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THE COMTHERM ON-LINE TECHNICAL MANUAL

FANS & BLOWER APPLICATION ENGINEERING

In most combustion installations the necessary air required for the combustion of the fuel is supplied by a centrifugal fan.
These fans basically consist of a fan wheel (impellor) fitted with blades that rotates within a snail shaped casing (scroll). Many different scroll shapes exist and many fan wheel designs are available.
The fan wheel assembly is driven by an electric motor which is normally flanged directly onto the scroll casing. Irrespective of the impellor or scroll type the following basic blower engineering rules apply for any particular point on a fan operating performance curve.
Fig 1. shows a typical cross section of a centrifugal fan.

burner

VARIATION OF ROTATION SPEED

Variation 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.

The volume of air produced is directly proportional to 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 volume of air produced is doubled. If V1 is the volume at speed R1 then the volume V2 at speed R2 can be calculated using the relationship:-

V2 = V1 × ( R2 ÷ R1 )

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.
If P1 is the pressure at speed R1 then the pressure P2 at speed R2 can be calculated using the relationship:-

V2 = V1 × ( R2 ÷ R1 ) 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:-

KW2 = KW1 × ( R2 ÷ R1 ) 3

VARIATION IN AIR DENSITY

Variation 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:-

P2 = P1 × ( S2 ÷ S1 )

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:-

P2 = P1 × ( A2 ÷ A1 )

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.
To calculate absolute temperature, which is graduated in the Kelvin scale, add 273.16 to the temperature in Centigrade (Celsius).
If a fan creating a pressure of P1 is operating at an air temperature T1 (Celsius) and the temperature at the inlet is changed to T2 (Celsius), then the pressure P2, produced by the fan at temperature T2 can be calculated using the relationship:-

P2 = P1 x ( T1 + 273.16) ÷ (T2 + 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.
If Kw1 is the power consumption at temperature T1 and Kw2 the power consumption at a temperature T2, then this relationship can be expressed by the formula:-

KW2 = KW1 × { ( T1 + 273.16 ) ÷ ( T2 + 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 many cases several factors have to be considered in fan selection; however the sizing of the fan must always be based on the weight of air required.

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.

Combustion air fans in general are fitted with motors that generate nominal rotation speeds of 2840 r.p.m. or 1420 r.p.m.; however this will vary depending on the nature of the electrical supply (50Hz or 60hz).
Naturally there exists many special variations but the fan engineering basic rules apply.
In some cases it is necessary to apply several of the rules, for example if a burner designed for the U.K. (sea level, 50Hz electrics) was transported to a U.S. city at an altitude (and with a 60Hz electrical supply.

When considering fan selection the performance curve of the fan must be considered. These curves (graphs) normally show the volume compared with the static pressure generated by the fan.
Power consumption curves are also normally transposed over the fan curve; these curves are required in order to select the power required from the drive motor.
Care should always be taken to ensure that the motors are not overloaded by operating the fans at too high a volume. It is always necessary to throttle or restrict the air flow to ensure overload does not occur.
Fig 2 shows the layout of a typical fan performance curve.

burner

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  Fan Application Engineering

  Variation of Rotation Speed
  Variation in Air Density



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