www.processingmagazine.com DECEMBER 2016 PROCESSING DECEMBER 2016 Filtration

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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. back to gas in the expander section ize the liquid. This consumes some energy as The term "pressure drop" refers to the dif- However, this calculation predicts pres- fan horsepower. When these drops enter the ference in static pressure between the gas sure drop reasonably well except at very high throat area and encounter the high-velocity at the inlet to the Venturi scrubber and the liquid-to-gas ratios (L/G.) Most Venturi scrub- gas stream, they explode into thousands of gas at the discharge from the Venturi scrub- bers are designed for an L/G between 7 and 10 smaller droplets (atomization). ber. Figure 3 is a typical pressure profile for a gpm per 1,000 acfm, and virtually no change in The second method is to atomize the liquid using spray nozzles. In this case, the energy used to atomize the liquid is provided by pump horsepower. No substantial energy sav- ings are realized by using either technique, but the high-pressure nozzle technique is limited to applications in which a clean liquid stream is fed to the Venturi, limiting its appli- cation when recirculating scrubber liquid.

Expander section As the gas exits the scrubbing throat, it car- ries with it all the liquid droplets, which have now achieved a velocity nearly that of the gas stream (see Figure 2). In the expander section, the gas is slowed as the cross-sectional area increases. Some of the kinetic energy from the liquid droplets transfers back to the gas Figure 3. Typical pressure profile for a Venturi scrubber performance occurs across this range of liquid At a given throat velocity, if all empirical frac- A properly designed Venturi scrubber rates when at a constant pressure drop across tional efficiency points are plotted versus par- includes a converging inlet, a defined Venturi the scrubber. Lower L/G does not properly ticle diameter, a collection efficiency curve for throat and an expander section as defined by distribute liquid across the throat. Higher L/G a given pressure drop or throat velocity is pro- physics. Shortcuts in design without all three wastes energy accelerating excess liquid with duced. Typically, collection efficiency curves critical design components create a Venturi no collection efficiency benefit. are plotted for varying pressure drops rather scrubber that is not as effective, and excess than throat velocities because this provides energy is wasted without achieving the desired Scrubber size information on fan requirements. PM collection. After the required pressure drop has been computed, the Venturi scrubber must be The importance of experience, Back to Sheldon sized. Usually, all dimensions of the scrub- testing & sampling Imagine two of the characters on The Big ber derive from the size of the scrubbing To design a Venturi scrubber properly for Bang Theory, Dr. Sheldon Cooper, physicist, throat. Therefore, a designer can use either the a specific application, previous experi- and Howard Wolowitz, engineer, discuss- Calvert equation or empirical velocity-versus­ ence with analogous applications, testing a ing Venturi scrubber physics and the engi- differential-pressure curves for a given scrub- Venturi scrubber on the source and/or con- neering and design of a Venturi scrubber. ber to size the throat for a given saturated gas ducting sufficient sampling of the source to Sheldon would be at his whiteboard explain- flow rate. Note that using the saturated gas determine the characteristics of the PM are ing to Howard the formulas and calculations flow rate, rather than the hot inlet gas flow required. With the particle size distribution for particle removal efficiency in a Venturi. rate, is imperative for sizing the throat. and specific gravity of the PM to be removed Howard would argue to Sheldon that short- As a hot flue gas stream enters the throat, from the gas stream, if collection efficiency cuts would need to be implemented to be it is immediately quenched to its saturation curves as a function of particle diameter for priced competitively in the marketplace. temperature, and the flow rate is reduced given pressure drops are available, the frac- Sheldon might say, “I have never said that substantially. If the throat were sized on the tion of PM that will pass through the scrubber you are not good at what you do. It’s just that unsaturated hot gas flow rate, it would be without being collected can be calculated. what you do is not worth doing.” And so the far too large for most applications, and the Manufacturers have developed their own banter would continue. required collection efficiency could not be design and sizing processes for their particu- achieved. A fan must also be selected which lar Venturi scrubbing systems. They involve will handle the required gas flow rate. The fan predicting fractional removal efficiencies at Dwayne Sanders is an air pollution control is sized on the actual gas flow rate at the fan, various diameter and specific gravity par- professional who serves as scrubber prod- not necessarily the saturated gas flow rate, ticles. The fractional removal efficiencies uct manager for Sly Inc. He is a degreed particularly on the forced draft side. Finally, are then plotted against a particle size dis- chemical engineer from Georgia Tech with pumps, piping, duct work and tankage must tribution to determine the differential pres- more than 30 years of experience in the air pollution industry also be designed to complement the design sure required to achieve a desired overall with expertise in wet scrubber applications. Sanders is a parameters of the Venturi scrubbing system. PM removal efficiency for that application. past U.S. Paralympic Cycling Team member. He may be For a given particle size and a given throat Venturi scrubber systems often include aux- reached at 800-334-2957, extension 506, or dsanders@ velocity, one can determine empirically the iliary components — such as recirculation slyinc.com. fraction of particles that will be collected by tanks, recirculation pumps, liquid treatment the Venturi scrubber. This represents the frac- systems, forced draft or induced draft fans, Sly Inc. tional efficiency for a given particle diameter. and pre-wired controls and instrumentation. www.slyinc.com

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].