1.9 Domestic fresh and salt water systems Open systems using gravity tanks, etc, are rarely met nowadays and figure 1. 8 shows a characteristic arrangement of pressure tanks, calorifiers and pumps for a large ship. The fresh water pumps are self-priming drawing from storage tanks and discharging through filters to a rising main, branched to give a cold supply and a hot supply in closed circuit through a calorifier. a tank kept under pressure by an external supply of compressed air rests on the system and provides the head necessary for constant supply throughout the ship1.9 Domestic fresh and salt water systems • Open systems using gravity tanks, etc., are rarely met nowadays and Figure 1.8 shows a characteristic arrangement of pressure tanks, calorifiers and pumps for a large ship. The fresh water pumps are self-priming, drawing from storage tanks and discharging through filters to a rising main, branched to give a cold supply and a hot supply in closed circuit through a calorifier. A tank kept under pressure by an external supply of compressed air rests on the system and provides the head necessary for constant supply throughout the ship