The correct answer is B) such a chiller can operate at a pressure below atmospheric pressure on the low side thus drawing in air through any low-side leaks. Low pressure centrifugal chillers operate on the low side at pressures below atmospheric pressure. This means that any leaks on the low side of the system will allow air to be drawn into the system, contaminating the refrigerant. A purge recovery unit is typically installed to remove this air and prevent it from accumulating in the system. The other answer choices are incorrect because: A) low pressure chillers do not necessarily require more frequent maintenance openings, B) is the correct explanation, C) the refrigerant boiling point is not the primary reason for the purge unit, and D) evacuation is possible but may not be sufficient to remove all air.