A Model for Pulse Jet Fabric Filters

ABSTRACT

A new model for pulse jet fabric filtration is proposed. In contrast to the earlier model of Ravin and Humphries,¹ which was formulated on the steady state assumption, the present study is aimed at developing a predictive capability for both transient and steady state operations, taking into account the compression effect of filter cakes. The model's relative simplicity allows frequent updating of the model parameter values, thus improving the accuracy of predictions. As a result, the model is particularly useful in developing control algorithms and designing controllers of pulse jet fabric filtration systems.     

Predicting Pressure Loss for Pulse Jet Filters

The analysis of pressure loss characteristics for pulse jet filters suggests that the relationship between dust adhesion to the fabric and the opposing force generated by pulse jet action plays a major role in dust removal. Hence, fabric cleanability is examined in terms of the adhesion-cohesion forces bonding the dust to the fabric vs. the intensity and frequency of the dust dislodgement forces produced by the high energy air pulses. The effect of jet size and location, jet air volume, and the intensity (pressure) and duration of the jet pulses is related to operating pressure loss.

The mechanics of energy transfer from the jet pulse to the dustladen fabric are explored in terms of jet pressure, solenoid valve action, the ratio of delivered pulse air volume to bag (tube) volume, and the elastic and flex properties of the felt bags. Effective and actual fabric dust holdings before and after cleaning are discussed with respect to steady-state dust deposition and removal rates, and operating pressure losses. Finally, predictive equations are proposed for estimating pressure loss over a broad range of design and operating parameters.     

Model calibration for pressure drop in a pulse-jet cleaned fabric filter

A model based on Darcy's law allows prediction of pressure drop in a pulse-jet cleaned fabric filter. The model considers the effects of filtration velocity, dust areal density added during one filtration cycle, and pulse pressure. Data used to calibrate the model were collected in experiments with three fabric surface treatments and three dusts conducted at three filtration velocities, for a total of 27 different experimental conditions. The fabric used was polyester felt with untreated, singed, or PTFE-laminated surface. The dusts used were granite, limestone and fly ash. Filtration velocities were 50,75 and 100 mm s⁻¹. Dust areal density added during one filtration cycle was constant, as was pulse pressure. Under these conditions, fabric surface treatment alone largely determined the values for two of the three constants in the model; the third constant depends on pressure drop characteristics of the venturi at the top of each filter bag.     

Dust Deposit Profiles in a High Velocity Pulse-Jet Fabric Filter

Dust deposit profiles in a pilot-scale pulse-jet fabric filter were measured using a beta gauge. Fly ash was collected on polyester needled felt bags, and the dust profiles were measured after the test system was operated to equilibrium at superficial filtration velocities of 50, 75,100,125, and 150 mm/s. The profiles measured show that a large mass of dust is retained on the fabric of a pulse-jet filter when operated at high filtration velocities. This dust mass retention can be caused by two mechanisms: the failure to remove dust from the fabric during a cleaning pulse, and the redeposition of suspended dust onto the fabric after a cleaning pulse. The dust deposit measured at the highest test velocity was found to be much different from the deposits measured at all other velocities. The deposit found at 150 mm/s had almost twice the average areal density and was more evenly distributed than the deposits found at lower velocities.     

Theory for Penetration in A Pulse-Jet Cleaned Fabric Filter

Presently, there is no effective way to interpret or predict dust penetration through a pulse-jet cleaned fabric filter. This paper presents a model which considers penetration straight through the filter and penetration by seepage. A considerable number of studies have been devoted to penetration by the straight through process; however, a comparison of data from the literature with the present model indicates that seepage and not straight through penetration accounts for virtually all penetrating dust. Although insufficient information is presently available to use the model to predict penetration, the model does show trends that should occur with changes in filter operating variables such as filtration velocity and pulse pressure, and suggests areas in which further research is necessary.     

Survey on the Use of Pulse-Jet Fabric Filters

Pulse-jet fabric filters rely on the filtration of dirty flue gas by the outside surface of the bags, which are then cleaned by a shock wave generated by an air pulse entering each bag from the top. As it travels down the length of the bag, the shock wave flexes the fabric and dislodges the dust cake. Enhancement of the pulse may be achieved by using a venturi, and cleaning may be on-line or off-line. This paper summarizes the results of an exhaustive study conducted for the Electric Power Research Institute to provide a convenient and versatile information base about the use of pulse-jet fabric filters on coal-fired boilers. Predominant features of the many pulse-jet installations identified by vendor survey and literature survey are shown in graphical and tabular form.     

