Become a TFP Supporter Pool Math Forum Rules Pool School
Results 1 to 9 of 9

Thread: The Physics of Freezing and Freeze Protection

  1. Back To Top    #1
    mas985's Avatar
    Join Date
    May 2007
    Location
    Pleasanton, CA
    Posts
    11,489

    The Physics of Freezing and Freeze Protection

    Recently, there have been a lot of questions from those living in moderate climates about using automatic freeze protection or running pumps to prevent freezing and what a pool owner should do when temperatures drop so I thought it might be useful to have a thread which dealt with the issue from both a practical and theoretical point of view.

    But first a few thoughts about using pumps to prevent freezing. Running the pumps is not fool proof and power outages can obviously be a problem. Also, some pools have a lot of features and some are manually controlled so automatic freeze protection may not always be a viable solution. Plus electricity is expensive in many areas so having the pumps run all night is not always an attractive option. Given all these factors, I am not a big fan of using the pumps to prevent freezing, but there are alternatives for those that are interested.

    Also, I wondered how long it would actually take for a pipe to freeze so I did some research into the physics of freezing pipes. Over the years, there have been several empirical university studies and the consensus is that for home plumbing in un-insulated spaces (e.g. attics), the "alert" temperature where pipe freezing and bursting can become a problem is below 20F. Above that temperature, freezing rarely occurs and bursting of the pipe is even rarer. But this is for small copper pipe in attics, not pool equipment so I had to look deeper.

    The time it takes a pipe to freeze is dependent on several factors:

    Size and dimensions of the pipe: More water in the pipe takes longer to freeze.
    Pipe thermal conductivity: Copper is about 2000x more thermally conductive than is PVC.

    But then there are also environmental conditions:

    Air Temperature: The lower the air temperature is below freezing the faster freezing will occur.
    Night Sky Exposure: Radiation losses can have a large impact on the heat loss.
    Wind Speed: Wind blowing over the pipe greatly increases the heat loss of the pipe.
    Location: Proximity to heat sources (e.g. side of house).

    But a freezing pipe does not always result a bursting pipe. In fact, it rarely does. When a pipe freezes it freezes from the outside inwards and as the ice expands, the excess pressure is relieved through the center of the pipe until the pipe freezes solid. This alone will not burst the pipe. However, once the pipe is fully frozen and blocks the relief of pressure, the freezing ice up stream of that section of pipe may not have a path to relieve the pressure and only then does the pipe burst. This process is fairly well described in the following sources:

    http://www.spokanecounty.org/data/bu...eBurstPipe.pdf
    http://www.iccsafe.org/cs/PMG/Docume...eBurstPipe.pdf
    http://scifun.chem.wisc.edu/radio/pipefrez.html

    So in order to take all these factors into account, I used standard thermal transfer equations to estimate the heat loss of a pipe when exposed to all these environmental conditions. I then ran a few experiments in my freezer to make sure the model was giving fairly accurate results, which it did.

    So here are a few examples:



    These results assume a minimum night time temperature of 20F, starting at 32F and the time to freeze only includes the time below 32F. So the average temperature is 26F over that time frame. There is some inherent margin included in these numbers because I assumed the water temperature started out a 32F which in most cases it is higher and it ignores the latent heat within the filter water which can act like a heat source for the rest of the plumbing.

    Wind has a fairly large affect but visibility to the night sky has an even larger effect. Covering the exposed equipment can reduce the heat loss considerably from both of these factors and freeze times are much much longer. So the easiest thing a pool owner can do is cover the pad equipment and plumbing with a tarp. Adding a heat source underneath the tarp or cover would add enough margin to eliminate the need for running the pump or at least provide enough backup should the automated freeze protection fail.


    The heat transfer model is available here for those who are adventurous:

    http://www.troublefreepool.com/threa...ull=1#post6231


    So if you would like to be able to go without automated freeze protection OR you would just like to have an extra layer of protection, then you can follow some of the suggestions listed below. The temperature ranges and time are using the thermal loss spreadsheet above which are fairly conservative freeze estimates (i.e. assumes 15 mph winds, full sky exposure with maximum radiation losses and ignores latent heat in filters and surrounding structures).

