Multiple tube bundle heat exchanger

A multiple tube bundle heat exchanger, comprising:

• a number of tube bundles, each of said tube bundles being formed as a self-contained integral unit having:
• two axially spaced tube sheets;
• and
• a plurality of heat exchanger tubes each with opposite axial ends, said two tube sheets mounted at said opposite axial ends of said heat exchanger tubes;
• a casing having:
• a tubular portion substantially coextending with said tube bundles and having:
• an interior subdivided by partitions into a plurality of axial chambers, a number of said axial chambers corresponding to said number of said tube bundles, each of said chambers receiving one of said tube bundles;
• two opposite axial ends;
• and
• flanges disposed at said two opposite axial ends;
• and
• head pieces removably mounted to said flanges, each of said head pieces having walls defining at least a first wall portion of a plenum chamber;
• and
• said two tube sheets of each of said tube bundles respectively sealing an adjacent one of said head pieces and forming at least a second wall portion of a respective plenum chamber of said adjacent one head piece.