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Water tank heat loss

Water tank heat loss

Water tank heat loss

(OP)
I have a water tank that is 24" X 96" (711 Liters at 10 Deg. C) wrapped in 2" of foilback insulation and heated by 2, 180w pad heaters. I am wondering at what Temp in Deg.C would this tank freeze?

Any info would be helpfull.
Steviewye.

RE: Water tank heat loss

Well the water would freeze when the bulk temperature reached 0°C, assuming, of course, you're reasonably close to atmospheric pressure.

It seems there is information missing in what you've asked, such as some temperature gradiant on the outside which causes you to need the insulation and the heaters.

Also, is this homework?

 

Patricia Lougheed

Please see FAQ731-376: Eng-Tips.com Forum Policies: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

RE: Water tank heat loss

(OP)
:) No, this is not homework. I'm involved with a project where a water storage tank will be in a outdoor enviroment (on a train) and I need to figure out if the two 180w pad heaters are enough to keep this volume of water from feezing. The tank is a 24" X 96" stainless steel cylinder. The temp. outside of the tank would vary from +20Deg.C to about -25Deg.C. SO I'm not sure if I need more heat or more/better insulation.

Thanks.

RE: Water tank heat loss

(OP)
The two Pad heaters would be installed on the underside of the tank (I thought to maximize the convection of heated water) enclosed in a protective skin made of steel. The train travels at a max of 60 MPH.

Thx.

RE: Water tank heat loss

(OP)
and yes they would be inside the insualtion.
 

RE: Water tank heat loss

now you are making sense smile Your initial question was ansvered correctly by Patricia - unless of course it not drinking water.

To answer your real quest you must know at least the inout that IRstuff comes up with.

However: Consider your situation: You are asking for free consultancy - all very good. But if somebody helps you - but gives an incroorect answer then what?

Best regards

Morten

RE: Water tank heat loss

(OP)
I guess what I am actually asking is, where is the point that the heat supplied by the heaters cannot keep up with the demands of the cold which would then bring the net temp of the water to 0Deg.C

I would be happy to be pointed in the right direction. I am more than willing to do my own legwork.

Thx.

RE: Water tank heat loss

As a very rough approximation, you can calculate what your inside temperature would be by using Q = hAΔT. (Just make sure you use all one system of units!)  It won't be really accurate as it will neglect both the train movement and any windspeed.

Patricia Lougheed

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RE: Water tank heat loss

Crude estimate:
Assume convective heat transfer coefficient on inside and outside = infinity (conservative)
Assume Insulation value of .3 Btu/hr in/ft2 (probably better, so this is conservative)
I calculate a total area of 53.4 ft2
Assume 1 heater fails (180 W only)
Assume temp must be 20 F above freezing (52 F)
Result: -25 F outside temp will allow water to remain at 52 F with just 1 heater on full time.
 

RE: Water tank heat loss

PS: I'd suggest neglecting convective heat transfer on the inside and outside surfaces and just look at conduction through your insulation to be sure the end result is conservative (ie: you don't want to freeze).  Heat transfer, Q, is the amount of heat you can allow to leave your water which is equal to the heat input from 180 Watt heaters.  Use the equation as shown here:
http://hyperphysics.phy-astr.gsu.edu/Hbase/thermo/heatra.html#c2

RE: Water tank heat loss

(OP)
Excellent, Thanks to everyone for the info. Much appreciated.

Steviewye.

RE: Water tank heat loss

As a simple assumption, start with the following:

area of the cylinder
air htc: 25 W/m^2-K
air deltaT = 0ºC - 25ºC
radiative emissitivity = 0.7
surfaceT = 273K
skyT = 220K

The result is 2.9kW of convective loss and 0.6kW radiated loss.  There are, of course, lots of caveats, since the emissitivity is unknown, and the htc is a guess.  So, then finding a surfaceT that results in 180W (for margin), we get 248K, or -25ºC.  

If you use a styrofoam insulation with a thermal conductivity of 0.08 W/m-K, with 2-in thickness, the heat loss for a 25ºC deltaT will be around 180W.  Given all the caveats, it looks to be doable, but I would probably go with 3-in thick styrofoam-like insulation.  Another insulation option would be http://www.glacierbay.com/products/ultra_R_specs.html, which has more than 10 times lower thermal conductivity.

TTFN

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