write a hypothesis about the effect of the fan speed

Fan Performance Characteristics at Various Rotational Speeds and Ambient Pressures 2014-01-2219

The scaling laws of fans express basic relationships among the variables of fan static pressure head, volume flow rate, air density, rotational speed, fan diameter, and power. These relationships make it possible to compare the performance of geometrically similar fans in dissimilar conditions. The fan laws were derived from dimensionless analysis of the equations for volumetric flow rate, static pressure head, and power as a function of fan diameter, air density and rotational speed. The purpose of this study is to characterize a fan's performance characteristics at various rotational speeds and ambient pressures. The experimental results are compared to the fan scaling laws.

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Write a hypothesis about the effect of the fan speed on the acceleration of the cart. Use the "if . . . then . . . because . . .” format and be sure to answer the lesson question: "How does an object's position and velocity change as the object accelerates?" If Intro

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9 Aug, 2019

Fan Laws and Fan Performance

This blog gives a general outline of the rules or laws which can be used to predict fan performance in a given system. Why are the fan laws important? As an example, let’s consider the fan curve typically provided by a manufacturer. This fan curve is usually measured at “standard” or other stated conditions. In real systems, it is unlikely that a fan will be spending its operating life at these identical conditions. Furthermore, suction pressure variations, density changes, composition changes, etc are common and can also affect how the fan will operate in the system. The fan laws help us estimate how a fan will operate in a system at different speeds, fluid density, impeller diameter, etc. Once we have a basic understanding of these laws, the performance of a fan can be calculated for various conditions. The performance of geometrically similar fans of different sizes or speeds can be predicted with reasonable accuracy for practical purposes using the fan laws. A higher level of accuracy would require the effects of say, the viscosity of the gas, surface roughness of the fan, scale effect also be considered. However, depending on the level of accuracy required, for many fan calculations, this may not be necessary. One point to note is that the laws apply to the same point of operation on the fan curve. They cannot be used to predict other points on the fan’s curve. The fundamental fan laws governing fan performance are generally only valid for a fixed system with no changes in airflow characteristics in the system or changes in aerodynamics. The term “system” refers to the combination of ducting, filters, grilles, dampers louvres, hoods, etc through which air is distributed.

As we know, the movement of air through a system causes friction/resistance between the air molecules and their surroundings and any other air molecules. Energy is therefore required to overcome this resistance. The faster the air moves through the system the greater the resistance imposed to flow and the more energy required to deliver the air through the system. This energy is described in terms of pressure. In general, the pressure required to overcome the resistance is referred to as static pressure. The pressure that results in the air/gas velocity is described as velocity pressure and the combination of these two values is often referred to as total pressure. Fans or blowers are often installed in the ventilation or industrial process systems to overcome the resistance. Fan performance is often represented in the form of fan curves. The curves are based on a specific set of conditions which typically include speed, volume, efficiency, static pressure and power required to drive the fan at the given set of conditions. Figure 1 provides a typical illustration of fan curves. The point where the static pressure curve intersects the system resistance curve represents the duty-point for the fan.

As noted earlier, as the airflow is increased in any fan system, the system resistance also increases. In a fixed system, it is said that the pressure required/system resistance varies with the square of the volume of air flowing through the system. The system resistance curve can be developed by determining the pressure required over a range of system flow rates. This resistance curve can then be plotted on the fan performance curve (also known as the fan capacity curve) to identify the actual duty-point. This is shown as point “1” in Figure 2 where the fan curve N1 and system resistance curve SC1 intersect. This duty-point is at airflow Q1 which is delivered against pressure P1.

Fans operate along a performance curve as provided by a manufacturer for a given fan speed. If we wish to reduce the air flow in the system, we could for instance partially close off a damper in the system or reduce the fan speed. Partially closing a damper will result in a new system resistance curve. This is shown as system resistance curve SC2 where the required pressure increases for any given air flow. The fan will now operate at duty-point 2 to provide the reduced air flow Q2 against the higher pressure P2.

On the other hand, we can reduce the fan speed from say N1 to N2 to reduce the airflow in the system and keep the damper in the fully open position. Under these conditions, the fan will now operate at duty-point 3 to provide the same airflow rate Q2 but at a lower pressure. Therefore, reducing the fan speed is a much more energy-efficient approach to reduce airflow since less power will be required resulting in less energy consumption.

