The paper discusses two mathematical models for the air flow through a plate heat exchanger with parallel plates. The first exhausts the used air and then supplies the fresh air. The second exhausts the used air above the plate and simultaneously supplies fresh air under it (counter-flow exchanger). In both cases, the exhaust air heat is used to heat the supply air. The purpose of the research is to verify which exchanger uses the exhaust air heat more efficiently. The method of the Trefftz function was used to determine approximate solutions of the analysed problems. The results obtained for 1.2 mm thick steel, aluminium, and copper plates and for external winter, summer, and spring–autumn temperatures are discussed. The results indicate that steel is the best material for a plate heat exchanger, and the counter-flow exchanger is more efficient of the two. Thanks to the use of thin steel plates and the reduction of the air exchange time to a few minutes, cheap and efficient counter-flow exchangers can be obtained.

This paper discusses a mathematical model for airflow through a cross-flow plate heat exchanger. The exhaust air is used to heat the supply air. Three kinds of plates are considered: made of aluminium, copper, and steel. The purpose of this research was to verify which material used to build the plate heat exchangers uses the exhaust air heat more efficiently. The method of the Trefftz function was used to determine approximate solutions to the analysed problem. The results obtained for 1.2 mm-thick plates and for external winter, summer, and spring–autumn temperatures are discussed. The results indicate that if the efficiency and price of the metals are considered, then steel is the best material for the plate heat exchanger. Thanks to the use of thin steel plates and the reduction in air exchange time to a few minutes, a cheap and efficient cross-flow heat exchanger can be obtained.