Helically coiled heat exchanger, with: a plurality of inlets which are connected in each case to at least one assigned tube of the heat exchanger, so that at least one stream of a first medium and one stream (S') of a second medium can be introduced in each case via at least one assigned inlet (30, 32, 36, 33, 35) in each case into the assigned at least one tube, the heat exchanger having a jacket surrounding a jacket space in which the said tubes are arranged, so that in particular, a stream (S"') of a medium routed in the jacket space comes into indirect heat exchange with a stream (S, S') of a medium routed in the respective tube, and the said tubes in this case being coiled helically around a core tube of the heat exchanger, each of the tubes assigned to the respective inlet defining a heating surface, characterized in that the heat exchanger has at least one changeover means which is designed to switch at least one of the inlets to and fro between at least one first operating state and one second operating state, so that, in the first operating state, the stream of the first medium and, in the second operating state, the stream (S') of the second medium can be introduced via the at least one inlet into the at least one assigned tube, in order, in the first operating state, to make more heating surface available to the stream of the first medium and make correspondingly less heating surface available to the stream (S') of the second medium, and in order, in the second operating state, to make more heating surface available to the stream (S') of the second medium and make correspondingly less heating surface available to the stream of the first medium. a plurality of inlets which are connected in each case to at least one assigned tube of the heat exchanger, so that at least one stream of a first medium and one stream (S') of a second medium can be introduced in each case via at least one assigned inlet (30, 32, 36, 33, 35) in each case into the assigned at least one tube, the heat exchanger having a jacket surrounding a jacket space in which the said tubes are arranged, so that in particular, a stream (S"') of a medium routed in the jacket space comes into indirect heat exchange with a stream (S, S') of a medium routed in the respective tube, and the said tubes in this case being coiled helically around a core tube of the heat exchanger, each of the tubes assigned to the respective inlet defining a heating surface, characterized in that the heat exchanger has at least one changeover means which is designed to switch at least one of the inlets to and fro between at least one first operating state and one second operating state, so that, in the first operating state, the stream of the first medium and, in the second operating state, the stream (S') of the second medium can be introduced via the at least one inlet into the at least one assigned tube, in order, in the first operating state, to make more heating surface available to the stream of the first medium and make correspondingly less heating surface available to the stream (S') of the second medium, and in order, in the second operating state, to make more heating surface available to the stream (S') of the second medium and make correspondingly less heating surface available to the stream of the first medium.