For detailed engineering - use manufacturers specifications for actual fans. The fan efficiency is the ratio between power transferred to airflow and the power used by the fan. The fan efficiency is in general independent of the air density and can be expressed as:.
Near all of the energy lost in a fan will heat up the air flow and the temperature increase can be expressed like. Add standard and customized parametric components - like flange beams, lumbers, piping, stairs and more - to your Sketchup model with the Engineering ToolBox - SketchUp Extension - enabled for use with the amazing, fun and free SketchUp Make and SketchUp Pro.
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Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It only takes a minute to sign up. What other parameters does it depend on? Assume I have all the fan properties, such as input power, number of blades, blade area, blade length and so on.
Also, assume the atmospheric pressure is known. Bear in mind this is an approximation. Hopefully it does help you reduce the fan noise in your system. There isn't a simple formula for fan noise, but the physics can be worked out from the fundamental equations of fluid mechanics and acoustics.
It isn't a simple problem however. The noise created by fans is complex and from several fundamental sources, and its amplitude depends on frequency. Here is an example of a fan noise frequency spectrum for a cooling fan.
That site also goes into a bit more detail about noise sources than is possible here, and has some references for further research. As can be seen in the diagram, there are several kinds of noise created by rotating fans.
These include narrow band noise at the fundamental frequency of fan blade rotation, and noise created by harmonics of that fundamental blade frequency.
This is fundamentally caused by the oscillating pressure field produced by the motion of the fan. There is also low and higher frequency noise created by boundary layer and turbulence effects at the fan's blades, and from the rotation of the fan shaft and motor. As discussed at the Wikibooks site, they produced the above diagram experimentally, and that is probably the best approach with the specific fan you're trying to model - capture the power spectrum experimentally and fit some empirical curve to it.
Solving the problem analytically or computationally is not simple. It can be done but computational fluid dynamics is an expensive problem both in modeling and computing effort.
Sign up to join this community. The best answers are voted up and rise to the top. Home Questions Tags Users Unanswered. Asked 4 years, 9 months ago. Active 1 year, 10 months ago. Viewed 4k times. Is there a simple formula for it? Alexandru Nedelcu Alexandru Nedelcu 27 1 1 silver badge 2 2 bronze badges. Which laws do you think I could use in this case? It's like -- "Oh, I'm feeling extra curious today.The fan curve is graph depicting the various points that the fan can operate.
It indicates the amount of CFM the fan will provide at a given total static pressure, which is dependent on the connected ducted system. Fans should be selected to operate at the stable region.
The stable region is the area on the fan curve where there is a single flow rate [CFM] value for ever pressure value. In the unstable region, a pressure value can have multiple CFM values, which will cause the fan system to surge. The stable region also has very little change in CFM for large changes in total pressure. The second curve that works in conjunction with the fan curve is the system resistance curve.
This curve is summation of all the friction losses in the ducting system at varying CFM's. Typically, the friction losses are summed up at the design CFM values, then this design point is connected to the 0,0 point by an upward sloping square polynomial curve, as shown below. If for example, the ducting system has a closed damper or dirty filter, this will cause the curve to shift to the left.
If a damper is opened or the dirty filter is cleaned then the curve will shift to the right. Combining the system curve with the selected fan curve, determines the operating point of the fan system, indicated in the figure below in green. Following the vertical line down determines the CFM and the horizontal line from that point indicates the operating total pressure. During system operation as dampers close, the system curve shifts toward the right in red.
This movement decreases the amount of CFM delivered by the fan. The opposite occurs as dampers open in the system, the amount of CFM delivered by the fan increases. It has been shown that the amount of CFM blown by a fan can be changed by shifting the system resistance curve.
However, the volumetric flow rate can also be changed by changing the speed of the fan, which shifts the fan curve. Exam Topics. Mechanical P. Feel free to email your questions to: contact engproguides.Fanno flow is the adiabatic flow through a constant area duct where the effect of friction is considered. For this model, the duct area remains constant, the flow is assumed to be steady and one-dimensional, and no mass is added within the duct. The Fanno flow model is considered an irreversible process due to viscous effects.