Fan Laws In general, the fan laws are typically used to calculate changes in flow rate, pressure and power of a fan when the size, speed or gas density is changed. In the fan laws outlined in Table 1 below, the subscript 1 represents the initial existing condition and subscript 2 represents the desired calculated condition.

The fan laws are a group of equations used to determine the effects of changes in the fan operating speed, the fan diameter or the density of the air in the system. The performance of a centrifugal fan, axial fan or blower is often given as a series of pressure, efficiency and shaft power characteristic curves plotted against air flow rate for specified values of speed, air density, and fan dimensions. It is, therefore, useful to determine the operating characteristic of the fan at other speeds and air densities. Using the fan law relationships, families of fan curves can be developed for operating the fan at different speeds, etc. The fan laws can also be utilized to consider test results obtained from smaller prototype fans to predict the performance of larger fans which are of course geometrically similar. Knowing the performance of a given fan under set specified operating conditions, variations in the performance can be predicted according to the fan laws. It should be noted however that adding or removing components of a fixed system such as dampers or incurring density changes will create an entirely different system resistance curve. Also, it’s worth noting that changing fan accessories such as inlet dampers, inlet boxes will change the fan performance curve from the standard. This should, therefore, be considered before considering or applying the fan laws. As part of the system design, the fan laws can be quite useful in determining alternate performance criteria or in establishing a minimum and maximum range. In the event that “safety factors” have been applied to system design calculations, it is worth noting that, based on the fan laws, a 10% increase in volume will result in a 33% increase in power requirement. Due consideration is therefore recommended on evaluating any applied “safety factors” against the actual cost penalty incurred.

In general, using these rules or fan laws, once we know the performance of a given fan under set specified operating conditions, variations in the performance can be predicted with reasonable levels of accuracy. FluidFlow software considers the effects of compressibility, suction pressure variation, etc whilst also solving the fan laws ensuring a high level of accuracy. References:

• Bureau of Energy Efficiency.

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Theoretical and experimental aspects of saving energy in fans

• Original Paper
• Published: 08 August 2012
• Volume 9 , pages 729–736, ( 2012 )

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• N. Dizadji 1 ,
• A. M. Mahmoudkhani 1 &
• N. Nouri 1  

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This study presents energy efficiency measures in fans as an important energy consumption facility in the industry and common usages by identifying the sources of energy loss and applying methods to reduce those losses, which are one of the critical issues in protecting the environment and in global warming. The carbon footprint can be lowered by reducing the energy consumption of a fan over its life cycle. The main sources of energy loss in fans such as noise, vibration, lubrication, temperature of the bearings, installation type, damper and filter, especially pulley and belt system compared to electrical variable speed drive, are theoretically and experimentally discussed. The laboratory results show that the mechanical variable speed drive is one of the critical sources of energy loss in centrifugal fans. The results also show that by changing the drive with an electrical variable speed drive, the energy usage can be substantially optimized. For instance, using an electrical variable speed drive has reduced the energy loss up to 38.5 % with regard to the speed and according to the different flow rates. Moreover, based on the results derived from the equations and figures, it can be concluded that a considerable amount of energy per year, as well as the related cost can be saved and this shall be noted particularly in industrial applications.

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Acknowledgments

The authors would like to thank Islamic Azad University; Science and Research Branch for their cooperation to do the experiment in Department of Mechanical Engineering’s Fluid Mechanics Laboratory.

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N. Dizadji, A. M. Mahmoudkhani & N. Nouri

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Dizadji, N., Mahmoudkhani, A.M. & Nouri, N. Theoretical and experimental aspects of saving energy in fans. Int. J. Environ. Sci. Technol. 9 , 729–736 (2012). https://doi.org/10.1007/s13762-012-0103-1

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Received : 13 June 2011

Revised : 10 November 2011

Accepted : 02 February 2012

Published : 08 August 2012

Issue Date : October 2012

DOI : https://doi.org/10.1007/s13762-012-0103-1

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Understanding and Applying the 3 Basic Fan Laws

When we are working on ventilation system redesign applications, the 3 Basic Fan Laws provide us the means by which we can correlate the relationship between fan air flow rate, static pressure, speed and horsepower.   They are useful to predict the outcomes when we want to change a known fan performance to a desired fan performance.   In this article, I’ll explain each of the fan laws, their limitations and how we would use them in a ventilation system redesign application.

