Abstract: A combustion air preheater for heating combustion air with heat energy provided from flue gas includes both a heat pipe type heat exchanger for effecting low temperature heat transfer between the flue gas and the combustion air and a tube-and-shell type heat exchanger for effecting higher temperature heat transfer. The preheater includes a first heat exchanger, having first and second heat transfer compartments with a plurality of heat pipes extending therebetween and a second, connected heat exchanger which includes a plurality of heat tubes supported by tube sheets within a shell. Ducting is provided to direct flue gas over the tubes of the second heat exchanger and then through one of the compartments of the first heat exchanger and additional ducting is provided to direct combustion air through the other compartment of the first heat exchanger and through the tubes of the second heat exchanger.

Abstract: Water is flowed through bayonet tubes where it is heated to steam which is superheated in other bayonet tubes all the bayonet tubes extending from a single tube sheet. Heated water enters the inner tube of a bayonet tube assembly so that it is heated in the annulus between the inner tube and its associated outer tube and then flows into the annular space between the inner and outer tube of the second bayonet tube assembly so that it is is heated to steam to leave by the inner tube of the second bayonet tube assembly to then flow into the annular space between the inner and outer tubes of the third bayonet tube assembly to be superheated and leave the third bayonet assembly by the inner tube thereof, the flow between one or more of the bayonet tube assemblies being connected by connecting tubes which connect with thermal sleeves covering the inner tubes of the bayonet tube assemblies below the single tube sheet.

Abstract: Water is heated to steam in bayonet tubes and the steam is superheated in bayonet tubes which are heated by helium and which extend from a single tube sheet in which there are no excessively large temperature gradients. Feed water enters the inner tube of a bayonet tube assembly so that it is heated in the annulus between the inner tube and its associated outer tube and then flows into the annular space between the inner and outer tube of a second bayonet tube assembly so that it is heated to steam to leave by the inner tube of the second bayonet tube assembly to then flow into the annular space between the inner and outer tubes of a third bayonet tube assembly to be superheated and leave the third bayonet assembly through the inner tube thereof.