Application status and improvement measures of the

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Application status and improvement measures of roof fans

1 types and characteristics of roof fans

roof fans are divided into two series: roof supply fans and roof exhaust fans. Their classification is generally divided into centrifugal and axial flow fans according to the form of fans; According to the specific form of rain cap, there are upper exhaust and lower exhaust; In terms of manufacturing materials, there are not only all metal structures, but also FRP structures and steel FRP composite structures

1.1 axial flow fan

1.1.1 roof exhaust fan

from the structural form of its rainproof part, roof supply fan mainly includes umbrella rainproof cap, conical rainproof cap, hemispherical rainproof cap and cylindrical rainproof cap, among which the exhaust above the conical and hemispherical rainproof cap is the main, the exhaust below the umbrella rainproof cap is the main, and the cylindrical rainproof cap can exhaust up and down. From the aspect of air diffusion performance and impact on the surrounding environment, the rain cap with the above exhaust dominated, and the upward exhaust outlet wind speed is 5.0 ~ 8.0m/s, which can blow the discharged polluted air to the high altitude (equivalent to connecting a 2 ~ 3M long air duct), which is conducive to the better diffusion of the discharged gas, especially when there is a roof blower near the exhaust fan, this characteristic is particularly obvious, which can reduce the interference of exhaust air to air supply, Therefore, the rain cap with lower exhaust has excellent performance. However, the upper exhaust roof fan also has the disadvantages of complex structure and high cost

1.1.2 roof ventilator

there are few types of roof ventilator, mainly including umbrella rain cap and louver rain cap

1.2 centrifugal fans

centrifugal roof fans are generally exhaust fans due to the limitations of impeller characteristics, which are characterized by high residual pressure and are mainly used in local exhaust systems. Because this kind of roof fan has no volute, the aerodynamic pressure generated by the high-speed rotation of the impeller cannot be effectively converted into static pressure, so the aerodynamic performance is very unsatisfactory and the power consumption is high. It is only used in systems with large exhaust resistance. Generally, this kind of roof fan should be avoided in the whole room ventilation system as far as possible

2 problems of roof fans

2.1 structural materials

roof fans mainly have all metal structure and FRP structure. All metal structure roof fans are mostly made of steel. The air duct and foundation are welded with 3 ~ 4mm thick hot-rolled steel plate, and the experimental performance meets the different requirements of different levels. The rain cap is welded with 1.0 ~ 1.5mm cold-rolled steel plate, and the surface is painted or baked after being coated with antirust paint. It is proved from several engineering practices that due to the poor air environment on the roof, especially after the plum rain season, the steel structure is seriously rusted, which reflects that the corrosion resistance of the steel structure rain cap is not ideal, which seriously affects the service life and beauty of the roof fan. For example, the steel roof fan installed in an automobile factory has lost most of its structural strength after 5 years of use. The all FRP roof fan can solve the corrosion problem well, but after long-term operation and exposure to wind, sun and rain, the air duct and base are easy to age and fall off, causing the air duct and motor to fall and cause accidents

2.2 in terms of design

2.2.1 many roof fans are not equipped with devices to prevent air backflow after shutdown, or the valves cannot be reliably closed after shutdown of roof fans. For workshops with large thermal pressure, the heat loss cannot be underestimated. You can often see that roof fans and axial flow fans without backstop devices rotate at high speed under the action of thermal pressure. This phenomenon shows that the wind speed caused by thermal pressure is quite large, and the wind speed in the air duct can reach more than 2.0 ~ 3.0m/s. The fan reversing is very unfavorable to the motor starting, and may be damaged due to excessive starting current

2.2.2 the anti condensation performance is poor. In winter, the indoor and outdoor temperature difference is large, and it is easy to condense on the air duct, causing water dripping, which will affect normal production

2.2.3 roof fans must have reliable basic performance of rain proof, snow proof and sand proof. However, at present, many roof fans only consider the rain proof function in their design, ignoring that snow and sand will not only enter the fan from the top, but also from the bottom when the outdoor wind speed reaches a certain intensity. Some roof fans using internal movable shutters as air check devices seem to prevent flying snow from falling into the room. In fact, flying snow accumulates on the movable shutters, which will cause dripping after the indoor temperature melts, and cannot play the role of preventing flying snow

2.2.4 in areas prone to typhoons and sandstorms, roof fans have poor strong wind resistance, high center of gravity, plane and concave structures increase wind resistance, and the strong wind resistance performance of umbrella rain caps is extremely unsatisfactory

