Heat exchangers

Claim 1

A heat exchanger comprising a steam generator shell, a plurality of substantially vertically disposed downcomer tubes mounted within said shell and having upper inlets for a first fluid, riser heat transfer tube bundles mounted in parallel spaced relationship with respect to said downcomer tubes, the lower ends of said downcomer tubes being connected to the lower ends of said tube bundles, said riser tube bundles having an upper outlet for said first fluid, said shell having an upper inlet for a second fluid and being in fluid flow relationship with the upper ends of said riser heat transfer tube bundles, said shell having a lower outlet for said second fluid, said lower outlet being in fluid flow relationship with the lower ends of said riser heat transfer tube bundles, a first cylindrically shaped adjusting shroud mounted adjacent said downcomer tubes and extending from the upper ends of the downcomer tubes to a location adjacent the second fluid outlet, said first adjusting shroud defining a first substantially vertically extending independent channel, a second cylindrically shaped shroud mounted in spaced relationship between the downcomer tubes and the riser tube bundles and extending substantially the length of the downcomer tubes, said first and second shrouds defining a second substantially vertically extending independent channel surrounding said downcomer tubes, means interconnecting the upper ends of said channels in fluid flow communication, whereby a portion of the second fluid can flow upwardly in the first channel and thence downwardly around said downcomer tubes in the second channel by means of natural circulation.