General ventilation, also known as dilution ventilation, is to ventilate the whole room.The principle is to use a certain amount of clean air to enter the room, dilute the indoor pollutants, make the concentration reach the allowable concentration of the sanitary specifications, and discharge the same amount of indoor air and pollutants to the outside.[1]
Natural ventilation driven by heat pressure or wind pressure is widely used by people.
(2) Mechanical ventilation
The method of making air flow through the action of fan to cause room ventilation.
(3) Natural and mechanical combined ventilation
Between the above two.
ventilation system
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The system using machinery to implement overall ventilation is divided into mechanical air supply system and mechanical exhaust system. For a room or area, there can be the following system combinations:
(1) There are both mechanical air supply system and mechanical exhaust system;
(2) There is only a mechanical exhaust system, and the outdoor air naturally penetrates through doors and windows;
(3) Combination of mechanical air supply system and local exhaust system;
(4) The mechanical air supply system is combined with the mechanical exhaust system and local exhaust system;
(5) Combination of mechanical exhaust system and air conditioning system;
(6) The mechanical air supply system is combined with the air conditioning system, or the air conditioning system realizes the task of overall ventilation.
Design principles
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1. For rooms with heat, moisture and harmful gases, when local exhaust cannot be used due to scattered or unfixed sources, or local exhaust cannot meet the sanitary requirements, comprehensive ventilation should be used or supplemented.
2. In addition to setting local air exhaust, the production plant that emits heat dissipation, steam and harmful gas at the same time, or only emits harmful gas with less density than air, should conduct natural or mechanical overall air exhaust in the upper zone, and its air exchange rate should not be less than once an hour.When the room height is more than 6 meters, the exhaust air volume can be calculated as 6 meters/hour per meter of ground area.
3. Comprehensive ventilation includes natural ventilation, mechanical ventilation or natural ventilation combined with mechanical ventilation.Natural ventilation shall be adopted as far as possible in the design to save energy, investment and noise.When natural ventilation is difficult to meet sanitary requirements, mechanical ventilation or a combination of mechanical ventilation and natural ventilation can be used.
4. For production rooms with central heating and ventilation, the possibility of natural air supplement should be considered first.The mechanical air supply system can not be set to compensate the exhaust air volume for the comprehensive exhaust system with the air exchange rate less than 2 hours or the local exhaust system with the operation time less than 2 hours per shift.When the natural air supplement cannot meet the indoor sanitary conditions, the production requirements or the technical economy are unreasonable, the mechanical air supply system should be set.
5. For rooms that require cleaning, when the surrounding environment is poor, the air supply volume should be greater than the exhaust volume to ensure positive room pressure;For rooms producing harmful gases, in order to avoid polluting adjacent rooms, the air supply volume should be less than the exhaust volume to ensure negative pressure in the room.Generally, the air supply volume can be 80%~90% of the exhaust volume.
6. When calculating the air balance and heat balance of general ventilation in winter, the following factors should be considered according to the specific situation:
(1) It is allowed to reduce the temperature for a short time or exhaust the air intermittently, and its exhaust air may not be considered in the air heat balance calculation;
(2) The inlet air of general ventilation for diluting harmful substances shall adopt the calculated temperature outside the heating room in winter to eliminate the residual heat and humidity.
7. For general ventilation, outdoor temperature calculation in winter ventilation can be adopted.
The following principles shall be followed when calculating heat dissipation of process and equipment:
(1) Winter
Calculated according to the heat dissipation of the process equipment of the minimum load shift;
The amount of heat dissipated infrequently shall not be included;
The hourly average value shall be used for the frequent but unstable heat dissipation.
(2) Summer
Heating shall be conducted according to the heat dissipation capacity of process equipment of the maximum load shift;
Frequent and unstable heat dissipation, heating according to the maximum value;
It shall be taken into account when the heat dissipation capacity of the day shift is relatively large.[2]
Air flow design principles
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(1) The air inlet and exhaust of general ventilation shall avoid the air containing a large amount of wet, hot or harmful substances flowing into the place without or with only a small amount of wet, hot or harmful substances.In general, the air inlet shall be as close as possible to the work site, and the air outlet shall be as close as possible to the source of harmful substances or the area with high concentration of harmful substances;
(2) When the required sanitary conditions in the workshop are higher than the surrounding sanitary conditions, keep the room in a positive pressure state.
(3) In a fully ventilated room, the inlet air flow should be evenly distributed as far as possible to reduce eddy currents and avoid the accumulation of harmful substances in local workplaces;
(4) The relative position of the air inlet and exhaust outlet shall be properly arranged to prevent the air inlet flow from being directly discharged out of the room without pollution, resulting in air flow short circuit.[3]
Calculation of overall air change
1. Air exchange volume required to eliminate waste heat
2. Air exchange volume required to eliminate residual moisture
3. Air exchange volume required for diluting hazardous substances
contrast
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General ventilation is to ventilate the whole room, dilute the concentration of harmful substances in the room to below the allowable concentration of the national health standard with the fresh air sent into the room, and release the polluted indoor air directly or through purification treatment into the outdoor atmosphere.Comprehensive ventilation can improve the indoor environment of the whole room, but it consumes a lot of air and wastes energy.
Local ventilation is the use of local air flow, so that the place where people work is not polluted by harmful substances, so as to create a good local working environment.Local ventilation has the advantages of good ventilation effect and air volume saving.It is suitable for large workshops, especially high temperature workshops with a large amount of waste heat, when full ventilation cannot ensure that all parts of the room are suitable.However, the local ventilation design needs accurate calculation, otherwise the ventilation degree cannot be guaranteed.
