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In most branches of industry there are processes in which one has the need to remove contaminants from a product, either from the finished product or from some intermediate stage in the production process. Some common methods used are washing with a suitable fluid, brushing, wiping, polishing, blow cleaning and vacuum extraction. There is an increased need to make this operation as gentle as possible due to higher demands in quality and at the same time there is a constant battle to reduce energy consumption making it more complicated to find a suitable process.
A very useful way to remove dry particles from a surface is to combine the blowing of air with vacuum extraction but to do this in a well-defined manner. The presentation contains a number of different application examples from different branches of industry to help illustrate the concept.
Why do particles stick to surfaces?
We all know that particles stick to surfaces – just think about the problems of dust being collected on surfaces in our home, in our car, or in a production process. Not only do particles stick to larger surfaces, they also stick to other particles. In this text we shall consider the electrostatic attractive forces and describe how they act to make particles move towards a certain surface and then electrostatically stick to the surface.
What is a particle? What does it look like?
There may be as many definitions of particles as there are unique production processes using different materials in different operations. In some cases particles can be large such as insects, confetti from punched out holes, textile or cellulose fibres, saw dust, human hairs, glass splinters etc. Yet in other cases they are very small such as pollen particles, viruses, dust from finishing processes or in sensitive semiconductor production even single atoms.
Particles also have differences in shape, density and the specific area which also influences the way the particle will interact.
In many cases there is a need to start by specifying the particles and then define the problem. Particles visible to the human eye are more easily described but smaller particles may need more or less complicated methods of analysis. Sometimes there is a need to use a high resolution electron microscope in combination with mass spectroscopy analysis to define size and shape and by atomic analysis get the necessary clues that can tell us the source of the particles. Samples may often be collected from the surface to be cleaned but in many cases it may be necessary to collect a large number of air samples to find out where the source of the contaminant is and the path it travels to reach its target.
What has static got to do with dust problems?
An electrostatic charge is generated as an imbalance in the charge distribution in a material, meaning that a lack of electrons will result in a positively charged surface and vice versa. The surface potential (E) of a charged-up surface will produce an electrostatic field (kV/m) which can be described by field lines oriented perpendicularly to each point of the surface and with the density of the field lines as a measure of the size of the field.
A charged-up particle will travel along a field line in the direction towards the oppositely charged surface with a speed, defined by size, shape and weight of the particle, distance between particle and surface and field strength as some basic factors. In the real world it gets more complicated since we do not have a constant steady state. In a production process there are movements both of the products being processed and of moving machine parts, or conveyors etc, as well as air flows which can be created for a particular reason or a side effect of movements or temperature gradients due to heating or cooling. When solvents are used the situation gets even more complex.
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Surface roughness and particle shape are important factors to explain why some particles seem more difficult to remove than others. A ”thin and flat” particle laying on a very smooth surface will be much more difficult to remove than an irregularly shaped particle on a rough surface at the same respective surface charges. This is due to the fact that the average distance between surface and particle will be much smaller with the smooth surfaces than the latter combination with rough surfaces.
Can killing the static field solve the problem?
By using an ionizing electrode designed for the specific product and/or application the static charge can be removed. The antistatic device produces a high concentration of ions, both positive and negative ions in order to discharge the positively and negatively charged surfaces on both product and particle thus reducing the electrical potential difference to a minimum. Once the potential difference is sufficiently low there will be no significant electrostatic attractive force to hold the particle onto the surface. So, having achieved this, why is the surface still full of particles? Apart from the difficult problem of moist or ”sticky” particles being ”glued” onto the surface or particles having been mechanically pressed into the surface there is a major amount of particles clinging to the surface for no obvious reason. The explanation is probably to be found in the way air flows across the surface.
Fluid dynamics and vacuuming
There are some basic facts that can be learned by studying fluid dynamics. In fluid dynamics we study what is happening when a fluid is in motion, it can be water flowing through a water pipe or air moving across a surface such as e.g. an airplane wing. Perhaps you studied the cv value when you bought your car to see that it had the aerodynamic properties that you were looking for?
When buying a new vacuum cleaner for your home the salesman will try to impress on you by telling how many watts the motor can give and vacuum cleaners are deliberately made noisier than needed to give an impression of being very efficient. It is of course a very good idea to equip the vacuum cleaner with a HEPA grade filter in order not to spread pollen and bacteria in the outlet air from the vacuum cleaner, but does not tell us anything about the efficiency of the unit with respect to collecting all particles from the surface we are vacuuming.
There are two different flow characteristics which play an important role when trying to remove particles by vacuum extraction;
There are many different factors influencing the formation and thickness of a laminar layer in a laminar air flow, but the main factors are surface speed and roughness. At high moving speeds it is very difficult to break through the laminar air flow layer and thus small particles get trapped inside the laminar layer. In order to be able to remove the particles from the material surface one must first break through the boundary layer.
Can killing the boundary layer solve the problem?
Perhaps and sometimes are two clever answers to a not quite as silly question as it seems at a first glance. Although breaking down the laminar air flow layer may expose the ”hidden” particles to an applied turbulent air flow from a vacuum extraction nozzle etc, even an extreme air flow may fail in lifting the particles from the surface. Due to strong electrostatic attractive forces many particles are still left on the surface because the attractive forces have not been overcome by the ”lifting forces”the vacuum extraction air flow.
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Blowing of compressed air onto a surface does apply a considerable force on the targeted particles, but the fast moving air containing particles will result in a strong charge build-up due to the ”rubbing” effect on the material surface. Thus most particles will be repositioned on the surface by being blown away from the original position, then being charged-up and deposited by a stronger static attractive force once they have travelled a short distance out of the focus of the strongest air flow.
How can we change the electrostatic properties?
Some materials can be modified to be less prone to electrostatic charge build-up. This is done by adding a suitable antistatic agent into the material. It may be possible to find a different material or modify the existing material.
Charge decay time is a measure of the time a certain material needs to bring down an initial peak voltage applied, in a controlled way to a sample, to a specified fraction such as for example 10%, 50% or 1/e of the peak voltage. A short decay time means that when a charge has been generated, the time that the charge will stay on the surface will be short. This means that a material with a short decay time will expose its electrostatic attractive field for a short time compared to a material with a long decay time and consequently attract less airborne particles.
The above discussion will help reduce problems with attracting foreign particles in many cases but is not a solution to all cases. A typical example where the material is conductive is in the production of steel sheet. Steel is of course a good conductor and the electrical potential in the finishing line is at ground potential, but removal of unwanted particles is both necessary and difficult.
Why is this? There are rich amounts of insulating particles that have been highly charged in the process and their decay time is long enough to create a serious problem.
A final conclusion; although there are applications to handle many problems with unwanted particles the best way to avoid problems would be to eliminate the particles at their source. If that is not possible, make the time a surface is exposed to a contaminant as short as possible and make all particles and surfaces free of static build-up by working on the electrostatic properties of the material or if that is not possible use discharging equipment.
Cleaning a surface from unwanted particles may be a complex task where it is necessary to combine a number of fairly basic operations into a practical working solution. By a well-balanced combination of using compressed air, vacuum extraction and discharging static build-up one can master the problem even ”when static goes dynamic”.
When static goes dynamic we have to avoid traffic jams during transportation of ions and blowing air into the target area at the same time as the vacuum extraction can remove the particles that have been made loose, efficiently enough in order not to deposit particles in new areas.
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