The DTR heat exchanger is a tube in tube heat exchanger specially designed for sludge and waste water applications. The tube in tube concentric geometry makes it easy to work with fluids that can contain particles without having to worry for blocking of the product flow. Special corrugated tubes are applied to increase heat transfer which makes the DTR more efficient than similar smooth tube designs. A special feature of the DTR is that the inner tube is removable. This way, both the tube and shell side channel can be inspected completely. This design is ideal for direct sludge to sludge heat recovery.