Nikon D780 Ken Rockwell, Nexgrill Replacement Parts, Maydis Leaf Blight Of Maize, Stokke Steps High Chair Sale, Why Are Barberry Bushes Illegal, What Country Has The Oldest Unchanged Borders, How To Draw A Statue Of Liberty, Virtual Reality Books Nonfiction, White Wisteria Tree Where Do They Grow, Zen Cart Vs Shopify, State Board Of Nursing, " />

centrifugal fan blade design

[citation needed]. Centrifugal fans displace air radially, changing the direction (typically by 90°) of the airflow. Dampers on the outlet side impose a flow resistance that is used to control gas flow. The centrifugal fan displaces a constant volume of air in a given system regardless of air density. Some common design criteria are the size of the motor, the shape of the fan blade, the direction of rotation, and the materials of construction. DYK York doesn’t require suction traps for R410a? Centrifugal fans consist of an impeller in a casing having a spirally shaped contour. [9] Thereafter, centrifugal fans gradually fell into disuse. The construction is often built to be installed in a wall or in an open ceiling or floor mount. ) The impeller is a double-width, double-inlet (DWDI) centrifugal type with two nonstaggered blade rows. Figure 2: FC centrifugal blower scroll housing. Air conditioning automatically provides faster speed because colder air is denser. To describe the flow using only two velocity triangles, we define mean values of velocity and their direction. [2], Centrifugal fans are constant-displacement or constant-volume devices, meaning that, at a constant fan speed, a centrifugal fan moves a relatively constant volume of air rather than a constant mass. As an integral part of a ventilation system layout, centrifugal inline Fan dampers are used to control gas flow into and out of the centrifugal fan. The gas from the impeller eye rushes in to normalize this. These fans increase the speed and volume of an air stream with the rotating impellers. They are suitable for high pressure applications as compared with axial flow fans. ", "TECHNICAL SPECIFICATION OF CENTRIFUGAL FANS DESIGN", "Overview of Blower Technologies and Comparison of High-Speed Turbo Blowers", "Value in the Air: Why Direct Drive Backward Curved Plenum Fans", Heating, ventilation, and air conditioning, High efficiency glandless circulating pump, https://en.wikipedia.org/w/index.php?title=Centrifugal_fan&oldid=983669630, Articles with Russian-language sources (ru), Articles needing additional references from July 2011, All articles needing additional references, Articles with unsourced statements from November 2018, Articles with unsourced statements from July 2015, Articles with unsourced statements from July 2011, Articles needing additional references from April 2015, Wikipedia articles needing clarification from April 2015, Creative Commons Attribution-ShareAlike License, = fan wheel speed, revolutions per minute, = motor nameplate speed, revolutions per minute, This page was last edited on 15 October 2020, at 15:24. • Common fan types: centrifugal (“squirrel cage”), axial, special designs (including radial) • Fan rotation direction (clockwise or counterclockwise) is important because the blades and housing are designed to In 1815 the Marquis de Chabannes advocated the use of a centrifugal fan and took out a British patent in the same year. Radial blowers, as in Figure 3(c), have wheels whose blades extend straight out from the centre of the hub. Completing this conversion in the most efficient manner requires an outlet duct of the same size as the fan outlet and of sufficient straight length to allo… Brad joined the cfm team in 2006, and now as Vice President of Sales, Applied Systems Group he focuses on business development, as well as helping contractors and engineers find creative and unique solutions to any size HVAC project. Leakage of some air and disturbance in the main flow field is caused due to the clearance provided between the rotating periphery of the impeller and the casing at the entry. Author content. Sometimes centrifugal fans are called radial flow type fans. The name, ‘Centrifugal Fan’ is derived from the direction of flow and how the air exits the fan impeller radially from the outer circumference of the fan. The centrifugal fan uses the centrifugal power supplied from the rotation of impellers to increase the kinetic energy of air/gases. Some electronics manufacturers have made centrifugal fans with external rotor motors (the stator is inside the rotor), and the rotor is directly mounted on the fan wheel (impeller). They are sturdy, quiet, reliable, and capable of operating over a wide range of conditions. https://resources.saylor.org/wwwresources/archived/site/wp-content/uploads/2011/09/Chapter-3.5-Fans-Blowers.pdf, https://www.cincinnatifan.com/blower-wheels.htm, https://www.pelonistechnologies.com/blog/axial-vs.