How do non-condensable gases affect condenser performance?

Non-condensable gases (air, CO2, N2) accumulate in condensers, especially at cold surfaces where vapor condenses away. Effects: create a gas film that adds mass transfer resistance, reduce local vapor partial pressure lowering condensation temperature, and decrease effective heat transfer surface. As little as 1% air can reduce heat transfer by 50%. Mitigation: venting systems to remove accumulated gases, vacuum pumps for vacuum condensers, proper deaeration of feedwater, and designing with adequate venting capacity.