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the presentation includes complete details about rotameters in a generic perspective. a sample problem is also added for a better comprehension of the topic

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  3. 3. AN INTRODUCTION ▪ Various flow measuring devices: – Vortex meter – Piston flow meter – Paddle wheel meter – PD meters – Magnetic meters – Ultrasonic meters. – Variable area meters (ROTAMETERS)
  4. 4. AN INTRODUCTION ▪ The first variable area meter with rotating float was invented by Karl Kueppers in Aachen in 1908. ▪ Felix Meyer found the first industrial company "Deutsche Rotawerke GmbH" in Aachen recognizing the fundamental importance of this invention. ▪ Kueppers invented the special shape for the inside of the glass tube that realized a symmetrical flow scale. They improved this invention with new shapes of the float and of the glass tube.
  5. 5. AN INTRODUCTION ▪ In orifice, nozzle or venturi the variation of flow rate through a constant area generates a variable pressure drop, which is related to flow rate. ▪ Another class of meters, called area meters, consists of devices in which the pressure drop is constant, or nearly so, and the area through which the fluid flows varies with the flow rate. ▪ This area is related, through proper calibrations to the flow rate. ▪ The most important area meter is the rotameter.
  6. 6. CONSTRUCTION & WORKING ▪ A rotameter consists essentially of a graduated tapered metering glass tube mounted vertically in a frame with the large end up. ▪ Inside the tube is the float which is an indicating element, the greater the flow rate, the higher the float rides in the tube. ▪ The fluid flows up through the tube and suspends freely a float (which actually doesn’t float but is completely submerged in the fluid). ▪ The entire fluid stream flows through the annular space between the float and tube wall.
  7. 7. MODIFICATIONS IN COMPONENTS ▪ Floats may be constructed of metals of various densities from lead to aluminum or from glass or plastic. ▪ Stainless-steel floats are common ones. ▪ Float shapes and proportions are also varied for different applications. ▪ For small flows floats are spherical in shape.
  8. 8. MODIFICATIONS IN COMPONENTS ▪ Metal tubes are used where opaque liquids are used or temperature or pressure requirement is quite high. ▪ Since float is invisible in metal tube means are to be provided for either indicating or transmitting meter reading. ▪ The task is accomplished by using a rod called extension, to the top or bottom of the float and using the extension as an armature. ▪ The extension is used enclosed in a fluid tight tube mounted on one of the fittings. ▪ The tube is surrounded by external induction coils, the length of extension exposed to coils varies with the position of float. ▪ This in turn changes the inductance of coil, and the variation is measured electrically to operate a control valve or to give recorder a reading.
  11. 11. FORMULAE,CALCULATIONS AND GRAPHS ▪ The coefficient CD depends on the shape of the float and the Reynolds number (based on the velocity in the annulus and the mean hydraulic diameter of the annulus) for the flow through the annular space of area A2. ▪ In general, floats which give the most nearly constant coefficient are of such a shape that they set up eddy currents and give low values of CD. ▪ The variation in CD largely arises from differences in viscous drag of fluid on the float, and if turbulence is artificially increased, the drag force rises quickly to a limiting but high value.
  13. 13. FORMULAE,CALCULATIONS AND GRAPHS ▪ As seen in Figure earlier, float A does not promote turbulence and the coefficient rises slowly to a high value of 0.98. ▪ Float C promotes turbulence and CD rises quickly but only to a low value of 0.60. ▪ The constant coefficient for float C arises from turbulence promotion, and for this reason the coefficient is also substantially independent of the fluid viscosity.
  15. 15. ADVANTAGES • A rotameter requires no external power or fuel, it uses only the inherent properties of the fluid, along with gravity, to measure flow rate. • A rotameter is also a relatively simple device that can be mass manufactured out of cheap materials, allowing for its widespread use. • Since the area of the flow passage increases as the float moves up the tube, the scale is approximately linear.
  16. 16. DISADVANTAGES • Due to its use of gravity, a rotameter must always be vertically oriented and right way up, with the fluid flowing upward. • Due to its reliance on the ability of the fluid or gas to displace the float, graduations on a given rotameter will only be accurate for a given substance at a given temperature. The main property of importance is the density of the fluid; however, viscosity may also be significant. Floats are ideally designed to be insensitive to viscosity; however, this is seldom verifiable from manufacturers' specifications. Either separate rotameters for different densities and viscosities may be used, or multiple scales on the same rotameter can be used.
  17. 17. DISADVANTAGES • Due to the direct flow indication the resolution is relatively poor compared to other measurement principles. Readout uncertainty gets worse near the bottom of the scale. Oscillations of the float and parallax may further increase the uncertainty of the measurement. • Rotameters normally require the use of glass (or other transparent material), otherwise the user cannot see the float. This limits their use in many industries to benign fluids, such as water. • Rotameters are not easily adapted for reading by machine; although magnetic floats that drive a follower outside the tube are available. • Usually rotameters aren't made in very large sizes (more than 4 inches/100 mm), but bypass designs are sometimes used on very large pipes. • Clear glass is used which is highly resistant to thermal shock and chemical action.
  18. 18. SAMPLE PROBLEM ▪ A rotameter tube is 0.3 m long with an internal diameter of 25 mm at the top and 20 mm at the bottom. The diameter of the float is 20 mm, its density is 4800 kg/m3 and its volume is 6.0 cm3. If the coefficient of discharge is 0.7 , what is the flowrate of water (density 1000 kg/m3) when the float is halfway up the tube?