ANS:A - get economical temperature difference by using moderate pressure steam.

When vacuum is maintained in the vapor space of an evaporator, it allows for the use of moderate pressure steam to achieve economical temperature differences during the evaporation process. Here's how it works:

1. Lower Boiling Point: By maintaining a vacuum, the pressure inside the evaporator is reduced, which lowers the boiling point of the liquid being processed. This means that the liquid can boil and evaporate at lower temperatures compared to atmospheric pressure conditions.
2. Use of Moderate Pressure Steam: Since the boiling point of the liquid is lowered due to the vacuum, it allows for the use of moderate pressure steam (rather than high-pressure steam) to provide the necessary heat for evaporation. Moderate pressure steam is typically more readily available and less expensive to produce compared to high-pressure steam.
3. Economical Temperature Difference: The use of moderate pressure steam in conjunction with the lowered boiling point of the liquid results in a more economical temperature difference between the heating steam and the boiling liquid. This means that less energy is required to maintain the temperature difference necessary for evaporation, leading to cost savings in energy consumption.