Fan curve

A fan that is built into a device or system must work against a flow resistance . The fan generates an overpressure (pressure increase). The fan characteristic curve (also fan characteristic curve , engl. Fan characteristics ), this interdependence of volume flow is and increasing the pressure. The course of the fan characteristic curve depends on the design of the fan.

The fan characteristics are given in the data sheets. They are measured by the manufacturer on the fan test bench. They are important for the design of cooling systems and devices (see also : Thermal management of electronic devices and systems ). It should be noted that the characteristic curves in data sheets were measured under ideal conditions - on individual free-standing fans with unobstructed flow. The real characteristics under installation conditions can deviate from this.

Typical profile of fan characteristics of various types. Axial fan, radial fan and diagonal fan.

Physical units

In the SI units , the pressure is given in Pascal (Pa) and the volume flow in m ³ / s. Other units are also used in the fan manufacturers' catalogs. The unit often used for pressure is mmH ₂ O (mm water column) or inH ₂ O (inch water column). The units m ³ / h (cubic meters per hour), l / s (liters per second), or cfm (cubic feet per minute) are used for the volume flow .

Conversion of numerical values for printing

1 Pa	= 0.1019716	mmH ₂ O
1 Pa	= 0.00401463	inH ₂ O

Conversion of the numerical values for the volume flow

1 m ³ / s	= 1000	l / s
1 m ³ / s	= 3600	m ³ / h
1 m ³ / s	= 2118.88	cfm

Operating point (operating point)

Fan curve, 2 device curves and their intersections

The operating point ( operating point , engl. Operating point ) of a fan which is installed in a device, obtained as the intersection of fan and system characteristic. At the operating point, the fan generates a pressure increase that precisely compensates for the pressure loss in the device (in the system). The actual volume flow through the device (through the system) is therefore determined by the operating point.

The characteristic curve A represents the behavior of a system with a high flow resistance (high pressure loss coefficient ), the pressure loss increases steeply. The characteristic curve B has a flatter course, it represents the behavior of a system with a small flow resistance (small resistance coefficient). With the same fan you will be able to generate a much larger volume flow in system B than in system A.

The operating point C results from a completely unhindered flow through the fan - fan blowing freely. The working point D would result from a completely blocked air flow.

Device or system characteristic

The device or system characteristic curve - also called system impedance - describes, analogous to the fan characteristic curve, the mutual dependence of volume flow and pressure increase in the flow through the system. The pressure losses in the system increase almost quadratically with the volume flow according to the following law

{\ displaystyle \ Delta p = {\ frac {1} {2}} \ cdot \ zeta \ cdot \ rho \ cdot v ^ {2}}

{\ displaystyle \ Delta p}

= Pressure loss

{\ displaystyle \ zeta}

= Pressure loss coefficient

{\ displaystyle \ rho}

= Air density

{\ displaystyle v =}

mean flow velocity

The system characteristic can be measured. As a rule, a pair of values consisting of volume flow and pressure drop is sufficient, since the system characteristic is a parabola with the apex in the coordinate origin. With the help of modern numerical methods, it is also possible to calculate the pressure losses through the system and to determine the system characteristic through numerical simulation ( numerical fluid mechanics ). However, the accuracy of such simulations is heavily dependent on how precisely all influencing parameters can be determined.

It should be noted that the air path within a device or system often does not have the same flow cross-section at every point. Therefore, the flow velocity averaged over the cross section used in the above equation is only defined locally or in sections in the same way. The quadratic dependence of the flow resistance (pressure loss) of the system on the volume flow remains unaffected.

Characteristic curve of a fan combination

Several fans are often used in a system at the same time. In analogy to electrical engineering, one speaks of the parallel arrangement ( parallel connection ) or the series arrangement ( series connection ). The characteristic curve of the fan combination can be derived from the characteristic curves of the individual fans.

If several fans are arranged too closely, however, the air flows obstruct each other and the volume flow of the fan combination does not reach the value that would result from the characteristic curves of the individual fans. The volume flow is also reduced if the flow is hindered in close proximity to the fan (e.g. by a protective grille).

Parallel arrangement

Fan characteristic with parallel operation of two fans and device characteristic

With the parallel arrangement (parallel connection) the fans are placed next to each other. The volume flow is multiplied. Two identical fans connected in parallel would ideally - free blowing fans - generate twice the volume flow. However, if the device characteristic is superimposed, a new operating point results and the actual volume flow through the device becomes smaller. The steeper the device characteristic, the smaller the gain of the parallel fan arrangement. The parallel fan arrangement is particularly suitable for devices with a flat characteristic curve.

Row arrangement

Fan characteristic for series operation of two fans and device characteristic

In the series arrangement (series connection), several fans are connected in series in the air flow ( push-pull arrangement ). With two identical fans connected in series, you would ideally achieve twice the pressure increase. When the device characteristics are superimposed, you can also see here that a new operating point is being set. How much the air flow through the device can be increased depends on the steepness of the device characteristic.

Modifications of the characteristic

When changing the fan speed

Fan characteristic at reduced speed and device characteristic

The characteristic curve changes with the fan speed according to the following laws (provided that all other parameters remain unchanged), also called proportionality laws (see also : Affinity laws ).

