Heavy duty and aeroderivative gas turbines for power generation and mechanical drive are installed inside an enclosure that is thermally and acoustically insulated and is equipped with fire extinguishing systems. The enclosure is bolted on the gas turbine basement and is made of a steel frame that supports removable side panels, doors and roof sections. All the fire extinguishing systems, lubricating oil pipes and electrical systems are fitted inside the enclosure and release, together with the gas turbine, a large amount of heat that has to be dissipated to avoid overheating of the gas turbine itself and its auxiliaries. Overheating of the enclosures is one of the main reasons for gas turbine unplanned stops. For avoiding high internal temperatures, the enclosure is equipped with a cooling system consisting of pipes and fans, that extracts heat from the enclosure and limits the internal temperature increase. This paper presents a calculation model developed for the study of the enclosure cooling system. Results are compared with experimental data measured on an aeroderivative gas turbine used as mechanical drive for a natural gas compression station, and acceptable correspondence is achieved.