[en] In practice it is very common to find heat exchangers designed for certain conditions which occasionally or permanently have to operate in different and even harsher conditions than those for which they were designed. In these cases it is useful to know whether or not the behaviour of these exchangers is acceptable, without having to turn to the original designer, by using a simple and sufficiently realistic method. This paper describes the solution to this problem based on the data sheet from the original design which defines at least the following parameters: Thermohydraulic parameters: q, t₁, t₂, T₁, T₂, Q, W, U, R_i, R₀. Geometric parameters A, number of tubes, type of tubes, number of passes. Based on the values indicated on the data sheet for previous parameters, the new parameters t'₂, T'₁, T'₂, U' can be calculated if, for example t₁, and Q change to values t'₁ and Q'. (Author)