The horizontal section of the kitchen exhaust duct should not be too long. Generally, the farthest horizontal distance should not exceed 15m and there should be a slope of more than 2%. A movable joint should be set at the horizontal end to facilitate the cleaning of oil stains. The exhaust speed shall not be less than 10m/s according to regulations to prevent oil from adhering to the flue due to low wind speed. The branch pipe connected to the exhaust hood should be equipped with an air volume regulating valve.
The design and installation of smoke exhaust ducts are unreasonable, often resulting in poor ventilation of kitchen smoke, directly affecting the use of oil fume purifiers. The main reasons for this are as follows:
(1) The ventilation area of the main smoke exhaust duct is insufficient, and the gas flow rate is too high. According to engineering experience, the wind speed inside the smoke exhaust main duct cannot exceed 15m/s. Otherwise, after a certain period of operation (such as one year later), it is easy to cause the smoke exhaust system to be blocked. In the operation practice of oil fume purifiers, most cases are caused by insufficient ventilation area of smoke exhaust pipes, resulting in blockage. The gas flow rate in the pipes is above 15m/s.
(2) Due to the need to pass through the building beam, the ventilation cross-sectional area of the smoke exhaust duct decreases, resulting in increased resistance on the cross-sectional area and obstructed airflow.
(3) When passing through building beams or avoiding other equipment pipelines, a section of the smoke exhaust pipeline will be lower than the overall plane of the smoke exhaust pipeline. After a period of use, the oil-water mixture will accumulate in the lower section of the smoke exhaust duct, directly affecting the smoothness of smoke exhaust.
(4) Kitchen oil fumes from different floors are discharged through the same main pipeline. The kitchens near the exit have better smoke exhaust effect, while the kitchens on lower floors have poor smoke exhaust.
There are three commonly used smoke exhaust fans: axial flow fan, fan box, and centrifugal fan.
(1) Axial flow fans were previously widely used, but due to their low total pressure, they were rarely used after installing oil fume purifiers.
(2) The fan box has a moderate total pressure, adjustable air volume, and low noise. It is often used in kitchen smoke exhaust systems where the distance between the hood and the oil fume discharge outlet is moderate.
(3) The total pressure and air volume of the centrifugal fan can be adjusted, but due to the high noise, noise reduction treatment is often required. When the kitchen is located at the bottom of the building, its exhaust fan should be placed on the roof to reduce indoor noise pollution, and the exhaust duct should be in a negative pressure state to avoid odor leakage. When placing fans on the roof, centrifugal fans are generally chosen.