An extended self-organizing map network for modeling and control of pulse jet fabric filters

Pulse jet fabric filters (PJFFs) have become an attractive option of particulate collection utilities, because they can meet stringent particulate emission limits regardless of variation in operating conditions. Despite their wide applications, the present control algorithm for PJFFs can best be described as rudimentary. In this paper, a modeling and control strategy based on the local model network (LMN) is proposed. An extended self-organizing map (ESOM) network is developed to construct the LMN model of the filtration process using the filter's input-output data. Subsequently, these ESOM local models are incorporated into the design of local generalized predictive controllers (GPC), and the proposed controller design is obtained as the weighted sum of these local controllers. Simulation results show that the proposed controller design yields a better performance than both conventional GPC and proportional plus integral (PI) controllers yield.     

脉冲喷吹扁式方框滤筒除尘器的清灰性能

为探究新型扁式方框滤筒的清灰性能,在自建的脉冲喷吹实验台上,测得不同电磁阀、开孔型号、喷吹距离、喷吹压力下的侧壁压力峰值大小及其分布规律。通过进一步工业附粉,监测除尘器的阻力和清灰后的粉尘残余量,验证找到扁式方框滤筒的最佳喷吹清灰参数组合。结果表明：扁式方框滤筒的最佳开孔型号是7个7 mm孔,喷吹距离为20 mm;对于玉米秸秆粉尘,当过滤风速低于0.6 m·min-1时,在1寸阀、0.2 MPa喷吹压力下可满足对其基本淸灰需求,且风速越小,清灰后的粉尘残余量越少,阻力也越小,低于150 Pa。另外,脉冲清灰过后,未被清除下来的粉尘多集中于扁滤筒的上部且是未正对喷吹孔的位置,如何进一步改善有待之后研究。     

脉冲喷吹2000mm长滤筒的清灰性能

为探究2 000 mm长滤筒滤筒在脉冲清灰时,不同喷吹条件(喷吹孔径、喷吹距离)对滤筒清灰性能的影响及沿长滤筒长度方向上侧壁压力峰值分布规律,设计脉冲喷吹实验,测定Φ147 mm×2 000 mm(覆PTFE膜)长滤筒在不同喷吹条件下的侧壁压力峰值.实验结果表明,2 000 mm长滤筒的最佳喷吹孔径为15 mm,最佳喷吹距离为240 mm,在0.5 MPa条件下,沿滤筒长度方向上各测点的侧壁压力峰值分别为P0 1 084、P1 1 898、P2 1 276、P3 1 556、P4 1 302和P5 3 258.并得出最佳喷吹距离随着喷吹孔径的减小而逐渐增大,以及沿长滤筒长度方向上侧壁压力峰值分布呈现先增加再减小再增加的分布规律.研究成果旨在为长滤筒过滤系统优化设计及滤筒除尘器代替滤袋除尘器的应用上提供指导.     

An Extended Self-Organizing Map Network for Modeling and Control of Pulse Jet Fabric Filters

ABSTRACT

Pulse jet fabric filters (PJFFs) have become an attractive option of particulate collection utilities, because they can meet stringent particulate emission limits regardless of variation in operating conditions. Despite their wide applications, the present control algorithm for PJFFs can best be described as rudimentary. In this paper, a modeling and control strategy based on the local model network (LMN) is proposed. An extended self-organizing map (ESOM) network is developed to construct the LMN model of the filtration process using the filter's input-output data. Subsequently, these ESOM local models are incorporated into the design of local generalized predictive controllers (GPC), and the proposed controller design is obtained as the weighted sum of these local controllers. Simulation results show that the proposed controller design yields a better performance than both conventional GPC and proportional plus integral (PI) controllers yield.     

Design Variables Associated with Fiberglass/Fly Ash Fabric Filter Cleaning

This paper documents the variation in pressure drop and collection efficiency for fiberglass/fly ash fabric filter systems caused by variations in cleaning intensity (reverse air and reverse pulse), air to cloth ratio, and dust loading. Reverse air rates greater than 2 fpm were required to produce stable pressure characteristics. Pulse jet pressure greater than 50 psi reduced collection efficiency. Increased air to cloth ratios produced decreased collection efficiency.     

