ANS:A - viscosity

The type of liquor circulation system to be employed in evaporators (natural or forced circulation) is determined mainly by the viscosity of the liquid. Viscosity is a measure of a fluid's resistance to flow. In evaporators, the circulation system needs to effectively transport the liquid through the evaporator tubes to ensure efficient heat transfer and evaporation. The viscosity of the liquid significantly influences its flow behavior:

1. Natural Circulation: Natural circulation relies on buoyancy forces generated by density differences within the fluid. Liquids with lower viscosity typically have better flow characteristics, facilitating natural circulation. Therefore, natural circulation is often preferred for liquids with lower viscosity, as it can efficiently circulate the liquid without the need for external pumps.
2. Forced Circulation: Forced circulation involves the use of external pumps to actively circulate the liquid through the evaporator. Liquids with higher viscosity may have difficulty flowing through the evaporator tubes, requiring the assistance of pumps to overcome resistance and ensure adequate circulation. Therefore, forced circulation is typically employed for liquids with higher viscosity to ensure consistent flow rates and efficient heat transfer.

While other factors such as density, thermal conductivity, and corrosive nature may also influence the choice of circulation system to some extent, viscosity is the primary factor that dictates the suitability of natural or forced circulation in evaporators.