Abstract: In a heat exchanger unit having a heating section and a regenerative section, the product to be heat treated is first passed through the regenerative section to preheat the product, thence through the heating section and then back through the regenerative section to recover heat from the product. In the regenerative section, the return flow of the product is counter-current to its incoming flow; but when the exchanger unit is to be heated, as for cleaning or sterilizing, a liquid other than the product is passed through the regenerative and heating sections in the same path as the product flow except that the incoming and return flows of the liquid through the regenerative section are concurrent to each other.