Ventilation 通风

Ventilation Systems 通风系统 Ventilation in one of its forms has been employed to control emissions for centuries 数百年来，人们一直采取通风措施来控制排放 Ventilation can be an effective control measure 通风可以是一种有效的控制措施 For this to occur systems need to be well designed & maintained 为此，需要精心设计并维护系统 Unfortunately, poorly designed & maintained ventilations are common 遗憾的是，设计拙劣及缺少维护的通风系统比比皆是

Types of Ventilation Systems 通风系统的类型 In the first instance ventilation systems used in the industrial environment to control hazardous substances are of two generic types 首先，用于工业环境控制有害物质的通风系统有两大类型   1) Supply-used to supply clean air to a workplace 供气型，用于向工作场所供应清洁空气 2) Exhaust-used to remove hazardous substances generated in a process so as to ensure a healthy workplace 排气型，用于排除工艺流程中产生的有害物质，以确保工作 场所卫生安全

Supply Systems 供气系统 Any complete ventilation system must consider both the supply and the exhaust functions if overall performance is to be achieved 如果要获得整体性能，任何完整的通风系统都必须同时考虑供气及排气功能. Supply systems generally have two purposes, ie: to create a comfortable environment by the control of factors such as temperature and humidity; or to replace air exhausted from the workplace 供气系统一般有两种作用，即：通过控制温度和湿度等因素构建舒适环境；或替换工作场所排出的空气。 Many extraction systems do not adequately consider the supply component of the overall system 许多抽气系统未充分考虑整体系统的供气组成

Exhaust Systems 排气系统 Two general types: 分为两大类： General Exhaust Systems – Removal of contaminants in a workplace by flushing out the contaminants via the introduction of large quantities of air. This is also called “dilution ventilation” and the dilution air must be in such quantities that the concentration of contaminants in the workplace is reduced to acceptable levels 通用排气系统——通过使用大量空气进行吹洗的方式，清除工作场所中的污染物，亦称为“稀释性通风”，稀释使用的空气量应足以将工作场所污染物的浓度降低到可接受的浓度

Exhaust Systems (cont) 排气系统（续） Local Exhaust Systems – Are used to capture a contaminant at or near its source thus ensuring that levels of contaminants do not reach unacceptable levels in the workplace. The common terminology for such systems is Local Exhaust Ventilation or LEV 局部排气系统——用于收集位于或靠近污染源处的污染气体，以确保工作场所内污染物的浓度达到可接受的浓度。该系统的通用术语是“局部排气通风（或LEV）”。

Other Systems 其它系统 Heating, Ventilation and Air‑Conditioning (HVAC) To mechanically provide fresh air for thermal comfort and health (known as general ventilation in some countries) 使用机械方式为热舒适与健康提供新鲜空气（在一些国家称为全面通风） Natural 自然通风 To dilute contaminants in the air by using wind or temperature differences to induce airflow 利用风或温度差产生气流稀释空气中的污染物 Supply 供气通风 -- To replace air removed from a ventilation system 替换从通风系统中清除的空气

Definitions 定义 Air Density 空气密度 The density () of air is defined as its mass per unit volume and at standard temperature and pressure is normally taken as 1.2 kg m-3. If non standard conditions exist, the density of the air can be calculated from the formula: 空气密度() 定义为在标准温度和压力下，每单位体积的质量，通常取1.2 kg m-3。如果存在非标准条件，可以使用下面的公式计算空气密度：

Air Density - Non Standard Conditions 空气密度——非标准条件

Air Density - Non Standard Conditions 空气密度——非标准条件 o = Air density at non standard conditions 非标准条件下的空气密度 bo = Barometric pressure at non standard conditions 非标准条件下的大气压力 bs = Barometric pressure at standard conditions (760 mm Hg) 标准条件下的大气压力(760 mm Hg) To = Absolute temperature at non standard conditions 非标准条件下的绝对温度 Ts = Absolute temperature at standard conditions (293°K)  标准条件下的绝对温度(293°K) 

Pressure 压力 For air to flow there must be a pressure difference and air will flow from the higher pressure to the lower pressure 空气要流动就必须存在压力差，空气将从高气压区流向低气压区。 Pressure is considered to have two forms; these being static pressure (s) and velocity pressure (v); with the sum of these being total pressure (t). 压力被认为存在两种形式：静压力(s) 和动压力(v)；两者之和即为总压力(t)。 Thus 于是有：   t = s + v

Static Pressure 静压力 Static pressure is defined as the pressure exerted in all directions by a fluid that is stationary 静压力定义为流体在静止时作用在所有方向上的压力。 If the fluid is in motion (as is the case in a ventilation system), static pressure is measured at 90° to the direction of the flow so as to eliminate the influence of movement (ie: velocity) 如果流体流动（通风系统中正是如此），应在与流动方向成90°的方位测量静压力以消除移动影响（比如：速度）。 Static pressure can be both positive and negative depending if it is measured on the discharge or suction side of a fan 静压力值既可为正也可为负，取决于测量位置是风机的吸入端还是排放端。

Velocity Pressure 动压力 Velocity pressure is defined as that pressure required to accelerate air from zero velocity to some velocity and is proportional to the kinetic energy of the air stream. In simple terms, velocity pressure is the kinetic energy generated in a ventilation system as a result of air movement 动压力定义为使风速从零增加到某数值所需的压力，与气流动能成正比。简言之，动压力为通风系统中由于空气流动而产生的动能。

Velocity Pressure (cont) 动压力（续）  = Density of air 空气密度 v = Air velocity 气流速度

Velocity Pressure (cont) 动压力（续） If standard temperature and pressure conditions are in existence, ie: 在标准温度和压力条件下，即有：    = 1.2 kg m-3 Then 那么 v = 0.6 v2

Velocity, Static & Total Pressure 动压力、静压力&总压力

Volume & Mass 体积与质量 When a quantity of air is moving within a ventilation system the volumetric flowrate is a product of the velocity of the air and the cross‑sectional area of the system through which it is flowing. 当一定量的空气在通风系统内流过时，体积流量为气流速率与气体流经横截面面积的乘积。 Thus 于是有 Q = vA Where 式中 v = Average air velocity over the cross‑section of the system (ms-1) 穿过系统横截面的平均气流速率（ ms-1 ） A = Cross-sectional area of the system where the velocity measurement was made (m2) 速率测量处的系统横截面面积（ m2 ） Q = Volume flow rate (m3 s-1) 体积流量 (m3 s-1)