The viscous friction causes the flow properties to change along the duct. The frictional effect is modeled as a shear stress at the wall acting on the fluid with uniform properties over any cross section of the duct. For a flow with an upstream Mach number greater than 1. On the other hand, for a flow with an upstream Mach number less than 1. Fanno flow is named after Gino Girolamo Fanno.
Assuming the Fanning friction factor is a constant along the duct wall, the differential equation can be solved easily. The left-hand side is often called the Fanno parameter.
Equally important to the Fanno flow model is the dimensionless ratio of the change in entropy over the heat capacity at constant pressure, c p. The above equation can be rewritten in terms of a static to stagnation temperature ratio, which, for a calorically perfect gas, is equal to the dimensionless enthalpy ratio, H :. According to the Second law of thermodynamicsentropy must always increase for Fanno flow.
This means that a subsonic flow entering a duct with friction will have an increase in its Mach number until the flow is choked.
Fan laws: terms and equations
Conversely, the Mach number of a supersonic flow will decrease until the flow is choked. Each point on the Fanno line corresponds with a different Mach number, and the movement to choked flow is shown in the diagram.
The Fanno line defines the possible states for a gas when the mass flow rate and total enthalpy are held constant, but the momentum varies. Each point on the Fanno line will have a different momentum value, and the change in momentum is attributable to the effects of friction. As was stated earlier, the area and mass flow rate in the duct are held constant for Fanno flow.
Additionally, the stagnation temperature remains constant. A stagnation property contains a 0 subscript. Differential equations can also be developed and solved to describe Fanno flow property ratios with respect to the values at the choking location. The ratios for the pressure, density, temperature, velocity and stagnation pressure are shown below, respectively. They are represented graphically along with the Fanno parameter. The Fanno flow model is often used in the design and analysis of nozzles.
In a nozzle, the converging or diverging area is modeled with isentropic flow, while the constant area section afterwards is modeled with Fanno flow.
For given upstream conditions at point 1 as shown in Figures 3 and 4, calculations can be made to determine the nozzle exit Mach number and the location of a normal shock in the constant area duct. Point 3 labels the end of the nozzle where the flow transitions from isentropic to Fanno.
With a high enough initial pressure, supersonic flow can be maintained through the constant area duct, similar to the desired performance of a blowdown-type supersonic wind tunnel. However, these figures show the shock wave before it has moved entirely through the duct.
The movement in Figure 4 is always from the left to the right in order to satisfy the second law of thermodynamics. The Fanno flow model is also used extensively with the Rayleigh flow model. These two models intersect at points on the enthalpy-entropy and Mach number-entropy diagrams, which is meaningful for many applications.
However, the entropy values for each model are not equal at the sonic state. If initial values of s i and M i are defined, a new equation for dimensionless entropy versus Mach number can be defined for each model. These equations are shown below for Fanno and Rayleigh flow, respectively. The intersection points are calculated by equating the new dimensionless entropy equations with each other, resulting in the relation below. The intersection points occur at the given initial Mach number and its post- normal shock value.What is a Fan Curve?
It is a characteristics curve, which shows the relationship between static pressure and the air flow rate of the fan. The fan manufacturers perform actual test to find out the static pressure value of the fan for the different air flow rates to plot the fan curve. Please note from the fan curve that, as the flow rate increases the static pressure decreases, means at higher flow rate the fan will be able to withstand lower system resistance or back pressure. I will write something next on how to draw a system curve.
I am not sure to well understanding the differences between fan curve and system curve. On the other hand,in fan curve,as the flow increases,the pressure decreases. Looking forward to hearing from you at the earliest. Best Regards Jon. You see for a fixed amount of input power for a fan, the pressure will decrease with the increase of flow rate. Now, for a system lets say a air tight compartmentas flow of air increases i.
Hope it clears. This site uses Akismet to reduce spam. Learn how your comment data is processed. Skip to content. Author Recent Posts. Shibashis Ghosh. Hi, I am Shibashis, a blogger by passion and engineer by profession. I have written most of the articles for mechGuru. Disclaimer: I work for Altair. Although i have tried to put my neutral opinion while writing about different competitor's technologies, still i would like you to read the articles by keeping my background in mind.
