A modern submarine such as the U.S. Navy's Seawolf (SSN-21), is the culmination of progressive developments tried and proven on prior submarines. Such vessels have evolved as ships that normally operate in a submerged condition, but when required will function like a ship on the surface. Because a modern submarine when operating in its normal submerged mode cannot exchange the air inside the hull with the clean air of the atmosphere above the sea, it must instead create an artificial atmosphere. Since the vessel must be capable of remaining submerged for extensive periods, the need for a suitable comfortable healthy environment is essential for the people living on board the submarine. Such is the challenge to those responsible for the design of the HVAC&R systems on board.