By Carlo Lupi
Due to ever-present dust and dirt particles, the air used for the pneumatic delivery of the material conveyed in a grain or feed mill has to be purified by jet filters integrated in the system. A filter chamber holds a required number of cylindrical filter sleeves supported by filter cages. The dirty air flows through the filter sleeves, which retain the particles. Depending on the environment, the air supply may contain oil particles and/or humidity. Routine cleaning is a must for economical operations. Very dirty filter sleeves increase the pressure required by the ventilators for air circulation. Also, the resistance of a dirty filter sleeve surface is considerable and may lead to trouble with product flow.
TYPES OF FILTER SLEEVES
Filter sleeves made of felt or filter cloth are normally obtained as tubular bodies fitted on sleeve cages. There are various types of filter sleeves: filter sleeves with bottom and stitched seams; filter sleeves with bottom and welded (glued) seam and seamless filter sleeves with bottom.
There are also bottomless filter sleeves supplied as hose and fitted over a sleeve cage with a metal bottom. How-ever, this version is controversial because of concerns about leakage and used very rarely because of leakage issues in fastening the filter to the cage. Still another variant comes with a felt ring at the upper edge. The felt ring is ‘squeezed’ into a matching device.
Choosing the most simple and economic type of filter sleeve for replacement also means to consider fabric type.
Special attention has to be given to the material of filter sleeves. On the one hand, all dust and dirt particles have to be retained; on the other hand a certain amount of purified air has to flow through the filter material. The porosity of the filter is a decisive factor for the volume of air to flow through a certain surface unit within a given space of time. The construction of the filter — be it felt or filter cloth — has to meet these requirements. It also directly defines the necessary pressure for the air volume per unit of time. Integrated ventilators produce this pressure, which determines the required air volume of the system.
In addition, the cloth must be protected from certain influences. Anti-static treatment of the fabric is necessary to reduce the risk of explosions. Further, a surface treatment is necessary to make the tissue resistant against oil and water residues carried by the airflow.
The filter units offered by various manufacturers are equipped with an automatic cleaning system. This allows the cleaning of filter sleeves through a device integrated in the jet filter installation. Control valves mounted at the head of the filter reverse the supply of compressed air into the filter sleeves, where the resulting pressure removes the collected particles to the outside. Long-term, additional cleaning attention is required as the filter sleeves from time to time have to be further cleaned or exchanged.
The most common cleaning methods are manual cleaning, replacement of filter sleeves instead of cleaning and cleaning machines.
To this day, manual cleaning is the most frequently used method. The filter sleeves, often together with their cages, are dismounted and manually cleaned. Particles may be removed with brushes, and washing of the sleeves is also quite common. However, these are both time-consuming. Dismounting and mounting of the sleeves as well as the cleaning operation itself are quite expensive. In addition, the facility is standing still during the cleaning phase; this, too, has its price.
A quicker and often more convenient method is to replace filter sleeves. Offsetting the additional filter costs is reduced operation downtime and costs associated with cleaning, including man-hours.
The third option is to use a filter cleaning machine, which can be a very efficient solution. Many are capable of cleaning up to 180 filter sleeves per hour, and the capital costs of the equipment are offset by reduced filter replacement costs, as well as reduced downtime and manual labor costs. Typically, filters can be cleaned 8 to 10 times before they need to be replaced.
Usually, these machines are designed to clean the sleeve without having to remove the filter cage. Cleaning machines use a dual cleaning approach for maximum effectiveness in the shortest time frame. Cleaning is achieved both mechanically with motor-driven brushes and pneumatically by aspiration. The concept provides easy, quick and dust-free cleaning.
The brushes have concave nylon bristles that adapt easily to the filter sleeves of varying diameters.
Mesma Trading’s filter cleaner features a solid machine frame; dust collector with aspirator connection; latch covers with rotary brushes for easy machine cleaning and brush changing; air consumption of 14 cubic meters per minute; and voltage that can be adjusted according to customer requirements.
CLEANING MACHINE RESEARCH
Mesma Trading conducted research on the cost-effectiveness of these three cleaning methods. The company believes the assumptions used for this comparison correspond approximately with data collected in European and German speaking areas. These situations are adaptable from case to case, illustrating the outline of occurring costs.
The following is based on the assumption that during the cleaning and milling process, a mill with a capacity of 300t/24h uses approximately 336 filter sleeves, each with a 120mm diameter and 2,500mm length.
• Price for filter sleeve: approx. €6.70/each (average standard price)
• Number of planned cleanings per year: 2
• Time consumption per filter with manual cleaning: approx. 10 minutes (incl. dismounting and mounting of filter sleeves)
• Time consumption for removal of filter with cage, introduction in cleaning machine and re-mounting: approximately 4 minutes
• Hourly rate of employee: approx. €30/h
According to the above scenario, the cost of manually cleaning 336 filter sleeves would take 3,360 minutes or 56 hours. With two cleanings a year totalling 112 hours, manpower costs would be approximately €3,360 year.
Cleaning the same amount of filters with a cleaning machine will annually take about 44.8 hours (including the removal, cleaning and remounting) for €1,344/year in costs. The annual savings of €2,016 over manual cleaning will quickly pay for the cleaning machine within as little as two years.
Assuming that filter sleeves are replaced twice a year, the cost of buying new filter sleeves would approach €4,500, in addition to €1,320 worth of manpower hours to replace them. With a total of €5,820/year, the exchange of filter sleeves is the most expensive option.
These calculations do not consider the machine downtime, which is estimated to be about the same for filter replacement or machine cleaning. With manual cleaning, it is about 2.5 times higher than with machine cleaning.
Although the time consumption, hourly rates of employees and purchase price of filter sleeves may vary, machine cleaning of filter sleeves seems in general to be the most cost-effective method.
Mr. Carlo Lupi is a mechanical engineer and owner of Mesma Trading AG, Switzerland. He can be reached at email@example.com.