Example

A solar still has water and glass temperatures equal to 55°C and 45°C, respectively. The constants C and n are determined experimentally and found to be C = 0.032 and n = 0.41. If the convective heat transfer coefficient from water surface to glass is 2.48 W/m2-K, estimate the hourly distillate output per square meter from the solar still.

Solution

From Eq. (5.21) and the temperatures of the water and glass, the partial pressures can be obtained as

Pw = 100(0.004516 + 0.0007178tw - 2.649x10-6 t2w + 6.944 x 10-7 t3w) = 100(0.004516 + 0.0007178 x 55 - 2.649x10-6 x 552

Pg = 100(0.004516 + 0.0007178tg - 2.649 x 10-6 t2g + 6.944 x 10-7 tg)

From steam tables, the latent heat of vaporization at 55°C (water temperature) is 2370.1 kJ/kg. From Eq. (8.30), m w = 3600qew = 3600

229.6

2370.1 x 1000

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