The volume flow changes proportionally to the speed:

{\ displaystyle {\ frac {{\ dot {V}} _ {1}} {{\ dot {V}} _ {2}}} = {\ frac {n_ {1}} {n_ {2}}} }

The pressure increase changes proportionally to the square of the speed:

{\ displaystyle {\ frac {p_ {1}} {p_ {2}}} = \ left ({\ frac {n_ {1}} {n_ {2}}} \ right) ^ {2}}

The power requirement of the fan and the flow rate introduced into the conveyed medium change proportionally to the third power of the speed:

{\ displaystyle {\ frac {P_ {1}} {P_ {2}}} = \ left ({\ frac {n_ {1}} {n_ {2}}} \ right) ^ {3}}

${\ displaystyle {\ dot {V}} _ {1}, {\ dot {V}} _ {2}}$ ... volume flow at speed n ₁ , n ₂ ... pressure increase at speed n ₁ , n ₂ ... power requirement at speed n ₁ , n ₂
${\ displaystyle p_ {1}, p_ {2}}$
${\ displaystyle P_ {1}, P_ {2}}$

When the air density changes

Fan characteristic at an altitude of 4000 m and device characteristic

The characteristic changes with changed air density according to the following laws (provided that all other parameters remain unchanged):

The volume flow is independent of the air density at the same speed:

{\ displaystyle {\ dot {V}} _ {1} = {\ dot {V}} _ {2}}

The pressure changes proportionally to the air density:

{\ displaystyle {\ frac {p_ {2}} {p_ {1}}} = {\ frac {\ rho _ {2}} {\ rho _ {1}}}}

The power requirement of the fan changes proportionally to the air density:

{\ displaystyle {\ frac {P_ {1}} {P_ {2}}} = {\ frac {\ rho _ {1}} {\ rho _ {2}}}}

${\ displaystyle {\ dot {V}} _ {1}, {\ dot {V}} _ {2}}$ ... volume flow for air density , ... pressure increase for air density , ... power requirement for air density , ${\ displaystyle \ rho _ {1}}$ ${\ displaystyle \ rho _ {2}}$
${\ displaystyle p_ {1}, p_ {2}}$ ${\ displaystyle \ rho _ {1}}$ ${\ displaystyle \ rho _ {2}}$
${\ displaystyle P_ {1}, P_ {2}}$ ${\ displaystyle \ rho _ {1}}$ ${\ displaystyle \ rho _ {2}}$

The air density decreases with increasing altitude above sea level . If the device to be cooled is operated at a greater height, the modified fan characteristic must be taken into account.

Dimensionless representation of the characteristic

As already explained above, different conditions lead to different characteristics. Dimensionless representations can be used to compare characteristics despite different conditions or between different fans. The following agreement is made:

Instead of the volume flow, the flow rate is used
The pressure number is used instead of the pressure

This representation allows the comparison of characteristics despite different speed, density or impeller diameter.

Operating points and characteristics

The operating point of a fan is where the pressure increase due to the fan is equal to the pressure loss in the system, i.e. where the fan characteristic and the system characteristic intersect.

This "natural" operating point of the fan is implemented by the complete ventilation system and by all individual ventilation components during initial operation. The actual volume flow measured here has adapted to the actual pressure loss of the system and reflects the actual pressure loss in the fan characteristic.
The desired operating point of a fan, deviating from the "natural" (with incorrect system / fan dimensioning), can only be shifted by using more energy.

literature

Siegfried Harmsen: Device fans for electronics cooling. Die Bibliothek der Technik Vol. 45. Verlag moderne industrie 1991, ISBN 3-478-93048-0 . (New edition in 2002 by PAPST-MOTOREN GmbH & Co. KG)
Woods: Guide to ventilation technology, with the assistance of WC Osborne, CG Turner. Edited by Woods of Colchester, England. Orell Füssli Verlag, Zurich 1972, DNB 573609179 .
Bruno Eck: fans. Design and operation of centrifugal, axial and tangential fans. 6th edition. Springer-Verlag, 2003, ISBN 3-540-44058-5 .

Web links

Wiktionary: Fan characteristic curve - explanations of meanings, word origins, synonyms, translations

General aspects on fan selection and layout electronics-cooling.com, accessed January 2, 2015
Cooling electronics at high altitudes electronics-cooling.com, accessed January 2, 2015
All you need to know about fans electronics-cooling.com, accessed January 2, 2015
Establishing Cooling Requirements: Air Flow vs Pressure comairrotron.com, accessed January 2, 2015
Fans: formulas and data klimapartner-berlin.de, accessed on January 2, 2015

Individual evidence

↑ Energy-saving potential of centrifugal fans in ventilation and air conditioning units ( memento of the original from January 2, 2015 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 5.5MB) opus.ba-glauchau.de, accessed on January 2, 2015 @1@ 2

^ Operating point www.schweizer-fn.de , accessed on January 3, 2014

[1] Energy-saving potential of centrifugal fans in ventilation and air conditioning units ( memento of the original from January 2, 2015 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 5.5MB) opus.ba-glauchau.de, accessed on January 2, 2015 @1@ 2

[2] Operating point www.schweizer-fn.de , accessed on January 3, 2014