while small generators may be cooled by air drawn through filters at the inlet, larger units generally require special cooling arrangements. hydrogen gas cooling, in an oil-sealed casing, is used because it has the highest known heat transfer coefficient of any gas and for its low viscosity which reduces windage losses. this system requires special handling during start-up, with air in the generator enclosure first displaced by carbon dioxide before filling with hydrogen. this ensures that the highly flammable hydrogen does not mix with oxygen in the air.