daniel,
If you are talking about a deaerator within a steam cycle for a power plant, the device must exist within a thermally optimized cycle.
In other words, the unit must provide deaerated water at a temperature and pressure appropriate to deliver water to the boiler(i.e. the equipment that is being protected form oxygen induced corrosion)
All power plant/heating boiler type deaerators operate in a pressurized condition (from about 5 to 30+psig)
Other types of deaerators for the chemical process industry may operate under vacuum conditions.......the design must service the process......not the other way around.
MJC