A heat recovery steam generator comprising: a housing having an inlet into which a hot gas is directed and an outlet through which the gas is discharged, whereby the gas flows through the housing from its inlet to its outlet; an economizer located in the housing such that the gas flows through it, the economizer being connected to a source of feedwater that is essentially in the liquid phase for elevating the temperature of the feedwater; an evaporator for converting water, which is primarily in the liquid phase, into saturated steam, said evaporator comprising: an upper drum oriented horizontally; a coil having tubes located in a flow of a hot gas and provided with inlets and outlets, with the outlets of the tubes being in communication with the upper drum; the evaporator having its coil located in the housing upstream, in the flow of gas, from the economizer, with the inlet line of the evaporator being connected to the economizer to receive from the economizer water that has undergone heating in the economizer; and a superheater located in the housing upstream, in the flow of the gas, from the evaporator, with the outlet line of the evaporator being connected to the superheater, all such that the superheater converts the saturated steam from the evaporator into superheated steam; and characterized in that the evaporator further comprises at least one lower drum oriented horizontally and located at an elevation less than the upper drum; a drain line connecting the lower region of the upper drum to the lower drum; an inlet line opening into one of the drums; an outlet line leading out of the upper region of the upper drum and wherein the inlets of the coil tubes are in communication with the lower drum. a housing having an inlet into which a hot gas is directed and an outlet through which the gas is discharged, whereby the gas flows through the housing from its inlet to its outlet; an economizer located in the housing such that the gas flows through it, the economizer being connected to a source of feedwater that is essentially in the liquid phase for elevating the temperature of the feedwater; an evaporator for converting water, which is primarily in the liquid phase, into saturated steam, said evaporator comprising: an upper drum oriented horizontally; a coil having tubes located in a flow of a hot gas and provided with inlets and outlets, with the outlets of the tubes being in communication with the upper drum; the evaporator having its coil located in the housing upstream, in the flow of gas, from the economizer, with the inlet line of the evaporator being connected to the economizer to receive from the economizer water that has undergone heating in the economizer; and a superheater located in the housing upstream, in the flow of the gas, from the evaporator, with the outlet line of the evaporator being connected to the superheater, all such that the superheater converts the saturated steam from the evaporator into superheated steam; and characterized in that the evaporator further comprises at least one lower drum oriented horizontally and located at an elevation less than the upper drum; a drain line connecting the lower region of the upper drum to the lower drum; an inlet line opening into one of the drums; an outlet line leading out of the upper region of the upper drum and wherein the inlets of the coil tubes are in communication with the lower drum.