The reduction of sizes of a series of types of chemical apparatuses due to the reduction of the variety of their construction parameters leads to a decrease in prices of the apparatuses but simultaneously produces an increase in the operating costs. The fact creates necessary conditions for the existence of an optimum series of types. The objective function for heat exchangers (as an example) is defined on the basis of a cost criterion. The function takes into account that a given heat exchanger belongs to a specific series of types which is more or less expanded. The average value of the objective function for a series of types makes it possible to choose the best series.