ANS:D - all (a), (b) and (c).

The actual temperature drop across the heating surface in an evaporator depends on all of the following:

• The feed: The temperature drop across the heating surface is influenced by the temperature of the feed entering the evaporator. A higher feed temperature typically results in a smaller temperature drop across the heating surface.
• Depth of liquid over heating surface: The depth of the liquid over the heating surface affects the temperature drop because it determines the contact area between the liquid and the heating surface. A greater depth of liquid generally results in a smaller temperature drop.
• Pressure difference between steam chest and vapor space: The pressure difference between the steam chest (where steam is introduced to heat the liquid) and the vapor space (where vapor is removed from the evaporator) affects the boiling point of the liquid and thus the temperature drop across the heating surface. A greater pressure difference typically results in a smaller temperature drop.