A multiple-tube evaporator with a vapor-liquid-solid flow of external natural circulating was established to investigate the characteristics of heat-transfer enhancement and prevention of fouling in the concentration of Gengnian'an extract, which is a kind of Chinese patent medicine. The evaporator is composed of five stainless steel heating tubes. The cylindrical solid particles are made of physiologically inert material. Steam was used as the heating medium. Heat-transfer enhancement was obtained with the three-phase evaporator. For the concentration of an alcohol extract of Gengnian'an, the overall average heat-transfer coefficients and the evaporation intensity with three-phase flow are all about 1.6-fold higher than those of the vapor-liquid two-phase evaporator under similar operating conditions. For the evaporation of a water extract of Gengnian'an, the evaporation intensity for the three-phase evaporator is about 1.3-fold higher than that of the vapor-liquid two-phase evaporator. Prevention of on-line fouling was also observed. After running more than 300 h, fouling was not seen on the surface of the heating tubes of the three-phase evaporator, although it does form in the evaporator with vapor-liquid flow when the operation time exceeds about 100 h under the same conditions. The concentration process can be simplified from the traditional two-step concentration to one-step concentration for such high viscous or easily fouled solutions. The quality of thick liquor was very stable according to the content analysis of the active ingredients, thus meeting the requirement for product quality standards. (C) 2005 American Institute of Chemical Engineers.