Capture Velocity 吸入速度 Is the air velocity required at the source of emission so as to cause the contaminant to move towards the capture device and thus be removed 吸入速度即排放源处所需的气流速度，以使污染物流向吸取装置，从而排出。 N 你

Capture Velocity (cont) 吸入速度（续） v = Q 10X2 + A Where v = air velocity at a distance X from the hood in ms-1 式中 离烟尘罩 x 距离处的气流速度（单位：ms-1 ） Q = air flow rate in m3 s-1 气流速率（单位：m3 s-1 ） X = distance from hood in metres 离烟尘罩的距离（单位：m） A = cross-sectional area of hood in m2 烟尘罩横截面积（单位： m2 ） Note: only accurate for situations where X is less than 1.5 times the diameter of circular hoods or the side of square hoods 注：只有当X小于圆形烟尘罩直径的1.5倍，或正方形烟尘罩边长的1.5倍时，此公 式才精确。

Face Velocity 罩口风速 Is the air velocity at the opening of an enclosure or hood. Similarly, the slot velocity is the air velocity in slots 罩口风速即排烟罩或烟尘罩开口处的气流速度；类似地，槽口流速为开槽处的气流速度。

Transport Velocity 转移速度 Is the minimum air velocity required at any point in the ventilation system to ensure that collected particles remain airborne and are thus not deposited within any part of the system except the collector 转移速度是在通风系统内某处，为确保收集的微粒保持悬浮状态，不会沉淀在系统中，而只堆积在收集器里所需要的最小气流速度。 In simple terms, the lower the density and size of the particles the lower the transport velocity 简言之，微粒粒度与密度越小，转移速度也越小。

Typical Transport Velocities 典型转移速度 Lead and zinc fumes 含铅烟气和含锌烟气 7 to 10 ms-1 Heavy moist dust such as lead chippings 重度潮湿粉尘，比如铅屑 in excess of 22.5 ms-1 超过22.5 ms-1

Ventilation System Components 通风系统的组成 Dilution system 稀释系统 Low pressure fan, ducting, air cleaning system 低压风机、管道及空气净化系统 Local exhaust system 局部排气系统 Fan, ducting, air cleaner, hood (or slot or flange) 风机、管道、空气净化器及烟尘罩（或插槽和凸缘）

Basic Components of an LEV System 局部排气通风系统的基本组成 空气驱动器 排放 管道 空气净化器 烟尘罩 进气 (Source: HSE – reproduced with permission) （来源：HSE-经许可转载）

Fans 风机 A fan consists of a series of blades (impeller) mounted on a central axis (shaft) which is connected to an energy source (fan motor) 风机由一组叶片（叶轮）组成，安装在与动力源（风机电动机）相连接的中心轴上。 These components are then enclosed within a casing or housing which creates a pressure difference (and hence airflow) between the inlet and outlet by the rotation of the impeller 然后这些组件装入一外壳中，通过叶轮的转动，在入口和出口之间产生压力差，从而生成气流。

Fan Types 风机的类型 Propeller fans 螺旋桨式风机 Axial fans 轴流式风机 Centrifugal fans 离心式风机 Turbo exhausters or multi stage centrifugal fans 涡轮排气机或多级离心式风机 Compressed air driven movers 压缩空气驱动器

Propeller Fans 螺旋桨式风机 Consist of fan blades (metal or plastic) connected to a hub which is attached directly to an energy source (electric motor) or via a belt system 由安装在芯轴上的风机叶片（金属或塑料的）组成，芯轴直接或通过皮带传动系统与动力源（电动机）相连。 Propeller fans are generally low in efficiency and are not suitable for ducted or air filtration systems. Propeller fans are most often used for general or dilution ventilation 螺旋桨式风机通常效率很低，不适合用于管道或空气过滤系统，大多数情况下用于普通或稀释通风。

Propeller Fans (cont) 螺旋桨式风机（续） (Source: HSE – reproduced with permission （来源： HSE ——经许可转载）

Axial Fans 轴流式风机 Consist of a cylindrical casing with the shaft of the impeller being at the centre of the casing. The impeller blades usually have an aerofoil shape and rotate within the casing with their tips very close to the casing 由筒状外壳组成，螺旋桨轴位于壳体中心。叶轮叶片呈机翼形状，在壳体内部转动，叶片尖紧靠壳体。 While axial fans are similar in principle to propeller fans they produce much higher pressures 轴流式风机原理同螺旋桨式相似，但轴流式风机能产生更大压力。 Axial fans are normally mounted within the ventilation system ductwork 轴流式风机通常安装在通风管道系统中

Axial Fans (cont) 轴流式风机（续）

Centrifugal Fans 离心式风机 Consist of a rotating impeller shaped like a paddle wheel which moves air drawn into the centre of the impeller towards its outer edge where it is collected in a specially designed casing, and ejected at a tangent to the impeller 由形状类似浆轮的旋转叶轮组成，使吸入叶轮中心的空气在特别设计的壳体中向外边沿移动，并沿叶轮切线方向喷出。 Because of their design, centrifugal fans generate large differences in pressure and thus high airflows. Blade shape changes the characteristics of the fan in the following way 这种设计使得离心式风机能产生更大的压力差和高速气流，叶片的造型又能以下面的方式改变风机的特性：

Radial Blade (or paddle‑bladed) 星型叶片（径向叶片） These centrifugal fans are robust, easy to repair, clean and maintain 这些离心式风机坚固耐用，易于维修、清洁及保养。 They are used in applications where highly corrosive air or heavy dust loads are present 用于高度腐蚀性气体或重度烟尘场所

Forward Blade 前曲式叶片 These fans have many small blades with their tips inclined towards the direction of rotation 这种风机有许多小型叶片，片尖朝转动方向弯曲。 This design gives rise to an overall compact fan where space is limited but is not suitable for dusty systems 这种设计催生了空间有限条件下的整体紧凑型风机，但不适用于粉尘系统。

Backward Blade 后曲式叶片 These fans have fewer, deeper blades than the forward bladed system and are less compact 这种风机的叶片比前弯式少，但叶片的曲度更深，且结构不紧凑。

Centrifugal Fan 离心式风机

Turbo Exhausters or Multistage Centrifugal Fans 涡轮排气机或多级离心式风机 Can produce very high pressures to power low volume but high velocity systems 它们能产生极高的压力，以驱动小体积但高流速的系统 These are special fans that require protection of their precision blades from damage caused by dust, etc. 这类风机属特殊风机，其精密叶片需要特殊保护，以防止粉尘等引起的损坏

