In parallel-flow heat exchangers, the two fluid streams enter together at one end, flow through in the same direction, and leave together at the other end. In counter flow heat exchangers, the two fluid streams flow in opposite direction. The flow arrangement depends on the temperature distribution along the length of the heat exchanger and the relative amount of heat transfer under given temperature specification for the heat exchanger surfaces. The parallel flow heat exchanger requires maximum area. The design of the parallel flow heat exchanger is advantageous when two fluids are required to be brought to nearly the same temperature.

Parallel flow burner has a central liquid fuel-atomizing nozzle, which is disclosed. Its inner member defines a steam chamber and is surrounded by a shell that defines a liquid fuel compartment. A conduit extends from the steam chamber forming a steam core flow. A first passage feeds liquid fuel about the steam core flow to form an annular liquid fuel stream thereabout. A second passage forms an outer steam low, which envelops the annular liquid fuel flow. The burner further includes ducts to form a first combustion airflow co-axially about the nozzle, a second combustion airflow which surrounds the first flow, and a third combustion airflow which envelops the second flow generally protects the furnace wall from coming into contact with the flame generated by the burner.