1、32.1CHAPTER 32INDUSTRIAL LOCAL EXHAUSTLocal Exhaust Fundamentals 32.2Air Movement in Vicinity of Local Exhaust 32.3Other Local Exhaust System Components 32.6Operation 32.9NDUSTRIAL exhaust ventilation systems contain, collect, andIremove airborne contaminants consisting of particulate matter(dusts,
2、fumes, smokes, fibers), vapors, and gases that can create ahazardous, unhealthy, or undesirable atmosphere. Exhaust systemscan also salvage usable material, improve plant housekeeping, andcapture and remove excessive heat or moisture. Industrial exhaustsystems must comply with ASHRAE Standard 62.1 a
3、nd other stan-dards as required e.g., National Fire Protection Agency (NFPA) stan-dards.Special Warning: Certain industrial spaces may contain flam-mable, combustible and/or toxic concentrations of vapors or dustsunder either normal or abnormal conditions. In spaces such as these,there are life safe
4、ty issues that this chapter may not completelyaddress. Special precautions must be taken in accordance withrequirements of recognized authorities such as the National Fire Pro-tection Association (NFPA), Occupational Safety and HealthAdministration (OSHA), and American National Standards Institute(A
5、NSI). In all situations, engineers, designers, and installers whoencounter conflicting codes and standards must defer to the code orstandard that best addresses and safeguards life safety.Local Exhaust Versus General VentilationLocal exhaust ventilation systems can be the most performance-effective
6、and cost-effective method of controlling air pollutants andexcessive heat. For many operations, capturing pollutants at or neartheir source is the only way to ensure compliance with occupationalexposure limits that are measured within the workers breathingzone. When properly designed, local exhaust
7、ventilation optimizesventilation exhaust airflow, thus optimizing system acquisition costsassociated with equipment size and operating costs associated withenergy consumption and makeup air tempering. Chapters 2 and 3in ACGIH (2013) also discuss this topic at length.In some industrial ventilation de
8、signs, the emphasis is on filteringair captured by local exhausts before exhausting it to the outdoors orreturning it to the production space. As a result, these systems areevaluated according to their filter efficiency or total particulateremoval. However, if an insufficient percentage of emissions
9、 arecaptured, the degree of air-cleaning efficiency sometimes becomesirrelevant.For a process exhaust system in the United States, the designengineer must verify if the system is permitted by the 1990 Clean AirAct. For more information, see the Environmental ProtectionAgencys web site (http:/www.epa
10、.gov/air/cca/).The pollutant-capturing efficiency of local ventilation systemsdepends on hood design, the hoods position relative to the source ofcontamination, temperature of the source being exhausted, and theinduced air currents generated by the exhaust airflow. Selection andpositioning of the ho
11、od significantly influence initial and operatingcosts of both local and general ventilation systems. In addition,poorly designed and maintained local ventilation systems can causedeterioration of building structures and equipment, negative healtheffects, and decreased worker productivity.No local ex
12、haust ventilation system is 100% effective in capturingpollutants and/or excess heat. In addition, installation of local exhaustventilation system may not be possible in some circumstances,because of the size, mobility, or mechanical interaction requirementsof the process. In these situations, gener
13、al ventilation is needed todilute pollutants and/or excess heat. Where pollutants are toxic orpresent a health risk to workers, local exhaust is the appropriateapproach, and dilution ventilation should be avoided. Air supplied bythe general ventilation system is usually conditioned (heated, humid-if
14、ied, cooled, etc.). Supply air replaces air extracted by local and gen-eral exhaust systems and improves comfort conditions in the occupiedzone.Chapter 11 of the 2013 ASHRAE HandbookFundamentals cov-ers definitions, particle sizes, allowable concentrations, and upperand lower explosive limits of var
15、ious air contaminants. Chapter 31 ofthis volume, Goodfellow and Tahti (2001), and Chapter 2 of ACGIH(2013) detail steps to determine air volumes necessary to dilute con-taminant concentration using general ventilations.Sufficient makeup air must be provided to replace air removedby the exhaust syste
16、m. If replacement air is insufficient, buildingpressure becomes negative relative to atmospheric pressure andallows air to infiltrate through open doors or window cracks, andcan reverse flow through combustion equipment vents. A negativepressure as little as 12 Pa can cause drafts and might cause ba
17、ck-drafts in combustion vents, thereby creating potential health andsafety hazards. From the sustainability perspective, a negative plantstatic pressure can also result in excessive energy use. If workersnear the plant perimeter complain about cold drafts, unit heaters areoften installed. Heat from
