Abstract: A heat exchanger having annular construction for a constant operation at high temperatures and pressures. A series of heating walls at high temperature surround the fluid to which heat is desired to be transferred in annular spaces. Counterbalancing pressures in areas filled with high pressure fluid and insulating gases allow use of thinwall construction for the heating walls to reduce cost and increase heat transfer. Spacers that are placed under compression are provided to maintain dimensional stability and transmit forces. Embodiments describing the use of the device as a heater or cooler and operation in either direction are illustrated. A second embodiment illustrates a heater with single sided heating of the fluid desired to be heated.