-centrifugal-fans, https://www.rs-online.com/designspark/fan-types-why-choose-a-forward-curved-centrifugal-fan, http://www.ijirset.com/upload/2017/july/11_Comparative.PDF, Design Tips for Sound Sensitive Applications, 60 Second Design Tip Videos |Brad Telker and Hank Kutilek. A Fair running static efficiency is 50-70 percent for both the straight radial blade and radial tip blade. [8], The fan wheel consists of a hub with a number of fan blades attached. and their performance is almost entirely governed by blade design and rotational speed, flow and head characteristics are generally governed by the fan's inlet and/or outlet ducts. CalQlata has tried to keep the operation of this calculation option as simple as possible, given that it is recommended for general purpose calculations only and not for actual purchase specifications (see Fan Calculator – Technical Helpbelow). Centrifugal fan which is also called as squirrel cage fan because of its hamster wheel design is widely used to move air/gas continuously in industries and commercial applications.The speed of the air stream entering into the fan, is increased by the impeller thereby it gains kinetic energy and accelerates radially changing the direction by 90°. As shown in the animated figure, the hub turns on a driveshaft mounted in bearings in the fan housing. These types of blowers are designed to handle gas streams with low to moderate particulate loadings[citation needed]. At higher-than-standard elevation (sea level) and higher-than-standard temperature, air density is lower than standard density. Sometimes centrifugal fans are called radial flow type fans. What type of chiller is ideal for your application? When the centrifugal fan performance is not at standard conditions, the performance must be converted to standard conditions before entering the performance tables. A wide variety of centrifugal fan blade design options are available to you, such as ce, rohs, and cb. Centrifugal fans often contain a ducted housing to direct outgoing air in a specific direction or across a heat sink; such a fan is also called a blower, blower fan, biscuit blower[citation needed], or squirrel-cage fan (because it looks like a hamster wheel). When he’s not at work, Brad enjoys reading, running and spending time with his family. Forward-curved blades, as in Figure 3(a), curve in the direction of the fan wheel's rotation. The design made a resurgence in the early 1800s. Burmeister, L.C., Elements of Thermal-Fluid System Design, Prentice Hall, 1998. There are two basic types of fan drives.[8]. First, the main dimensions of the fan were calculated by using the conventional one-dimensional method. Instead of pulling air through the fan parallel to the axis, the centrifugal fan moves air radially, from the center. A centrifugal fan resembles a paddle wheel. Whilst most fan designs (axial, Sirocco, centrifugal, etc.) Vane Axial- These fans are a variation of the tube-axial fans with air straightening vanes added in front of or behind the propeller blades. The efficiency of the optimized backward-curved blades centrifugal fan through the design optimization process of Total FAN-Turbo® was increased by 5% in comparison with reference design. This cycle repeats and therefore the gas can be continuously transferred. D Register now to receive HVAC design and application tips that are not brand specific, delivered approximately twice per month. They are the most budget friendly and the least efficient blade design. Centrifugal fans can be classified by their wheel like: F-wheel - Curved forward blades. Because of the low-pressure high-volume airflows they create, axial fans are best suited for general purpose applications. fan Centrifugal Fan - Centrifugal fans discharge air perpendicular to the axis of the impeller rotation. [12] A centrifugal fan was invented by Russian military engineer Alexander Sablukov in 1832, and was used both in the Russian light industry (such as sugar making) and abroad. Propeller- A propeller axial fan consists of a propeller shaped blade mounted on a flat frame. As a result, the kinetic energy of gas is measured as pressure because of the system resistance offered by the casing and duct. For CFD simulation, it is needed to have a closed watertight model (sometimes called waterproof, or model negative, or wet surface) of the fan inner parts where the air flows. A minimum number of data are required to draw a velocity triangle at a point on blade. Vice President of Sales, Applied Systems Group, How to use Fan Laws for design/build retrofits, Refrigeration System Installation Defrost Schedule Tips. How a Centrifugal Fan Works. The fan blades on the hub can be arranged in three different ways: forward-curved, backward-curved or radial.