A heat exchanger block comprising: a plurality of first air flow passages and a plurality of second air flow passages adjacent said plurality of first air flow passages for exchanging heat energy between first air flows passing through said plurality of first air flow passages and second air flows passing through said plurality of second air flow passages; a first cross flow region where flow passages of said plurality of first air flow passages and adjacent flow passages of said plurality of second air flow passages are arranged in crosswise relationship with respect to each other, said first cross flow region having a first opening region including ends of said first air flow passages and having a second opening region including ends of said second airflow passages; a parallel flow region where flow passages of said plurality of first air flow passages and adjacent flow passages of said plurality of second air flow passages are arranged in parallel relationship with respect to each other; a second cross flow region where flow passages of said plurality of first air flow passages and adjacent flow passages of said plurality of second air flow passages are arranged in crosswise relationship with respect to each other, said second cross flow region having a first opening region including the opposite ends of said second airflow passages and having a second opening region including the opposite ends of said first airflow passages; said first cross flow region, said parallel flow region and said second cross flow region being arranged in series along said first air flow passages and along said second air flow passages, with said parallel flow region being located between said first cross flow region and said second cross flow region along said first air flow passages and along said second air flow passages; characterized in that the total flow cross-sectional area of the first opening region and of the first air flow passages (AFP1-CF1) in said first cross flow region is greater than the total flow cross-sectional area of the second opening region and of the second air flow passages (AFP2-CF1) in said first cross flow region; and the total flow cross-sectional area of the first opening region and of the second air flow passages (AFP2-CF2) in said second cross flow region is greater than the total flow cross-sectional area of the second opening region and of the first air flow passages (AFP1-CF2) in said second cross flow region. a plurality of first air flow passages and a plurality of second air flow passages adjacent said plurality of first air flow passages for exchanging heat energy between first air flows passing through said plurality of first air flow passages and second air flows passing through said plurality of second air flow passages; a first cross flow region where flow passages of said plurality of first air flow passages and adjacent flow passages of said plurality of second air flow passages are arranged in crosswise relationship with respect to each other, said first cross flow region having a first opening region including ends of said first air flow passages and having a second opening region including ends of said second airflow passages; a parallel flow region where flow passages of said plurality of first air flow passages and adjacent flow passages of said plurality of second air flow passages are arranged in parallel relationship with respect to each other; a second cross flow region where flow passages of said plurality of first air flow passages and adjacent flow passages of said plurality of second air flow passages are arranged in crosswise relationship with respect to each other, said second cross flow region having a first opening region including the opposite ends of said second airflow passages and having a second opening region including the opposite ends of said first airflow passages; said first cross flow region, said parallel flow region and said second cross flow region being arranged in series along said first air flow passages and along said second air flow passages, with said parallel flow region being located between said first cross flow region and said second cross flow region along said first air flow passages and along said second air flow passages; characterized in that the total flow cross-sectional area of the first opening region and of the first air flow passages (AFP1-CF1) in said first cross flow region is greater than the total flow cross-sectional area of the second opening region and of the second air flow passages (AFP2-CF1) in said first cross flow region; and the total flow cross-sectional area of the first opening region and of the second air flow passages (AFP2-CF2) in said second cross flow region is greater than the total flow cross-sectional area of the second opening region and of the first air flow passages (AFP1-CF2) in said second cross flow region.