In this column, I will discuss the different types of filtration that are available for cleaning oil and polymer quenchants.
Introduction
As an oil is used, particulate from scale or oxidation products build up in the quenchant. Filtration is a common method to remove particulates, including oxidation products from quench oil. Following are brief discussions of several filtration methods.
The dirt load, including particulate from oxidation products, ranges in size from tiny (sub-micron) to large. Particle size in general is inversely proportional to the amount of particulate present. In other words, there is a greater amount of fine particulate to be removed than large particulate (Figure 1).
These oxidation by-products change the heat transfer characteristics of the quench oil [1][2]. This contamination can lead to increased distortion and cracking, part cleanliness, and plugged heat exchangers. Reduction of heat exchanger efficiency can increase cycle times.
Removal of staining on a part can be an expensive process. It requires additional effort such as shot blasting to obtain a clean surface. If the parts are not properly cleaned, then surface appearance can be objectionable (Figure 2). Lack of adhesion for subsequent coating processes such as painting, nitriding, or plating can become an issue.
Bag filters
Bag filtration systems are common in the heat-treating industry. They are an inexpensive method of particulate removal but fall short in cleaning the oil to a level that would extend the oil life. Typically, bag filters are used at 10-20 microns. As can be seen from Figure 1, this size will only remove about 10 percent of the particulate present. Bag filters have the advantage of being inexpensive, but suffer the drawback of limited particulate retention. This results in replacing the filter element at a short interval, increasing maintenance costs. Replacement of the filters is a messy job.
While there have been improvements in reduction of pore size, poor dirt load retention results in rapid clogging of the filter. Often, a manufacturer will spend the money on a filtration system but maintenance will fail to replace the bags because of the short replacement interval. Bags can also be replaced with a much bigger pore size to reduce the replacement interval. This is a false economy, as the particulate that is responsible for part staining and sludge is fine and passes through the larger pore size.
Pleated filter elements
Pleated filter elements are either cellulose or micro-fiberglass fiber. This type of filtration is similar to that used in automotive oil filters. These filters have the capability to remove very fine particles, down to less than one micron. However, the problem with these filters is the amount of dirt that the filter can hold. An industry-standard pleated micro-glass filter can only hold around 140 grams of dirt per filter element. With the high number of particles that are inherent in a quenching system, these filters would not last very long and would be a very expensive solution to the problem. In addition, the micro-glass filters would only be able to remove particles and would have no impact on the water, acid, or soft contaminants (that lead to varnish). In the presence of water, the pleated cellulose elements will swell and become clogged very quickly. As an example, a commercial heat treater purchased and installed a pleated filtration system. They stopped using the system shortly after installation due to blinding of the filters, which had resulted in changing the filter elements daily. While the intent was there, and the filters were capable of very tight filtration, the system was just not economically feasible to use due to the low dirt retention of the filter system.
Offline depth filtration
The best method for capturing and retaining fine particles, water, and varnish, is by installing an offline depth filter. This system uses a dense cellulose filter medium capable of filtration down to three microns absolute or 0.8 micron nominal (Figure 3). The system circulates the oil continuously. Depth filters provide a large surface area to capture fine particles as well as soft contaminants and water.
Debris is captured on the surface of the filter and in the interior filter material. This enables the depth filter to have the highest dirt-holding capacity of any type filter insert or element. This high dirt-holding capacity makes the depth filter the best solution for filtering quench oils with a high dirt load. Not only can the oil get to the cleanliness level necessary to prevent the sludge and carbon deposits, but this can be done with an insert that can hold several pounds of contaminants. The high dirt-load capacity will result in long filter change intervals. Depending on the cleanliness of the oil and the size of the filtration system, filter change intervals can be extended to months between changes.
The typical dirt-holding capacity for this type of filter is four liters of evenly dispersed solids (depending on size of filter element). The filtration element will retain approximately four kg of oxidation products, such as sludge and the oligomers from oxidation. Wood cellulose is the most-used filter material and can retain up to 50 percent of the total retention capacity.
However, deep filtration elements can often be blinded by large particulate that prevent fine particulate from penetrating the element. It is often necessary to use bag filtration or other types of filtration upstream of the deep filter element to capture the large particulate, so that the fine particulate can be captured by the depth filtration.
Sand filtration
For polymer quenchants, a sand-type swimming pool filter is recommended. This type of filtration is economical and provides excellent filtration to 6 to 8 µm. Once the filters become saturated, they can be readily backflushed. Filter sand is inexpensive to replace and readily available. One advantage of sand filters for polymer quenchants is the high velocity through the filter. This can prevent biological buildup and growth.
Conclusion
In this column, I have described the typical filtration types that are available to help keep the heat treater’s quenchant clean and (mostly) free from particulate contamination.
Should there be any questions regarding this article, or if you have suggestions for further columns, please contact the editor or myself.
References
- B. L. Ferguson and D. S. MacKenzie, “Effect of Oil Contamination on Pinion Gear Distortion,” in Proceedings from the 6th International Quenching and Control of Distortion Conference, 9-13 September, Chicago, IL, 2012.
- D. S. MacKenzie, “Effect of Contamination on the Heat Transfer of Quench Oils,” in 6th International Conference on Quenching and Control of Distortion, 9-11 September, Chicago, IL, 2012.
- CC Jensen, “Application and Selection Catalogue — Quench Oil Systems,” CC Jensen, Svendborg, Denmark, 2017.
