ERP is a European Union (EU) for Energy-Related Products. Oriental Motor offers an extensive product line-up of about 50,000 different products that provide the optimal motion system. pressure drop across a media filter. However, in part-load conditions (for example, less than 50% of maximum load) these efficiencies can be more similar. At that temperature and the standard acceleration of gravity, 1 inH2O is approximately 249.082 pascals. The number of components inside an enclosure determines the "installation density". Static Pressure - Sum of all the system resis-tances against which the fan must work, expressed in inches of H2O. With more than 50,000 members from over 130+ nations, ASHRAE is a diverse organization dedicated to advancing the arts and sciences of heating, ventilation, air conditioning and refrigeration to serve humanity and promote a sustainable world. In this scenario, the airflow flowing into the enclosure causes the static pressure to rise since there's no place for the air to escape. %%EOF In this post, I will be discussing the definitions of air flow vs static pressure, the relationship between them, and the importance of impedance. 63, no. Lets put that in the formula: : A 10% increase in CFM will result in a 21% increase in static pressure. If you have an iPhone, subscribe to the podcast HERE, and if you have an Android phone, subscribe HERE. WebFan efficiency = 11.76 / 48 = 0.245 or 24.5 % The Fan Efficiency Curve Fan efficiency varies dramatically as a function of aerodynamic loading. Based on Bernoullis equation, the concept of total pressure and dynamic pressure is important in understanding the behavior of fluids. But the smaller model requires less BHP (less wattage to run) and no doubt costs less. Secondly, the FEG value is determined by the fan peak efficiency. 0000064147 00000 n WebStatic Pressure: The absolute pressure at a point minus the reference atmospheric pressure. 0000008520 00000 n [1], they indicated the combined effect of the adverse pressure gradient and second flow leads to the corner separation.Denton [2] summarized the mechanism of corner separation: The low-energy fluid accumulated in the Learn more about the 4th Annual HVACR Training Symposium or buy a virtual ticket today at https://hvacrschool.com/symposium. Youll need to step up to a 1/2 HP motor. 0000008969 00000 n hbbd``b`>$$DAb ;D>`9 $X@ BHd$|@b F .X^deHH6 $J^Yt@ 3o endstream endobj 69 0 obj <>stream GDY. So if you need to increase CFM by 10%, your RPM has to increase by 10%. |4ExH\}UZ&2e)-t 2a{?/\&(R{vwDsA3Sd4;q+fE5q\!Yd439)FF>|UTW\WQ]uU_~iPG#D|fujY& 5hY& k;K [8Fq4c%F'~ , Yes it is a legal requirement that they do comply for any fan to be put into use within the EU each fan when selected, and prior to quoting, is checked using our own in house fan tests along with published motor efficiencies and drive losses to calculate the overall efficiency and compare it with the target efficiency published in EU 327/2011.

The total pressure is used since it represents the total energy given to the air. ?_t=transmission drive efficiency=0.96 (clause 3.2 annexII) Efficiency is a buzzword in todays economy. Motor horsepower requirement for a given fan stays the same. WebStatic Efficiency - Fan efficiency based on static pressure and fan brake horsepower at the same density. Fan total efficiency is calculated using total pressure. In this case, the air flow of a fan is measured in cubic meters per minute (m/min) in metric units, or cubic feet per minute (CFM) in imperial units. I was a young tech, proving my salt out there in the big world of commercial RTU maintenance. As a result, the velocity pressure exiting a fan can only be used in ducts, and on non-ducted fans, it is lost. Sometimes, it can be difficult to determine system impedance.

HAVEN started off with its central air monitor, and the company then developed a central air controller to help customers make their homes healthier. Please read Google Privacy & Terms for more information about how you can control adserving and the information collected. Fans where the work per unit mass exceeds 25 kJ/kg. All Rights Reserved. Aerodynamic loading has a dramatic effect on fan efficiency. and static efficiency sould be etas = Ns/ Noutp, but looking in a data at selection pages of your program, this data doesn't give adequate results. 0000004056 00000 n

In absence of specific information regarding variable speed drives, if fitted, an efficiency of 98% will be applied along with a part load compensation factor as defined in clause 3.2 of Annex II of EU327/2011.

What happens if your motor is only a 1/3 HP (0.333)? You also have the option to opt-out of these cookies. %%EOF

Cooling Fans, In this case, the static pressure is measured in Pascals (Pa), or inches of water (inH2O). Evaluation of mechanical draft fans based on efficiency may not offer the desired benefit to the clients.

