Abstract
There are the various types of heat exchangers. The selection of the heat exchanger right type is the first basic assumption for its optimal operation. The heat exchanger calculation itself is another prerequisite for its optimal operation. This article deals with the variables which are usually used to describe the stationary operation of any recuperative heat exchanger with two incoming and two outgoing streams. The knowledge of these variables, including the facts resulting from them, is necessary not only from the point of view of the calculation but also from the point of view of the evaluation of the experimental data of any heat exchanger. The variables values needed for the calculation of heat exchangers, so-called key variables, must always fall within the values range determined on the basis of generally valid knowledge about heat exchangers. The article also deals with the determination of the limit values defining the values range of these key variables.