There is no standard for this. The main functions of this hat are: 1 to prevent smoke from rushing up; 2. Prevent foreign objects from falling into the chimney; 3. Prevent acid rain from corroding chimneys (that is, sulfur dioxide in flue gas generates sulfuric acid water after encountering rain, and falls back to chimneys, and covers the diffusion with a hat);
Square and round mainly depend on the matching with the shape of chimney, with emphasis on corrosion resistance and small resistance (upward resistance of flue gas, natural wind resistance, etc.). )