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Re: [TRNSYS-users] Tank loss coefficient



I have a question concerning the tank loss coeff. of Type4.

Is the tank loss coefficient in TRNSYS also the u-value of the insulation??

Using just the U-value of the insulation will under-predict the u-value of the tank. You're not accounting for the convective resistance of the water-side heat transfer, the resistance of the tank wall, and the combined radiative and convective air-side resistance. But all of that assumes the tank is ideally insulated and that's really never the case. There are quite a few "thermal shorts" that will reduce the effective u-value of a tank from that calculated by simply using the insulation. There are algorithms to calculate the u-value of a tank if you know the energy factor or have access to heat loss data.

I switched the unit of the tank loss coefficient to W/m^2K. It´s the
same unit as the u-value. I knew knew the heat conductivity of my
insulation (lambda = 0.04W(mK)) and its thickness (100mm), i was able
to calculate the u-value by using u = 1/(0.1m/(0.04(W/mK))) = 0.4.

That formula only applies to 1-d conduction and does not hold for cylindrical tanks - you'll have to use a radial calculation for cylindrical tanks.

Jeff


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Jeff Thornton
President - TESS LLC
22 N. Carroll Street, Madison WI USA 53703
Office: (608) 274-2577  Fax: (608) 278-1475
www.tess-inc.com
E-Mail: thornton@tess-inc.com