局部通风设计

1、局部通风系统,Source Path Worker,Local Exhaust Ventilation 局部排风 (Ref. ACGIH - Industrial Ventilation Manual) General Ventilation 全面通风 (Ref. ASHRAE - 62),ENGINEERING CONTROLS VENTILATION 通风,General and Local Ventilation,General Ventilation 全面通风,Local Exhaust Ventilation 局部通风,全面通风,LOCAL EXHAUST VENTILATION (

2、LEV) 局部排风,October 2, 2020,7,SOME COMMON MYTH 常见问题,LEV SYSTEMS 局部排风系统,1. Fans 风机 2. Hoods 排风罩 3. Ducts 风管 4. Stacks 排气筒 5. Air Cleaners 空气净化器,LOCAL EXHAUST SYSTEM局部排风系统,Types of Hood排风罩的类型,Total enclosures 全密闭罩 Partial enclosures 局部密闭罩 Captor hoods 收集式吸气罩 Receptor hoods 接受式排风罩,TYPES OF HOODS 排风罩类型,密闭

3、罩,半密闭罩,October 2, 2020,16,捕集罩,Types of Local Exhaust Hood and the Effectiveness,收集罩,半密闭罩,全密闭罩,DESIGNING HOODS排风罩设计,1. Enclosed the process as much as possible while not interfering with worker. 在不影响工人操作的情况下，排风罩尽可能包括整个操作范围。 2. Place the hood so that contaminants are drawn away from the breathing zone

4、 of the worker. 排风罩的位置适当，应能将有害物从工人呼吸带处排出。 3. Locate the hood as close as possible to the point of generation of contamination (to minimize air volume and for effective capture). 排风罩应尽可能靠近有害物的产生点，可以减少所需风量、提高捕集效率。,WORKER POSITION EFFECT工人位置的影响,Velocity Terms 风速,Capture Velocity 控制风速 Face Velocity 面风速

5、Duct Velocity 风管风速,CAPTURE VELOCITY控制风速,The velocity of air induced by a hood to capture emitted contaminants from the source external to the hood. 在排风罩外部的污染源处，捕集逸散污染物的气流速度。,CAPTURE VELOCITY 控制风速,The velocity of air induced by a hood to capture emitted contaminants from the source external to the ho

6、od. 在排风罩外部的污染源处，捕集逸散污染物的气流速度。,Not face velocity! 罩口风速这种情况下有意义吗?,How to Assess its Effectiveness?,October 2, 2020,24,控制风速要求,Capture Velocity,不同形状排风罩的风量计算,October 2, 2020,28,不同形状排风罩的风量计算,Maximum Capture Distance Should Not Be More Than 1.5 Times the Duct Diameter最大捕集距离不超过风罩直径的1.5倍。,Rule of Thumb for S

7、imple Capture Hoods 排风罩的基本规则,Relationship Between Capture , Working & Breathing Zones 排风罩的有效区域,DUCT VELOCITY 风管风速,Air velocity through the duct cross-section. 风管横截面的风速 When solid material is present in the air stream, duct velocity must be equal to or greater than the minimum air velocity required t

8、o move the particles in the air stream. 当风管气流中存在固体颗粒时，风管风速应不小于能将颗粒物移动的最小风速。,RANGE OF MINIMUM DUCT DESIGN VELOCITIES 风管最小设计风速的范围,SHORT CIRCUITING 短路,Minimum Stack Height in Relation to Immediate Roof Line or Center of Any Air Intake on the Same Roof 屋顶进风口与屋顶排气口的高度要求,ENERGY LOSSES IN THE LEV SYSTEM局部排

9、风系统能力损失,HOOD ENTRY LOSS 排风罩进口阻力 ELBOW LOSS 弯头阻力 BRANCH ENTRY LOSS 支管入口阻力 WEATHER CAP LOSS 风帽阻力 FRICTION LOSS 摩擦阻力 DUCT CONTRACT LOSS 管径改变导致的阻力,排风罩进口阻力 (hood entry loss),October 2, 2020,36,弯头阻力,October 2, 2020,37,支管入口阻力,October 2, 2020,38,摩擦阻力,October 2, 2020,39,选择合适的风管风速,通风系统的阻力与风管风速的平方成正比 高风速： 风道截面