    1. Air Temp 32F-25F for <10 hours. Plumbing 1" and above is safe. Tarp or drain for smaller pipes.

    2. Air Temp 32F-20F for <10 hours. Plumbing 1.5" and above is safe. Tarp or drain for smaller pipes.

    3. Air Temp 32F-15F for <10 hours. Plumbing 2" and above is safe. Tarp or drain for smaller pipes.

    4. Air Temp 32F-10F for <10 hours. Plumbing 2.5" and above is safe. Tarp or drain for smaller pipes.

    5. Air Temp 32F-0F for <10 hours. Tarp equipment and use an incandescent light bulb underneath OR use heat tape available at your local hardware store.

    6. Air Temp <0F. Drain all equipment and plumbing.

    Note that when air temps get close to 0F, water temps are likely to be below 40F so there is no harm in draining the equipment and letting the pool go idle since algae is fairly rare at those water temperatures. Also, opening all valves so that all ports have pressure relief to the pool will virtually eliminate any chance of the pipes bursting.

    But just so you know, I do practice what I preach. My situation is #2 above and I have not bothered to use automated freeze protection for 8 winters now without incident. But I do not have any plumbing less than 2".
    Mark
    Hydraulics 101; Pump Ed 101; Pump/Pool Spreadsheets; Pump Run Time Study; DIY Acid Dosing; DIY Cover Roller
    18'x36' 20k plaster, MaxFlo SP2303VSP, Aqualogic PS8 SWCG, 420 sq-ft Cartridge, Solar, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater

  2. Back To Top    #2
    JasonLion's Avatar
    Join Date
    May 2007
    Location
    Silver Spring, MD
    Posts
    37,879

    Re: The Physics of Freezing and Freeze Protection

    Some other factors to consider:

    1) Weather reports may not be accurate. A weather report predicting lows around 25 might turn out to be wrong and temperatures might actually go below 20. This isn't common, but it certainly can happen.

    2) The largest risk is when day time high temperatures do not go above freezing. This allows ice to accumulate over several days, and greatly increases the odds of pipe damage.

    3) I favor "defense in depth" approaches, having several different things that can each separately prevent any damage. Plumbing damage is unusual, but it can be very expensive to fix (imagine you need to tear up the deck to get to the broken pipe). It is often worth some extra effort to eliminate even small risks because of the risk of relatively high costs to repair.

    4) If it gets cold enough, running the pump won't be sufficient. It has to get seriously cold for this to be an issue, but given a deep enough cold snap that lasts several days simply running the pump won't always be enough.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
    Creator of PoolMath and Pool Calculator. Other handy links: Support this site, TF Test Kits, Pool School

  3. Back To Top    #3

    Join Date
    Sep 2013
    Location
    Dallas-Ft. Worth Metroplex, TX
    Posts
    810

    Re: The Physics of Freezing and Freeze Protection

    X 2

    Those test results -- that is without the water moving through the pipes? This means your pipes wont freeze nearly within that time frame if the pump is running, correct?
    14K Freeform Gunite w/60% Blue Quartz plaster, Quad DE filter, Intelliflo VS Pump + Booster for Cleaner, Aquacal Heat Pump

  4. Back To Top    #4

    Join Date
    Mar 2007
    Location
    San Rafael, CA USA
    Posts
    12,082

    Re: The Physics of Freezing and Freeze Protection

    Quote Originally Posted by mas985 View Post
    The time it takes a pipe to freeze is dependent on several factors:

    Size and dimensions of the pipe: More water in the pipe takes longer to freeze.
    Pipe thermal conductivity: Copper is about 2000x more thermally conductive than is PVC. Although the difference is large, this actually has a fairly small affect on heat transfer.
    Mark,

    I earlier wrote this post where I did a very rough and simple (conservative) calculation for moving water taking a 1" length volume of water moving through the pipe and calculating what temperature would be needed with ideal heat transfer given the thermal conductivity. This would be worst case assuming plenty of wind. So how can it be that with copper being so much more thermally conductive and also being thinner than the PVC has a small affect on heat transfer? I did finally figure out that once a thin layer of ice forms then it becomes the limiting factor since it's thermal conductivity is 2.18 W/(m•K) compared to 401 for copper (PVC is 0.19), but while the copper can pretty much be ignored since the ice is limiting, the PVC should be noticeably more limiting in heat transfer than the ice or copper. So while with bare copper pipe with near freezing water only needs a slightly lower outside temperature to start forming ice at the surface of the pipe, this becomes self-limiting very quickly and the water moving through the pipe prevents it freezing solid for any reasonable temperature difference (unless the pipe run is extremely long).

