4.3　Mechanical Ventilation

4.3.1　Positive pressure shall be maintained in the rooms where air requires to be kept clean.Negative pressure shall be maintained in the rooms where dust，harmful gas or explosive harmful substance is emitted.If several rooms having different required air cleanliness levels are jointed through access doors（holes），air shall flow from cleaner rooms into more contaminated rooms.

4.3.2　General or local exhaust systems shall handle the air directly from harmful substance sources or the most contaminated indoor zones，and the contaminated airflow shall not pass through operation zones or frequently occupied areas.

4.3.3　For production buildings containing explosive hazardous substances or harmful gas or vapor，if it is difficult to determine the harmful substance sources，airflow rate distribution of general exhaust should satisfy the following requirements:

a）If the gas or vapor at emission temperature is lighter than indoor air，or the sensible heat emitted from production building will lead to stable upward airflow throughout the year even though the gas or vapor at emission temperature is heavier than indoor air，air should be exhausted from the upper position of the building.

b）If the gas or vapor at emission temperature is heavier than indoor air and the sensible heat emitted from production building will not lead to stable upward airflow，or volatile liquid（e.g.，benzene，ether and etc.）is emitted and will cause the gas or vapor being accumulated in the lower position of the building due to absorption of the heat in the air during volatilization，2/3 of the total air rate should be exhausted from the lower position of the building and 1/3 of the total air rate should be exhausted from the upper position of the building，and air change rate shall not be less than 1AC/h.

c）For the rooms with height less than or equal to 6m，floor area less than or equal to 150m2 or air change rate larger than or equal to 8AC/h，air may be exhausted only from the upper position of the rooms，if Item a）is satisfied，and air may be exhausted only from the lower position of the rooms if Item b）is satisfied.

Notes:1　Gas with relative density less than or equal to 0.75 is assumed to be lighter than air，while gas with relative density larger than 0.75 is assumed to be heavier than air.

2　The zone within 2m above ground is regarded as the lower zone.

3　The exhaust air rate from the upper position or the lower position of the room includes the local exhaust rate in this room.

4.3.4　The layout of inlets for general exhaust system shall meet the following requirements:

a）For the general exhaust system used to remove the flammable gas or vapor，the distance from the upper edge of air inlets located in the upper position of the room to the ceiling plane or building roof shall not be more than 0.4m.

b）For the general exhaust system used to remove hydrogen and air mixture，the distance from the upper edge of air inlets to the ceiling plane or building roof shall not be more than 0.1m.

c）The distance from the lower edge of air inlets located in the lower position of the room to the floor shall not be more than 0.3m.

d）Guide devices shall be provided at the blind angle of production buildings where explosive hazardous gas may be accumulated and escaped due to the structure of the building.

4.3.5　For underground or semi-underground production buildings or pits located in the plant，where explosive hazardous gas or harmful gas or vapor is not generated but possibly accumulated，their lower positions shall be mechanically ventilated，and the air change rate should be 6AC/h.

4.3.6　Enclosed type local exhaust hoods should be used，otherwise，suitable exhaust hoods may be selected according to process requirements and techno-economic comparison.

4.3.7　Local exhaust hoods shall be installed at the locations emitting harmful substances（e.g.，feed nozzle，discharge nozzle and etc.），and should be of enclosed type integrated with process equipment.

4.3.8　Air containing extremely toxic substances from general exhaust systems or local exhaust systems shall be purified before venting.

4.3.9　Air from local exhaust systems shall be vented to a height above the aerodynamic shadow zone and positive pressure zone of production buildings via tall exhaust stack or high velocity flow（i.e.，ejection exhaust）for any one of the following cases:

a）Containing extremely toxic substances or the substances with very awful odor.

b）It is located in non-explosive hazardous area，and exhaust air contains relatively high concentration of explosion hazardous substances.

Notes:1　The aerodynamic shadow area of buildings refers to the negative pressure zone（static pressure is smaller than the atmospheric pressure）which is resulted from that the outdoor atmospheric airflow impacts the windward side of building causing airflow bending phenomenon，as a result，the static pressure at roof and the less side of the building is decreased.

