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标题:固体流动对喂料效率的影响:你知道多少?

1楼
上海北京顺德 发表于:2010/5/8 20:29:00

What You Don’t Know About Solids Flow Can Hurt Your Feeding Efficiency<!-- InstanceEndEditable -->

By Joseph A. Grande, Contributing Editor

 

Plastics processors who want to specify an optimal resin feeding and conveying system need to know about the handling characteristics of their materials. That’s the expert opinion of Jenike & Johanson, Tyngsboro, Mass., a leading supplier of laboratory and engineering services for powder and bulk solid processing (jenike.com). Too often, J&J sources say, processors are not informed about the flow properties of their materials, thus putting their operations at a severe disadvantage and potentially exposing them to quality problems and downtime.

 

塑料加工者要想确定最优的树脂喂料和传送系统,就得知道他们加工材料的输运特征。这是Jenike & Johanson, Tyngsboro, Mass公司(简称J&J)的专家意见。该公司是粉料和实心材料实验与工程服务的顶尖供应商。J&J公司的资料宣称,加工者常常不被告知他们的材料的流动性能,因此他们的操作处于严重的错误境地,并且易于出现质量问题而稀里糊涂(纠正:downtime不是糊涂的意思,而是停机。2010年5月11日)。

[此贴子已经被作者于2010-5-11 12:23:27编辑过]
2楼
上海北京顺德 发表于:2010/5/8 20:41:00

“A quick analysis reveals that there is a lot more than most people realize to understanding how bulk solid materials convey and flow and how that affects the design of a materials handling system,” says Brian Pittenger, senior consultant for Jenike & Johanson. “Processors on the whole know very little when it comes to material flow characteristics.” J&J provides extensive testing services to ensure that bulk raw materials move through a system smoothly. In nearly 50 years in business, the company has tested over 10,000 bulk solids.

 

“一种快速的分析显示,有许多人认识到实心材料如何输运和流动,并且知道这些性能如何影响材料输运系统的设计的,”Brian Pittenger说,Brian Pittenger是J&J公司的高级顾问。“当涉及到材料的流动特征时,总的来说,加工者知之甚少。”J&J公司提供昂贵的测试服务,以确保原材料平稳地在一个系统中进行输运。在近50年的业务中,该公司测试了10000多个固体材料。

3楼
上海北京顺德 发表于:2010/5/8 20:41:00
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You need to understand the flow characteristics of your bulk solid materials in order to specify a material handling system that will perform efficiently. Testing your materials in advance at a properly equipped lab can prevent costly mistakes.
4楼
上海北京顺德 发表于:2010/5/8 20:56:00

“Too often, the design effort consists of sending out bids to equipment vendors with little detail on the solids being handled and a focus on the lowest price,” says Pittenger. Often, processors pay little or no attention to solid handling before a system is installed and then must deal with flow problems when the system starts up, he adds. At that point, they’ve spent their capital budget and they must deal manually with problems at the floor level until they become such a severe quality or product issue that changes are made through the floor operating budget.

 

情况经常是这样,设计工作包括给设备批发商发出投标,这些批发商对如何处理固体的细节知之甚少,设计工作还有一个关心的是最低价格,”Pittenger说。加工者经常对固体材料的输运不加注意就把设备安装好了,最后当系统启用时,加工商不得不面临流动问题。这时,他们既花了钱,还得手工处理问题,弄得很没水平。直到这些问题成为严重的质量和产品问题,他们才通过一个最低的操作预算来做出改变。

5楼
上海北京顺德 发表于:2010/5/8 21:02:00

Pittenger cites a Rand Corp. study, which reveals that manufacturing plants generally perform poorly in the area of “mechanics of flow,” with most plants never reaching their nameplate capacity. Flow problems give rise to throughput variation and product non-uniformity and, at the extreme, can cause plant downtime, equipment failure, and product rejects.

 

Pittenger引用Rand 公司的一个研究,该研究表明,制造厂在流动机理方面一般表现不佳,大多数厂从没有达到额定产能。流动性能引起产量变动和产品部均匀。极端时,会引起工厂停产、设备失效和产品废弃。

[此贴子已经被作者于2010-5-8 21:02:30编辑过]
6楼
上海北京顺德 发表于:2010/5/8 21:21:00

Tailoring a material handling system to the bulk material’s flow characteristics can influence specifications for a wide range of system components. These include hopper sizes and shapes, conveying line sizes and construction, vacuum pumps, chutes, feeders, dust collection, drive sizing, and operating and data-collection systems.

 

按照材料的流动特征设计一个材料的输运系统会对系统的许多部件产生影响。这包括料斗尺寸和形状、输送线的大小和结构、真空泵、送料斜槽、喂料设备、集尘器、电机大小、操作和数据收集系统。

 

The key to reliable handling of materials is to design the handling system equipment based on the measured flow properties of the material to be handled. At the time that a handling system is specified and ordered, data on powder flowability are generally incomplete, unavailable, or non-representative of the actual material to be processed, notes Pittenger. Flow property testing should be part of routine pre-engineering work, he says. By obtaining flow properties beforehand, processors can make important engineering decisions.

 

可靠的材料输运的关键问题是,基于要加工的材料的流动性能的测量去设计输运设备。当要确认输运系统时,粉体流动性的数据一般不够完备、找不到数据或者与真实的加工材料相比没有代表性。Pittenger如是说。流动性是日常先期工程工作的一部分,他说。通过预先获得流动性能,加工者可以做出重要的工程决定。

7楼
上海北京顺德 发表于:2010/5/8 21:49:00

WHY MATERIALS WON’T FLOW材料为什么不流动
Plastic resin, either in pellet or granular form, is susceptible to flow problems such as blocking, sticking, erratic flow, inconsistent discharge, poor mixing, and lack of first-in/first-out flow. All of these problems stem from an incompatible combination of solid flow properties and equipment design details.

塑料树脂,无论是粒子还是颗粒,都易于存在流动问题,如不下料、发粘、不规则流动、非持续喂料、混合不好、不能先进先出。所有这些问题起源于材料的固体流动性与设备设计细节的不和谐。

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Among the most common flow problems, funnel flow occurs when the sloping hopper walls of a silo are not steep enough and sufficiently low in friction for material to flow along them. Particles slide on themselves rather than the hopper walls, and an internal flow channel develops. Mass flow is the desired flow pattern where upon withdrawal of any material, all of the material in a silo moves.

Two of the most common flow problems experienced in an improperly designed silo, bin, tank, or hopper are no-flow and erratic flow. No-flow from a silo is typically due to either arching (bridging) or rat-holing. Arching or bridging occurs when an obstruction in the shape of an arch or a bridge forms above the outlet of a silo and prevents further discharge. It can be an interlocking arch or a cohesive arch. An interlocking arch, where the particles mechanically lock to form the obstruction, occurs when the particles are large compared with the outlet size of the hopper. A cohesive arch occurs when particle-to-particle bonds form, allowing the material to pack together and obstruct flow.

 

在不恰当设计的料仓、罐、箱上常见的两个问题是不流动和不规则流动。料仓中出现的不流动一般是由于架桥或出现空洞(鼠洞)。当料仓出口上方出现弧形或桥型障碍时,就出现架桥,它阻碍进一步卸料。它可能是一个互锁弧形或者一个粘连弧形。在互锁弧形中,粒子通过机械力锁住而形成障碍。当粒子聚集体的尺寸与料斗出口尺寸可以相比拟时,互锁弧形就出现了。当形成粒子对粒子的粘连,从而使材料粘成一体并阻碍流动时,粘连弧形就出现了。

8楼
上海北京顺德 发表于:2010/5/8 22:19:00

Rat-holing occurs in a silo when flow takes place within a narrow channel above the outlet, while material on the periphery remains stationary. If the material being handled has sufficient cohesive strength, the stagnant material outside of this channel will not flow into it. Once the flow channel has emptied, all flow from the silo stops.