    I just find it hard to believe that under extreme conditions with wind that even with still water that the insulating effect of PVC has virtually no effect. With still water in a copper pipe, shouldn't it freeze nearly as fast as ice cubes in a freezer? Or is the point that the main limiting factor in time is the slowness of freezing, not so much the limit of heat transfer at least for copper, ice, or PVC (insulation on copper pipe might be different since that is 6 times more insulating than PVC comparing typical thickness for 2" PVC pipe vs. 3/8" polyurethane)?

    Richard
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

  5. Back To Top    #5

    Re: The Physics of Freezing and Freeze Protection

    Pipe heating tape/cable can be helpful assuming no power loss. Having a backup generator can be helpful if power is lost. Having a plan to drain in case of power loss is a good idea.

  6. Back To Top    #6
    mas985's Avatar
    Join Date
    May 2007
    Location
    Pleasanton, CA
    Posts
    11,489

    Re: The Physics of Freezing and Freeze Protection

    Quote Originally Posted by zethacat View Post
    Those test results -- that is without the water moving through the pipes? This means your pipes wont freeze nearly within that time frame if the pump is running, correct?
    Yes, without water running and yes, time to freeze can be much longer with the pump running but only for the plumbing that has water running through it. The intent of the table was to show how long to freeze if you cannot run the pump or do not want to run the pump.



    Quote Originally Posted by chem geek
    So how can it be that with copper being so much more thermally conductive and also being thinner than the PVC has a small affect on heat transfer?
    The ice does help slow the heat transfer (~4x) but there is another factor. Take the un-insulated attic example. The primary heat transfer mechanism will be convection to the air and radiation to the roof above and those have much slower heat transfer characteristics than the pipe conductivity. So it doesn't matter if the pipe surface can conduct heat that quickly, convection and radiation will dictate the total heater transfer rate out of the pipe. This basically means that the temperature on each side of the pipe boundary will be about the same. Lower temperature delta for copper and a little higher for PVC.
    Mark
    Hydraulics 101; Pump Ed 101; Pump/Pool Spreadsheets; Pump Run Time Study; DIY Acid Dosing; DIY Cover Roller
    18'x36' 20k plaster, MaxFlo SP2303VSP, Aqualogic PS8 SWCG, 420 sq-ft Cartridge, Solar, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater

  7. Back To Top    #7

    Join Date
    Mar 2007
    Location
    San Rafael, CA USA
    Posts
    12,082

    Re: The Physics of Freezing and Freeze Protection

    OK, so if exposed outside to wind then we can expect to see the differences manifest themselves more, in particular where a bare copper pipe would freeze faster than PVC which would freeze faster than the copper pipe having polyurethane insulation. That's pretty much what I was calculating in the other post -- worst case assuming theoretical heat transfer rates.

    Hence the advice of covering which cuts down the wind factor (convection) and provides a modicum of insulation and limits radiation.
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

  8. Back To Top    #8
    mas985's Avatar
    Join Date
    May 2007
    Location
    Pleasanton, CA
    Posts
    11,489

    Re: The Physics of Freezing and Freeze Protection

    Exactly! When covered, freeze time would be about 5% longer for PVC vs Copper but in a 15 mph wind with full sky exposure, PVC would take about 50% longer to freeze than copper. This is why pipe wrap works well for copper pipes but probably wouldn't help much for a PVC pipe unless it was a foam type wrap.

    BTW, I had to update the table. I inadvertently used 1/2" numbers instead of 3/4". It should read correctly now.

    I would also add heat tape as an effective method of freeze protection. It is probably more effective than running the pump (except maybe water features), works at much lower temps and it costs a whole lot less to run.

    http://www.homedepot.com/p/Frost-Kin...C18A/100196820
    Mark
    Hydraulics 101; Pump Ed 101; Pump/Pool Spreadsheets; Pump Run Time Study; DIY Acid Dosing; DIY Cover Roller
    18'x36' 20k plaster, MaxFlo SP2303VSP, Aqualogic PS8 SWCG, 420 sq-ft Cartridge, Solar, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater

  9. Back To Top    #9
    Patrick_B's Avatar
    Join Date
    Jun 2011
    Location
    Midland TX
    Posts
    15,001

    Re: The Physics of Freezing and Freeze Protection

    Great thread all! Very interesting and helpful information.
    TFP Moderator
    Essential Links:
    ABC's Of Pool Chemistry, Test Kits, SLAM Your Pool
    28K Gal IG FreeForm, CLI Quartz, Pentair 36"SF & VS Pump, Dolphin M5, Rheem

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •