Design of a filtration system mostly is the last priority for any process designer or turnkey solutions provider. Most of the time and effort is spent in design and engineering a process that helps in increasing product yield, maximizing utilization, minimizing wastages & optimizing process economics. Needless to mentions design of a proper filtration system goes a long way in achieving the above objectives as well. We will discuss in this chapter various parameters and principles which need to be considered for the design of a cartridge filtration system. A properly designed filtration system will help in reducing downtime, improving filtrate quality, and optimizing the direct cost of filtration.
In most process applications, filters are used to protect the process or a product. It can be RO membrane in a desalination plant or alkanolamines in a refinery /gas processing plant. Filters also play a vital role in protecting heat exchanger fouling’s too controlling particulates contamination in a CED paint bath system in a paint booth. Several basic factors play a critical role in selecting the right cartridge filtration system. Let’s discuss the same one after another.
Nature of Contaminants
The nature and composition of contaminants play a very significant role. Contaminants/particles which are easily deformable or gel-like in a structure are the hardest ones to remove. Whereas angular and rigid ones are easier to remove. Further, the concentration level of each type of contaminant also plays a significant role in selecting a proper filtration system. In certain process fluids, we can have a mixture of different types of contaminants both in size and type. That’s where selecting and designing a filtration system gets more tricky and the knowledge and experience of the applications engineers come very handy. In these kinds of cases having a stage-wise filtration train with a proper mix of Graded depth filter media with Surface filter media works well.
Filter media selection depends on operating parameters like chemical compatibility, temperature, pressure, and mechanism of filtration. Broadly filter media can be grouped into two types-Non-rigid e.g Polypropylene, Polyester, cotton, glass fiber etc., and Rigid- like steel, ceramic, etc. The filtration mechanism can be grouped into three types – Surface, Depth & Adsorptive. The surface types remove contaminants on the surface of the filter media. Mostly the surface mechanism is used in the pleated form and with uniform porosity. Whereas the depth media removes within the depth of the media, particles get trapped within the tortuous path of depth media. In contrast to all these the adsorptive mechanism removes contaminants by adsorption to the media. Adsorptive filter media are capable of retaining particles smaller than their pore size. Great examples are coalescer or carbon filters.
Flow rate is the rate at which the fluid passes through the filter. The dirt holding capacity and efficiency are directly linked to the filter flow rate. Flow rate plays a critical parameter for deciding the length, number of filters, and inlet/outlet connection sizes of housings. As a normal rule of thumb, most manufacturers recommend 1m3/hr flow rate per 10” cartridge (depth filter). In certain cases the system is designed at higher flow rates per 10 “ cartridge, which affects the dirt holding capacity of the filter cartridge, as flow rate and dirt-holding capacity is inversely proportional. Lower dirt holding capacity means increasing the frequency of change-outs. In a lot of other instances, in order to minimize the diameter of housings (cost saving on housings), there is an increasing tendency to use longer-length cartridges like 50 ” /60″. But we need to keep in mind the typical cartridge filters have 27mm/28mm inner core so passing more volume of fluid will lead to higher velocity in the inner core. That will lead to higher turbulence, increasing pressure drop, and lower efficiency. At filter concept we have developed cartridges with larger inner core to support customers who want to go for longer lengths of cartridges necessitated either due to the requirement of higher filtration area or smaller diameter housings with longer length cartridges.
The micron rating of the filter is related to the degree of filtration. The degree of filtration reflects the level of cleanliness of filtrate. The degree of cleanliness has two aspects – the number of particles and the size of particles. Typically filters are given Nominal or Absolute ratings. In common parlance, any filter which has an efficiency of below 99% removal ratings is taken as a Nominal rating filter and Any filter above 99% efficiency is rated absolute. Filter ratings depend on the test protocol employed. ASTM F795(4) is commonly used for such testing. However different manufacturers have their own way of defining this as well. That makes it difficult to compare two filters from different manufacturers. However, filter rating acts as a guide towards actual performance.
The filter cartridge and the fluid must be compatible. Variables affecting compatibility include service temperature, duration of exposure, and concentration of any component capable of causing filter degradation. Most of the filter manufacturers suggest a compatibility chart for use. But notable compatibilities can be complete enough to cover all fluids or conditions of exposure. Experience in identical or similar systems can be highly valuable. Without such information, some sort of testing is required. “Bucket testing” — in which a cartridge is put into a pail of the process fluid for a relevant amount of time — can be useful. The challenge in running the test with hazardous fluids, at elevated temperature, from a change in fluid composition with time and from being static rather than a flow-through test. If the filter cartridge seems somewhat affected but not destroyed, further testing is essential to see if it can provide adequate service life or if a more chemically resistant filter is necessary.
It is always advisable to position the filter near the source of contamination. That makes the particles easier to be separated, as the filter gets farther from the source chances of particles getting smaller due to friction and an increase in concentration can happen. This typically applies to viscous solutions like paint. While deciding the locations it also makes sense to have enough space for filter change-out, venting, and sample collection. A cramped place also is a safety risk.
Most of the filter systems are designed with positive pressure operations. The Differential pressure is a critical parameter while deciding on filter cartridge change-out also the construction of housings is dependent on the same. While calculating pressure drop across the system both the pressure drop due to cartridge and housings have to be considered. As a general rule, most of the cartridge filters are designed with a working pressure of a maximum 6 Bar and a change out the pressure of 1.5 Bar.
To sum up, a properly designed cartridge filter system goes long way in trouble-free performance and better filtrate quality. Further, an optimized design with balanced capital cost Vs consumable cost also makes the life of plant operation engineers easier. We at Filter concept have the knowledge and skill-set for assisting our entire customer in filter system design. Please get in touch with us at firstname.lastname@example.org