18、these units often is drawn into the plantinterior, overheating the interior. Too often, this overheating isaddressed by exhausting more air from the interior, causingincreased negative pressure and more infiltration. Negative plantpressure reduces the exhaust volumetric flow rate because ofincreased
19、 system resistance, which can also decrease local exhaustefficiencies or require additional energy to overcome the increasedresistance. Wind effects on building balance may also play a role,and are discussed in Chapter 24 of the 2013 ASHRAE HandbookFundamentals.Positive-pressure plants and balanced
20、plants (those with equalexhaust and replacement air rates) use less energy. However, if thereare clean and contaminated zones in the same building, the desiredairflow direction is from clean to dirty, and zone boundary construc-tion and pressure differentials should be designed accordingly.Exhaust s
21、ystem discharge may be regulated under various fed-eral, state, and local air pollution control regulations or ordinances.These regulations may require exhaust air treatment before dis-charge to the atmosphere. Chapter 30 of the 2012 ASHRAE Hand-bookHVAC Systems and Equipment provides guidance andre
22、commendations for discharge air treatment.The preparation of this chapter is assigned to TC 5.8, Industrial VentilationSystems.32.2 2015 ASHRAE HandbookHVAC Applications (SI)1. LOCAL EXHAUST FUNDAMENTALSSystem ComponentsLocal exhaust ventilation systems typically consist of the follow-ing basic elem
23、ents:Hood to capture pollutants and/or excessive heatDucted system to transport polluted air to air cleaning device orbuilding exhaustAir-cleaning device to remove captured pollutants from the air-stream for recycling or disposalAir-moving device (e.g., fan or high-pressure air ejector), whichprovid
24、es motive power to generate the hood capture velocity plusovercome exhaust ventilation system resistanceExhaust stack, which discharges system air to the atmosphereSystem ClassificationContaminant Source Type. Knowing the process or operationis essential before a local exhaust hood system can be des
25、igned.Hood Type. Exhaust hoods are typically round, rectangular, orslotted to accommodate the geometry of the source. Hoods areeither enclosing or nonenclosing (Figure 1). Enclosing hoods pro-vide more effective and economical contaminant control becausetheir exhaust rates and the effects of room ai
26、r currents are minimalcompared to those for nonenclosing hoods. Hood access openingsfor inspection and maintenance should be as small as possible andout of the natural path of the contaminant. Hood performance (i.e.,how well it captures the contaminant) should ideally be verified byan industrial hyg
27、ienist.A nonenclosing hood can be used if access requirements makeit necessary to leave all or part of the process open. Careful attentionmust be paid to airflow patterns and capture velocities around theprocess and hood (under dynamic conditions) and to the processcharacteristics to make nonenclosi
28、ng hoods effective. The use ofmoveable baffles, curtains, strip curtains, and brush seals may allowthe designer to increase the level of enclosure without interferingwith the work process. The more of the process that can be enclosed,the less exhaust airflow required to control the contaminant(s).Sy
29、stem Mobility. Local exhaust systems with nonenclosinghoods can be stationary (i.e., having a fixed hood position), move-able, portable, or built-in (into the process equipment). Moveablehoods are used when process equipment must be accessed for repairand loading and unloading of materials (e.g., in
30、 electric ovens formelting steel).The portable exhaust system shown in Figure 2 is commonlyused for temporary exhausting of fumes and solvents in confinedspaces or during maintenance. It has a built-in fan and filter and anexhaust hood connected to a flexible hose. Built-in local exhaustsystems are
31、commonly used to evacuate welding fumes, such ashoods built into stationary or turnover welding tables. Lateral ex-haust hoods, which exhaust air through slots on the periphery ofopen vessels, such as those used for galvanizing metals, are anotherexample of built-in local exhaust systems.Effectivene
32、ss of Local ExhaustThe most effective hood design uses the minimum exhaust air-flow rate to provide maximum contaminant control without com-promising operator capability to complete the work task. Captureeffectiveness should be high, but it is difficult and costly to develophoods with efficiencies a
33、pproaching 100%. Makeup air supplied bygeneral ventilation to replace exhausted air can dilute contaminantsthat are not captured by the hood. Enclosing more of the processreduces the need to protect against contaminant escape throughcross drafts, convective currents, or process-generated contaminant