2　The positive pressure zone of building refers to the area on the windward side of building where the static pressure is higher than the atmospheric pressure due to the impacting effect of atmospheric airflow.

4.3.10　For the production buildings in which the explosion hazardous substances or highly toxic substances emit，if the horizontal distance between the vent of centralized general mechanical exhaust system and the air inlet of mechanical air supply system is greater or equal to 20m，the vent outlet shall be higher than 1m above the roof of building，and if it is smaller than 20m，it shall be higher than 6m above the air inlet.

4.3.11　Separate exhaust systems shall be provided for the following rooms:

a）Rooms containing explosive hazardous substances and normal rooms.

b）Rooms containing extremely toxic substances，rooms containing highly toxic substances，and rooms containing moderately toxic substances.

4.3.12　Separate local exhaust systems shall be provided for any one of the following consequences if two or more substances contained in the air are mixed:

a）To cause fire or explosion.

b）To generate more harmful mixture or compound.

c）To tend to settle or condensate.

4.3.13　Backup ventilators with automatic switchover function shall be provided，and signals indicating operation conditions of ventilators shall be provided in control rooms，operation rooms or operation posts for any of the following cases:

a）The local exhaust and overall exhaust system of the plant building diffusing extremely toxic substances.

b）The local exhaust system for removing the highly toxic substances in the air.

4.3.14　Power load rating for exhaust systems to handle the air containing extremely or highly toxic substances shall be the same as that for process systems.

4.3.15　Air inlet positions of mechanical ventilation supply system shall comply with the following requirements:

a）It shall be set up at the location with relatively clean outdoor air，and the provision of Areicle 4.1.7 of this specification shall be complied with.

b）The air inlet shall be positioned on the windward side of the exhaust outlet and be lower than the exhaust outlet.

c）The distance between the bottom of air inlet and the outdoor ground shall not be less than 2m.When it is located in the green zone，it shall not be less than 1m.

d）Short-circuit of air inlets and air draft shall be avoided.

e）Air inlets for cooling should be arranged in the shades of buildings.

4.3.16　For production buildings and auxiliary buildings where central heating system and exhaust system are designed，if natural air makeup does not satisfy the indoor health requirements and process requirements or is not techno-economically justified comparison，mechanical air makeup system shall be provided.And when mechanical air makeup system is to be provided，calculations shall be made for air flow balance and heat balance.

4.3.17　Air supply into production buildings emitting harmful gas or vapor shall be sent to the operation zones.If air supply to operation zone may accelerate volatilization and dispersion of harmful substances or dispersion of dust，air supply should be sent to the upper positions of operation zones in production buildings.

4.3.18　Dust collection system shall not be oversized，and should be provided based on the production line defined.And the exhaust air rate of dust collection system may be calculated on a basis that all air intakes of the dust collection system are operated simultaneously.Dust collection system provided for each production line should be designed with programmable controlled interlock，and shall be started before startup of the production line and stopped after shutdown of the production line.

4.3.19　Dust or dust-laden wastewater from dust collectors shall be recovered or properly treated.Drainage of dust-laden wastewater shall conform to GB 8978 and GBZ 1.

4.3.20　If the collected dust is allowed for directly sending into the process flow，dust collectors should be arranged above the process equipment（e.g.，belt conveyor，silo and etc.），otherwise，dust hoppers shall be provided and recovery or disposal provisions shall be made.

4.3.21　For production buildings where dust is emitted，wet flushing method should be used，and if wet flushing method is not allowed by a process and dust emission is strictly controlled，vacuum dust cleaning system should be used.

4.3.22　If two and more exhaust systems are installed in one room of the laboratory，all of them shall be in operation.

4.3.23　Where one ventilation system is equipped with several fume hoods，variable air volume（VAV）system should be used.

4.3.24　Where central air conditioning system or central air makeup system is equipped in the central lab，VAV system should be used for air supply and exhaust.