2.21. Compressibility Error. Air that is subjected to a pressure to force it through a series of pipes, a mass of grain, or a heat exchanger is compressed so that yi is not equal to 72. In most calculations where drying and ventilation problems are being considered, air is assumed to be incompressible to simplify calculations, atmospheric pressure being used throughout. Pressures under these conditions seldom exceed 10 in. of water and are usually in the order of 4 or less. The error resulting by neglecting compression is dependent upon the absolute pressures. It would be only 2.5 per cent if 10 in. of water were the operating pressure