34、momentum. In turn, this reduces the exhaust airflow required tocontrol the contaminant(s).Capture Velocity. Capture velocity is the air velocity required toentrain contaminants at the point of contaminant generation up-stream of a hood. The contaminant enters the moving airstream nearthe point of ge
35、neration and is carried along with the air into thehood. Designers use a designated capture velocity Vcto determine avolumetric flow rate to draw air into the hood. Table 1 shows rangesof capture velocities for several industrial operations. These figuresare based on successful experience under idea
36、l conditions. Oncecapture velocity upstream of the hood and hood position relative tothe source are known, then the hood flow rate can be determined forthe particular hood design. Velocity distributions for specific hoodsmust be known or determined.Hood Volumetric Flow Rate. For a given hood configu
37、rationand capture velocity, the exhaust volumetric flow rate (the airflowrate that allows contaminant capture) can be calculated asQo= VoAo(1)whereQo= exhaust volumetric flow rate, m3/sVo= average air velocity in hood opening that ensures capture velocity at point of contaminant release, m/sAo= hood
38、 opening area, m2Low face velocities require that supply (makeup) air be as uni-formly distributed as possible to minimize the effects of room aircurrents. This is one reason replacement air systems must bedesigned with exhaust systems in mind. Air should enter the hoodFig. 1 Enclosing and Nonenclos
39、ing Hoods(Adapted from ACGIH, Industrial Ventilation: A Manual of Recom-mended Practice, 27th ed. Copyright 2010. Reprinted with permission.) Fig. 2 Portable Fume Extractor with Built-in Fan and FilterIndustrial Local Exhaust 32.3uniformly. Hood flanges, side baffles, and interior baffles are some-t
40、imes necessary (Figure 3).Airflow requirements for maintaining effective capture velocityat a contaminant source also vary with the distance between thesource and hood. Chapter 3 of ACGIH (2013) provides methodol-ogy for estimating airflow requirements for specific hood configu-rations and locations
41、 relative to the contaminant source.Airflow near the hood can be influenced by drafts from supplyair jets (spot cooling jets) or by turbulence of the ambient air causedby jets, upward/downward convective flows, moving people,mobile equipment, and drafts from doors and windows. Processequipment may b
42、e another source of air movement. For example,high-speed rotating machines such as pulverizers, high-speed beltmaterial transfer systems, falling granular materials, and escapingcompressed air from pneumatic tools all produce air currents. Thesefactors can significantly reduce the capturing effectiv
43、eness of localexhaust systems and should be accounted for in the exhaust systemdesign.Exhausted air may contain combustible pollutant/air mixtures. Ifit does, the amount by which the exhaust airflow rate should beincreased to dilute combustible mixture must be verified to meet therequirements of Nat
44、ional Fire Protection Association (NFPA) Stan-dards 86 and 329.Principles of Hood Design OptimizationNumerous studies of local exhaust systems and common prac-tices have led to the following hood design principles:Hood location should be as close as possible to the source of con-tamination.The hood
45、opening should be positioned so that it causes the con-taminant to deviate the least from its natural path.The hood should be located so that the contaminant is drawn awayfrom the operators breathing zone.Hood size must be the same as or larger than the cross section offlow entering the hood. If the
46、 hood is smaller than the flow, ahigher volumetric flow rate is required.Worker position with relation to contaminant source, hooddesign, and airflow path should be evaluated based on the princi-ples given in Chapters 3 and 10 of ACGIH (2013).Canopy hoods (Figure 4) should not be used where the oper
47、atormust bend over a tank or process (ACGIH 2013).2. AIR MOVEMENT IN VICINITY OF LOCAL EXHAUSTAir capture velocities in front of the hood opening depend on theexhaust airflow rate, hood geometry, distance from hood face andsurfaces surrounding the hood opening. Figure 5 shows velocitycontours for an
48、 unflanged round duct hood. Studies have establishedthe similarity of velocity contours (expressed as a percentage of thehood entrance velocity) for hoods with similar geometry (Dalla-Valle 1952). Figure 6 shows velocity contours for a rectangularhood with an aspect ratio (width divided by length) o
49、f 0.333. Theprofiles are similar to those for the round hood but are more elon-gated. If the aspect ratio is lower than about 0.2 (0.15 for flangedopenings), the flow pattern in front of the hood changes fromapproximately spherical to approximately cylindrical. Velocitydecreases rapidly with distance from the hood; per DallaValle,velocity decreases on the order of 1/(distance from suction inletsquared).The design engineer should consider side drafts and othersources of air movement close to the capture area of a local exhausthood. Caplan and Knutson (1977, 1978) found that air m
