www.processingmagazine.com DECEMBER 2016 PROCESSING DECEMBER 2016 Filtration FEATURED PRODUCTs Colorimetric Analyzer The CA6 Colorimetric Analyzer from Electro-Chemical Devices provides accurate and reliable measurement of manganese or iron levels in municipal drinking water production, food and beverage processing, and other high-purity water processes. The analyzer is an on-line sequential sampling analyzer. A sequence of PROCESSINGMAGAZINE.com sampling, analysis and results processing is performed and repeated using colorimetric methods. Electro-Chemical Devices www.ecdi.com 502 TEMPERATURE MEASUREMENT SOLUTION Emerson Process Management introduces Rosemount X-well COVER STORY Technology, a surface-sensing, temperature measurement solu- PAGE 8 tion that eliminates the need for thermowell process penetration Breakthrough Products when measuring process temperatures in pipe applications. 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Endress+Hauser www.us.endress.com 501 PR_1216_Cover.indd 1 11/22/16 2:06 PM Venturi Scrubbers, physics & THE Big Bang Theory Effective design and testing define the success of a filtration system By Dwayne Sanders, Sly Inc. ans of the TV show The Big Bang Theory are familiar with the ban- F ter between the characters regard- ing the alleged superiority of physicists to engineers. One-liners delivered Inlet by physicist Dr. Sheldon Cooper purportedly In the wetted approach Venturi scrubber as boasting of a physicist’s intellectual superi- shown in Figure 1, the gas is introduced in ority over engineers, such as “Engineering is a radial fashion and scrubbing liquid is pro- merely the slow younger brother of physics,” vided to completely wet the inlet section. The are intended to provide humor. In all serious- gas is introduced in such a way that, once it ness, however, the laws of physics dictate leaves the inlet gas nozzle, it never contacts a the formulas used by engineers to predict dry wall. The only surfaces the gas can touch the performance of Venturi scrubbers for the are already wetted with a liquid film so solids removal of particulate matter (PM) from a gas deposition does not occur. stream. In a Venturi scrubber, the collection object Figure 1. Wetted Venturi Throat is a liquid drop. Its diameter is a complicated scrubber operation Venturi scrubbers can be furnished as either function of velocity, liquid rate and fluid prop- All graphics courtesy of Sly Inc. fixed-flow or variable-flow devices. In the erties. Normally, a centrifugal fan is installed fixed-throat Venturi, gas flow must be con- upstream (forced draft) or downstream stant to maintain a steady differential pres- (induced draft) from the Venturi scrubber. laws of physics in an effort to produce a com- sure and collection efficiency. In the variable- The fan provides the motive force to the gas petitive product. The laws of physics, how- flow Venturi, the throat is adjustable. When stream carrying the PM. The gas is acceler- ever, require that a properly designed Venturi reduced gas flow conditions are encountered, ated to the throat velocity in the converging scrubber must include three critical ele- the position of the throat damper can be inlet. The gas then passes through the throat, ments: a converging inlet, a defined Venturi changed to hold constant differential pres- where the PM encounters the liquid drops throat and an expander section. Whenever sure, and collection efficiency can be main- (obstacles). shortcuts in design exclude any of the three tained. The particles that impact liquid drops can critical elements, the Venturi scrubber will be The physical mechanisms affecting PM col- be easily separated from the bulk of the gas less effective and will consume excess energy lection in Venturi scrubbers include inertia stream by collecting them in a cyclonic (cen- without achieving the intended PM collection (inertial impaction), diffusion, electrostatics, trifugal) or chevron (impaction) mist elimina- efficiency. This article highlights the impor- Brownian motion, nucleation and growth, tor. Particles that do not impact liquid drops tant design factors for wet Venturi scrubbers and condensation. While all of these mecha- "penetrate" the scrubber and are exhausted and explains why shortcuts in design create nisms affect collection, the predominant phe- with the gas stream. Venturi scrubbers that are less effective at nomenon is inertial impaction. Some Venturi scrubbers are designed by capturing PM. When capturing a particle of a given well-intentioned engineers to shortcut these diameter and density in a gas stream of a Copyright © 2017 by Grand View Media. All rights reserved. No part of this publication may be reproduced, distributed, or transmitted in any form or by any means, including photocopying, recording, or other electronic or mechanical methods, without the prior written permission of the publisher, except in the case of brief quotations embodied in critical reviews and certain other noncommercial uses permitted by copyright law. The material in all Grand View Media publications is copyrighted and may be reprinted by permission only. Reprinted with the permission of Grand View Media and the author[s]. given viscosity, two main variables affect Venturi scrubber. As the gas accelerates, the the collection efficiency (allowing, as stated pressure in the gas stream decreases to its earlier, that impaction is the predominant lowest point in the throat. As the gas begins mechanism): to slow down in the expander section, pres- • Relative velocity between the gas and the sure rises and reaches a level only slightly collection object lower than the pressure at the inlet. The dif- • Characteristic dimension of the collec- ference between A and D (inlet and outlet tion object pressures), or the pressure drop, represents For inertial impaction to occur, the dust the energy expended in the scrubbing pro- particle must run into a liquid droplet. How cess. If the expander section is omitted, the does this occur? Simply. The greater the rela- energy expended is greater and shown as the tive velocities (momentum) between the difference between A and C. dust particles relative to the liquid drop- The Calvert equation can be used to predict lets, the greater probability that the dust the actual pressure drop for a given throat particles will collide with the liquid drop- velocity. The Calvert equation states: lets and be captured. To illustrate: Someone P = (5 x 10-5) V2 L/G drives a car down a road, and the air is filled Where: with flying insects. The faster the car travels, P = pressure drop in inches water column the more likely that insects will hit the wind- V = gas velocity in feet per second shield. In this analogy, the bugs are the dust L/G = liquid-to-gas ratio in gallons per min- particles, and the car is the liquid droplet Figure 2. A properly designed Venturi scrubber includes a ute (gpm) (collector). converging inlet, a defined Venturi throat and an expander section. / 1,000 actual cubic feet per minute Without all three critical design components, the scrubber is not as A Venturi is a well-known device for accel- effective, and excess energy is wasted without achieving the desired (acfm) erating a fluid stream to a high velocity and PM collection. returning it to its original velocity with a At saturated conditions, the pressure drop minimum loss of energy. This is why a Venturi stream, resulting in a recovery of part of the is equal to the power required to acceler- is an integral component in high-velocity energy required to accelerate the gas to throat ate the liquid to the gas velocity. This is not wind tunnels. It is therefore only natural that velocity. This energy regain is what distin- 100 percent accurate because it does not the Venturi was chosen as the most efficient guishes a Venturi scrubber from any other type account for: method for contacting a gas and a liquid for of wet scrubber. Once the gas has been slowed • Frictional losses in the Venturi maximum PM removal from the gas. sufficiently to minimize additional turbulent • The possibility that liquid is not acceler- Liquid drops can be created in two different losses, it is directed to a cyclonic separator/ ated to the full gas velocity ways in the scrubber. The most common is mist eliminator where it is separated from the • When the liquid transfers momentum simply to allow the high-velocity gas to atom- gas stream.