A special apparatus permitting to determine maintaining time necessary for the metallic matrix of the heat wheel to absorb maximum energy from the flue-gas was invented. In order to obtain a good accuracy in determining the maintaining time, in this apparatus and in the rotary Ljungstrom type pre-heaters it must realize a similitude of flue- gas flow. This paper presents a procedure to obtain the similitude of flue-gas flow in the rotary Ljungstrom type pre-heater and the special apparatus presented above.