We continue our discussion by considering variable-area flowmeters.
Variable-area flowmeters are simple and versatile devices that operate at a relatively constant pressure drop and measure the flow of liquids, gases and steam. The popularity of this type of flowmeter in the heat-treat shop can be explained by their direct-view design, where flow is indicated mechanically, which makes it easy to understand the operating principle.
Several different designs of variable-area flowmeters used in industry are shown in Fig. 1: (A) glass/plastic tapered-tube rotameter; (B) slotted metal cylinder; (C) metal tapered-tube rotameter; (D) vane-type; (E) piston meter with spring-loaded orifice piston over a tapered plug; and (F) tapered-tube with spring.
The glass or plastic rotameter (Fig. 1A) is the most widely used because of its low cost, low pressure drop, relatively wide range, linear visual flow indication and simplicity of operation. To pass through the tapered tube, the fluid flow must raise the float. The greater the flow rate, the higher the float is lifted. In liquid service, the float rises due to a combination of the liquid’s buoyancy and its velocity. With gases, buoyancy is negligible, so the float responds mostly to velocity.
The float moves up and down in proportion to the fluid-flow rate and the annular area between the float and the tube wall. As the float rises, the size of the annular opening increases. As this area increases, the differential pressure across the float decreases. The float reaches a stable position when the upward force exerted by the flowing fluid equals the weight of the float. Thus, every float position corresponds to a specific flow rate for a particular fluid’s density and viscosity.
This is why it is necessary to size the rotameter for each application. When sized correctly, the flow rate can be determined by matching the float position to a calibrated scale on the outside of the instrument. Many rotameters come with a built-in valve for manually adjusting flow. Several shapes of floats are available for various applications.
Both glass and metal rotameters are available. Glass or plastic rotameters cost less and are more accurate than metal tapered-tube rotameters (Fig. 1B), but they may not be able to provide the durability and reliability needed in a manufacturing environment. Metal rotameters are reliable, but the machined tapered tube limits the flow measurement range (turndown). Another limitation is that metal rotameters typically have brass or aluminum bodies, which may make them unsuitable for use in certain gases (ammonia, for example).
A flowmeter used in the process industries substitutes a slotted cylinder for the tapered tube (Fig. 1C). Compared with a metal rotameter, a greater selection of construction materials and a flow turndown of at least 25:1 (vs. 3.6:1) are provided.
The lower portion of the float is a piston that can “plug” the slot in the cylinder wall. The float rises until enough of the slot has opened to create equilibrium between the two upward-acting flow forces and the single downward-acting force. As for rotameters, when in this equilibrium position, float height is proportional to flow rate.
The basic equations for tapered-tube and slotted-cylinder flowmeters are similar, with their flowmeter coefficients (K factors) accounting for any differences.
- Braziunas, Vytas and Daniel H. Herring, “A Flowmeter Primer,” Heat Treating Progress, March/April 2004.
- Encyclopedia of Fluid Mechanics: Solids and Gas-Solids Flows, N.P. Cheremisinoff (Ed.): Gulf Professional Publishing, Elsevier Science/Harcourt, Burlington, Mass., 1986, ISBN 0872015149, 1506 pages.
- Herring, Daniel H., “Frequently Asked Questions About Flowmeters,” Heat Treating Progress, Vol. 2, No. 7, October 2002, p. 23–25.
- Anderson, John D.,Modern Compressible Flow: With Historical Perspective, McGraw-Hill Companies, New York, 1990, ISBN 0070016739, 650 pages.
- National Institute of Standards and Technology (www.nist.gov), Gaithersburg, Md.
- Fan, Liang-Shih and Chao Zhu, Principles of Gas-Solid Flows, Cambridge University Press, New York, 1998, ISBN 0521581486, 575 pages.
- Useful websites: http://apps.fss.gsa.gov (Federal Supply Service, U.S. General Services Administration); www.flowmeterdirectory.com; www.manufacturing.net (Reed Business Information, Div. Reed Elsevier Inc.); www.omega.com (Omega Engineering Inc.); www.sensorsmag.com (Sensors, Advanstar Communications Inc.); www.yokogawa.com (Yokogawa Electric Corp.).
- DeFasselle, C.R., “Variable-Area Flowmeters,” Measurement & Control, 1994.