Filters used to filter air, water and chemicals in industrial processes can cost thousands of dollars and more. Some filters are intended to be renewable by cleaning. Other filters, even some considered "disposable," can be recovered for re-use using ultrasonic cleaning techniques. A filter, as we discussed in earlier blogs (search "filter") is a device intended to prevent the passage of particles depending on the size of the particles. There are many types of filters ranging from those with only one filter stage (similar to a window screen) to those which capture particles using progressively smaller passages. These multi-layer filters are commonly called "depth filters." Filters also use different filter media which include, paper, natural and artificial fibers, metal and other materials. Because of the diversity of filter media and filter design, it follows that some are suitable for cleaning while others are not. Filters that are most likely to be successfully cleaned are single pass filters which capture particles based on size. Since the particles that are too large to pass remain on one side of the filter media it is quite simple to physically remove them. Ultrasonic techniques are particularly well suited to do this as scraping, brushing and other techniques may not only damage the filter media but may also lodge the captured particles more securely in the passages of the filter. Ultrasonics has the capability to dislodge the particles without further embedding them. Metallic filters including screens, perforated metal and other configurations are usually an easy task for ultrasonic cleaning. Other metallic filters that are cleaned relatively easily are those that capture grease, oil and other airborne contaminants by forcing them to negotiate a complex labyrinth (matrix) as they are propelled by a fan. These are the filters often found in range hoods and suspended over frying stations at fast food restaurants. The labyrinth is usually constructed of narrow ribbons of metal which are intertwined and partially compressed to hold together as a thin sheet. As the fast-moving suspended particles collide with the walls of the labyrinth, they are captured either by their own adhesion (in the case of grease and oil) or by an adhesive which is sprayed on to the filter prior to use. Since this type of filter captures contaminants based on adhesion and not on size, removing the captured particles is not much different than cleaning a flat metallic sheet. The particles easily fall out once they are released by a suitable chemistry. Once clean, the filters can be re-coated with adhesive (if necessary) and returned to use. Filters are difficult or impossible to clean are depth filters including those made out of wound cotton yarn and any number of other natural and man made fibers. There are two strikes against cleaning in these cases. First, the smaller particles are buried deeply within the depth of the filter and are usually lodged securely in place behind progressively larger particles. In most cases, a spent filter appears as a solid mass of particles which may be several inches thick depending on the filter design. The second problem is that the flexible fibrous material effectively prevents penetration of ultrasonic energy through the filter to loosen the captured particles. Back flushing (running liquid through the filter in a direction opposite to the normal flow) may improve the chances of cleaning this type of filter as the captured particles are partially dislodged by the reverse flow. Cleaning and recovering filters is an application which can be highly successful saving big dollars as costly filters are re-used instead of being replaced. BUT, it is an application which should be thoroughly proven before investing money in an ultrasonic filter cleaning system. Filters, because of their intended purpose of efficiently capturing particles and other contaminants, are often more difficult to clean than one might expect.
- FJF -