Clamshell Heat Exchangers

A clamshell heat exchanger for use in a combustion furnace, the clamshell heat exchanger comprising:

• a first clamshell half;
• a second clamshell half;
• wherein, when coupled, the first clamshell half and the second clamshell half form the clamshell heat exchanger;
• a first passageway coupled to an inlet for receiving combustion products from a burner assembly, the first passageway having a longitudinal length in a first direction and comprising a first portion of a flow pathway;
• a turnaround passageway coupled to the first passageway and comprising a second portion of a flow pathway that receives fluids going in a first direction and turns the fluids at least 120 degrees;
• a second passageway coupled to the turnround passageway and having a longitudinal length in the first direction and comprising a third flow pathway that is opposite in direction to the first flow pathway and that comprises two flow streams;
• and
• wherein the first passageway has a lateral cross section resembling an airfoil with its leading edge closest to the second passageway.

A clamshell heat exchanger for a combustion furnace, the clamshell heat exchanger comprising:

• a heat exchanger body formed with an inlet and an outlet;
• a first passageway, turnaround passageway, and a second passageway formed in the heat exchanger body that together define a flow pathway from the inlet through the first passageway, then the turnaround passageway, then the second passageway, and finally to the outlet;
• wherein an outer wall portion and an inner wall portion of the first passageway diverge from one another;
• and
• wherein the first passageway is not symmetric about a lateral centerline.