[8]. These are especially sensitive to particulates and commonly are only specified for clean-air applications such as air conditioning. They are more expensive to build than FC and BI fans. Designed for applications with high airflow volumes and also fairly high-pressure adaptabilities. The rather sharply rising static pressure curve of the radial blade centrifugal fan allows for small changes in volume as the resistance of the system changes considerably. Flow at the intake and its turning from axial to radial direction causes losses at the intake. Centrifugal fans are capable of generating relatively high pressures. Airflow is directed through a duct, which creates a higher pressure air flow than an axial fan, despite its lower flow rate. New content will be added above the current area of focus upon selection D - An axial fan discharges air parallel to the axis of the impeller rotation. Their compact design gives designers an excellent alternative to conventional blowers. Axial fans are typically used in low pressure applications. Ratings found in centrifugal fan performance tables and curves are based on standard air SCFM. The air enters the impeller in an axial direction and is discharged at the impeller outer periphery. The Guibal fan had a spiral case surrounding the fan blades, as well as a flexible shutter to control the escape velocity, which made it far superior to previous open-fan designs and led to the possibility of mining at great depths. As a general rule, axial fans are preferred for high volume, low pressure, and non-ducted systems. [citation needed], Centrifugal fans are not positive-displacement devices and centrifugal fans have certain advantages and disadvantages when contrasted with positive-displacement blowers: centrifugal fans are more efficient, whereas positive-displacement blowers may have a lower capital cost. Source: ebm-papst Inc.A substantial portion of the total energy of the air leaving an FC centrifugal blower is in the form of velocity pressure. A scroll housing (Figure 2) is required to convert that velocity pressure into usable static pressure that can move air against the system resistance present in the application. [3][4][5][6][7], The centrifugal fan has a drum shape composed of a number of fan blades mounted around a hub. The propeller or axial flow fan propels the air in an axial direction (Figure 1a) with a swirling tangential motion created by the rotating impeller blades. [13], One of the most important developments for the mining industry was the Guibal fan, which was patented in Belgium in 1862 by the French engineer Théophile Guibal. You can include thi… Forward Curve- Sometimes referred to as a “squirrel cage” fan, forward curve fans are used in most residential and light commercial HVAC equipment. They are made to be durable, resilient, and powerful enough to move large amounts of air – made possible by achieving the highest pressures of virtually any industrial fan configuration. High efficiency, small dimensions, changing in pressure have little influence on pressure head. [citation needed], Backward-curved fans are much more energy efficient than radial blade fans and so, for high power applications may be a suitable alternative to the lower cost radial bladed fan.[20]. [18] Forward-curved blades provide a low noise level and relatively small air flow with a high increase in static pressure. To get the actual pressure required at 200 Â°F (93 Â°C), the designer must multiply the pressure at standard conditions by an air density correction factor of 1.25 (i.e., 1.0/0.8) to get the system to operate correctly. As with the backward curved centrifugal fan, the suction side of the impeller blade draws air from the centre of the fan which results in a directional change of the airflow between the inlet and the exhaust of 90 o. In order to effectively manage the craft fuel consumption, a reduction in fan’s operating power is … The property that distinguishes a centrifugal fan from a blower is the pressure ratio it can achieve. After the gas is thrown-off, the gas pressure in the middle region of the impellers decreases. [further explanation needed] These impeller blade losses are also included in the category. motor Positive Displacement Pump", "What’s better, a PD or Fan Combination Unit? Each impeller blade row has backward-swept blades mounted between a common back plate and shrouds. These fans are general built to be installed in low pressure duct work. Axial-flow fans produce lower pressure than centrifugal fans, and exhibit a dip in pressure before reaching the peak pressure point. All content in this area was uploaded by Vineet Ahuja on Oct 20, 2015 . Airfloil fans operate with clean air/gas and tend to be the quietest of all centrifugal … When a centrifugal fan is specified for a given CFM and static pressure at conditions other than standard, an air density correction factor must be applied to select the proper size fan to meet the new condition. Heat is conducted through the shaft and into the oil, which must be cooled to prevent overheating the bearing. fan Bearings are an important part of a fan. Some sleeve-ring bearings may be water-cooled. [16], The fan drive determines the speed of the fan wheel (impeller) and the extent to which this speed can be varied. Usually they are tested and rated as one of four standard installation types as designated in AMCA Standard 210.