错流内滤式滤芯脉喷清灰的数值模拟

错流式过滤除尘技术利用切向风流抑制尘饼生长变厚,具有过滤阻力上升缓慢的优点,采用脉喷气流撞击设置在滤芯外的横向挡板并产生局部静压可实现内滤式滤芯的清灰。构建CFD数值模拟考察了喷吹气流的分布,分析了喷嘴入口处的喷吹压力和挡板位置对脉冲喷吹性能的影响,设计并考察了挡板往复式喷吹策略。结果表明：滤芯压力随喷嘴入口压力升高而增加,确定了最佳喷嘴入口压力为5 000 Pa;挡板位置的变化显著影响清灰的有效区,对于1 000 mm长的滤芯,挡板位置为500 mm和350 mm分别实现350~500 mm和0~350 mm区域的有效清灰;相比普通喷吹,设计的挡板往复式喷吹 (挡板位置依次为350、500、650、500 mm) 可实现滤芯喷吹压力增大1.57倍、均匀性提升7.17倍。该研究结果可为内滤式滤芯脉喷清灰提供参考。     

生物制剂对MBR中无纺布膜组件过滤性能的影响

研究了生物制剂对浸渍式膜生物反应器(submerged membrane bioreactor,SMBR)中聚丙烯无纺布(non-wov-en fabric,NWF)膜组件过滤性能的影响。结果表明,能减少膜组件表面附着污泥胞外聚合物(extra-cellular polymeric sub-stances,EPS)的含量及污泥的沉积,减缓膜通量的衰减速率和膜污染,无纺布膜组件的过滤性能得到明显改善,表现出一定的耐污染性。生物制剂能改善污泥的沉降性能,有效防止污泥膨胀;对MBR的COD去除率基本没有影响,但略微增大了处理水的浊度。     

The design of an aerosol test tunnel for occupational hygiene investigations

An aerosol test tunnel which provides large working sections is described and its performance evaluated. Air movement within the tunnel is achieved with a powerful D.C. motor and centrifugal fan. Test dusts are dispersed and injected into the tunnel by means of an aerosol generator. A unique divertor valve allows aerosol laden air to be either cleaned by a commercial pulse jet filtration unit or recycled around the tunnel to obtain a high aerosol concentration. The tunnel instrumentation is managed by a microcomputer which automatically controls the airspeed and aerosol concentration.     

响应面法优化袋式除尘器脉冲清灰性能

基于计算流体动力学的方法采用三维、可压缩、非稳态流动数学模型对袋式除尘器脉冲清灰过程进行了数值模拟,得到了滤袋内外压差,并与文献实验值进行了比较,验证了仿真模型的可靠性。基于响应面法研究了喷吹压力、喷吹高度、滤袋直径和滤袋长度对脉冲清灰性能的影响,得到这4个影响因子的二次多项式预测模型,并进行优化。结果表明,喷吹压力为0.3 MPa,喷吹高度为0.2 m,滤袋直径为0.16 m,滤袋长度为6 m时,内外压差峰值最优,优化结果与仿真模拟结果相差小于3%。研究结果为袋式除尘器脉冲清灰系统的设计与优化提供了重要参考。     

脉冲喷吹大风量滤筒除尘器的清灰变化过程研究

褶皱比高、过滤面积大的大风量滤筒除尘器存在不完全清灰的随机性更大,因为脉冲清灰过程中脉冲气流在滤筒内的流场分布不均引起滤筒侧壁压力的不均匀分布。主要通过检验大风量滤筒(325×1 000 mm)侧壁压力峰值变化过程和探寻引起侧壁压力峰值分布的气流动态变化过程。实验结果表明,冷态实验时,有诱导喷嘴时,滤筒侧壁压力峰值呈现从上到下减小的趋势,滤筒侧壁压力变化过程与滤筒上粉尘剥落的过程相一致;动态实验时,随着过滤风速增大,滤筒侧壁压力峰值出现增大的现象,因为脉冲气流受到系统过滤气流的影响,脉冲气流在轴向的速度降低的要缓慢一些,动压转化为静压的速度要缓慢一些,出现滤筒650 mm位置侧壁压力峰值明显增大,而在滤筒850 mm位置侧壁压力峰值并未增大,将动态实验用于实际现场,结果表明,长时间运行滤筒底部出现清灰效果差的现象。     

