The vacuum maintained in the salt water feed heater shell of a flash-type evaporator is generally accomplished by __________.

Correct Answer: Option C — by an external line from the first stage via a fixed orifice

Maintaining vacuum in the feed heater shell is achieved by connecting it to the first-stage flash chamber via a fixed orifice. This arrangement allows the lower pressure of the flash chamber to draw off non-condensable gases and maintain the required vacuum level, ensuring efficient heat transfer within the heater.