[21]. Further losses due to incidence occur if the device is working beyond its design conditions. Radial bladed wheels are often used on particulate-laden gas streams because they are the least sensitive to solid build-up on the blades, but they are often characterized by greater noise output. [11] Similarly, in 1832, the Swedish-American engineer John Ericsson used a centrifugal fan as blower on the steamship Corsair. FanMechanics.com is owned by H & R Resources - created in 2001 to supply the fan industry with fan design and engineering software at affordable prices. Our application engineers design all of the centrifugal fan and blower components to accommodate an application. Friction and flow separation losses also occur due to the volute passage. The angle α is the angle made by the absolute velocity with the axial direction and angle β is the angle made by blade with respect to axial direction. Fan manufacturers define standard air as clean, dry air with a density of 0.075 pounds mass per cubic foot (1.2 kg/m³), with the barometric pressure at sea level of 29.92 inches of mercury (101.325 kPa) and a temperature of 70 Â°F (21 Â°C). Centrifugal fan systems are designed to handle low to medium static pressures, up to a few inches, and are very cost effective with ducted systems. Airfoil fans have the highest efficiencies and are named after the style of blades they utilize. The centrifugal fan was invented in 1556 to ventilate mines. Fig 2shows the pressures through a fan, each of which is described below: Inlet Pressure; is the static pressure on the inlet side of the fan. Impeller Design of a Centrifugal Fan with Blade Optimization.pdf. For this reason, fans must be rated in standardized SCFM. The fan wheel can be linked directly to the shaft of an electric motor. With an axial fan, the air passes through the device parallel to the shaft, and generally travels in a straight but twisting line. Hence an infinite number of velocity triangles are possible for a given blade. Selecting a centrifugal fan to operate at conditions other than standard air requires adjustment to both static pressure and power. A diagram called a velocity triangle helps us in determining the flow geometry at the entry and exit of a blade. To get the actual power at 200 Â°F (93 Â°C), the designer must divide the power at standard conditions by the air density correction factor. They may be installed on the inlet side or on the outlet side of the fan, or both. Friction and flow separation cause impeller blade losses since there is change in incidence angle. The air flow moves along the centrifugal direction (or radial direction). It disappeared from history after that. Wang et al. Many turbo blowers use either an air bearing or a magnetic bearing.[17]. A Backward curved centrifugal fan is characterised by its cylindrical shape, several large curved blades and a conical inlet nozzle. Generally centrifugal fans have three types of blade: forward blade, backward blade and radial blade. Centrifugal Fan Design A typical input for a detailed simulation analysis is a watertight (wet) surface model in form of STL surface. High speeds, low volumes, and high pressures are common with radial blowers[citation needed], and are often used in vacuum cleaners, pneumatic material conveying systems, and similar processes. Centrifugal fan systems are very cost-effective when it comes to ducted systems and are widely used in industrial and power generation applications. The Benefits of Radial Blade Centrifugal Fans Radial blade fans commonly feature the strongest structural design in the field. The high flow-rate centrifugal fan needs a three-dimensional impeller to achieve a high efficiency. Backward Inclined- The performance of the backward inclined wheel is similar to the forward curve, however BI fans tend to be non-overloading, meaning their fan curves do not tend to increase hp as static pressure changes, but cfm falls off. [1], Centrifugal fans use the kinetic energy of the impellers to increase the volume of the air stream, which in turn moves against the resistance caused by ducts, dampers and other components. Fan Characteristic. This means that the fan wheel speed is identical to the motor's rotational speed. These fans are for low to medium static applications. Air Movement and Control Association (AMCA), CS1 maint: multiple names: authors list (. {\displaystyle {\text{RPM}}_{\text{fan}}={\text{RPM}}_{\text{motor}}\,{\bigg (}{\frac {\,D_{\text{motor}}}{D_{\text{fan}}}}{\bigg )}}. centrifugal fans for the utility and industrial fields and discuss the considerations taken into account in selecting and designing the fan fur that particular service. motor RPM Centrifugal fans create … 113 AERODYNAMIC DESIGN Considerable literature exists on the theory of fan design which, when applied, can result in a … Understanding Centrifugal Fans In the broadest sense, what sets axial and centrifugal fans apart is the way in which air passes through the casing. AC fans can be connected directly to a DC power source, such as solar panels or batteries. Blades mounted to the circular fan hub spin, essentially catching