18. If the main condenser were operating at a vacuum of 28.7"Hg, a condensate discharge temperature of 81°F, a seawater inlet temperature of 72°F, and a seawater outlet temperature of 79°F, what would be the condensate depression

The condensate depression is 4.0F, calculated as the difference between the saturation temperature at 28.7″ Hg vacuum (approximately 85F) and the condensate temperature of 81F. Seawater temperatures are irrelevant to this calculation; condensate depression is a temperature difference, not a pressure measurement.