Developments in geothermal energy in Mexico—part thirty-eight. Heat transfer assessment of a geothermal steam gas removal reboiling process

Abstract

The performance of the upstream reboiling gas removal process, previously evaluated at the Geysers geothermal field, U.S.A. (Coury and Associates, Report EPRI AP-2100, 1981) and at the Cerro Prieto geothermal field, Mexico (Angulo et al., Report EPRI AP-5124, 1987; Angulo et al., Heat Recovery Systems & CHP 6, 295-303, 1986; Lam et al., EPRI project 1197-6, 1990), was re-evaluated. Special emphasis was placed on the heat transfer process occurring during the condensation of steam in the presence of non-condensable gases. This was to validate the design of similar equipment of higher capacity. The most important aspects of this recent assessment, as well as the results of a comparison made against an empirical heat transfer process model (Lam et al., EPRI project 1197-6, 1989), are described. The gas removal efficiency, the overall heat transfer coefficient and the amount of condensate produced were measured in the process equipment operating at a high overall temperature difference, ΔT (10 to 20°C), across the heat exchanger tubes, using high gas content geothermal steam (5 to 10% by weight). To compare the experimental data with the basic relationships of heat transfer for this type of process, an empirical model, applied to the reboiler, was developed (UGAS model). It was found that the local heat transfer coefficient for steam condensation, in the presence of non-condensable gases in the operating ranges described above, can be expressed by a relationship between the gas content of the steam and two adjustable parameters. From the UGAS model calculation, the best average shell and tube temperature difference to calculate overall heat transfer coefficients was found. For the condensation of steam with non-condensable gases, where the cooling agent is a boiling liquid, the condensate-weighted average temperature difference is more appropriate than the logarithmic mean temperature difference commonly used.