An apparatus and a process are described for preparation of nitric acid from ammonia and oxygenous gas by the single pressure or dual pressure process, in which the oxidation of the ammonia used is accomplished by means of compressed process air which has been compressed in at least one compressor over a catalyst, and the nitrous gas formed by the oxidation is at least partly absorbed by water, forming nitric acid, and the unabsorbed residual gas is expanded for the purpose of recovering compressor work in at least one multistage residual gas turbine (1). The characteristic features of the process and the apparatus are that, at least during the startup and/or shutdown of the nitric acid plant, in the multistage residual gas turbine (1), a substream (3) of the medium (4) flowing through the multistage residual gas turbine (1) is withdrawn and that a substream (2) of the medium (4a) supplied to the multistage residual gas turbine (1) is optionally withdrawn upstream of the multistage residual gas turbine (1), and that the medium withdrawn is supplied to a chimney (5), the withdrawal of the substream (3) taking place between two or more residual gas turbine stages. This measure can effectively prevent freezing of the residual gas turbine.