3 Basic Fan Laws

Before explaining each law, I want to talk about their limitations.   These laws are only applicable when we are not changing the configuration of the fan propeller (diameter, number of blades, blade pitch angle, and hub size) and the geometry of the fan inlet or outlet.   Think of the limitations as constants in a science experiment where we would hold some variables constant and change only one variable to observe the changes in outcomes. 

Here are brief explanations of each law:

Fan Law 1 tells us that the change in air flow rate of a fan is proportional to the change in speed of the propeller.   If the propeller speed is increased by 10%, the air flow rate will also increase by 10%. Fan Law 2 tells us that the change in total static pressure of the ventilation system will increase by the square of the change in propeller speed of the fan.If the propeller speed is increased by 10%, the total static pressure will increase 21%. Fan Law 3 tells us that the change in horsepower required by the fan to turn the propeller will increase by the cube of the change in propeller speed of the fan.   If the propeller speed is increased by 10%, the horsepower required to turn the propeller will increase 33.1%.

Applications

We are most likely to use the 3 Basic Fan laws for a ventilation system redesign where the customer desires more air flow but doesn’t want to increase the size of the fan.   This happens often in process ventilation system redesigns where the cost of changing the ductwork limits the options for increased air flow to changing the propeller speed.   But the laws can also apply to a general ventilation redesign application.   An example of a situation we have encountered:    

• Customer desired more air flow from their belt drive exhaust fans. 
• Their exhaust fans were moving 15,000 CFM of air each with a propeller speed of 1000 RPM.
• The exhaust fan motors were rated at 1750 RPM, 5 HP but were only using peak HP of 3.75.
• The air supply to the building was through louvers with a static pressure of .15 in WG.
• The customer didn’t want to increase the negative static pressure in the building.
• They asked us:   How much air flow can each fan provide without going into the motor safety factor if they were only to change the sheave sizes in each fan?

We started with Fan Law 3 and solved for RPM 2 :

RPM 2 = (HP 2 /HP 1 ) 1/3 x RPM 1

RPM 2 = (5.0/3.75) 1/3 x 1000

RPM 2 = 1101    

Then we applied Fan Law 1 to determine the new air flow from each exhaust fan:

CFM 2 = CFM 1 x (RPM 2 /RPM 1 )

CFM 2 = 15,000 x (1101/1000)

CFM 2 = 16,515

Fan Law 2 told us that the impact of additional air flow into the customer’s building would result in an increase in static pressure:

SP 2 = SP 1 x (RPM 2 /RPM 1 ) 2

SP 2 = .15 x (1101/1000) 2

Having correctly applied the 3 Basic Fan Laws, we were able to confidently tell the customer that they could expect to get a maximum of 16,515 CFM from their belt drive fans by changing only the sheave sizes.   However, we also needed to tell the customer that they would need to increase the area of their supply louvers in order keep the static pressure below their desired level.

Even though in this situation the customer decided to go a different route, our expertise in knowing how to apply the 3 Basic Fan Laws gave the customer valuable information to use in deciding how much money needed to be spent to redesign their ventilation system.

If you are in need of a ventilation system redesign, whether it as simple as changing sheaves or as complex as adding more fans, Eldridge can provide you with the options that best fit your financial situation.

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Weathering the Windchill: How Does Wind Speed Affect How Quickly an Object Cools?

David Whyte, PhD, Science Buddies

Use an anemometer and an infrared thermometer to determine how wind speed affects the rate of cooling of an object.

