q o = operating volume flow (m 3 /s)
t r = reference temperature ( o C) - in general 20 o C NTP conditions
t o = operating temperature ( o C)
A drying process requires 1 m 3 /s air at normal conditions 20 o C . The fan operates when the air is heated to 80 o C . The required volume capacity in the fan at operating conditions can be calculated using 1b as
q o = (1 m 3 /s) (273 + (80 o C)) / (273 + (20 o C) )
= 1.2 m 3 /s
The fan must be selected for volume flow 1.2 m 3 /s (and whatever the pressure loss in the system is).
The ratio between developed pressure at different temperatures can be expressed as:
dp o / dp r = (273 + t o ) / (273 + t r ) (2)
dp o = dp r (273 + t o ) / (273 + t r ) (2b)
dp r = reference pressure developed (Pa) - in general at NTP conditions
dp o = operating pressure developed (Pa)
With temperature higher than the reference temperature - the volume flow is higher and the required pressure developed in the fan is higher.
The ratio between power consumption at different temperatures can be expressed as:
P o / P r = (273 + t r ) / (273 + t o ) (3)
P o = P r (273 + t r ) / (273 + t o ) (3b)
P r = reference power consumption (W)
P o = operating power consumption (W)
The volume, pressure and power ratios are expressed in the chart below. The chart is based on a NTP reference with temperature of 20 o C .
The calculator below can be used to estimate the volume, pressure and power ratios at different temperatures. The default values are based on NTP conditions.
A fan delivers 10000 m 3 /h of hot air at 60 o C . The total pressure loss in the system at this volume is estimated to 500 Pa .
Decide the correct air volume and pressure for choosing a fan from the manufacturers data. Decide the power consumption.
Since the air volume is estimated for the hot air, the correct volume for the fan is 10000 m 3 /h .
The pressure coefficient is approximately 1.15 for air at 60 o C according the chart. The correct pressure in the manufacturing data sheet should be
500 x 1.15
= 575 Pa
The power consumption according the manufacturing data is 2.5 kW . The power coefficient is approximately 0.88 for air at 60 o C according the chart. The correct power consumption should be
2.5 kW x 0.88
= 2.2 kW
Note! - do not compensate the pressure developed by the fan if the pressure loss in the system is estimated on the basis of normal charts based on air with density 1.2 kg/m 3 .
10000 m 3 /h of normal standard air at 20 o C shall be transported at an operating combustion air temperature of 180 o C . The total pressure developed at 180 o C is estimated to 500 Pa .
Decide the correct air volume and pressure for choosing the fan from the manufacturers data and decide the total pressure for selecting the fan!
The volume coefficient in the chart above is 1.55 at 180 o C . The operating volume flow for selecting the fan would be
10000 x 1.55
= 15500 m 3 /h
The pressure coefficient for air at 180 o C is approximately 1.55 according the chart. The correct pressure used in the manufacturing data sheet should be
500 x 1.55
= 775 Pa
The power consumption according the manufacturing data is 4 kW . According the chart the power coefficient is approximately 0.65 . The correct power consumption should be
4 kW x 0.65
= 2.6 kW
Note! - the power consumption is lower in operating condition than during start up. A motor (and the motor protection) should in general be big enough to handle higher start up power consumption.
Remember! If a fan starts with temperatures below 20 o C (NTP) - the power consumption will be higher than specified in the catalogue - and the fan may be stopped by the electrical overload protection. The power consumption during start up can be reduced by limiting the volume flow with a closing damper on the outlet of the fan.
Systems for ventilation and air handling - air change rates, ducts and pressure drops, charts and diagrams and more.
Fan classification established by AMCA.
Pressure, head, air flow volume and fan capacity diagrams.
The motor must be capable of accelerating the fan wheel to it's operating speed.
AHP - Air Horse Power and BHP - Brake Horse Power.
Power consumption and typical efficiencies of fans
The definition of STP - Standard Temperature and Pressure and NTP - Normal Temperature and Pressure.
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