In a cryogenic plant utilizing a liquid sparged into a cold vapor in heat exchange relationship with hotter vapor, variations in the load in terms of the volume of hot vapor are compensated for by a stepwise complete closing of a uniformly-spaced-apart fraction of the cold vapor passageways....http://www.google.com/patents/US4208198?utm_source=gb-gplus-sharePatent US4208198 - Stepwise turndown by closing heat exchanger passageways responsive to measured flow
