The DTI type heat exchanger is a double tube heat exchanger (tube within a tube). The inner tube is corrugated for increasing the heat transfer. The product flows through the inner tube and the service fluid through the annulus between the inner and outer tube. Because of its geometry, the DTI heat exchanger is a true counter current heat exchanger. An expansion joint (bellow) is fitted in the shell to allow for differential expansion of the inner and outer tube during operation.