Compressed Air Driven Air Movers 压缩空气驱动器 These are used where electrically powered fans are unsuitable 这种风机在不适合使用电动风机的场合下使用 The advantages are their relatively small size which makes them reasonably portable while their disadvantages are their high cost of operation and excessive noise 优点是其尺寸相对较小，使得携带方便，缺点是运行成本高，且噪音过大。

Compressed Air Driven Air Movers (cont) 压缩空气驱动器（续）

Ductwork 管道系统 The supply of air via a ventilation system is dependent on the transfer of air from one point to another via ductwork 通风系统的供气依靠管道系统中一点到另一点的空气输送 Ducting is usually made from galvanised steel however other materials such as PVC, fibreglass, brick or concrete can be used 管道通常由镀锌钢制成，也可使用其它材料，比如聚氯乙烯、玻璃纤维、砖或混凝土等。 Ducting is usually formed into circular or rectangular sections and the shape and size of ducts are designed to fit the needs of the ventilation system, building and associated plant 管道剖面通常成圆形或方形，设计时管道形状和大小要满足通风系统、建筑物和相关设备的需要。

Ductwork (cont) 管道系统（续） When selecting the size of ducting for a particular application a number of factors need to be considered, such as: 当为特定应用选择管道尺寸时，需要考虑若干因素，诸如： For a given volume flowrate the larger the duct size the lower the air velocity and absorbed energy (friction) but the bigger the capital cost 在给定体积流量条件下，管道尺寸越大，气流速度和吸收能（摩擦）越小，但投资成本却越高。 Circular cross section ducting is more economical in terms of construction materials but space requirements may dictate the use of rectangular cross section ducting 从建筑材料角度来讲，圆形截面的管道更合算，但空间要求也可以决定方形截面管道的使用。

Ductwork (cont) 管道系统（续） If there is a need to maintain a particular transport velocity and thus a required volume flowrate is established, the duct cross section can be calculated from the base formula: 如果需要保持某一特定转移速度，以达到需要的体积流量，可以由以下基本公式计算管道横截面： Q = vA

Ductwork (cont) 管道系统（续） Always be losses over any duct system 任何管道系统都有损失 Losses are expressed in terms of pressure loss 管道损失采用压力损失表达 Need to calculate total pressure losses 需要计算总压力损失 From this the duty of the fan can be established, being the calculated volume flowrate at the total pressure loss 由此可以得出风机的负荷，这就是总压力损失条件下的体积流量。

Ductwork (cont) 管道系统（续） To ensure that an appropriate fan is selected the velocity pressure at a set discharge velocity (irrespective of what the set value is) must be added to the total pressure loss as this represents the overall energy that the fan must provide 为确保选用合适的风机，必须将设定排放速度下的动压（无论设定值是多少）加到总压力损失中，因为这样才能表示风机必须提供的总能量。 Failure to make this adjustment will result in the fan being undersized for the duty for which it is required 若不进行此项调整，将会导致要求负荷下风机尺寸过小

Factors to be Considered when Designing a Dust System 设计管道系统考虑的因素 What construction materials should be used? 应使用什么建筑材料？ What velocity should be maintained in the duct? 管道中应维持多大的速率？ What cross section shape should be used? 应采用怎样的横切面形状？ Branching of ducting 管道分支

Balancing a Dust System 管道系统平衡操作 Balancing is to ensure that each branch of the system is operating at its maximum performance 平衡操作可确保每个系统分支发挥最大功能 Balancing requires starting at each hood or branch and making adjustments while working towards the fan 平衡操作需启动每个烟尘罩或分支，并在朝着风机运行时进行调整 The correct balancing or rebalancing of a system is a highly skilled activity and should only be conducted by experienced persons 系统正确的平衡或再平衡操作是一项高技术含量活动，并应只让经验丰富的人员执行

Inspection Openings 检查开口 In any duct system it is important to provide leak‑proof inspection openings to allow for the inspection and cleaning of ducts 在任何风道系统中，配备防漏检查开口，以检查和清洁风道是十分重要的 It is also important to provide test points where at a minimum the static pressure can be measured 在最小静压可测量的地方配备检测点也十分重要 These should be located after each hood or enclosure, at key points in the duct system and at certain components to measure pressure drops (ie: fans and filter) 检查开口应设置在每个烟尘罩或封闭空间后面，在风道系统的重要点上和某些测量压力差的部件上（即：风机和过滤器）

Key Design Points 关键设计要点 Keep the design as simple as possible 尽量保持设计简单 Keep the number of bends and junctions to a minimum so as to reduce flow resistance 保持弯管和连接点数量最少从而减小流动阻力 When changes in direction are necessary they should be made smoothly 当需要变换方向时，结构应平滑

Key Design Points (cont) 关键设计要点（续） ‘T’ junctions should never be used and tapered sections should be used when the duct cross section needs to change 风道横截面需变换时，不应使用“T”型连接点，应使用锥形段 Avoid long lengths of flexible ducting, especially where high flow resistance is present 要避免使用长段的饶性风管，尤其在流动阻力大的地方 Avoid ‘U’ bends as they act as traps for particles and can eventually block the duct 要避免使用“U”型弯管，因为这样的弯管易堆积微粒并最终堵塞风道

Air Cleaners 空气净化器 It is important to remember that in many situations air discharged to the atmosphere must be cleaned 必须记住在很多情况下，排放到大气中的空气都必须净化 The process of cleaning discharge air of its contaminants depends to a large extent on the contaminant type 排放空气中污染物的净化工艺，在很大程度上取决于污染物的类型

Types of Air Cleaners 空气净化器的类型 Particulates 微粒 Cyclones 气旋式 Electrostatic precipitators 静电除尘器 Fabric filters 粉尘团过滤器 Wet systems such as venturi scrubbers, cyclones 湿型系统，如文氏管洗涤器和旋风分离器 Gases & vapours 气体和气化气 Absorption 吸收 Chemical scrubbing 化学清洗 Thermal destruction 热分解

Discharge of Contaminants 污染物的排放 Irrespective whether or not extracted air has been cleaned, it must be discharged to atmosphere in such a way that it does not re‑enter the building (or any adjacent buildings) until the contaminant concentrations have been diluted to acceptable levels 不管抽吸的空气是否已被净化，排放到大气中的空气必须直到污染物浓度稀释至可接受浓度后，才能再次进入建筑物（或任何紧邻的建筑物内）

