Walker Process heat exchangers are concentric tube counterflow type, that are proven to be the most efficient design available.

Walker Process sludge-to-sludge heat exchangers are concentric tube counterflow type that are the same as water-to-sludge heat exchangers but with a larger annular area between tubes.  Sludge-to-Sludge heat exchangers are used with almost any temperature phased anaerobic digestion system.