Latest posts by Shibashis Ghosh see all. Leave a Reply Cancel reply. Sorry, your blog cannot share posts by email.They apply to pumpsfansand hydraulic turbines. In these rotary implements, the affinity laws apply both to centrifugal and axial flows.
The affinity laws are useful as they allow prediction of the head discharge characteristic of a pump or fan from a known characteristic measured at a different speed or impeller diameter. The only requirement is that the two pumps or fans are dynamically similar, that is the ratios of the fluid forced are the same. It is also required that the two impellers' speed or diameter are running at the same efficiency.
How to Read a Fan Curve
Law 1a. Flow is proportional to shaft speed: . Law 2. With shaft speed N held constant geometrically scaling the entire pump : . Law 2c. Power is proportional to the fifth power of the impeller diameter assuming constant shaft speed :. The exact relationship between speed, diameter, and efficiency depends on the particulars of the individual fan or pump design. Product testing or computational fluid dynamics become necessary if the range of acceptability is unknown, or if a high level of accuracy is required in the calculation.
Interpolation from accurate data is also more accurate than the affinity laws. When applied to pumps the laws work well for constant diameter variable speed case Law 1 but are less accurate for constant speed variable impeller diameter case Law 2. For radial flow centrifugal pumps, it is common industry practice to reduce the impeller diameter by "trimming", whereby the outer diameter of a particular impeller is reduced by machining to alter the performance of the pump.
In this particular industry it is also common to refer to the mathematical approximations that relate the volumetric flow rate, trimmed impeller diameter, shaft rotational speed, developed head, and power as the "affinity laws".
Because trimming an impeller changes the fundamental shape of the impeller increasing the specific speedthe relationships shown in Law 2 cannot be utilized in this scenario. In this case the industry looks to the following relationships, which is a better approximation of these variables when dealing with impeller trimming.
With shaft speed N held constant and for small variations in impeller diameter via trimming: . The volumetric flow rate varies directly with the trimmed impeller diameter: . The pump developed head the total dynamic head varies to the square of the trimmed impeller diameter: . The power varies to the cube of the trimmed impeller diameter: . To learn how these forces combine, e. From Wikipedia, the free encyclopedia. This article's factual accuracy is disputed.
The system head visualized in the System Curve above is a function of elevation - or the static head and the major and minor losses in the system and can be expressed as:.
Increasing the constant - k - by closing some valves, reducing the pipe size or similar - will increase the head loss and move the system curve upwards. The starting point for the curve - at no flow, will be the same. The pump characteristic is normally described graphically by the manufacturer as the pump performance curve.
The pump performance curve describes the relation between the flowrate and the head for the actual pump.
Other important information for a proper pump selection is also included - like efficiency curvesNPSH r curvepump curves for several impeller diameters and different speedsand power consumption. Increasing the impeller diameter or speed increases the head and flow rate capacity - and the pump curve moves upwards. The head capacity can be increased by connecting two or more pumps in seriesor the flow rate capacity can be increased by connecting two or more pumps in parallel.
The proper pump can be selected by combining the System Curve and the Pump Curve :. Special considerations should be taken for applications where system conditions frequently changes during operation - like applications for heating and air conditioning systems or water supply systems with variable consumption and modulating valves.
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System Curve and Pump Performance Curve Utilize the system curve and the pump performance curve to select the proper pump for a particular application Sponsored Links. Pump Performance Curve The pump characteristic is normally described graphically by the manufacturer as the pump performance curve. Selection of Pump The proper pump can be selected by combining the System Curve and the Pump Curve : The operating point is where the system curve and the actual pump curve intersects.
Carry Out When a pumps operates in the far right of its curve with poor efficiency - the pumps Carry Out.Titus Timeout Podcast - What is a Fan Curve
Shutoff Head The Shutoff Head is the head produced when the pump operates with fluid but with no flow rate. Churn A pump is in Churn when it operates at shutoff head or no flow. Tag Search en: pump system curve es: curva del sistema de bomba de: Pumpensystem Kurve. Privacy We don't collect information from our users.
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