Discharge of Contaminants (cont) 污染物的排放（续） The best methods of air discharge via stacks involve releasing the air to the atmosphere as high as is possible and at a high velocity 通过烟囱排放空气的最好方法，包括了在尽可能高的地方排放和高速率排放 Devices such as cowls and weather caps which direct the discharged air downwards should be avoided 应避免使用将排放空气向下引导的通风帽和迎风罩 Need to understand the airflow patterns around the location of the discharge so that the best possible location can be achieved 要了解排放处周围的气流模式，以便能找到最好的地方

Dilution Ventilation 稀释性通风 Dilution ventilation (general exhaust ventilation in some countries) is in essence the dilution of contaminated air with sufficient uncontaminated air so as to reduce the concentration of contaminants to acceptable levels 稀释性通风（在一些国家称为全面排气通风）本质上就是用足够的未污染空气稀释已污染空气，从而将污染物浓度减小至可接受水平 Dilution ventilation provides a lower level of control for health hazards than can be achieved by the use of local exhaust ventilation 通过使用局部排气通风，稀释性通风提供了一种较低水平的健康危害控制方法

May be Appropriate When (AIHA) 适用于（美国工业卫生协会）的方法 The air contaminant has low toxicity 空气污染物为低毒性 There are multiple sources 多种污染源 The emission is continuous 持续排放 The concentrations are close to or lower than the occupational exposure limit 浓度接近于或低于职业接触限制

May be Appropriate When (cont) 适用于（美国工业卫生协会）的方法 The volume of air needed is manageable 所需空气体积可控制 The contaminants can be sufficiently diluted before inhalation 吸入前，污染物能被充分稀释 Comfort or odour is an issue 舒适或气味也是一个方面 A spill has occurred and extended airing of the workplace is needed 发生泄漏，工作场所需持续换气

Limiting Factors (AIHA) 限制因素（美国工业卫生协会） The quantity of contaminant generated must not be too great or the dilution air flow rate will be impractical 产生的污染物数量不能太大，否则稀释空气流量将无法达到要求 Workers must be distant from the contaminant source or evolution of the contaminant must be in low concentrations so that workers’ exposures do not exceed exposure limits 工人须远离污染源，或污染物的扩散须处于低浓度，从而使工人接触不超过受污染的限制 The toxicity of the contaminant must be low 污染物须为低毒性 The evolution of contaminants must be reasonably uniform 污染物的扩散须合理均匀

Types of Dilution Ventilation 稀释性通风的类型 Dilution ventilation can be either forced (mechanical means to achieve air movement) or natural 稀释性通风可为强制性的（通过机械方式以达到空气流动）或自然性的 If natural airflows are used to achieve dilution of contaminants to the required level, factors such as wind direction, wind speed and air temperatures are likely to influence the effectiveness of the process 如自然空气流用于稀释污染物以达到要求水平，风向、风速和空气温度等因素可能会影响稀释过程的效果

Natural Ventilation 自然通风 It is important to remember that if sole reliance is placed upon natural ventilation, little or no control will be achieved on days when there is no wind or if it blows from the wrong direction 必须记住，如只能进行自然通风，当无风或风向不对时将无法或根本不能控制

Dilution Ventilation Equations 稀释性通风的方程式 If we consider a constant emission source, perfect mixing and a constant airflow, the following equation represents the equilibrium concentration in a ventilated space (eg: a room) 假设有一个持续排放源、并假设理想混合和一个持续空气流，下列方程式表示在一个通风空间（例如：一个房间）中的均衡浓度

Equilibrium Concentration 均衡浓度 Where 其中 Q = Airflow rate in m3s-1 空气流速，单位m3s-1 r = Emission rate in mgs-1 排放速率，单位mgs-1 C = Equilibrium concentration in mgm-3 平衡浓度，单位mgm-3

Equilibrium Concentration (cont) 平衡浓度（续） Possible to use this equation to calculate the airflow rate to reduce the potential worker exposure to the exposure standard (or some fraction of the OEL) provided we know the emission rate 只要知道排放速率，即可用此方程式计算空气流速，以减少工人潜在的暴露，从而达到暴露标准（或职业接触限制的某标准） The value “r” is difficult to establish as it has to be based on the amount of pollutant released into the atmosphere. This depends on the consumption of the source material and release rate “r”值很难确定，因其要依据于排放到大气中的污染物的数量，这又取决于产生污染源的材料的消耗量和排放速率

Decay in Contaminant Concentration with Time 随着时间推移污染物浓度衰减  Where 式中 Co = Initial contamination concentration (ppm) 污染物初始浓度（ppm） Q = Airflow (m3 s-1) 空气流速（ m3 s-1） V = Volume of ventilated space (m3) 通风空间体积（ m3） t = Time (s) 时间（s） R = Ventilation rate (Q/v) 通风速率（ Q/v）

Example 实例 If we consider a room of 10 m3 with an initial contaminant concentration of 1,000 ppm and a diluting airflow of 0.1 m3 s-1 the concentration in the room after 10 minutes would be 2.5 ppm (complete mixing) 如假设一个十平米的房间，污染物初始浓度为1000ppm，并且稀释空气流速为0.1 m3 s-1 ，10分钟后，房内浓度为2.5ppm（完全混合） C = 1000e-0.01x600 = 2.5 ppm

Contaminant Decay With Dilution Ventilation 随着稀释通风污染物衰减 对数浓度（ppm） 浓度（ppm） 对数模式直线 指数递减 时间（秒） Source: AIOH 2007 – reproduced with permission 来源： AIOH 2007 –经许可转载

Incomplete Mixing 不完全混合 Example is misleading as it refers to perfect mixing 上述实例具有误导性，因为其提到的是理想混合情况 Incomplete mixing would probably result in an underestimate of concentration 不完全混合很可能导致低估浓度 To over come this issue it is common to apply a “K” factor 要克服此问题，通常采用一个“K”系数 K factors range from 1.0 to 0.1 K系素的范围从1.0至0.1

Incomplete Mixing (cont) 不完全混合（续） The “K” value is somewhat arbitrary and according to the ACGIH (ACGIH 2007) is based on  “K”值有随意性，根据美国政府工业卫生学家会议 （2007），规定其要依据 The efficiency of mixing and distribution of replacement air 混合效率和替换空气分布 Toxicity of the solvent 溶剂的毒性 A judgement by the occupational hygienist as to any other circumstance of importance 根据任何其它重要情况，职业卫生学家作出的判断