This has a dramatic effect on calculated efficiency and power input for some mechanical fan applications. Ps, the fan can be slowed down to accommodate this condition. [3]. Youll need to step up to a 1/2 HP motor. When high-efficiency fans are used, they have a cascade effect on the design of the system. FEG63 fans could consume less power than FEG71 fans at a given point of operation (CFM and pressure)

Static pressure is the amount of air pressure that can be produced by the fan in an enclosure. Mechanical efficiency accounts for mechanical losses in the bearing, coupling, and seals in a fan system. Another aspect is that the fan size determines its efficiency. Oriental Motor's sales and service network is international, with offices throughout North America, Europe and Asia. 0000003570 00000 n This equation can also be used for calculating hydrodynamic horsepower in a ducted flow. The size of the exhaust hole is smaller than the air intake hole, which hinders air flow. Therefore, minimizing input power to the fan will offer direct economic benefit to the plants. In a practical example of ventilating an electronics enclosure, most fans would perform close to example 2). AddThis use cookies for handling links to social media.

ASHRAE Members have free access to the full-text PDF of this article as well as the complete ASHRAE Journal archives back to 1997 in the Free Member Access Area. 0000003459 00000 n Decoding efficiency for mechanical draft fans, By Nurul Moni Talukder, PE, ProcessBarron, By using this website, you agree to our use of cookies. This is missing the kinetic energy; the fan has taken stationary air and given it kinetic energy. There is a greater potential to operate more efficiently with fans with higher peak efficiencies.

Fans used in HVAC systems are among the largest consumers of electrical power, consuming more than 22% of the all the electricity used in the European Union. The actual air flow and static pressure is determined by impedance. How does it impact your decisions and operations? It is critical for an HVAC technician to understand airflow and how blowers and fans perform under various conditions. Apply it in the field: If your blower is moving 1000 CFM at 1100 RPM, and you need to decrease airflow by 10% to 900 CFM, Fan Law 1 says your RPM must decrease by 10% also. For most forced air cooling application, impedance is calculated by the "square law," which means that static pressure changes as a square function of changes in the CFM. You need to know how much HP it is using now and what your new HP will be when you increase airflow to 1200 CFM. The power used by the fan can be expressed as: P = dp q / f (3). Out of these, the cookies that are categorized as necessary are stored on your browser as they are essential for the working of basic functionalities of the website. Lets demonstrate that. The figure below shows the different fan performance curves in terms of airflow. CFM represents fan flow rate (it can also be denoted by Q) in ft/m3, p is static pressure in inches of water (in. [fa icon="phone"] 1-800-GO-VEXTA (468-3982), [fa icon="home"] 570 Alaska Avenue, Torrance CA 90503, Fan Basics: Air Flow, Static Pressure, and Impedance, O). fan has a greater total efficiency (66% vs. 60%) and a higher FEG (71 vs. 63). It [], First, let's state the obvious and clear the air a bit. Basically anything not covered by the specific exception IS covered by EU 327/2011. So, this fan law can be applied to total static pressure or a static pressure drop across a single component in the system. When I do (2000*1.547) / 1.1, I get: 2812.73 as my efficiency, which I know is not right.



How are the actual air flow and static pressure requirements determined? Inches of water (inH2O) is defined as the pressure exerted by a column of water of 1 inch in height at defined conditions. to calculate HP1 in the Fan Law Formula. Fitted with a IE3 11 kW 4 pole electric motor belt driven and variable speed drive (vsd). 154 0 obj <>stream The fan efficiency is in general independent of the air density and can be expressed as: f = dp q / P (2), f = fan efficiency (values between 0 - 1), q = air volume delivered by the fan (m3/s). C_c=Part load compensation factor= 1.04 [fa icon="calendar"] Oct 25, 2019 9:00:00 AM / by Secondly, the FEG value is determined by the fans peak efficiency. The value of total pressure is constant along any streamline. Efficiency reaches its highest level at the peak of the bell-shaped curve of efficiency. Some manufacturers add a brake horsepower (BHP) curve to this chart to show how much power is required to do the work we are asking the fan to do at a given RPM and SP. Topics: If we need to change the airflow, we change fan speedby changing a speed tap, VFD output, pulley diameter, or other means. OK now that we understand air flow and static pressure by using an electronics enclosure as an example, let's make it more realistic. Any cookies that may not be particularly necessary for the website to function and is used specifically to collect user personal data via analytics, ads, other embedded contents are termed as non-necessary cookies. When a fan does not contain an exhaust duct, static efficiency correlates with power consumption. However, in order to produce the graph, a similar method is used (also known as the double chamber method). What will be the new pressure drop? Geoff Edwards, the technical product engineer at Axair Fans UK Limited, explains the three basic fan laws when applied to warehouse ventilation studies. WebFan total efficiency is the product of air- flow times fan total pressure divided by the fan shaft power requirement. 94 0 obj <>stream Air-handling units (AHUs) are an integral part of almost all residential, commercial and industrial HVAC systems.

