Heater to prevent sump water from freezing HELP!!!

[thien2002]

I have (3) 21' x 7' x 6' Sump

We are being told to prevent this sump water from freezing. I don't know how to spec a heater that would be enough to keep this sump water around 40F when ambient temperatures fall below 40F. I think it will have a thermostat to control the heating operation. This sump water comes from the cooling tower.

The middle sump will not require heating since it will share temperature its 2 neighbor sumps.

Do anyone have any experience as far as sizing the heater and any recommendation action? Someone told me about 15KW will do the job. Thank you.

 

[TD2K]

The chromalox catalog has some data on this. For example, heat losses from an uninsulated open tank to ambient air for a 10F differential (80F water, 70F ambient) varies from 20 to 50 watts per ft2 depending on the air's relative humidity and velocity.

You might want to track down a copy to take a look at it. I'd use 35F to 40F as my maintenance temperature and then whatever minimum air temperature you have.

 

[MintJulep]

That's a lot of water. Its not like it will instantly freeze if the ambient temperature reaches 32F. There is this thing called the latent heat of fusion.

 

[thien2002]

Do you know if there are any source of reference for calculation? Thank you very much and special thanks to TD2K

 

[25362]

To thien2002, answers to these questions may help clarifying the situation:

1. Does any of the sumps contain stagnant water; are operations of an intermittent character ?
2. Are the sumps located beneath the CT ?
3. How do you control your cooling tower's (CT) operation in cold weather (louvres, fans, etc.) ?
4. What is the returning circulating cooling water (ccw) "warm" temperature ?
5. Is there a possibility of by-passing the CT putting some of the returning warm ccw straight into the sump(s) under temperature control ?
6. What is the ccw flow rate ?
7. What is the "exposed" surface of the sumps ?
8. What is the lowest air temperature ?

 

[thien2002]

To 25362,

1. Does any of the sumps contain stagnant water; are operations of an intermittent character ? Stagnant
2. Are the sumps located beneath the CT ? Beneath
3. How do you control your cooling tower's (CT) operation in cold weather (louvres, fans, etc.) ? Fans
4. What is the returning circulating cooling water (ccw) "warm" temperature ? 40F
5. Is there a possibility of by-passing the CT putting some of the returning warm ccw straight into the sump(s) under temperature control ? No
6. What is the ccw flow rate?
7. What is the "exposed" surface of the sumps ? Air
8. What is the lowest air temperature ? 19F

 

[25362]

My question 4. wasn't clear, it should be: what is the temperature of the returning warm ccw entering, not leaving, the CT ? Q 6.: does the lack of an answer means the CT isn't working ? Q 7.: are the air-exposed surfaces three times 21' x 7' ? Thanks.

 

[25362]

Freeze protection in stagnant systems may have to be extended to piping, the CT itself, etc. Sumps may be heated for example, by electricity as you suggest, steam coils, direct steam injection.
Various solutions can be found doing a web search. In the meantime, I found these two sites:

http://www.jlhermon.com/freezing_weather.htm

http://www.ogontz.com/freeze.htm

that may be of help.

 

[25362]

Freezing could happen anywhere in the system, even on the (stopped) wet fan blades and CT filling. Freezing on the sumps' would happen on the surface by the cooling effect of water evaporation (~605 kcal/kg) since the heat of vaporization comes from the sensible heat of the water and from the latent heat of ice formation (~80 kcal/kg) driven by the drier air around, rather than by direct air convection cooling. Snow may be another factor, the soil's low temperature may also affect albeit in a lesser degree.

Once formed ice (having a lower vapour presure) may become an efficient physical barrier for cold dry air.

Coming back to your original question, a 15 kW (12900 kcal/h) heater would take care of melting some 350 lb/h of potential ice. For the cooling effect of the cold (dry) air follow TD2K's advice, and multiply the heat in W/ft[sup]2[/sup] by the exposed area, to obtain the wattage needed. Good luck.

 

[25362]

Consider where you install the temperature measuring instrument(s) because of possible "layering" of the stagnant water in the sumps.

 

[thien2002]

25362 - Thank you very much for your accurated information.
TD2K - Thank you very much for your valuable advice.

This type of helps meaning you are so dedicated in your career and man, "god bless you". I got me out of the problem. I am sure it will come some more as I live. Thank you.