Fans - Influence of Air Temperature and Air Density on Volume Flow, Pressure Head and Power Consumption

The temperature and density of air will influence on volume flow, pressure head and power consumption of fans compared with manufacturing data

Since density of air vary with temperature and air pressure (or altitude and elevation above sea level), a fan will not deliver according manufacturing specification if the operating conditions are outside NTP - Normal Temperature and Pressure conditions.

NTP - Normal Temperature and Pressure Conditions

Manufacturers specification of fans are in general based on the

• NTP - Normal Temperature and Pressure Conditions - 20^oC, 101.6 kN/m², 1.204 kg/m³ (68^oF, 29.92 inches Hg, 0.075 pounds per cubic foot).

A fan is a "constant volume" device where the transported volume always is the same, no matter the air temperature or density, if all other things are equal. Only the mass flow through the fan vary with air temperature and air density.

• With hot air and lower air density - less mass will be transported through the fan
• With cold air and higher air density - more mass will be transported through the fan
• With equal speed and dimensions - the volume flow remains equal

When selecting a fan it is important to know if the specification of the system is based on operating conditions or NTP conditions. The formulas below can be used to calculate the volume flow, pressure head and power consumption at NTP conditions if the operating conditions are known, or vice versa if the NTP conditions are known.

The examples below may clarify the procedures:

Volume Flow

The ratio between volume flow at different temperatures can be expressed as:

q₁ / q₂ = (273 + t₂) / (273 + t₁) (1)

where

q₁ = reference volume flow (m³/s)

q₂ = operating volume flow (m³/s)

t₁ = reference temperature (^oC)

t₂ = operating temperature (^oC)

Pressure Head

The ratio between developed pressure at different temperatures can be expressed as:

dp₂ / dp₁ = (273 + t₁) / (273 + t₂) (2)

where

dp₁ = reference pressure developed (Pa)

dp₂ = operating pressure developed (Pa)

Power

The ratio between power consumption at different temperatures can be expressed as:

P₁ / P₂ = (273 + t₂) / (273 + t₁) (3)

where

P₁ = reference power consumption (W)

P₂ = operating power consumption (W)

Volume, Pressure and Power Ratio Chart

The volume, pressure and power ratios are expressed in the chart below. The chart is based on a NTP reference of 20 ^oC.

Volume, Pressure and Power Ratios Calculator

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.

t₁ - reference temperature (^oC)

t₂ - operating temperature (^oC)

Example - Fan with Hot Air

A fan delivers 10,000 m³/h of hot air at 60^oC. 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 10,000 m³/h.

The pressure coefficient is approximately 1.15 for air at 60^oC 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^oC according the chart. The correct power consumption should be 2.5 kW x 0.88 = 2.2 kW.

Note! Don't 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/m³.

Example - Fan with Combustion Air

10,000 m³/h of normal standard air at 20^oC shall be transported at an operating combustion air temperature of 180^oC. The total pressure developed at 180 ^oC 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^oC. The operating volume flow for selecting the fan would be 10,000 x 1.55 = 15,500 m³/h.

The pressure coefficient for air at 180^oC 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 conditions than during start ups. A motor should in general be big enough to handle the higher power consumption at start ups.

Remember! If fans starts with temperatures below 20^oC (NTP), the power consumption will be higher than the catalogue specification. In this situation a fan may be stopped by the electrical overload protection. Higher power consumption during start ups may be compensated by reducing the volume flow through the fan with a closing damper on the fan's outlet.

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