2.2.5 the safety of some roof fans is not only to choose their own products, and the appropriate size of the pneumatic range can be well guaranteed. The original intention of not focusing on promoting biological materials and biological matrix composites in fan Xinda is not only to occupy the highland of new material innovation, but also to set solid and reliable safety in the lower part. Decorative air vents alone cannot guarantee that the falling objects will not fall and hurt people when the motor falls off and the wind blade breaks

2.3 engineering application

2.3.1 the use places of roof fans vary widely. For example, some projects require connecting some short pipes to achieve better ventilation effect, some users require less noise and need to install simple silencers. These additional equipment require roof fans to provide a certain residual pressure to ensure normal operation, and some roof fans provide a small residual pressure on the premise of ensuring the rated air volume, Affect the normal operation of the system

2.2.2 in engineering application, circular foundation should be adopted for concrete roof to reduce weight, circular foundation should be adopted for steel structure to facilitate construction, and the connection between some roof fans and foundation is not convenient and reliable. The power distribution can only be one-way line, and the junction box is set outdoors, which brings trouble to the installation. The indoor interface of the roof fan is not provided with a flange, so the user cannot easily connect the air duct

2.2.3 some roof fans are equipped with air filters that require frequent manual cleaning. From the perspective of the use effect, it is unrealistic to expect the operation and management personnel to often climb on the roof to clean the filters, so the operation result is very poor

2.2.4 the roof fan is installed at high altitude, which is an ideal bird nest, and bird prevention must be set

3 improvement measures and design problems of roof fan

3.1 materials

in order to improve the anti-corrosion performance of roof fan with all metal structure, the whole machine can be hot-dip galvanized, and the rain cap can also be made of aluminum alloy; In order to solve the aging problem of all glass fiber reinforced plastic roof fans, it is usually strengthened by increasing the thickness of glass fiber reinforced plastic, which will undoubtedly increase the production cost. Therefore, the material cost of all glass fiber reinforced plastic roof fans is higher than that of all metal roof fans. According to estimates, the difference between the two is about 30%; The best way is that the roof fan adopts a steel FRP composite structure, and the rain cap and check valve are made of FRP. Give full play to the excellent anti rust ability and easy modeling characteristics of glass fiber reinforced plastics, roof fans in various conditions can be made according to the air distribution and aesthetic requirements, so that the fan resistance is minimized and the appearance is more beautiful; The air duct and foundation are made of steel structure and hot-dip galvanized, which can ensure the structural strength and anti-corrosion requirements. Some connectors are directly welded to the air duct, simplifying the manufacturing process

3.2 residual pressure

on the premise of ensuring the rated air volume of the roof fan, the residual pressure of not less than 50pA should be guaranteed for users

3.3 check and safety aspects

roof fans are equipped with check devices that can be reliably closed during shutdown. Loose leaf dampers can be installed. There are three kinds of loose leaf dampers: gravity type, electric type and manual type, which can be selected according to the engineering requirements. If the roof fan used in the design does not have backstop, the indoor hot air loss and some heat loss caused by the invasion of outdoor cold air should be fully considered when calculating the heating heat load. For severe cold areas, the impact on the on duty heating should also be considered. A solid and reliable safety device shall be set at the lower part of the fan to ensure safety

3.4 anti condensation

when designing the roof fan, the area of the common contact part between indoor and outdoor air should be reduced as much as possible to reduce the amount of condensation. For the sake of safety in the engineering design, the installation position of the roof fan should be avoided directly above the process equipment and work posts as far as possible to reduce the impact of dripping water on production

3.5 in terms of strong wind prevention

roof fans with good typhoon resistance performance should be selected in typhoon and strong wind prone areas, and their center of gravity should be as low as possible. Streamlined structures should be used to reduce wind resistance. The wind resistance performance of umbrella rain caps is extremely unsatisfactory, so this structure should be used as little as possible. The strong wind is abnormal, and the rain cap design can be considered according to the shutdown condition. Automatic ailerons can also be considered for roof fans used in typhoon and strong wind prone areas. Even in strong wind weather, the installation of automatic ailerons will reduce the entry of wind and rain. The aileron will return to its original position through the spring mechanism. At the same time, the aileron can also play the role of a flow sheet to reduce air resistance and achieve the effect of increasing air volume

3.6 noise

roof fans should have good acoustic indicators. In order to control noise, the fan speed should not be higher than 1500r/min, and 960r/min fan is better. The roof fan noise of the world's latest large passenger aircraft below 5000m3/h should be controlled below 50dB, and the noise greater than 5000m3/h should be controlled below 60dB, and high-frequency noise should be avoided

3.7 in terms of operation and maintenance

roof fans should not be equipped with air filters that need frequent manual cleaning. If the air supply has cleanliness requirements, other air treatment schemes should be adopted

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