USEFUL DATA

THE COMTHERM ONLINE TECHNICAL MANUALFANS & BLOWER APPLICATION ENGINEERING
In most combustion installations the necessary air required for the combustion of the fuel is supplied by a centrifugal fan. VARIATION OF ROTATION SPEEDVariation of the rotation speed (rpm) of the blower affects the volume and pressure of the air produced by the fan.
The power consumption (Kw) is also affected. V_{2} = V_{1} × ( R_{2} ÷ R_{1} )The pressure of the outlet air produced is proportional to the square of the rpm of the fan. In simple terms this means that if the
speed of a fan is changed from say 1420 rpm to 2840 rpm (doubled) then the pressure of the air produced is increased fourfold. V_{2} = V_{1} × ( R_{2} ÷ R_{1} ) ^{2}The power consumption of the blower is proportional to the cube of the rpm of the fan. In simple terms this means that if the speed of a fan fitted with a 4Kw motor is changed from say 1420 rpm to 2840 rpm (doubled) then the new power consumption would be increased eightfold to 32kw. If Kw1 is the power consumption at speed R1 then the power consumption Kw2 at speed R2 can be calculated using the relationship: KW_{2} = KW_{1} × ( R_{2} ÷ R_{1} ) ^{3}VARIATION IN AIR DENSITYVariation in density of the inlet air influences the outlet pressure obtained from a fan. The density of the inlet air can vary depending on type of gas (molecular weight), the altitude of the fan installation and the temperature of the inlet air. The pressure of the outlet air is proportional to the density of the inlet air. In simple terms this means that if a fan designed for operation with air (s.g. =1.0) is used with say a natural gas (s.g. 0.5) then the outlet pressure will be reduced by 1/2. If P1 is the pressure with inlet gas at s.g. = S1, then the pressure P2 produced with inlet gas s.g. = S2, can be calculated using the relationship: P_{2} = P_{1} × ( S_{2} ÷ S_{1} )The altitude dictates the barometric pressure at the fan inlet and therefore the density of the inlet air. The density is proportional to the barometric pressure, so in simple terms if the barometric pressure reduces by 20% then the fan outlet pressure will reduce by 20%. If P1 is the pressure at altitude A1 then the pressure P2 at altitude A2 can be calculated using the relationship: P_{2} = P_{1} × ( A_{2} ÷ A_{1} )Charts and tables are available specifying standard barometric pressure at stated altitudes. As a point of fact, the standard barometric pressure at sea level is 1013.2 mbar and the on the South African high veld (Johannesburg) at a nominal altitude of 1760 metres (5764 ft) barometric pressure is a nominal 833mbar. The density of a gas is inversely proportional to the absolute temperature of that gas.
It follows therefore that the pressure produced by a fan is inversely proportional to the absolute temperature of the air.
In simple terms, if the absolute temperature doubles, then the density is halved. P_{2} = P_{1} x ( T_{1} + 273.16) ÷ (T_{2} + 273.16 )The power consumption (kw) of a fan is directly proportional to the density of the air. A change in inlet air temperature
from say 0C (273.16K) to 100C (373.16K) would change the power consumption by a factor of 273.16 : 373.16, = 0.73. KW_{2} = KW_{1} × { ( T_{1} + 273.16 ) ÷ ( T_{2} + 273.16 ) }Combustion air blowers are sized to supply sufficient air for the combustion of the full fuel rating of the burner. A burner of stated
thermal capacity requires the same amount of air, irrespective of the altitude or temperature (density) of the inlet air. In general burner suppliers’ size combustion air fans assuming normal ambient inlet air temperature and sea level altitude and
will only involve themselves in the above fan design rules when considering fan rotation speed and power consumption.
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