A compact heat exchanger for interchanging thermal energy between at least two fluids, one of which fluids may be a refrigerant in hot or cold form or in a liquid/vapor phase, and another of which fluids is a thermal transfer fluid. The heat exchanger may incorporate an internal heating element. The thermal transfer fluid is transported between two concentric metal tubes, while the refrigerant moves along a tubing helically wrapped about or between the tubes and is in thermal contact therewith.