Introduction

The windchill factor describes what happens to an object (like your body) when it is cold and windy outside. As wind increases, heat is carried away from the body at a faster rate, driving down both skin temperature (which can cause frostbite) and eventually the internal body temperature (which, in extreme cases, can lead to death). Windchill charts are useful to help predict when a person is most in danger of frostbite, which damages skin and other tissues due to extreme cold. The chart in Figure 1, below, shows how wind speed affects the time it takes to develop frostbite at various temperatures. On the left side of the chart, the temperatures are relatively high, meaning there is no danger of frostbite (light blue). As it gets colder, the danger of frostbite increases. If you are outside when the temperature is 0°F and the wind is blowing at 30 miles per hour (mph), you could develop frostbite within 30 minutes on exposed parts of your body (usually your fingers and face). If it is minus 25°F and the wind is blowing at 40 mph (this is in the purple part of the chart), you could get frostbite in as little as 5 minutes!

Figure 1. Windchill chart. (Wikipedia, 2009.)

There are two ways that the wind cools things off. The first way is by simply blowing away the warm air that is next to your skin and replacing it with cold air. This is called convective cooling. Another way it cools is by evaporating the moisture on your skin. As the moisture evaporates, it causes the surface to cool. This is called evaporative cooling. Both kinds of cooling occur on human (and animal) skin that is exposed to cold wind.

For an inanimate object that has a dry surface, only convective cooling occurs. In convective cooling, the object can only be cooled to the temperature of its surroundings. For example, if you put a cup of hot coffee on a table, it will eventually cool to room temperature. If you blow on it with a fan, it will cool faster, but it will only cool to room temperature and no further. What happens if you spray the cup of hot coffee with a mist of water while you are blowing on it with a fan? It is then being cooled by both convective and evaporative means; thus, it will cool faster than the dry cup and will also cool to a temperature a little below room temperature.

In this science fair project, you will measure how wind speed affects the rate of cooling of an object. The speed of the wind will be measured using a handheld device called an anemometer. The anemometer measures wind speed by measuring how fast the wind makes a fan blade turn. As the wind blows, it spins the fan blades and a tiny generator to which the fan blades are attached. The generator subsequently produces a voltage that is proportional to the speed of the wind, measured by an electronic circuit that gives an instant readout of the wind speed on a digital display. The surface temperature of the object will be measured using an infrared thermometer.

Terms and Concepts

• Convective cooling
• Evaporative cooling
• In what areas of the world are you most likely to get frostbite?
• Based on your research, what are some different ways scientists have developed to measure wind speed?

Bibliography

• National Oceanic and Atmospheric Administration (NOAA). (n.d). Wind Chill Chart . Retrieved April 14, 2018.
• HowStuffWorks, Inc. (2009). How does the windchill factor work? . Retrieved July 15, 2009.

For help creating graphs, try this website:

• National Center for Education Statistics. (n.d.). Create a Graph . Retrieved June 2, 2009.

Materials and Equipment

• Handheld anemometer; available online from Amazon.com
• Infrared thermometer; available online from Amazon.com
• Large electric fan, 3-speed
• Tape measure
• Masking tape
• Washable marker
• Lab notebook
• Graph paper
• Coffee cups (identical), with handles (2)
• Liquid measuring cup
• Saucers that match the coffee cups (4)
• Microwave or stovetop to heat water
• Stool, or other object about 1 meter (m) high to hold the coffee cup

Disclaimer: Science Buddies participates in affiliate programs with Home Science Tools , Amazon.com , Carolina Biological , and Jameco Electronics . Proceeds from the affiliate programs help support Science Buddies, a 501(c)(3) public charity, and keep our resources free for everyone. Our top priority is student learning. If you have any comments (positive or negative) related to purchases you've made for science projects from recommendations on our site, please let us know. Write to us at [email protected] .

Experimental Procedure

Note: Prior to testing, you should read the instructions for the anemometer and the infrared thermometer. Practice using both of them.

Setting Up the Test Area

• Place the fan in a room with at least 3 meters (m) of open space in front of the fan.
• Point the fan in the horizontal direction you will use for the experiments.
• If you are not able to adjust the height to 1 m, use the available height and modify the procedure accordingly.
• Make a 3-m line on the floor with masking tape, starting at and leading away from the fan.
• For the procedure below, you will just use the mark at 1 m. Use the other markings if you choose to collect more data.