the incoming air and throwing it away from the hub at a 90° angle. Since 200 Â°F (93 Â°C) air weighs only 80% of 70 Â°F (21 Â°C) air, the centrifugal fan creates less pressure and requires less power. [14][15], Other components used may include bearings, couplings, impeller locking device, fan discharge casing, shaft seal plates etc. Velocity triangle of any turbo machine has three components as shown: These velocities are related by the triangle law of vector addition: This relatively simple equation is used frequently while drawing the velocity diagram. Fans (axial and centrifugal) Subject. Airfoil- these blades are shaped like that of an airplane wing and offer slightly higher efficiencies and lower noise levels than BC fans, however are more expensive to build. Fans have a specific ratio of up to 1.11, blowers from 1.11 to 1.20 and compressors have more than 1.20. In a centrifugal fan the air enters the impeller axially and is accelerated by the blades and discharged radially (Figure 1b). Inlet dampers (inlet vanes) reduce fan energy usage due to their ability to affect the airflow pattern into the fan. [15] optimized the fan design based on an orthogonal experimental design method and CFD numerical simulation and obtained the relative optimal combination model of the impeller geometric parameters and speed. The gas enters from the side of the fan wheel, turns 90 degrees and accelerates due to centrifugal force as it flows over the fan blades and exits the fan housing. In a centrifugal fan the air flows is in a radial direction relative to the shaft. are suitable for many applications including; extraction, distribution, compression, etc. The fan wheel speed depends upon the ratio of the diameter of the motor sheave to the diameter of the fan wheel sheave and can be obtained from this equation:[8], RPM Flow from the impeller or diffuser expands in the volute, which has a larger cross section leading to the formation of eddy, which in turn reduces pressure head. Sleeve-ring oil bearings are used extensively in fans. Centrifugal fans rated by the Air Movement and Control Association (AMCA) are tested in laboratories with test setups that simulate installations that are typical for that type of fan. 912 centrifugal fan blade design products are offered for sale by suppliers on Alibaba.com, of which axial flow fans accounts for 3%, fans accounts for 1%. In comparison to an axial design, a centrifugal fan: Has a greater range of static pressure capabilities (from moderate low to high static pressure) Is more energy efficient over greater ranges Content uploaded by Vineet Ahuja. Larger backward-inclined/-curved blowers have blades whose backward curvatures mimic that of an airfoil cross section, but both designs provide good operating efficiency with relatively economical construction techniques. Backward Curved- these wheels are extremely efficient, more robustly built, and very quiet. Per the American Society of Mechanical Engineers (ASME), the specific ratio - the ratio of the discharge pressure over the suction pressure – is used for defining fans, blowers and compressors. [20], Backward curved fans can have a high range of specific speeds but are most often used for medium specific speed applications—high pressure, medium flow applications such as in air handling units. These are the most efficient design of centrifugal fan blade and can easily match or exceed the efficiency of axial flow fans when operating at medium pressures and flow rates. This is why centrifugal fans are very popular in supply fan sections for small to medium size rooftops, splits systems, and some smaller custom air handlers. History of Centrifugal Fans. As the main parameter of impeller, the blade outlet angle of centrifugal fans has also been studied by scholars. Centrifugal fans suffer efficiency losses in both stationary and moving parts, increasing the energy input required for a given level of airflow performance. When the impellers rotate, the gas particles near the impellers are thrown off from the impellers, then move into the fan casing. Some component of velocity varies at different point on the blade due to changes in the direction of flow. Smaller blowers may have backward-inclined blades, which are straight, not curved. For example, they excel at moving air from one place to another, cooling confined spaces such as computers, and cooling larger spaces such as work spaces. Such fans were used extensively for mine ventilation throughout Britain. Centrifugal fan design is usually given when the conditions are: the volume flow rate, total pressure, the working medium and its density (or the working medium temperature), and sometimes there are structural requirements and special requirements. [19] They are typically used in fan coil units. In this paper, the design procedure of a high-efficiency three-dimensional centrifugal fan is presented. ( Viscous drag on the back surface of the impeller disc causes disc friction losses. The centrifugal fan operates a bit differently than the