An evaporator apparatus for receiving liquid water from a steam drum and providing at least one of steam and heated liquid water to the steam drum, the evaporator apparatus comprising: a steam drum; a gas duct a first evaporator (EVAP-1, EVAP-1A, EVAP-1B) for receiving liquid water at a first inlet (10, 10a, 10b), the first evaporator (EVAP-1, EVAP-1A, EVAP-1B) having at least one first evaporator conduit, the first evaporator conduit defining a first evaporator passageway (14, 14a, 14b) extending from the first inlet (10, 10a, 10b) through the gas duct to a first outlet (12, 12a, 12b) of the first evaporator (EVAP-1, EVAP-1A, EVAP-1B) for transferring heat, during operation, from gas passing within the gas duct to water within the first evaporator passageway (14, 14a, 14b); a second evaporator (EVAP-2, EVAP-2A, EVAP-2B) for receiving liquid water at a second inlet (20, 20a, 20b), the second evaporator (EVAP-2, EVAP-2A, EVAP-2B) having at least one second evaporator conduit defining a second evaporator passageway (24, 24a, 24b) extending from the second inlet (20, 20a, 20b) through the gas duct to a second outlet (22, 22a, 22b), the second evaporator passageway (24, 24a, 24b) being arranged for transferring heat from the gas to water; and a combined output conduit in communication with the first outlet (12, 12a, 12b) of the first evaporator (EVAP-1, EVAP-1A, EVAP-1B) and the second outlet (22, 22a, 22b) of the second evaporator (EVAP-2, EVAP-2A, EVAP-2B) for outputting at least one of steam and heated liquid water from both the first and second evaporators (EVAP-1, EVAP-1A, EVAP-1B, EVAP-2, EVAP-2A, EVAP-2B); wherein the first evaporator passageway (14, 14a, 14b) only makes a single pass through the gas duct and wherein the gas will flow through the gas duct along a gas flow axis in a direction that is substantially perpendicular to a direction water flows through a length of the first evaporator passageway (14, 14a, 14b) that extends through the gas duct to define the single pass, wherein the first evaporator (EVAP-1, EVAP-1A, EVAP-1B) is positioned adjacent the second evaporator (EVAP-2, EVAP-2A, EVAP-2B) in the gas duct such that water in the first evaporator (EVAP-1, EVAP-1A, EVAP-1B) is exposed to hotter gas for heat transfer than water in the second evaporator (EVAP-2, EVAP-2A, EVAP-2B), characterized by a first feed conduit (9, 9a, 9b) for transporting the liquid water from the steam drum to the first inlet (10, 10a, 10b); a second feed (11, 11a, 11b) conduit for transporting the liquid water from the steam drum to the second inlet (20, 20a, 20b) that is separate from the first feed conduit (9, 9a, 9b). a steam drum; a gas duct a first evaporator (EVAP-1, EVAP-1A, EVAP-1B) for receiving liquid water at a first inlet (10, 10a, 10b), the first evaporator (EVAP-1, EVAP-1A, EVAP-1B) having at least one first evaporator conduit, the first evaporator conduit defining a first evaporator passageway (14, 14a, 14b) extending from the first inlet (10, 10a, 10b) through the gas duct to a first outlet (12, 12a, 12b) of the first evaporator (EVAP-1, EVAP-1A, EVAP-1B) for transferring heat, during operation, from gas passing within the gas duct to water within the first evaporator passageway (14, 14a, 14b); a second evaporator (EVAP-2, EVAP-2A, EVAP-2B) for receiving liquid water at a second inlet (20, 20a, 20b), the second evaporator (EVAP-2, EVAP-2A, EVAP-2B) having at least one second evaporator conduit defining a second evaporator passageway (24, 24a, 24b) extending from the second inlet (20, 20a, 20b) through the gas duct to a second outlet (22, 22a, 22b), the second evaporator passageway (24, 24a, 24b) being arranged for transferring heat from the gas to water; and a combined output conduit in communication with the first outlet (12, 12a, 12b) of the first evaporator (EVAP-1, EVAP-1A, EVAP-1B) and the second outlet (22, 22a, 22b) of the second evaporator (EVAP-2, EVAP-2A, EVAP-2B) for outputting at least one of steam and heated liquid water from both the first and second evaporators (EVAP-1, EVAP-1A, EVAP-1B, EVAP-2, EVAP-2A, EVAP-2B); wherein the first evaporator passageway (14, 14a, 14b) only makes a single pass through the gas duct and wherein the gas will flow through the gas duct along a gas flow axis in a direction that is substantially perpendicular to a direction water flows through a length of the first evaporator passageway (14, 14a, 14b) that extends through the gas duct to define the single pass, wherein the first evaporator (EVAP-1, EVAP-1A, EVAP-1B) is positioned adjacent the second evaporator (EVAP-2, EVAP-2A, EVAP-2B) in the gas duct such that water in the first evaporator (EVAP-1, EVAP-1A, EVAP-1B) is exposed to hotter gas for heat transfer than water in the second evaporator (EVAP-2, EVAP-2A, EVAP-2B), characterized by a first feed conduit (9, 9a, 9b) for transporting the liquid water from the steam drum to the first inlet (10, 10a, 10b); a second feed (11, 11a, 11b) conduit for transporting the liquid water from the steam drum to the second inlet (20, 20a, 20b) that is separate from the first feed conduit (9, 9a, 9b).