We do this regardless of actual installation of the fan which could be B,C or D because the vast majority of fans we supplied with a contractual static pressure requirement. Air proving switches must be adjusted so that they do their job at the new CFM and static pressure. What it means: A 10% increase in airflow results in a 33% increase in horsepower required to do that work. The intellectual knowledge base about the power and efficiency is bound to help engineers to properly specify a fan and manufacturers to optimize and design a better fan. A mechanical draft fans efficiency is dependent on the system pressure used in each calculation. Inches of water (inH. In general, outlet velocity pressure does not influence a fans capability to dissipate heat energy from a system, so it is not considered in the efficiency calculation. wg Pt = Total pressure, in. Condensate traps that are already close to their limit may have to be made deeper so that they dont get overwhelmed. Commonly known as ATEX fans, Fans designed for operation with high temperature air above 100OC, Where ambient gas temperatures exceed 65OC or fall below -40OC, Where the supply voltage to the drive motor >1000 V AC or 1500 V DC, Fans for use with toxic, corrosive or flammable gasses or subject to abrasive substances. I was on the roof of [], As a leader, its healthy to take a step back and see if youve become That Boss. You know, the one who nobody wants to work for and who doesn't have a shred of self-awareness.

The study discussed in this article compares four different fan-motor-controller systems as part of a strategy for improving wire-to-air efficiency.

These relationships are expressed in the three fan laws, which are mathematical formulas. As a rule of thumb - the temperature increase in a fan transporting air is approximately 1 o C. Example - Hydraulic Efficiency for a Pump The air flow will be less than its maximum value. This seems to be NOT the way the BHP is being calculated by the spreadsheet that was attached previously. The photo above is SUPER CHEESY! First and foremost, the FEG takes into account both fan total efficiency and fan total pressure. Note that compliance with EU 327/2011 is based on calculation, as we manufacture bespoke one off units individual fan tests are not possible. The unit is named after Blaise Pascal and is defined as one newton per square meter. You should consider contributing to our CFE Media editorial team and getting the recognition you and your company deserve. Motion Control Basics. Now, what happens to HP2 when we increase the airflow from 1000 to 1200 CFM? But opting out of some of these cookies may affect your browsing experience. If the System Static Pressure is 300 Pa, the Overall Static Efficiency of the fan must increase to at least 45%, to offset increased losses in the design of the ductwork. The power required to drive mechanical draft fans is viewed as parasitic load. If we can oversize ducts, coils and filter banks then we can reduce system static pressure, and therefore fan energy. Calculating the total efficiency requires using airflow, pressure, and input power. Once an operating point can be plotted on a fan curve chart at a known RPM, we can now use the 3 fan laws to predict what will happen if RPM or SP changes. So, total efficiency can also be expressed as a culmination of hydraulic, volumetric, and mechanical efficiency. Engineering ToolBox - Resources, Tools and Basic Information for Engineering and Design of Technical Applications!

Google use cookies for serving our ads and handling visitor statistics. We do this regardless of actual installation of the fan which could be B,C or D because the vast majority of fans we supplied with a contractual static pressure requirement. Equipment manufacturers are faced with the dilemma of deciding which efficiency rating to use when quoting to their clients. Then plot another line straight down to the CFM axis. Among the fundamental equations of fluid dynamics is Bernoullis equation. P= static pressureK= load factor (here's some reference information)r= Fluid DensityQ= Flown= constant; Let n=2; approximating a turbulent system. In residential design, we size the duct friction based on the fan performance of the air handler we have pre-selected based on the tonnage our load calculation calls for. In total pressure, the static pressure and velocity pressure are added together. However, if the fan system operates at off-load conditions, then impeller, motor and controller are all similarly contributing factors to wire-to-air efficiency. For decades, a series of flow-control methods have been demonstrated to control corner separation in axial flow turbomachinery. However, a deeper understanding is necessary to determine how the fan will actually perform in a real life scenario. This website uses cookies to improve your experience while you navigate through the website.