Measuring the Wind Speed

• Turn the fan on at its lowest setting.
• Use meters per second (m/s) as the units for wind speed.
• Measure the wind speed at the height of the top of the stool.
• Repeat step 1 of this section for the fan's middle and top speeds.
• Make a data table that contains the fan settings and corresponding wind speeds at 1 m from the fan.
• Graph the data, with the fan setting on the x-axis and the wind speed on the y-axis.

Measuring How the Wind Affects the Cooling Rate of a Cup of Coffee

Note: It may take several tries to get this procedure to go smoothly. A helper can read the temperatures as you record the data.

• Label the two cups "1" and "2" using the masking tape and marker.
• The stool should be 1 m away from the fan.
• Fill two identical coffee cups with equal volumes of water (200 mL, for example) and microwave them until they are boiling.
• Take the cups out of the microwave and place them on saucers.
• Place another saucer on top of each of the cups to prevent evaporation.
• Start the timer.
• Use the infrared thermometer to read the temperature of the side of the cup.
• Read the temperature on the side of the cup opposite from the handle.
• Try to be consistent in the angle and distance used to take readings with the thermometer.
• Record the time that you took the temperatures in your lab notebook.
• The starting temperatures of the cups should be the same.
• Place coffee cup #2 on a saucer on a surface that is away from any drafts.
• Place coffee cup #1 on a saucer on the stool.
• The handle of the cup should face away from the fan.
• Turn the fan to its lowest setting.
• Read the temperature on the side opposite the handle.
• Feel free to modify the times that you take the temperatures.
• Repeat steps 3–13 of this section two more times. This enables you to show that your results are repeatable.
• Repeat steps 3–13 of this section with the fan set to medium and high speed.
• Graph the data.
• Calculate the average temperatures for each time point.
• Label the graphs with the fan setting (low, medium, and high) and the wind speed (in meters per second).
• Put the time on the x-axis and the average temperature of cups 1 and 2 on the y-axis.
• How does wind speed affect the rate of cooling?
• Graph time on the x-axis and the average temperature on the y-axis.
• Compare the rates of cooling.

Ask an Expert

Global connections.

The United Nations Sustainable Development Goals (UNSDGs) are a blueprint to achieve a better and more sustainable future for all.

• Measure how evaporation affects the rate of cooling. Repeat the procedure above, but add a third cup that is placed in the wind and is sprayed with water mist. Hint: Don't forget to add a control cup of hot water that is sprayed with water, but not placed in the wind.
• Pick a temperature (for example, 60°C) and note how long it took to reach this temperature for cups 1 and 2 in the different wind speeds. Graph the wind speed on the x-axis (the x-axis will have cups 1 and 2 at the lowest speed, cups 1 and 2 at the medium speed, and cups 1 and 2 at the high speed), and the time to time it took to reach this temperature on the y-axis. Discuss the results.
• Devise windchill experiments that are performed at cold temperatures. For example, place cups of room-temperature water in a refrigerator and blow wind on one of them with a battery-powered fan. Graph time on the x-axis and temperature on the y-axis.
• Build your own anemometer, as shown in the Science Buddies project How Does a Wind Meter Work? and use it to do this science fair project.

If you like this project, you might enjoy exploring these related careers:

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The fan effect: a tale of two theories

Affiliation.

• 1 Department of Psychology, University of Notre Dame, Indiana 46556, USA. [email protected]
• PMID: 10406105
• DOI: 10.1037//0096-3445.128.2.198

This article addresses J. R. Anderson and L. M. Reder's (1999) account of the differential fan effect reported by G. A. Radvansky, D. H. Spieler, and R. T. Zacks (1993). The differential fan effect is the finding of greater interference with an increased number of associations under some conditions, but not others, in a within-subjects mixed-list recognition test. Anderson and Reder concluded that the differential fan effects can be adequately explained by assuming differences in the weights given to concepts in long-term memory. When a broader range of data is considered, this account is less well supported. Instead, it is better to assume that the organization of information into referential representations, such as situation models, has a meaningful influence on long-term memory retrieval.

Publication types

• Research Support, U.S. Gov't, P.H.S.
• Association Learning / classification*
• Memory / classification*
• Models, Psychological*

Grants and funding

• 1RO3 MH56315-01/MH/NIMH NIH HHS/United States

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