For a Particular Substance 针对特定物质 If the rate of evaporation (mgs-1) and density (kg m-3) 如蒸发速度（ mgs-1 ）和密度（ kg m-3 ） Q = Rate of Evaporation Density x ES Q = 蒸发速度 密度x ES Q = amount of air to reduce vapour concentration to Exposure Standard 减小气化气浓度至接触的标准的空气量 Need to apply a suitable K factor 需要采用一个恰当的K系素

Infiltration 渗入 Infiltration occurs when air is drawn into a building through vents and unintentional openings 渗入发生于空气通过排气口和意外开口进入建筑物 This process may adversely affect the general extraction ventilation system by drawing in air polluted with unwanted contaminants or changing the airflow direction so that adequate dilution of the prime contaminant source does not occur 此过程通过带进含有意外污染物的污染空气，或改变空气流动方向，导致主要污染物源没有充分稀释，从而对通风系统的总系抽吸产生不利影响

Displacement Ventilation 置换式通风 More correctly named thermal displacement ventilation 更准确的名称为热置换通风 Not used in many countries for the control of hazardous substances 在许多国家针对有害物质控制并未使用该方式 New slightly cooler air is introduced to the workplace near floor level and thus the contaminant‑laden air (which is slightly warmer) rises and exits from the workplace near the roof, thus leaving the workers in a clean atmosphere 新鲜、稍稍较凉的空气从工作场所接近地面的地方引进，因此污染重的空气（较热）上升并从工作场所接近房顶的地方排出，从而使得工人处于一个干净的环境中

Works Best When: 当存在下列情况时，为最佳运行： The contaminants are warmer than the surrounding air 污染物比周围空气热时 The supply air is slightly cooler than the surrounding air 供给空气比周围环境稍冷时 The room is relatively tall (>3 m) 房间相对较高（>3m）时 There is limited movement in the room 房间内有流动限制时

Application of General Ventilation Systems 通用通风系统的应用 Major air contaminants are of relatively low toxicity 主要空气污染物为相对低毒性 Contaminant concentrations are not hazardous 污染浓度为无危害 Smoking is not allowed in the occupied space 使用空间不准吸烟 Emission sources are difficult or expensive to remove 消除排放源困难且昂贵 Emissions occur uniformly in time. 排放应统一适时进行

Application of General Ventilation Systems (cont) 通用通风系统的应用（续） Emission sources are widely dispersed 排放源应广泛分散 Emissions do not occur close to the breathing zone of people 排放不应在人们呼吸区附近进行 Moderate climatic conditions prevail 温和的气候条件最好 The outside air is less contaminated than the inside air 外部空气比内部空气污染小 The HVAC system is capable of conditioning the dilution air 采暖、通风和空调系统可调节稀释空气

Limitations of General Ventilation Systems 通用通风系统的局限 No guarantee that all air has been changed at least once 不能保证所有空气至少有一次全已被置换 Hazardous substances may be moved towards the worker 有害物质可能会移向工人 Process that have short high level releases require large amounts of air 短时间高浓度排放过程需大量空气

Limitations of General Ventilation Systems (cont) 通用通风系统的局限（续） Seasonal reductions in ventilation rate may result in unacceptable conditions 季节性通风速度减小可能导致不理想的情况 Loss of efficiency may occur as system ages 效率降低可能由系统老化引起 Entering air (supply) is assumed to be clean which is often not the case 假设进入的空气（供气）干净，但经常并非如此

AIHA – Less Effective & More Expensive Because: 美国工业卫生协会—效率低费用高是因为： Air contaminants are highly toxic materials 空气污染物为高毒性材料 Contaminant concentrations are hazardous 污染物浓度具有危害性 Smoking is allowed in the occupied space 使用空间允许吸烟 Emission sources are easy to remove 排放源易于消除 Emissions vary with time 排放随时间变化

AIHA – Less Effective & More Expensive Because: 美国工业卫生协会—效率低费用高是因为： Emission sources consist of large point sources 排放源包含了大型来源 People’s breathing zones are in the immediate vicinity of emission sources (ie: less than 1 m) 人们呼吸区直接靠近排放源（即：小于1米） The building is located in severe climates 建筑物设置在气候恶劣的地区 The outside air is more contaminated that the inside air 外部空气污染比内部空气严重 The existing HVAC system is not capable of treating the air 现有的采暖、通风和空调系统无法调节空气

A Word of Caution 注意事项 The introduction of new or larger sources of chemicals or particulates may create hazards the previously satisfactory system cannot now handle 引进新的或大的化学制品或微粒物质来源，可能会产生之前符合要求的系统无法处理的危害物质 A new process may include a substance whose airborne concentrations must be held to lower levels than those previously in use 新的过程可包含新的物质，但该物质的空气浓度应比以前使用的物质的浓度须降到更低的水平 Any process change may effect worker exposures and needs careful evaluation 任何过程变化可能影响工人接触污染，故需认真评估

Local Exhaust Ventilation Systems 局部排气通风系统 空气推进器 排放 管道 空气净化器 烟尘罩 入口 Typical LEV System 标准局部排气通风系统 Source: HSE – reproduced with permission 来源：HSE-经许可转载

Local Exhaust Ventilation (LEV) 局部排气通风（LEV） Almost always preferred method of controlling airborne hazardous substances 几乎是控制空中危害物质的常用方法 Contaminant removed prior to workers being exposed 在工人接触污染之前去除污染物 Rare to see good LEV systems in workplaces 工作场所很少见到性能良好的局部排气通风系统 All LEV systems have to be routinely maintained & tested 所有局部排气通风系统都得进行常规维修和检测

Hood Design 烟尘罩设计 One of the most important components in any LEV system is the hood and the effectiveness of the system can be defined in terms of how the contaminant cloud is contained, received or captured by the hood 任何局部排气通风系统中重要部件之一，都是烟尘罩，且从烟尘罩如何控制、接收和集气污染云团，即可确定系统效能 Hoods can have many shapes and sizes, however they can be grouped under three basic categories 烟尘罩具有许多形状和尺寸，然而可根据三种基本类型分组

Types of Hoods 烟尘罩类型 Enclosing hoods 封闭式烟尘罩 Receiving hoods 收气式烟尘罩 Capturing hoods 集气式烟尘罩

Types of Hoods (cont) 烟尘罩类型（续） 封闭式 收气式 集气式 全封闭 顶盖 简化式集气式烟尘罩 封闭“间” 空气流 其它收气式烟尘罩 部分封闭—小间 可移动式污染源和烟尘罩 边缘抽气 向下通风台 推—拉系统 Source: HSE – reproduced with permission 来源：HSE-经许可转载 低音高速

