A heat exchanger has been designed to obtain 250,000 Btu/hr from a 20-in.-diameter geothermal well at various brine temperatures. The system consists of a 10-in.-diameter plastic pipe to promote convective flow in the well and a 4-in.-diameter, Schedule 40 steel U-tube containing distilled water to extract the energy. Subject to the validity of the major assumptions, the required lengths of one leg of the U-tube at various brine temperatures are 34 ft (150/sup 0/F), 42 ft (140/sup 0/F), 54 ft (130/sup 0/F), and 75 ft (120/sup 0/F) for a mean working fluid temperature of 90/sup 0/F.