Engineering task and calculation objective
The QVEN module calculates the power requirement of fans according to the Energietechnische Arbeitsmappe (14th edition), a standard German energy engineering reference. From volume flow, pressure rise and fan efficiency it determines the shaft power to be supplied — the basis for motor selection and for assessing operating costs.
Being able to calculate fan power is part of everyday work in energy and process engineering: for forced-draft and induced-draft fans of combustion plants, for cooling tower and air cooler fans, and in drying and exhaust air systems. Temperature and inlet pressure enter the analysis because they determine the gas density and hence the relationship between volume flow, mass flow and pressure rise.
Since fans generate only small pressure ratios, the medium may be treated as incompressible — the power then follows directly from the product of volume flow and total pressure rise, divided by the efficiency. For larger pressure ratios, a compressor calculation with compression work is required instead.
Standard and calculation basis: Energietechnische Arbeitsmappe 14. Auflage: 1995
Calculation workflow
- Record the operating data: The inputs are the volume flow, the temperature and inlet pressure of the gas, and the required pressure rise. Volume flow and pressure rise must refer to the same state — usually the suction state.
- Use the total pressure rise: The governing quantity is the fan's total pressure rise, i.e. the sum of the static pressure rise and the change in dynamic pressure between suction and discharge. It results from the pressure drop calculation of the connected duct or flue gas path.
- Form the ideal fan power: The loss-free power is the product of volume flow and total pressure rise. Because of the small pressure ratio of fans, no compressibility correction is required.
- Account for the efficiency: Dividing by the fan efficiency yields the power requirement at the fan shaft. For motor sizing, gearbox, belt and motor efficiencies plus a design margin are added on top.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Volume flow | V∙ | m³/s |
| Temperature | ϑ | °C |
| Pressure at inlet | p1 | Pa |
| Pressure increase | dP | Pa |
| Fan/blower efficiency | n | – |
Calculated results
| Quantity | Symbol | Unit |
|---|---|---|
| Power consumption | P | W |
Worked example
An induced-draft fan handles V̇ = 8 m³/s of flue gas and must provide a total pressure rise of Δp = 2,500 Pa. The fan efficiency is η = 0.78. Calculate the power requirement at the shaft in this worked example.
Given values
| Volume flow V̇ | 8 m³/s (= 28,800 m³/h) |
| Pressure rise Δp | 2,500 Pa |
| Fan efficiency η | 0.78 |
Solution
Loss-free fan power
Pideal = V̇ · Δp = 8 · 2,500 W = 20,000 W = 20.0 kW
Power requirement at the shaft
P = V̇ · Δp / η = 20,000 / 0.78 W
P ≈ 25,641 W ≈ 25.6 kW
For motor selection, the motor efficiency and a design margin are added; typically a 30 kW standard motor would be chosen here.
Result
| Power requirement P | 25.6 kW |
All values are illustrative. The applicable standard and project-specific boundary conditions remain authoritative.
Frequently asked questions
Up to what pressure rise may I treat a fan as incompressible?
As a rule of thumb, the simple relation P = V·Δp/η holds with negligible error up to pressure rises of about 3,000 Pa; up to roughly 10,000 Pa (pressure ratio approx. 1.1) the error usually stays below one percent. Beyond that, the compression work should be used, as compressor modules do — the density change during the pressure rise is then no longer negligible.
Which efficiency is meant — and what typical values are realistic?
It is the overall efficiency of the fan (internal losses plus mechanical losses), referred to the shaft power. Centrifugal fans with backward-curved blades achieve about 0.75 to 0.85, simple forward-curved (squirrel cage) rotors considerably less, and large axial fans up to more than 0.85. The motor efficiency is not included and must be considered additionally for the electrical input power.
Why does the gas temperature matter even though it does not appear directly in P = V·Δp/η?
Temperature and inlet pressure determine the gas density. The pressure rise a fan produces at fixed speed is proportional to the density — an induced-draft fan designed for hot flue gas produces a significantly higher pressure rise during a cold start with dense air and draws correspondingly more power. The motor sizing must cover this cold-start case.