Explanation:

The overall heat transfer coefficient (U) in a vertical tube evaporator is affected by the liquor level. Let's understand the effect of liquor level on U in detail.

Effect of Liquor Level on Overall Heat Transfer Coefficient:

As the liquor level in the evaporator increases, the following changes occur:

1. Increase in Film Coefficient:

The film coefficient (h) is the heat transfer coefficient between the liquid film and the heating surface. As the liquor level increases, the liquid film thickness also increases. This results in an increase in the film coefficient. However, this increase is limited, and beyond a certain liquor level, the film coefficient becomes constant.

2. Decrease in Fouling Coefficient:

The fouling coefficient (Rf) is the resistance to heat transfer due to the formation of a layer of impurities on the heating surface. As the liquor level increases, the flow velocity of the liquid also increases. This results in a decrease in the fouling coefficient as the impurities are washed away from the heating surface.

3. Increase in Convective Coefficient:

The convective coefficient (hc) is the heat transfer coefficient due to the convective flow of the liquid. As the liquor level increases, the velocity of the liquid also increases, resulting in an increase in the convective coefficient.

Overall, the effect of liquor level on U is determined by the combined effect of the film, fouling, and convective coefficients. In most cases, the increase in the film and convective coefficients is offset by the decrease in the fouling coefficient, resulting in a decrease in U with an increase in liquor level.

Conclusion:

In conclusion, the overall heat transfer coefficient (U) in a vertical tube evaporator decreases with an increase in liquor level due to the combined effect of the film, fouling, and convective coefficients.