 

当流动只发生于料仓出口上方的一个窄的通道中,鼠洞就出现了,这时,鼠洞周边的材料仍然静止不动。假如这种待加工材料具有足够的粘附力,通道外面的静止材料将不会流入通道。一旦流动通道被排空,料仓中的所有流动就停止了。

 

Erratic flow is often the result of an obstruction alternating between an arch and a rat hole. A rat hole may fail due to an external force, such as ambient plant vibrations, vibrations created by a passing train, or vibrations from a flow-aid device such as an air cannon or mechanical vibrator. While some material may discharge as the rat hole collapses, falling material often gets compacted over the outlet and forms an arch. This arch may break due to a similar external force, and material flow resumes until the flow channel is emptied and a rat hole forms again.

 

不规则流动常常由于弧形障碍和鼠洞障碍的交替出现。由于外力,鼠洞会垮塌下来,如周边工厂的振动、通过的火车的振动、或者辅助流动设备的振动(如空气枪或机械振动器的振动)。当鼠洞垮塌卸料时,落下的材料常常在出料口上方变得致密而形成拱弧。这种拱弧由于类似的外力可能垮塌。材料接着流动,直到流动通道被排空,又重新形成鼠洞。

 

These problems occur in silos where flow takes place through a channel formed within stagnant material. This is called a “funnel flow” pattern, in which some material moves while the rest remains stationary during discharge. Funnel flow occurs when the sloping hopper walls of a silo are not steep enough and insufficiently low in friction for material to flow along them. Under these conditions, particles slide on themselves rather than the hopper walls, and an internal flow channel develops. The desired flow pattern, called “mass flow,” is a the flow pattern when, upon withdrawal of any material, all of the material in a silo moves.

 

出现这些问题,是由于在静止的材料通道中发生流动。这叫做“通道流动”模式。其中,一些材料输运的同时,另一些材料在卸料时静止。当料仓的坡形料斗壁不够陡并且材料沿着壁面的摩擦不是足够低时,通道流动就出现了。在这些条件下,粒子不是沿着料斗壁面滑动,而是沿着粒子之间的壁面滑动,这样一个内部通道就形成了。我们需要的流动模式,叫做“群体流动”,是这样的一种流动模式:当其中的任何材料卸料时,料仓中的材料都发生移动。

9楼
上海北京顺德 发表于:2010/5/8 22:22:00

HOW TESTING CAN HELP
Knowledge of solids flow properties is crucial to understanding and solving these costly flow problems. To fix flow problems in existing solids handling systems or to develop recommendations for new systems, Jenike & Johanson uses a scientific approach based on fundamental flow properties of bulk materials, including cohesive strength, internal and wall friction, compressibility, and permeability. At Jenike’s fully equipped laboratories, bench-scale tests on small quantities of materials can be used to design large and complex handling systems that will perform reliably. When evaluating bulk solids properties, it is vital to replicate actual or expected process conditions.

The minimum arching and rat-holing dimensions for a given bulk solid can be determined by measuring the cohesive strength of the material. This is done using Jenike Direct Shear testing methodology (ASTM D 6128). By measuring cohesive strength, the minimum outlet size to prevent cohesive arching or rat-holing can be calculated.

With the direct shear tester, J&J also recommends measuring the internal friction and wall friction. The internal friction may be used to understand how the material shears on itself in an internal flow channel, while wall friction is the shearing along the equipment surface. Both internal and wall friction measurements are used to calculate the necessary hopper angle to achieve flow along the walls and ensure mass flow to eliminate dead and stagnant regions.

Compressibility, or bulk density as a function of consolidating pressure, is another important flow property to be tested. Using a compressibility tester, Pittenger recommends material testing at seven pressure levels. The compressibility value is a useful tool in optimizing material handling systems. It allows processors to take into account feed rate, storage capacity, blend sizes, and mechanical considerations such as torque, horsepower, and loads.

Permeability, or the movement of air or gas through a settled solid bed, is an important property that enables calculation of the limiting discharge rates from hoppers as well as the limits of gas injection in processing vessels. The calculation will affect decisions on hopper size, outlet size, feeder size, and possibly the number of process trains. When dosing additives or fine powders, a consideration of the permeability can help to prevent problems like flooding and uncontrolled discharge.

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10楼
上海北京顺德 发表于:2010/5/8 22:22:00
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The Jenike Direct Shear tester is a valuable tool for measuring a range of flow properties such as cohesive strength, internal friction, and wall friction.
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