The design of mechanical systems involving heat transfer, such as direct-use geothermal systems, is heavily influenced by temperature. Temperature difference--what engineers refer to as Delta T--is particularly important as it frequently governs feasibility, equipment selection and flow requirements for the system. Important though it is, the gruesome intricacies of heat transfer can be a yawner for even the most ardent engineer, but acquiring familiarity with a few key principles is relatively painless