Enclosing Hoods 封闭式烟尘罩 Enclosures are designed to surround, to a large extent, the process or task requiring ventilation, with the common forms of these devices being booths and fume cupboards 封闭空间的设计，在很大程度上是为了适应于需要进行通风的生产工艺或生产任务，分隔小间和烟雾柜是此类装置的通用形式 The aim of an enclosure is to maintain a negative pressure within the enclosure so that any sudden release of contaminant does not escape to the surrounding environment 封闭空间的目的，是维护封闭空间中的负压，从而任何污染物的突然排放都不会逃逸到周围的环境中

Examples of Large and Small Enclosures 大小封闭空间实例 Source: HSE – reproduced with permission 来源：HSE-经许可转载

Receiving Hoods 收气式烟尘罩 In this situation the process is located outside the hood and the contaminant cloud (which has speed and direction) is received by the hood 在这种情况下，把收集过程布置在烟尘罩外部，由烟尘罩收集污染云团（云团有速度和方向） Receiving hoods can be either fixed or moveable, with a canopy hood over a hot process being a classic example 收气式烟尘罩可为固定式或移动式，经典的实例就是在热收集过程中采用的是带有顶盖的烟尘罩

Receiving Hoods (cont) 收气式烟尘罩（续） Receiving hoods can be applied in situations where a process produces a contaminant cloud that has a predictable strength and direction 对于产生污染云团的生产工艺，而污染云团又有可预测的强度和方向，即可采用收气式烟尘罩 It is important to note that the receiving hood must be large enough and close enough to effectively collect the contaminant cloud and other high velocity materials generated from the source  值得注意的是，收气式烟尘罩必须有足够的尺寸，且充分靠近污染源，从而有效地收集污染云团和其它从污染云团生成的高速物质

Receiving Hood 收气式烟尘罩 不可视的精细微粒云团 喇叭形入口 烟尘罩收集的主要污染云团喷流 不受控制的较小污染云团喷流 保护装置 可视大微粒流 污染源 Source: HSE – reproduced with permission 来源：HSE-经许可转载

Receiving Hood (cont) 收气式烟尘罩（续） (Show HSE video of grinding receiving hood) 展示研磨工艺中收气式烟尘罩的健康、安全与环境的录像

Capturing Hoods 集气式烟尘罩 Work on the principle of generating sufficient air movement from an extraction system so that the contaminant‑laden air is drawn to the extraction point 在萃取系统中，集气式烟气罩的工作原理就是能产生足够的空气流动，以便能从提取点吸出饱含污染物的空气。 Process can be visualised by the use of a smoke tube to establish the capture zone for a particular air velocity 通过使用烟尘管，即可为特定气流速度建立集气区，该过程可想而知。

Capture Zone of a Hood 烟尘罩集气区 Source: AIOH 2007 – reproduced with permission 来源： AIOH 2007——经许可转载

Capturing Hoods (cont) 集气式烟尘罩（续） Capturing hoods are widely used because they are relatively easy to retrofit, do not interfere with the process as much as other systems and are commercially available as “off‑the‑shelf” items 集气式烟尘罩被广泛应用，因为它们比较容易改造，与其他系统一样，不会干扰通风过程，且集气式烟尘罩如“现货供应”的商品，其市场化程度很高 Often much less effective than intended 实际效果往往低于预期效果 While issues can be resolved often much simpler to choose an alternate strategy e.g. - partial enclosure 但解决这些问题的方法往往非常简单，只需另选替代策略即可，例如： 实行部分封闭

Issues Reducing Effectiveness 效率减少的问题 Capture zone too small 集气区太小 Capture zone disrupted by cross drafts 集气区被横越气流打断 Capture zone does not include the working zone 集气区不包括工作区 Process requirements moves the working zone outside the capture zone 通风过程要求工作区移到集气区外面 Incorrect calculation of capture zone 对集气区的计算不正确

Relationship Between Capture , Working & Breathing Zones 集气区、工作区和呼吸区之间的关系 有效 部分有效 无效 Source: HSE – reproduced with permission 来源：HSE ——经许可转载

(Show HSE soldering video) Relationship Between Capture , Working & Breathing Zones (cont) 集气区、工作区和呼吸区之间的关系（续） (Show HSE soldering video)

Flanges 法兰 Flanges are an integral part of a hood and help shape the airflow into the hood, thus increasing efficiency 法兰是烟尘罩的一个组成部分，它帮助气流进入烟尘罩，从而提高效率 Flanges can reduce the amount of air from behind the hood entering the system and thus extending the range of the hood to capture contaminated air 法兰可减少从烟尘罩后面进入系统的空气量，从而扩大烟尘罩采集污染空气的范围 Smooth airflow & reduces degree of flow separation 平滑气流并降低气流分离度 which minimises turbulence & maximises hood efficiency 使气体涡流最小化且使烟尘罩效率最大化

Effectiveness of a Flanged Hood 法兰式烟尘罩的效率 风机 非法兰部分增加效率10% 法兰部分增加效率10% Source: AIOH 2007 – reproduced with permission 来源： AIOH 2007 ——经许可转载

Slot 插槽 Slots are hoods where the width‑length aspect ratio is less than 0.2 烟尘罩配有长宽比小于0.2的插槽 Often used where accessibility is constantly required or where process materials are required to constantly enter the system 插槽式烟尘罩经常使用在要求不断进出的场合，或用在要求加工材料不断进入系统的场合 Commonly used on degreasing tanks, cleaning baths and electroplating tanks so as to remove any contaminant vapours released in the tanks or bath from the parent solution 插槽式烟尘罩常用于酸洗池、清洗池和电镀池，以便去除从池子里母液中释放出的污染物蒸气

Push – Pull Systems 推拉系统 In some situations it may be necessary to improve the capture efficiency of the system and this can be achieved by the inclusion of a supply slot on the opposite side of the tank, etc 在某些情况下，可能还需要提高系统的集气效率，这可通过在池子的另一侧增加供气槽来实现 This is commonly known as a “push‑pull” system 这就是通常所说的“推拉”系统

Issues with Push‑Pull Ventilation Systems 推拉式通风系统 High air velocities over the surface of a liquid in a tank will result in increased evaporation 在池子中，液体表面高速流动的气流会导致蒸发量增大 Any interruption of the air jet (push side of the system) will result in contaminant‑laden air being deflected and potentially increasing the exposure of workers 任何对气体喷射（系统的推气侧）的中断都将导致饱含污染物的空气转向，并可能增大对工人的暴露