axial fan. Centrifugal fans are sometimes called blowers because they create a high-pressure stream of air. The purpose of AMCA Standard 210 is to define exact procedures and conditions of fan testing so that ratings provided by various manufacturers are on the same basis and may be compared. In general, a blower can produce a higher pressure ratio. In a centrifugal system, the fan pulls air through a side intake. The gas is then guided to the exit via outlet ducts. Backward curved wheels are often heavier than corresponding forward-curved equivalents, as they run at higher speeds and require stronger construction. Javascript must be enabled for the correct page display. Centrifugal Inline Fan CIS inliner fans are widely used in square ducts as clean air boosters in both supply and exhaust systems where the installation of conventional blowers is impractical. The optimum operating area for a forward curved centrifugal fan is when it is operating at higher pressure. This should also include the velocity pressure on the inlet side (if known) that is constant and in-line with the fan. Georgius Agricola with Herbert Clark Hoover and Lou Henry Hoover, trans., sfn error: no target: CITEREFWallace1988 (, ANSI/AMCA Standard 210-99, "Laboratory Methods Of Testing Fans for Aerodynamic Performance Rating", Learn how and when to remove this template message, Three-dimensional losses and correlation in turbomachinery, "Advantages of Rotary Positive Displacement Blowers Versus Centrifugal Blowers", "The Great Debate: Centrifugal Fan vs. It wasn't until the early decades of the nineteenth century that interest in centrifugal fans revived. Friction and flow separation also causes losses in the diffuser. Fan wheel speeds in belt-driven fans are fixed unless the belt(s) slip. Axial Fan Types Propeller, Tube Axial and Vane Axial. A centrifugal fan is a mechanical device for moving air or other gases in a direction at an angle to the incoming fluid. [10] In 1827, Edwin A. Stevens of Bordentown, New Jersey, installed a fan for blowing air into the boilers of the steamship North America. The centrifugal fan performance tables provide the fan RPM and power requirements for the given CFM and static pressure at standard air density. ASHRAE Handbook: HVAC Systems and Equipment, 1992. Air enters near the centre of the wheel, turns through a right angle and moves radially outward by centrifugal action between the blades of the rotating impeller. Among centrifugal fan designs, aerofoil designs provide the highest efficiency (upto 10% higher than backward curved blades), but their use is limited to clean, dust-free air. They can be easily fitted with wear protection but certain blade curvatures can be prone to solids build-up. Lower-speed fans have bearings in hard-to-reach spots, so they use grease-packed bearings. [8], The earliest mention of centrifugal fans was in 1556 by Georg Pawer (Latin: Georgius Agricola) in his book De Re Metallica, where he shows how such fans were used to ventilate mines. Air density corrections must account for centrifugal fans that are specified for continuous operation at higher temperatures. With this type of fan drive mechanism, the fan speed cannot be varied unless the motor speed is adjustable. Water-cooled sleeve bearings are often used when the fan moves hot gases. = Tube Axial – These fans consist of propeller shaped blades mounted in a tube. Backward-curved blades, as in Figure 3(b), curve against the direction of the fan wheel's rotation. This paper takes centrifugal fan as the research object and establishes five impeller models with different blade outlet angles. Dampers on the inlet side (inlet vanes) are designed to control gas flow by changing the amount of gas or air admitted to the fan inlet. Belt slippage can reduce the fan wheel speed by several hundred revolutions per minute (RPM). The velocity diagram for the forward, backward face blades shown are drawn using this law. This means that the air velocity in a system is fixed even though the mass flow rate through the fan is not. Centrifugal fans come in a variety of designs. Those blades may be straight or curved either backwards or forwards with … A set of sheaves is mounted on the motor shaft and the fan wheel shaft, and a belt transmits the mechanical energy from the motor to the fan. A standard AC model is energy efficient, using no more than 100 watts when on high speed. Shrouded Radial Blade blower wheels are designed to deliver higher pressures for pressurizing, … AMCA Standard 210 defines uniform methods for conducting laboratory tests on housed fans to determine airflow rate, pressure, power and efficiency, at a given speed of rotation.

Nikon D780 Ken Rockwell, Nexgrill Replacement Parts, Maydis Leaf Blight Of Maize, Stokke Steps High Chair Sale, Why Are Barberry Bushes Illegal, What Country Has The Oldest Unchanged Borders, How To Draw A Statue Of Liberty, Virtual Reality Books Nonfiction, White Wisteria Tree Where Do They Grow, Zen Cart Vs Shopify, State Board Of Nursing,

Leave a Reply