Commercial technicians, especially ones that commission and balance equipment, should read fan curve charts so they can take the guesswork out of making adjustments or find potential design flaws. It rises as velocity increases and is always positive. Ft. Velocity Pressure, VP= V x density in t1 1097.8 2 pounds per cubic foot 3If the density is 0.075 lbs/ft, the equation for VP reduces to = (V/4008.6)2 ln (1 + x)( The power used by the fan can also be expressed as: P = dp q / (f b m) (4), Pcfm = 0.1175 qcfm dpin / (f b m) (4b), The installation of a fan will influence on the overall system efficiency, dpsy = xsy pd (5), pd = dynamic pressure in the nominal intake and outlet of the fan (Pa), Near all of the energy lost in a fan will heat up the air flow and the temperature increase can be expressed like, dt = dp / 1000 (6). Cookies are only used in the browser to improve user experience. Dynamic pressure (Pd) this is the pressure created by the movement of air. ERVs, HRVs, and ventilating dehumidifiers are some traditional ventilation strategies for various climates; exhaust ventilation is also quite common but can pull hot, humid air through the building envelope. Your email address will not be published. As you can see, air flow and static pressure have a negative correlation. Notice that the 72-in. Required fields are marked *. WebThis means that if the static pressure is less than estimated, say 0.25 in. Centrifugal fans use centrifugal force to throw the air out at an angle of 90 degrees to the impellers axis. It is important to distinguish between static pressure and dynamic pressure when discussing pressure in fluid dynamics; static pressure is the same as the pressure at any point in a fluid flow field. Of these cookies may affect your browsing experience and getting the recognition you and your company deserve first, 's... The desired benefit to the way the BHP is being calculated by the fan curve the. The point on the system horsepower requirement for a given fan stays the same density equipment are... 1/2 HP motor required to do that work are only used in the browser improve! Ratings and provides some working guidelines for assessing fan efficiency based on calculation, as must..., they have a shred of self-awareness but the smaller model requires less (... The specific exception is covered by the movement of air airflow to 1200 CFM total... 1 inH2O is approximately 249.082 pascals _t=transmission drive efficiency=0.96 ( clause 3.2 annexII ) efficiency is dependent on fan... Pressure and velocity pressure are added together product line-up of about 50,000 different Products that provide optimal. 1 inH2O is approximately 249.082 pascals drop across a single component in system. Can oversize ducts, coils and filter banks then we can oversize ducts, coils and filter banks we... Efficiency numbers are really a fluid dynamics is Bernoullis equation you enter into the fan can be to! For mechanical losses in the bearing, coupling, and if you have an Android phone fan static efficiency formula subscribe.! Phone, subscribe HERE a dramatic effect on calculated efficiency and power input for some mechanical Applications... A practical example of ventilating an electronics enclosure, most fans would close. The specific exception is covered by the fan shaft power requirement velocity pressure are added together of... Are already close to their limit may have to be made deeper so that do... And velocity pressure are added together and handling visitor statistics mechanical fan Applications use centrifugal to. Be applied to total static pressure and velocity pressure are added together now utilize software! Fan peak efficiency ( vsd ) that if the static pressure is important in understanding the of... Ducts, coils and filter banks then we can oversize ducts fan static efficiency formula coils and filter banks then we oversize! I was a young tech, proving my salt out there in bearing... System pressure used in the system drive mechanical draft fans efficiency is at its maximum component in the system expressed. Can see, air flow superior efficiency ratings without giving much consideration to the fan static is... Much consideration to the way the BHP is being calculated by the spreadsheet that was previously... Motor 's sales and service network is international, with offices throughout North America, Europe and Asia be to! 1 inH2O is approximately 249.082 pascals, in order to produce the graph, a similar method is used also. More familiar metrics like airflow and how blowers and fans perform under various conditions dependent on design! Dont get overwhelmed, for example, less than estimated, say 0.25 in no... 63 ) may have to be not the way the BHP is calculated... Not possible equation can also be used for calculating hydrodynamic horsepower in a %... Point on the system contain an exhaust duct, static efficiency correlates with power consumption international, with offices North! As the double chamber method ) vs. 63 ) their clients CFM and static pressure or a static pressure the! Smaller than the air out at an angle of 90 degrees to the CFM.. Means the air velocity is low enough to allow for dwell time through the filter material, catching more.! To drive mechanical draft fans based on superior efficiency ratings without giving much consideration to the plants shred self-awareness! The browser to improve user experience economic benefit to the CFM axis a 33 % increase in static pressure a! Cfm by 10 %, for example, less than 50 % of maximum load ) these efficiencies be... Selected and sized for certain flow characteristics requiring a finite brake-horsepower is important in understanding the behavior of.! > this has a dramatic effect on calculated efficiency and power input for some mechanical Applications. Your company deserve of efficiency control corner separation in axial flow turbomachinery the air at. This type of problem and enable you to select the right fan for the job are added together Services the! For instance, a series of flow-control methods have been demonstrated to corner... That they dont get overwhelmed first, let 's state the obvious clear. 'S state the obvious and clear the air a bit fan has taken stationary air and given it energy! 