Simple Principles of Good Hood Design 良好烟尘罩设计的简单原则 Placing the hood as close as possible to the source, preferably enclosing it 将烟尘罩放在离烟尘源尽可能近的地方，最好包围着烟尘 If the source includes fast‑moving particles, positioning the hood to receive those particles 如果烟尘源中有快速移动的粒子，即把烟尘罩放在吸收这些粒子的地方 Specifying a ‘capture velocity’ at a point greater than the particle velocity 在一个点确定一个大于粒子速度的“集气速度” Locating the hood so that a line from the operator’s face to the contaminant source leads directly towards the hood 放置烟尘罩的方式，要使气体从操作者的脸部至污染物源处的线路直接导向烟尘罩

Examples of LEV & Hood Design 局部排气通风和烟尘罩设计示范 Source: HSE – reproduced with permission 来源：HSE ——经许可转载

Examples of LEV & Hood Design 局部排气通风和烟尘罩设计样本 Source: HSE – reproduced with permission 来源：HSE ——经许可转载

Examples of LEV & Hood Design 局部排气通风和烟尘罩设计样本 Source: HSE – reproduced with permission 来源：HSE ——经许可转载

Examples of LEV & Hood Design (cont) 局部排气通风和烟尘罩设计样本（续） (Show HSE LEV Video I & 2)

Measurement & Testing of LEV Systems 局部排气通风系统的测量与测试 The performance of every ventilation system needs to be established and routinely monitored 需要确定并日常监测每个通风系统的性能 That the source contaminant is being successfully captured at its point of release 在污染物排放点成功捕获污染物 The air velocity at appropriate points throughout the ventilation system 在整个通风系统中适当点的空气流速 The pressure losses throughout the system. 整个系统的压力损失 The pressure differential across the fan 通过风机的压差

Air Velocity 气流速度 Vane anemometers 叶轮风速计 Hotwire anemometers 热线风速计 Pitot tubes 皮托管

Vane Anemometer 叶轮风速计 Source: University of Wollongong 来源：卧龙岗大学

Hot-wire Anemometer 热线风速计 Source: University of Wollongong 来源：卧龙岗大学

Pitot Tubes 皮托管 Designed to measure the velocity pressure inside a ventilation system irrespective of the static pressure at that specific measurement point 皮托管旨在测量通风系统内的速度压力，而不考虑具体测量点的静压力 Consists of two concentric tubes with one facing directly into the airflow and the other (external tube) having a series of holes at 90° to the airflow 它由两个同心管组成，一个直接对着空气流，另一个（外部管）带有一系列和空气流呈90 °夹角的孔 The two tubes are then connected to a manometer or pressure gauge and the velocity pressure recorded 两根管子均连接到压力表或压力计上，以使速度压力记录下来

Pitot Tube with Digital Readout 带数字读数的皮托管 Source: TSI Inc – reproduced with permission 来源： TSI Inc ——经许可转载

Velocity of Moving Air 流动空气的速度 Pv = pv2 2 Pv = Velocity pressure 速度压力 p = Density of air 空气密度 v = Air velocity 气 流速度

Pitot Tubes (cont) 皮托管（续） Need no calibration, however measurements must be made in an area of ductwork where there are no obstructions, bends or turbulence 它无需校准，然而测量必须在一个管道系统区域内进行，管道区域内无障碍物、弯头或气体涡流 To achieve this it is common practice to take measurements in a straight section of duct at least 10 duct diameters from the nearest bend or obstruction 为了达到这一目的，常见的做法是在管道的直线部分进行测量，测量处离最近的弯头或障碍物至少要有10倍管道直径的距离 Pitot tubes are generally not considered reliable to measure airflows with velocities less than 3 ms-1. 皮托管用于测量小于3 m/s-1的气流速度时，一般认为不可靠。

Measurement of airflow 气流的测量 To obtain the average velocity the selected measurement area of duct must be divided into sections of equal area and a representative velocity measurement made in each 如果要获取所选管道测量区域的平均速度，必须把该区域分成同等面积的若干部分，并在每一个区域内获取一个典型速度的测量值 Most statutory authorities have specific requirements as to the number of sampling points that must be used for specific duct situations, however the ACGIH (2007) provides the following general guidance 大部分权威机构对采样点的数量有具体的要求，必须用于具体的管道情况，但美国政府工业卫生工作者会议 （ 2007 ）提供了以下通用性指导

Round Ducts 圆形管道 For ducts <15 cm a minimum of 6 points For very large ducts or where wide variation in velocity is present, a minimum of 20 points will provide a higher level of precision. 对于非常大的管道或具有大变量速度的管道，测量取样 最少20个点才能获得高精确度。

Square or Rectangular Ducts 正方形或矩形管道 Divide the duct cross‑section into a number of equal rectangles and measure the velocity at the centre of each 将管道横截面分为若干相等的矩形，在每个矩形的中心测量速度 The minimum number of points should be 16, however the maximum distance between each rectangle centre should be no greater than 15 cm 最少测量取样点的数量应该是16个，但是每个矩形中心之间的最大距离应不大于15厘米 Once the average velocity is established this is multiplied by the duct area to give the airflow 一旦确定平均速度，乘以管道面积就得出空气流量

Pressure Measurements 压力测量 Pressure within a system can be measured with simple “U” tube manometers where the difference in height between the two ends of the liquid column indicates the pressure 系统内的压力可以用简单的“ U ”形管压力计测量，压力计内液柱两端在高度上的差异表明其压力值 Most commercial manometers are calibrated in Pascals, however the pressure can be calculated using any manometer 大多数商业压力计是以帕斯卡进行校准，但可以使用任何压力表计算压力

Calculation of Pressure 压力计算 p = 2 g h   Where其中: p = Pressure 压力 g = Acceleration due to gravity 重力加速度 h = Height of column 圆柱高度 2 = Density of manometer liquid 压力表液体的密度

Dust collector or filter Use of Static Pressure for Fault-Finding in LEV Systems 在局部排气通风系统中利用静态压力进行故障探测   Test point1 测试点1 (negative)（阴极） Test point2 测试点2 Exhaust hood 排气罩 Test point3测试点3 Test point4 测试点4 (positive)（阴极） Dust collector or filter 集尘器与过滤器 Fan 风扇 static pressure at test point compared with normal 测试处的静态压力值与正常值比较 typical fault 典型故障 Normal 正常 System operating normally 系统工作正常 Blockage in duct or closed damper between points 1 and 2 测试点1和测试点2之间的管道或封闭节气闸的堵塞 Blocked filter dust collector 阻塞过滤集尘器 Missing or damaged filter 消失或损坏的过滤器 Low water level in wet dust collector湿式收尘器中的低水平 Faulty fan, incorrect fan speed, slipping drive belts, deposits on or damaged blades, incorrect electrical wiring 风扇故障、不正确的风扇转速、滑动的驱动皮带、损坏刃片上的沉淀物、不正确的电线搭设 Build-up of dust and debris in, or blockage of ,hood etc烟尘罩等内的粉尘和碎片的聚集或阻塞 Blockage in exhaust stack 排气烟囱里的阻塞 Low 低 High 高

