8.3　Air and Flue Gas System

8.3.1　The types,number,air flow,and pressure heads of forced draft（FD）fans shall be determined in accordance with the requirements below:

1　FD fans should be of moving vanes adjustable axial flow type.Instead,speed regulation centrifugal type may be adopted as well.

2　Each boiler should be equipped with two FD fans and no standby fan is required.

3　The air flow and pressure head of FD fans shall be determined in accordance with the requirements below:

1）The basic air flow of FD fan shall be calculated assuming that the boiler operates on design coal type and considering the corresponding excess air coefficient,and shall include the secondary air flow required when boiler operates under the BMCR condition and the net air leakage at the air supply side of air preheater after operation for one year as guaranteed by the manufacturer.The basic pressure head of FD fan is calculated assuming that the boiler operates on design coal type under the BMCR condition,and shall include the resistance at the air side of boiler proper（including the stack draft）as guaranteed by the manufacturer,system resistance,and the static pressure（negative）of furnace at the position of burners.

2）If trisector air preheater is used,the FD fans should be selected with an air flow margin of no less than 5%plus a temperature margin（which can be determined based on the"outdoor air temperature used for ventilation calculation during summer"）,as well as a pressure head margins of no less than 15%.For units incorporating foreign advanced technologies,the air flow and pressure head margins of FD fans should be determined using the applicable calculation standards based on the actual project conditions.

3）If bisector or tubular air preheater is used,the FD fans should be selected with an air flow margin of 10%plus a temperature margin（which can be determined based on"outdoor air temperature used for ventilation calculation during summer"）and a pressure head margin of 20%.

4）If hot air recirculating system is used,the air flow margin of FD fans shall not be less than the hot air recirculating flow under operating conditions in winter.

4　For boilers burning low calorific value or low volatile coals,if each boiler is equipped with two FD fans,the selected fan margins shall be checked such that the boiler minimum stable load without oil supporting can be reached when only one FD fan is put into service.

8.3.2　The types,number,gas flows,and pressure heads of induced draft（ID）fans shall be determined in accordance with the requirements below:

1　Axial flow fans should be selected for 300MW and above units.Speed regulation centrifugal fans may be selected for units below 300MW,but in this case,safety assessment shall be necessary to prevent boiler implosion.

2　If the test block capability of ID fan at ambient temperature is higher than the designed transient withstanding pressure of boiler furnace,then centrifugal fans shall not be adopted.

3　Each boiler should be equipped with two FD fans and no standby fan shall be provided.

4　The gas flows and pressure heads of ID fans should be determined in accordance with the requirements below:

1）The basic gas flow of ID fan should be determined to be the sum of the flow of flue gas produced when the boiler operates on design coal under the BMCR condition,the leakage at flue gas side of air preheater after operation for one year as guaranteed by the manufacturer,and the leakage of the boiler flue gas system.The basic pressure head of ID fan shall be calculated assuming that the boiler operates on design coal type under the BMCR condition,and shall include the resistance at the flue gas side of boiler proper as guaranteed by the manufacturer（including the stack draft and the negative pressure at the starting point of furnace）,the resistance of denitration reaction system,desulphurization system（when integrated with the booster fan）and precipitator,and the system resistance.

2）ID fans should be selected with a gas flow margin of no less than 10%plus a temperature margin of 10-15℃and the pressure head margin of ID fans should be no less than 20%.For units incorporating foreign advanced technologies,the gas flow and pressure head margins of ID fans may be determined using the applicable calculation standards based on the actual project conditions.

5　For boilers burning low calorific value or low volatile coal,if each boiler is equipped with two ID fans,the selected fan margins shall be checked such that the boiler minimum stable load without supporting oil can be reached when only a single ID fan is put into service.

8.3.3　The air heating system shall comply with the following requirements:

1　The air heating system shall be provided according to meteorological conditions and coal quality conditions.Hot air re-circulating system,steam air heater or other air heating systems may be selected through techno-economic analysis.

2　If the coal quality is favorable,the ambient temperature is relatively high,or the cold end of air preheater is made of corrosion resistant materials such that air preheaters can really be immune from corrosion and ash clogging,the heating system may be eliminated.

3　For regenerative trisector air heaters,if they first heat the primary air,then air heating system may not be provided at the primary air side.

4　Hot air recirculation system should be adopted for tubular air preheater,low-sulfur and low-ash coal and in areas the ambient temperature is relatively high.If hot air recirculation system is used for regenerative air preheaters,it shall be necessary to take into account the requirement on anti-corrosion of fans and air ducts.The hot air recirculation rate should not be larger than 8%.The hot air extraction point shall be arranged at places where the ash content is low.

5　The steam air heater system shall comply with the following requirements:

1）The installation location of steam air heater shall be determined reasonably；in extremely cold regions the steam air heater should be arranged at the inlet side of fan.

2）The steam air heater shall be in such a structure and arrangement that permit resistance to be reduced.For steam air heaters with low annual utilization hours,they may be of movable structures.

3）The ambient temperature used for steam air heater selection should be taken as the outdoor air temperature adopted for heating calculation in winter for heating regions or the average temperature of the coldest months in winter for non-heating regions.Appropriate heating area margin should be reserved.

8.3.4　The types,number,air flows,and pressure heads of flame detector cooling fans should be determined in accordance with the requirements below:

1　The flame detector cooling fans should consist of two centrifugal fans,one in operation and the other for standby.

2　The air flow and pressure head margins of cooling fans shall accommodate the requirements for cooling of boiler flame detectors.