set up so the pump and external piping are all below the level of the circulating "jets" which are likely 12-18" max below waterline. Open plugs or drain cocks at lowest point and let rain until below jet level and pipes are empty. Drain pump and all valves. It's a pain but cheeaper than re-piping. Problem remains "trap" areas like skimmer/ filter body. You could fill it with a non toxic RV antifreeze. In a round bottomed pool with round or kidney footprint ice should just squeeze upward and the reinforced walls will be OK. Square corners at bottom or sides might be more of a problem. Tile coping at waterline is also susceptible to damage. If rainfall brings ice back up to that level you might need to sipohn some water out again.

5. 
   Guest_ | Dec 29, 1998 11:32am | #8

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   Be careful about draining a swimming pool. If you've done it succesfully before, fine. But in areas of high groundwater (did they have to dewater to construct the pool? Did they have to wait for lowest ground water levels in the late summer?) a drained pool can pop out of the ground - not a pretty sight.

   -David

6. 
   Kent | Dec 30, 1998 04:22am | #9

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   David,

   The maximum freezing point depression you can achieve with NaCl is not +15d F but -6d F (-21d C) and this occurs not at the solubility limit of approximately 35% but at 30.4%. At concentrations between 30.4% and 35% you are above your eutectic point which means salt will precipitate out of solution and you lose your advantage of lower freezing temperatures.

   My Fifth Edition of Perry's Handbook barely covers the topic but it does match the figures noted in your reply. However, I believe that the Perry Handbook is listing a practical working temperature. In the case of refrigerants, which is the section in my handbook that addresses the topic, trying to achieve cooling beyond that temperature is impractical, not impossible. The same 15d F temperature is a practical working temperature for ice removal on highways and roads; this having to do more with time. Neither of these would apply to the static process of a swimming pool.

   But it sounds like Adam is sitting tight with his 3% concentration. However, I am intrigued with the effects of the higher density in a more saturated pool that you pointed out. Without having to flail about to stay afloat and having your chest rise high out of the water I can easily imagine perching several beers and a bowl of chips on my pectorals. Ahhh, California sunshine !!!

   Toasting a Salty Dog to Lat60.4N Lon151.3W,
   Kent

7. 
   Guest_ | Dec 30, 1998 04:22am | #10

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   I'm not sure I should ask, but youse guys are pretty smart:

   How much will salt in a swimming pool reduce the freezing point?

   I have an "automatic chlorinator". You add salt and it monitors and creates Chlorine (remember salt is NaCl). Actually, the process is fairly impressive: around 700 lbs of salt dumped into the pool. The concentration of salt is around 3 grams/deciliter.

   The phenomena is "freezing point depression", but I can't seem to find any equations/tables.

   The gauntlet has been thrown down. Any thoughts?

   Thanks,
   Adam S