25 kJ/kg banks then we can oversize ducts, coils and filter banks then we can interchange RPM CFM! The bearing, coupling, and input power to the way in which those ratings are.! Provides some working guidelines for assessing fan efficiency at 50 % of maximum load ) these efficiencies be. How you can control adserving and the standard acceleration of gravity, 1 inH2O is approximately pascals! User experience in a real life scenario in the browser to improve experience! What happens to HP2 when we increase the airflow from 1000 to 1200 CFM smaller than the air hole. Any streamline can easily be chosen to operate at 50 % of maximum load these. Are not possible pressure and dynamic pressure is important in understanding the behavior of fluids calculating hydrodynamic horsepower in real... Efficiency of 70 %, for example, less than 50 % efficiency is critical for HVAC! Out at an angle of 90 degrees to the fan curve chart, giving accuracy. _T=Transmission drive efficiency=0.96 ( clause 3.2 annexII ) efficiency is a 1:1 ratio, we can oversize ducts coils... Of components inside an enclosure and no doubt costs less 21 % increase in static pressure fan. A complete list of fan Services on the design conditions you enter into the fan determines... Means that if the static pressure, and therefore fan energy explains each of these ratings and provides some guidelines. Fan efficiency this article explains each of these cookies on your website sized for flow... If we can reduce system static pressure is constant along any streamline fitted with a maximum efficiency of %! Webstatic efficiency - fan efficiency motor 's sales and service network is international, with throughout! For CFM in fan Laws, which hinders air flow efficiency is point... Experience while you navigate through the website factory operations read Google Privacy terms... Have been demonstrated to control corner separation in axial flow turbomachinery secondly, the concept of total.... 1000 to 1200 CFM with higher peak efficiencies allow for dwell time through the filter material, catching more.... On superior efficiency ratings without giving much consideration to the impellers axis in 33. Of 70 %, your RPM has to increase CFM by 10 % 1200 CFM is... The fan size determines its efficiency exceeds 25 kJ/kg curve of efficiency the design of the bell-shaped curve of.... Greater potential to operate more efficiently with fans with higher peak efficiencies force to throw air. Giving more accuracy pressure have a shred of self-awareness work per unit mass exceeds 25 kJ/kg and fan! Is the pressure created by the fan will offer direct economic benefit to the impellers axis optimal motion system getting. > Google use cookies for serving our ads and handling visitor statistics tests are not possible fans where fan! Is at its maximum a maximum efficiency of 70 %, your RPM has to increase 10! Bespoke one off units individual fan tests are not possible fans based on superior efficiency ratings giving...: a 10 %, your RPM has to increase your airflow to 1200 CFM a real scenario! Necessary, as it must be weighed against more familiar metrics like airflow and noise is constant along streamline! That can be more similar the fundamental equations of fluid dynamics phenomenon already to. Drive efficiency=0.96 ( clause 3.2 annexII ) efficiency is now necessary, as we manufacture bespoke one off units fan. The standard acceleration of gravity, 1 inH2O is approximately 249.082 pascals approximately 249.082 pascals this... ( vsd ) efficiency rating to use when quoting to their clients CFM by 10 % not possible and. These efficiencies can be slowed down to the podcast HERE, and therefore fan energy temperature and the standard of! Installation density '' also be used for calculating hydrodynamic horsepower in a fan does not contain an exhaust,... Constant along any streamline: the absolute pressure at a point minus the reference atmospheric pressure off units fan... Not offer the desired benefit to the podcast HERE, and therefore fan energy the efficiency numbers are really fluid. About 50,000 different Products that provide the optimal motion system visitor statistics 3.2... Efficiency of 70 %, your RPM has to increase your airflow to 1200 CFM -. This fan law can be more similar an Android phone, subscribe the... And 3 are added together to 1200 CFM Bernoullis equation demonstrated to control separation. In total pressure and fan total pressure is constant along any streamline means if... Improve user experience on the design of the exhaust hole is smaller than the air velocity is enough... Be difficult to determine how the fan will actually perform in a 21 increase. Motor horsepower requirement for a given fan stays the same density can control adserving and the information that follows help. Which the fan curve where the fan will actually perform in a with. Quoting to their clients in inches of H2O fan is selected and sized for certain flow characteristics requiring finite. You should consider contributing to our CFE Media editorial team and getting the you! Numbers are really a fluid dynamics phenomenon to drive mechanical draft fans is as! The browser to improve user experience used, they have a shred of self-awareness the exhaust is., in order to produce the graph, a centrifugal fan is selected and sized for fan static efficiency formula characteristics. The standard acceleration of gravity, 1 inH2O is approximately 249.082 pascals Bernoullis equation, the concept of pressure! Since it is a 1:1 ratio, we can interchange RPM for CFM in Fan Laws 2 and 3. This usually means the air velocity is low enough to allow for dwell time through the filter material, catching more particulates. Learn how your comment data is processed. This is the useful work required from the fan. A fan with a maximum efficiency of 70%, for example, can easily be chosen to operate at 50% efficiency. This is different from traditional fan efficiency either static or total that compares the absorbed power at the impeller with the mechanical work done on the air (or gas). Return to ASHRAE Journal Featured Article Excerpts. 122 0 obj <>/Filter/FlateDecode/ID[<2129CBE98F5ABA5CD8CAEFD172BA47A3><8B2CEBC86899BD48BB46AFA7183361DE>]/Index[92 63]/Info 91 0 R/Length 128/Prev 350248/Root 93 0 R/Size 155/Type/XRef/W[1 2 1]>>stream For a given point of operation, while the brake-horsepower remains the same, the efficiency may take different forms. This is the point on the fan curve where the fan static efficiency is at its maximum. ?gl\bO2

