A heat exchanger comprising: a first passageway housing defining a first fluid passageway; a first plurality of heat exchanging features positioned within the first fluid passageway, each of the first plurality of heat exchanging features defining a first thickness and a first feature density; a second passageway housing defining a second fluid passageway in thermal communication with the first fluid passageway, the first passageway housing and the second passageway housing being additively manufactured as a single, integral piece; and a second plurality of heat exchanging features positioned within the second fluid passageway, each the second plurality of heat exchanging features defining a second thickness and a second feature density, the heat exchanger being characterized in that at least one of the first thickness and the second thickness is between about 0.0127 cm and 0.0254 cm and at least one of the first feature density and the second feature density is between about two and thirteen heat exchanging features per centimeter, wherein the first passageway housing comprises a first wall and the second passageway housing comprises a second wall, the first plurality of heat exchanging features being oriented at a first angle relative to the first wall and the second plurality of heat exchanging features being oriented at a second angle relative to the second wall, and wherein the magnitude of at least one of the first angle and the second angle is less than ninety degrees, wherein the heat exchanging features (160, 170) are straight, wherein the heat exchanging features are staggered, wherein an inlet plenum and an outlet plenum are separated along a Y-direction by a divider wall such that the inlet plenum and outlet plenum are in fluid communication only through the plurality of first fluid passageways, where the divider wall extends from a top side along a Z-direction toward a bottom side in order to separate the plurality of first fluid passageways and define an inflow segment and an outflow segment, wherein fluid entering the heat exchanger through inlet plenum is directed by the first fluid passageways downward along the Z-direction in the inflow segment, wherein the fluid then passes around the divider wall and travels upward along the Z-direction in the outflow segment toward the outlet plenum. a first passageway housing defining a first fluid passageway; a first plurality of heat exchanging features positioned within the first fluid passageway, each of the first plurality of heat exchanging features defining a first thickness and a first feature density; a second passageway housing defining a second fluid passageway in thermal communication with the first fluid passageway, the first passageway housing and the second passageway housing being additively manufactured as a single, integral piece; and a second plurality of heat exchanging features positioned within the second fluid passageway, each the second plurality of heat exchanging features defining a second thickness and a second feature density, the heat exchanger being characterized in that at least one of the first thickness and the second thickness is between about 0.0127 cm and 0.0254 cm and at least one of the first feature density and the second feature density is between about two and thirteen heat exchanging features per centimeter, wherein the first passageway housing comprises a first wall and the second passageway housing comprises a second wall, the first plurality of heat exchanging features being oriented at a first angle relative to the first wall and the second plurality of heat exchanging features being oriented at a second angle relative to the second wall, and wherein the magnitude of at least one of the first angle and the second angle is less than ninety degrees, wherein the heat exchanging features (160, 170) are straight, wherein the heat exchanging features are staggered, wherein an inlet plenum and an outlet plenum are separated along a Y-direction by a divider wall such that the inlet plenum and outlet plenum are in fluid communication only through the plurality of first fluid passageways, where the divider wall extends from a top side along a Z-direction toward a bottom side in order to separate the plurality of first fluid passageways and define an inflow segment and an outflow segment, wherein fluid entering the heat exchanger through inlet plenum is directed by the first fluid passageways downward along the Z-direction in the inflow segment, wherein the fluid then passes around the divider wall and travels upward along the Z-direction in the outflow segment toward the outlet plenum.