ANS:B - decreases

In a vertical tube evaporator, the overall heat transfer coefficient may be affected by the liquor level, but the direction of change depends on various factors. Generally, with an increase in liquor level in a vertical tube evaporator:

• Increase in overall heat transfer coefficient: This can occur if the increased liquor level leads to better wetting of the tube surfaces by the liquid film, resulting in improved heat transfer due to enhanced surface area for heat exchange and improved convective heat transfer.
• Decrease in overall heat transfer coefficient: Conversely, an increase in liquor level might lead to increased liquid holdup, which can reduce the effectiveness of the vapor phase in contacting the tube surfaces for heat transfer, thus decreasing the overall heat transfer coefficient.
• No significant effect: In some cases, the change in liquor level may not significantly affect the overall heat transfer coefficient, particularly if other factors such as flow rates, fluid properties, and operating conditions remain relatively constant.