10、积较小，系统造价低； 但系统阻力大，设备能耗大； 设备内部易受磨损 低风速： 系统阻力小，设备能耗小； 风道截面积较大，系统造价高； 风速过低易导致粉尘沉积，堵塞风道,October 2, 2020,40,FAN LAWS 风机定律,WHAT DO THE FAN LAWS TELL US? 风机定律告诉了我们什么？,The relationship between volumetric air flow and HP is cubic 风量和风机功率的关系是三次方的关系 Therefore, to double the air flow you must increase the horse

11、power by 因此，为了让风量增加一倍，风机功率要变为原来的8倍 Cost for electricity is proportional to fan HP. 所需电费与风机功率成比例 Illustrates importance of proper design and low losses 说明优化设计和低能量损失的重要性,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 空气的流

12、动需要有压力差, 空气从压力高的位点移向压力低的位点 Pressure is considered to have two forms: 压力通常分为两种: static pressure (Ps) 静压 velocity pressure (Pv) 动压 With the sum of these being total pressure (Pt). 静压和动压之和为全压,Static Pressure静压,Static pressure is defined as the pressure exerted in all directions by a fluid that is stati

13、onary 静压定义为静止气流向各个方向施加的压力 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) 如果气流是运动的 ( 如在通风系统中），静压为垂直于气流运动方向测得的压力，以消除气流运动的影响（如风速）,Static Pressure (Cont)静压,FAN,(

14、-) SUCTION SIDE,(+) PRESSURE SIDE,Can be both positive and negative depending if it is measured on the discharge or suction side of a fan 在风机的进风侧为负压，出风侧为正压。,October 2, 2020,46,Velocity Pressure动压,Defined as that pressure required to accelerate air from zero velocity to some velocity and is proportio

15、nal to the kinetic energy of the air stream 定义为将气流速度从零加速到一定流速所需要的压力，与气流的动能成正比。 In simple terms, velocity pressure is the kinetic energy generated in a ventilation system as a result of air movement 简单的说，动压就是通风系统中气流运动的能量的动能。,Velocity Pressure (cont)动压, = Density of air (kgm-3) v = Air velocity ms-1 P

16、v = Velocity pressure (Pa ie Nm-2),Velocity Pressure (cont),If standard temperature and pressure conditions are in existence, ie: =1.2 kg m-3 Then Pv=0.6 v2,Relationship is: Pt = Ps + Pv总压= 静压+动压,全压测定,局部通风系统效果评估,October 2, 2020,52,Smoke Tubes Indicating Air Movement发烟管显示气流的运动,Smoke Tubes发烟管,测量控制风速 （

17、非罩口风速）,October 2, 2020,55,Measuring Face Velocity测量罩口风速 （仅适用于密闭罩、半密闭罩）,Hot-wire Anemometer热线风速仪,Pitot Tubes皮托管 （风管风速，动压、静压测量）,Pitot Tube and Manometer with Digital Readout皮托管和压力计,1. Blocking off hood or inlet 排风罩或进气口阻塞2. Leakage (discoloration at seam and turns) 接合处或转弯处漏风3. Fans wired incorrectly 风机

18、连接不当4. Slipping drive belts 传动带松弛5. Burned out motors 电机烧毁,IDENTIFYING LEV PROBLEMS识别局部排风系统存在的问题,Reduced Flow Rate 风量降低的可能原因 Plugged or dented ducts 风管阻塞或管壁有凹陷 Slipping fan belts 风机传动带松弛 Open access doors 检查门未关闭 Holes in ducts, elbows 管道、弯道有漏洞 Closed blast gate to branch, or open blast gates to other branches, or corroded and stuck blast gates 调风阀关闭,或调风阀指向其它支道,或调风阀被腐蚀或受阻 Fan turning in reverse direction 风机反转 Worn out fan blades 风机叶轮破损 Additional br