A tubeless heat exchanger for heating a production fluid disposed in a vessel, including: a tubeless heat exchanger core disposed at least partially within the vessel, the tubeless heat exchanger core comprising an inner casing and an outer casing disposed around the inner casing, the inner and outer casings defining therebetween a flow passage for a thermal transfer fluid to flow, the tubeless heat exchanger core further comprising a core inlet arranged to receive the thermal transfer fluid and a core outlet arranged to provide the thermal transfer fluid, the core inlet and core outlet being fluidically connected to the flow passage, wherein each of the outer casing and the inner casing has an inner surface and an outer surface, wherein the respective inner surfaces face each other and define therebetween the flow passage for the thermal transfer fluid to flow and wherein at least a portion of the respective outer surfaces are arranged to be contacted by the production fluid, wherein the flow passage defines a path comprising at least one pass around a perimeter of the heat exchanger core, and wherein, in use, the thermal transfer fluid in the heat exchanger core transfers heat from the thermal transfer fluid to the production fluid through at least a portion of both the inner and outer casings.