ANS:A - increases

In a single evaporator system, the steam economy increases by creating a vacuum in the evaporator. Here's why:

1. Steam Economy: Steam economy is a measure of the efficiency of the evaporation process and is defined as the amount of vapor produced per unit of steam used. A higher steam economy indicates that more vapor is produced for the same amount of steam input, resulting in greater efficiency.
2. Creating Vacuum: By creating a vacuum in the evaporator, the boiling point of the liquid to be evaporated decreases. This means that the liquid can boil at a lower temperature, requiring less energy input (lower steam temperature) to achieve the desired evaporation. As a result, the amount of vapor produced per unit of steam used increases, leading to a higher steam economy.