高温袋式除尘器金属滤袋脉冲喷吹清灰性能分析

为解决高温袋式除尘器在应用中因压缩空气温度偏低导致金属滤袋糊袋,以及因清灰参数选取不当导致滤袋残余阻力上升和除尘器运行阻力升高的问题,使用滤料微观模型分析过滤状态下颗粒在清洁金属和有机纤维层的穿透过程;并结合物理模型与数值模拟正交实验研究不同因素对不锈钢金属滤袋脉冲喷吹清灰过程的影响;以传统有机滤袋做对照,通过数值模拟实验研究在高温状况下脉冲喷吹清灰气流对金属滤袋袋口区域结露的影响。结果表明,在相同的颗粒粒径条件下,清洁金属滤料纤维的颗粒穿透量和穿透距离更大,拦截效应低于有机纤维滤料。滤袋清灰效果整体呈现上部>底部>中部的趋势,且金属滤袋清灰评价指标测试值整体低于传统有机滤袋;显著影响滤袋清灰的因素均为喷吹压力和喷吹孔径,金属滤袋最佳喷吹距离为200 mm,喷吹时间为100 ms。当脉冲喷吹气流温度为0 ℃时,滤袋袋口0.5~2 m处有结露风险,升高至50 ℃可有效防止工业窑炉烟气滤袋袋口区域结露现象的产生。该研究结果可为高温袋式除尘器金属滤袋的喷吹参数设计和结构优化提供参考。     

New Fabrics and their Potential Application

Considering fabric filtration, particularly the fabrics and their ability to handle submicron particulate and fumes, it is noted that the value of new developments are measured in terms of both improved operational and economic efficiency. The types of available fibers dictate fabric production characteristics as well as the limits of application for the manufactured fabric.     

Performance of a Pulse-Jet Filter at High Filtration Velocity III. Penetration by Fault Processes

Performance data for fabric filters using either woven or felt bags can be better understood when fault processes such as pinhole bypass and seepage are considered. Penetration straight through the dust cake and fabric may not be important by comparison. Observed trends of increased penetration with increased filtration velocity, constant or slightly increased penetration with increasing particle diameter, and constant penetration with additional dust loading can be explained by fault processes. The pulse-jet experimental work described here, done over many filtration and cleaning cycles, shows that penetration increases substantially with increasing filtration velocity and that this increase is due entirely to seepage.     

On the forming mechanism of the cleaning airflow of pulse-jet fabric filters

To reveal the formation mechanism of a pulse-jet airflow’s cleaning effect in a filter bag, a theoretical model is built by using the theory of the gas jet and unitary adiabatic flow according to given specifications and dimensions of the bags and resistance characteristics of the cloth and dust layer. It is about the relationship between cleaning system structure and operating parameters. The model follows the principle that the flow and kinetic energy of jet flow injected into a filter bag should be consistent with the flow of cleaning airflow in the bag and the pressure drop flowing through the filter cloth and dust layer. The purpose of the model is to achieve the peak pressure of cleaning airflow, which dominates the effect of the pulse-jet cleaning process. The cleaning system structure includes air pressure in the nozzle, structure and size of nozzle, exit velocity of nozzle, jet distance, and diameter of jet cross section. Based on the condition of the cleaning system structure and operating parameters established by using the theoretical model, Fluent software is applied to carry out a numerical simulation of the jet airflow field at the nozzle’s outlet, jet airflow field between nozzle and bag top, and cleaning airflow field in the filter bag. Experimental results are used to verify the reliability of the theoretical model. They are obtained in a pilot-scale test filter with a single bag, with length 2 m and in general full-scale dimensions of the cleaning system. The results show that when any rectification measure is not installed at the bag opening, the cross-sectional area covered by the jet gas is hardly sufficient to cover the entire area of the bag opening. A large portion of the gases injected into the filter bag will overflow reversely upward from the edge due to pressure differences between the upper area and lower area inside the bag opening. This led to a serious shortage of the cleaning airflow and ar limited increase in static pressure. When a venturi-type rectifier tube is installed at the bag opening, the jet flow is converted to funnel flow for which the cross-section velocity distribution is more uniform at the throat of the rectifier tube due to the guided effects of the upper tapered pipe. Then it is transited to stressful flow below the bag opening via rectified effects of the lower dilated pipe. The results show that the gap between the static pressure of gas in the bag and the expected value is significantly reduced. The theoretical value of the nozzle diameter is enlarged to compensate for two aspects of adverse effects of cleaning airflow and energy. This is because the flow is not a purely free-form jet from the nozzle to the entrance of the rectifier tube and because the gas suffers from local resistance while flowing through the rectifier tube. The numerical simulation and experiment show that the peak pressure of cleaning airflow in the filter bag is able to reach the expected value. The results confirm that the mechanism of the pulse-jet cleaning airflow and the calculation method of the pulse-jet cleaning system structure and operating parameters offered in this study are correct. The study results provide a scientific basis for designing the system of pulse-jet fabric filters.

Implications: Pulse-jet cleaned fabric filters are commonly used for air pollution control in many industries. Pulse-jet cleaning is widely used for this purpose as it enables frequent cleaning while the filter is operating. However, the theoretical system of the forming mechanism of the pulse-jet cleaning has not formed so far. This indicates the theoretical model plays an important role in designing effective pulse-jet cleaned fabric filters.