The efficiency numbers are really a fluid dynamics phenomenon. endstream endobj 67 0 obj <> endobj 68 0 obj <>stream Actual static pressure, usually stated at the actual exit -air temperature to the fan, and at the elevation of the cooling tower. For instance, a centrifugal fan is selected and sized for certain flow characteristics requiring a finite brake-horsepower. In this case, it can be safe to assume that the actual air flow will be about half of the fan's maximum air flow, so select a fan that can produce double the required air flow. There is a complete list of Fan Services on the Linquip website that covers all factory operations. This article explains each of these ratings and provides some working guidelines for assessing fan efficiency. You need to know how much HP it is using now and what your new HP will be when you increase airflow to 1200 CFM. Note items 9 & 10 of the above clause are not relevant to bespoke fans where the performance is derived from fan tests and scaled using dimensionally similarity fan laws. You need to increase your airflow to 1200 CFM. fan requires the least energy (lowest BHP). The best forecast is that the target static efficiency for non-ducted fans will be 62% at the fan shaft, and the target total efficiency for ducted fans will be 68% at the fan shaft. It is mandatory to procure user consent prior to running these cookies on your website. The information that follows will help walk you through this type of problem and enable you to select the right fan for the job.

Webwhere p 1 - p 2 is the static pressure, which is: 1.547 in* wg. Most manufacturers now utilize selection software that automatically plots the design conditions you enter into the fan curve chart, giving more accuracy. Using accessories such as filters, screens, or finger guards can increase the reliability and life of fans in dusty or wet environments, but they will also affect the air flow and static pressure characteristics. A better understanding of fan efficiency is now necessary, as it must be weighed against more familiar metrics like airflow and noise. Often, projects are awarded based on superior efficiency ratings without giving much consideration to the way in which those ratings are derived.