Abstract: Disclosed herein is a once-through evaporator comprising an inlet manifold; one or more inlet headers in fluid communication with the inlet manifold; one or more tube stacks, where each tube stack comprises one or more substantially horizontal evaporator tubes; the one or more tube stacks being in fluid communication with the one or more inlet headers; one or more outlet headers in fluid communication with one or more tube stacks; an outlet manifold in fluid communication with the one or more outlet headers; and a plurality of flow control devices to dynamically control the fluid flow to a respective inlet header.