Study and testing of direct contact heat exchangers for geothermal brines. Final report, June 1975--July 1976

Abstract

The object of the work reported herein was to assess the technical and economic feasibility of preheating and evaporating a secondary fluid via direct contact with hot geothermal brine. The work covered a period of 12 months and included the design, construction, and testing of a unit which heats and vaporizes 10 gpm of isobutane by direct contact with 325/sup 0/F brine. The analytical and experimental efforts explored design and economic characteristics, including anticipated problem areas such as working fluid loss in the brine, production of a stable dispersion of the working fluid in brine, fluids separation, axial mixing and carry-over of water vapor with the working fluid. Isobutane was selected as the working fluid for tests primarily because of the favorable amount of net work produced per pound of geothermal brine and the low amount and cost of working fluid lost in the heat exchange process. The Elgin Spray Tower concept was selected for the preheater and boiler. The test apparatus includes a separate boiler and a separate preheater, each 6'' diameter by 6' high. Brine enters the top of each vessel and leaves the bottom. Isobutane enters the bottom of the preheater through a distributor plate to produce 0.15 inch diameter drops. The experimental unit operated with no major problems and demonstrated its hydraulic and thermal capabilities. Volumetric heat transfer coefficients obtained ranged up to 4000 BTU/hr /sup 0/F ft/sup 3/. Boiling heat transfer coefficients of as high as 17,000 BTU/hr /sup 0/F ft/sup 3/ were obtained with a design value of 10,000 BTU/hr /sup 0/F ft/sup 3/. Amount of isobutane in a 21 percent NaCl solution leaving the preheater was less than 40 ppM. A conceptual design and cost estimate was prepared for a direct contact heat exchange system sized for a 50 MW power plant.