A radiant heat recovery heater includes U-shaped heat transfer tubes each including a first path and a second path arranged on a mounting section. The U-shaped heat transfer tubes are housed in a container fixed to the mounting section. The first paths and the second paths of the U-shaped heat transfer tubes are arranged on the mounting section at equal intervals with a pitch angle . The first paths are each arranged on the mounting section at a position offset from the pitch angle for the associated second path by a predetermined angle , so as not to completely overlap a projection of that second path, the projection extending from the container toward the center C of the container.

A monolithic heat exchanger body for inputting heat to a closed-cycle engine includes heating walls and heat sink, such as heat transfer regions. The heating walls are configured and arranged in an array of spirals or spiral arcs relative to a longitudinal axis of an inlet plenum. Adjacent portions of the heating walls respectively define corresponding heating fluid pathways fluidly communicating with the inlet plenum. At least a portion of the heat sink is disposed about at least a portion of the monolithic heat exchanger body. The heat sink includes working-fluid bodies including working-fluid pathways that have a heat transfer relationship with the heating fluid pathways. Respective ones of the heat transfer regions have a heat transfer relationship with a corresponding semiannular portion of the heating fluid pathways. Respective ones of the heat transfer regions include working-fluid pathways fluidly communicating between a heat input region and a heat extraction region.

A constant density heat exchanger and method of operating are provided. The constant density heat exchanger includes a housing extending between a first end and a second end and defining a chamber having an inlet and an outlet. A first plate is positioned at the first end of the housing and rotatable about an axis of rotation such that the first plate selectively allows a working fluid to flow into the inlet of the chamber. A second plate is positioned at the second end of the housing and rotatable about the axis of rotation such that the second plate selectively allows the working fluid to flow out of the outlet of the chamber. The first plate and the second plate are rotatable about the axis of rotation so as to hold a volume of the working fluid at constant density as a heat source imparts thermal energy thereto.

A burner-heat exchanger assembly (100) includes a burner (1) with a tubular diffuser wall (9), a combustion chamber (13) formed inside the diffuser wall (9), a heat exchanger (14) arranged in the combustion chamber (13) and having one or more heat exchange surfaces (31) exposed in the combustion chamber (13). A minimal diffuser-exchanger distance (C) between the diffuser wall (9) and the corresponding heat exchange surface (31) in the combustion chamber (13) ranges from 20 mm to 40 mm.

A radiant heat recovery heater includes U-shaped heat transfer tubes each including a first path and a second path arranged on a mounting section. The U-shaped heat transfer tubes are housed in a container fixed to the mounting section. The first paths and the second paths of the U-shaped heat transfer tubes are arranged on the mounting section at equal intervals with a pitch angle . The first paths are each arranged on the mounting section at a position offset from the pitch angle for the associated second path by a predetermined angle , so as not to completely overlap a projection of that second path, the projection extending from the container toward the center C of the container.

A new thermal engine design and novel hot air inlet provide a high-efficiency hot-gas free-piston Stirling engine design.