Filtration–Powerful Tool with Some Perils
“It is important to remember that filtration is a science and not black magic.”
HAROLD D. FITCH
Filtration is one of the key tools of the contamination control engineer. It is an important method of removing or separating solids from other solids, liquids and gases. There are two basic methods of filtration. One is the screening process, where undesirable solids are removed by passing material through a filter medium that removes solid materials above its pore size.
The second process is depth filtration, where the undesired solid material becomes trapped in the filter structure by a series of mechanisms such as molecular attraction, static charge, and electrostatic charge. This process is very long compared to the size of the material being removed. Depth filtration is used in the oil filter on your car. In the contamination control field, it is used for some types of gas filtration, and in HEPA filters for filtering air.
This month, I will concentrate primarily on surface filtration and its use in screening out (removing) solids from liquids and gases. I will look at the mechanism, consider some real-life filtration problems, and provide a checklist for you to consider for your own filtration applications.
The Screening Process
The screening process is fairly simple in principle. The filter, or screen, is a thin layer of material with many openings of a known size on its surface. In general, the surface area of the holes, or free space, is much larger than the surface area of the solid material making up the screen.
In the common screens we are acquainted with, such as screen doors and windows, the solid material is a series of metal or plastic threads running vertically and horizontally, forming a criss-cross pattern to keep out insects and other undesirables. The pitch of the threads in the vertical and horizontal directions determine the size of the openings and therefore control the size of the material or insect that can pass through.
Screen filters are often considered absolute; that is, nothing larger than the pore-size can pass through. However, this is not quite true in either the case of screen doors and windows or the fine pore-size filters we use in contamination control. Insects that cannot fly through may still squeeze through or find a defective pore because of variation in wire or thread diameter or broken or stretched wires or threads.
Similar problems can occur in high-tech filters. Membranes can be stretched, enlarging pore size, and material can be forced through by a mechanism such as a water hammer. Bacteria or viruses can actually grow through, and some sliver-shaped materials can pass through even though they are much larger than overall pore size.
The membrane filters used in high-tech micro-filtration applications are mostly made up of some type of plastic membrane, with a closely controlled pore-size determined by the manufacturing process. The available pore sizes run down to submicron level and are specified to a statistical control level.
Real-life Filtration Problems
Let`s review a few real-life filtration problems to get an idea of what problems can arise. The first example is a contamination problem with parts from a support area. A review of the process showed that it was very dependent on clean solvent. I asked about the control on the solvent and was told the process filtered all of the solvent through 0.5-µm filters. When asked, no one knew how often filters were changed. It was stated that they had never had a problem with the filter setup before, but when I checked further, I found out why. The filter media selected was incompatible with the solvent being filtered and was probably dissolved in the first few minutes of contact with the solvent. When I took the filter housing apart, there was no trace of any filter in the housing. Installing a correct filter with proper testing and maintenance cleared up this contamination problem.
A second problem that was much more complicated involved an effort to install continuous recirculation/filtration on a precision etching process. In this case, the etchant contained a wetting agent of with limited solubility. When the filtration setup was installed, the filtration process removed the wetting agent from the solution and affected the etching control. The eventual solution to this problem was replacement of the wetting agent with one that went totally into solution and was not removed by the filtering process.
Some other filter problems we encountered have included:
lack of filter support screens
use of filter membrane separators from packages used instead of filters
leaky filter housings, allowing by-pass
incompatible seals, support screen, and adhesives
poor filter-housing threads, allowing by-pass
support screens on top of filters or housings installed upside down, preventing proper support.
It is important to remember that filtration is a science and not black magic. It is a tool that will serve you well if you understand it and utilize it correctly. Filter suppliers are well equipped to help you with your applications and provide technical help if you request it.
The following checklist will help you get off on the right foot with filtration applications.
1. What size solid material do you want to remove from your application?
2. What are the chemical and physical properties of the material to be filtered?
3. What are the chemical and physical properties of the filter media?
4. What are the chemical and physical properties of the filter setup, support screen, seals, housing and plumbing?
5. What is your filter test procedure?
6. What is your filter maintenance schedule?
7. Who is responsible for filter testing and maintenance?
8. Are filter changes and filter checks clearly posted and dated so they can be easily confirmed?
Being able to answer these questions and having a good two-way relationship with your filter supplier go a long way toward making filtration work for you! The perils of poor filtration can be avoided by proper understanding and preparation.n
Harold Fitch is president of Future Resource Development, a consulting firm in Burlington, VT, specializing in cleanroom education and problem-solving. He conducts international training seminars for CleanRooms` shows and seminars.