Apparatuses for maintaining a solid as a melt (e.g., for storage or transport) may comprise: a closed vessel having an upper section and a lower section; a pool of a heat-receiving fluid located in the lower section; at least one product circulation conduit located in the lower section and at least partially immersed in the pool of the heat-receiving fluid; and a cooling fluid conduit located in the upper section and spaced apart from the pool of the heat-receiving fluid, the cooling fluid conduit being in thermal communication with vapor produced from the heat-receiving fluid; wherein a cooling fluid is present in the cooling fluid conduit.