The total thermal resistance of the vertical wall of the tank is:

(1)

The convection coefficient in air hair

Nuair = 0 if Ra < 104

Nuair = 0.59 ■ Rar0f if 104 < Ra < 109

Nuair = 0.13 ■ Ra033 elsewhere

The convection coefficient in water hwater is obtained from the next relationship:

г -|2

Supplementary losses appear for the first and the last element, at the top and the bottom of the tank. Different relationships are employed for these two horizontal surfaces in contact with ambient air depending on the temperature gradient. If heat is recuperated from the environment next correlations are computed:

Nuair = 0 if Ra < 104

Nuair = 0.54 • Ra0 25 if 104 < Ra < 109 (4)

Nuair = 0.54 • Ra0 33 elsewhere

And, if heat is lost to the environment, the correlations are:

Nuair = 0if Ra < 104 Nuair = 0.27 • Ra0 25 elsewhere