Abstract: An evaporation-cooled gas insulated electrical apparatus comprising, in a housing, an electrical device generating heat when in operation, a condensable refrigerant convertible between liquid and vapor phases, and a noncondensable, electrically insulating gas. The condensable refrigerant and the noncondensable gas are selected so that the ratio V.sub.g /V.sub.1 of the gas phase volume V.sub.g and the liquid phase volume V.sub.1 is between 1 and 10, and so that the specific weight of the noncondensable gas is smaller than the specific weight of the vapor of the condensable refrigerant during operation, and so that the noncondensable gas and the condensable refrigerant vapor are separated due to the difference in their specific weights.

Abstract: A plurality of upstanding cooling ducts in a condenser are closed at their upper ends and only a lower header which is disposed at the lower ends of the cooling ducts communicates a tank with the cooling ducts. In another embodiment, a check valve and a gas pump are disposed in an upper conduit connecting the tank to a common upper header communicating the cooling ducts with each other at their upper ends so as to discharge the noncondensable gas from the condenser to the tank. Further, a sensing device may be disposed to sense the interface between the noncondensable gas and the vapor refrigerant in the cooling ducts, and a controller may be disposed to compare the interface level sensed by the sensing device with a reference interface level set in the controller to control the gas pump such that the actual interface level is in conformity with the reference interface level.