The fin-and-coil air coolers are investigated in following conditions: at weak for-mation of a condensate, condensate formation on a part of a surface and on all surface. The error of three known methods of calculation of air coolers in the investigated condi-tions of formation of a condensate is defined. It is found out that a calculation error is maximum for heat exchangers with the most developed external surface, working in the conditions of condensate formation on a part of a surface. The error reason - unsatis-factory calculation of a humidity ratio of air on an exit from the heat exchanger. Expe-rimental data for working out of a method of calculation of air coolers taking into ac-count a transitive zone, in which plates are covered by a condensate only partially.