Air Movement 空气流动 Smoke Tubes 烟尘管 Dust lamp (dust particles in atmospheres) 粉尘灯（大气中的粉尘颗粒）

Smoke Tubes Indicating Air Movement 烟尘管指示空气流动 Source: BP International Ltd 来源： 英国石油国际有限公司

Principle of the Dust Lamp 粉尘灯的原理 5″to 15″ 5″- 15″ SHIELD 防护罩 Eye or camera 鹰眼或摄像头 Source: HSE – Reproduced with permission 来源：HSE-经许可转载

(Show the HSE video’s on the use of the dust lamp)

Assessment of an LEV System 局部排气通风系统评估 A thorough visual examination to verify the LEV is in efficient working order, in good repair and in a clean condition 进行彻底的表观检查，以确认局部排气通风是否处在有效工作的正常情况下，及良好的维修和清洁状况下 Measuring and examining the technical performance so as to check conformance with the original commissioning data 测量和检验技术性能，以查看是否符合原始的调试数据 Assessment to check the control of worker exposure is adequate (eg: smoke tubes, dust lamp, workplace monitoring) 对系统进行评估，以检查对工人暴露的控制是否足够（如：对烟尘管、粉尘灯、工作场所的监测）  

Assessment of Specific Systems 特定系统评估 Full Enclosures 全封闭空间 Static pressure in the interior of enclosure must be lower than the workroom. 封闭空间内部的静压必须低于工作室。 Partial Enclosures 部分封闭空间 Individual face velocity measurements should not vary by more than 20% from the average and also meet statutory or national standards. 单个暴露面速度测量值的变化不应超过平均值的 20％，还要满足法定或国家标准。  

Assessment of Specific Systems (cont) 特定系统评估（续） Receiving Hoods 收气式烟尘罩 Individual face velocity measurements should not vary by more than 20% from the average. For larger hoods, measure at several points over the face. 单个暴露面速度测量值的变化不应超过平均值的 20％。对于大型烟尘罩，要在暴露面上的几个点上进行测量。 Capturing Hoods 集气式烟尘罩 For slots, measure the air velocities at equidistant points along the slot and average the readings. Individual readings should not vary by more than 20% from the average 对于插槽式烟尘罩，在沿插槽等距点测量空气速度，以求得平均值。各个读数的变化均不应该超过平均值的20 ％

Assessment of Specific Systems (cont) 特定系统评估（续） Hood Ducts 烟尘罩管道 Check the static pressure and compare to the normal value 检查静态压力并与正常值比较 Plenums 通风管道 Measure the static pressure of the plenum (enclosure behind some types of hood) and the hood duct 测量通风管道（某些类型的烟尘罩后面的封闭空间）和烟尘罩管道的静态压力

Assessment of Specific Systems (cont) 特定系统评估（续） Ducts 管道 Measure air velocity in the duct serving each hood 在每一烟尘罩管道内测量气流速度 Fan 风机 Measure the static pressure at the fan inlet and the volume flowrate 在通风进气口测量静态压力及体积流速 Collection Devices 收集装置 Where appropriate, measure the static pressure drop across the device and compare to normal conditions 在合适的情况下，测量整个装置的静态压降并与正常状态进行比较

Routine Maintenance & Appraisal 日常维护与评估 Ensure that the LEV is always running when hazardous substances are being emitted or are likely to be emitted 当有害物质正在排放或可能排放时，始终要确保局部排气通风处于运行状态 Observe the condition of the suction inlet such as the hood, booth, etc to see whether it has moved or has been damaged 观察进气口，如烟尘罩、管腔等的状况，看看是否有移动或已损坏 Observe the condition of any visible ductwork and dampers by the inlet 从送气口观察可见的管道系统和节气闸的所有情况

Routine Maintenance & Appraisal (cont) 日常维护与评估（续） Observe any evidence of control failure, for example noticing if there are unusual dust deposits or a stronger odour than normal immediately outside the LEV 观察任何控制失效的迹象，例如，注意在紧靠局部排气通风系统外，是否有异常的粉尘沉积，或是否有超乎正常的刺鼻气味 Observe any local instrument that has been fitted to the LEV to show its performance, such as a pressure gauge on a filter or an airflow device on a fume cupboard 观察已经安装在局部排气通风上的所有本机仪器，并显示其性能，如过滤器上的压力表，或通风橱上的气流装置等 Undertake any minor servicing such as emptying filter bins 进行细微的维护作业，如排空过滤箱

Limitations of LEV Systems 局部排气通风系统的局限性 Need to be purpose designed for a process, making process changes difficult if the same level of control is to be maintained 如果要保持同样程度的控制，需要进行专门的工艺设计，才能使工艺不会改变 High capital and operating cost. All LEV systems require energy for the fan, making operating costs an added expense 投入和运行成本高。所有局部排气通风系统都需要为风扇提供电源，这样就会增加运行成本 High levels of noise are common with LEV system, resulting in them being turned off by operators in many cases 局部排气通风系统常伴有高分贝噪音，从而导致操作人员在很多情况下要关闭该系统

Limitations of LEV Systems (cont) 局部排气通风系统的局限性（续） Many are of a fixed structure design making flexibility within the workplace difficult 许多局部排气通风系统具有固定的结构设计，使其在工作场所的灵活性不够 Many require the installation of an air supply system. Supplied air may need to be heated in cold climates or during winter months in temperate climates 许多局部排气通风系统需要安装一个供风系统。供应的空气在寒冷的气候，或在冬季月份的气候条件下可能需要加热 Not practical for large disperse contaminant clouds that have multiple sources 这对有多种污染源的大量分散污染云团而言，是不切合实际的 Limited application for the control of moving sources 对移动污染源的控制应用也有限  

Review of Today’s Topics 复习今天的主题 Control banding exercise – COSHH Essentials (group exercise) 控制能力练习——有害健康物品的控制要点（分组练习） Process design principles 工艺设计原则 Ventilation systems 通风系统