A heat exchanger apparatus for receiving water from a steam drum (1) and providing steam and heated unevaporated liquid water to the steam drum includes a first evaporator (EVAP-1) and a second evaporator (EVAP-2). The first evaporator can receive water from a steam drum via a first feed conduit (9) and the second evaporator can receive water from a second feed conduit (11). Both evaporators can output heated fluid to the steam drum via a combined evaporator output conduit (13). Each first evaporator passageway (14) only makes a single pass through a gas duct (15) having a heated gas flow (7) passing therethrough while each second evaporator passageways (24) can make one or more passes through the gas duct for transferring heat from the gas to the fluid within the evaporators. A portion of the first feed conduit can also have a pre-specified volume a pre-specified height below the first inlet (10).