There are two main types of Expansion Vessel dependent on what application you have. You will normally find a Red Vessel (Heating Vessels) to be used on a Sealed System Heating application whilst the most common use for a Blue Vessel or White Vessel (Potable Vessels) would be in a potable application such as an Unvented Water Heater or a Pumped System.
Potable Expansion Vessels are built from different materials than Heating Expansion Vessels to ensure that the water is contaminant free and safe for human consumption. Expansion Vessels operate on the basis that the internal chamber is separated by a rubber membrane, inside this membrane sits the water and surrounding this sits a pocket of air. As the water inside the sealed system is heated the water pressure increases, as water is non compressible the air inside the air chamber of the Expansion Vessel will become compressed thus leaving the system safe from over pressurisation. As water can increase by 4.5% volume when heated from 0-100 DegC an Expansion Vessel is a vital part of any sealed heating system.
Expansion Vessels are commonly used on pumped systems to remove the requirements of a continuous pump. This works as the user will set a low and high pressure on the pump settings, when the water pressure reaches the lowest setting the pump will engage and fill the Expansion Vessel to the high pressure point and stop. Once this cycle has been completed the Expansion Vessel puts the stored water under pressure ensuring higher pressure water. This will continue until the Expansion Vessels pressure drops to the lower point and the pump starts its cycle again