By: Mike Click
A comprehensive, 13-step procedure for selecting components and determining an optimum, cost-effective filtration system design for single-pass, process-fluid filtration systems.
Optimum filtration depends on how equipment is used and its operating environment. Since these parameters frequently change, process engineers must frequently review filtration system design features, and modify them if necessary, to provide a desired level of contamination control.
Single-pass filtration systems are those in which process fluids pass through the system only once and are not recirculated. The major objective in single-pass systems is the reduction of contamination; however, other important single-pass system objectives can be clarification or classification. Clarification means reducing the effects of particles on clarity due to their scattering or blocking of light. Classification is the removal of selected particle sizes while intentionally leaving others.
Filtration system design, or the upgrading of existing filtration, consists of two phases. First, filter locations are determined and the media that will be used in them is chosen. Then, actual filter assemblies are selected based on manufacturers published data.
A quantitative, 13-step model, or procedure, is a powerful and practical tool for selecting filters and media for process filtration systems. In this model, Fig. 1, the first eight steps lead to a selection of locations for filters and appropriate filter media. The last five steps involve selecting filter assemblies and performing a cost/benefit analysis. These steps are repeated until an optimum cost/benefit tradeoff is obtained. A computer program is usually necessary to perform the iterative calculations.Planning for Filtration
A general filtration plan should be developed prior to initiating the 13-step design and selection procedure. This plan should, at least, address the considerations in Table 1.
When the filter also has to protect system components while achieving adequate contaminant reduction for the process, both the process and component sensitivities must be examined to determine which will govern removal efficiency. Also, a maximum system pressure-drop limitation will be a major determinant of housing size and number of cartridges.
In many applications the type and number of solid contaminants can affect the type of cartridge and system required. Applications that require removal of solid contaminants require different filter media than do applications where gels or agglomerates are to be removed.Simplifying Assumptions
Typically, a model calculates the filter Beta ratio based on required fluid cleanliness level, or vice versa. Beta ratio (also known as the filtration ratio) for a specific particle size is the ratio of number of particles of a given size and larger upstream of a filter to the number of particles of a given size and larger downstream.
Filter element Beta ratios, usually determined under controlled laboratory flow conditions, must be derated to compensate for cyclic or surge flow conditions. This derating factor should be based on testing and guidelines supplied by the filter element manufacturer.
Another possible assumption is that no fluid bypasses the housing separator plate or filter elements. This assumption should be verified by measuring the contaminant concentrations both upstream and downstream of the filter housing. If the downstream concentration is equal to the upstream concentration the cause may be that there is fluid by passing around the element seal or the housing separator plate, or the element is unloading.Step 1: Material and Design Compatibility
Compatibility issues require a review of all filter materials. The compatibility review looks at the effects of operating variables, fluid characteristics, and compliance with appropriate codes and standards. Primary operating variables are flow, pressure, temperature, and viscosity.
Compliance with codes and standards considers potential hazards associated with product consumption, biological exposure to filter materials, filter housing design codes, and suitability of the filter based on operating variables.
Guidelines frequently used to select the number and location of filters include: (1) amount of contaminant in the fluid; (2) element service life needed; (3) point of fluid use or packaging; (4) contaminant source.Step 2: Filter Locations
If the fluid is heavily contaminated with a wide range of particle sizes, a single filter may not be practical. One solution is to use two filters installed next to each other in the fluid line. In this two-stage system, relatively coarse media is installed in the first filter to capture larger particles. The second stage, or polishing filter, has tighter media to capture smaller particles.
Side-stream, or off-line, filtration is used in some applications. In this configuration, the filter is located in a loop off the main fluid line. Side-stream filtration provides the added benefit of preventing contaminant build-up in storage tanks.Steps 3 through 6: Operating Conditions
These steps are statements or measurements of process operating conditions. The variable values in these steps must be found before specifying media and the size of the housing. In a batch-flow situation, it may be useful to determine both average and peak flow conditions, and to determine the total volume of each batch to be filtered. This information may lead to selection of a less expensive filter system than a continuous-flow situation would allow.
Flow and viscosity, major determinants of initial pressure drop in a filter system, influence element service life, type of media, and housing size. Upstream and downstream contaminant concentrations and particle size and distribution are very helpful in determining the most economical filter media.
Variables can be measured with in-line instruments, or by taking fluid samples and having the filter manufacturer analyze the samples in their laboratory and make system recommendations. Care must be taken to provide representative samples of the process for analysis.Step 7: Particle Removal Efficiency
For ideal particle control or clarification, the minimum acceptable media efficiency must be determined. Removal efficiency can be furnished for each of the particle size ranges of interest. These efficiencies are determined from manufacturers published data or from sample analysis. The upstream counts divided by the downstream counts determine the minimum filtration ratio (or Beta ratio).
When visual clarification is the objective, turbidity measurement can usually be substituted for particle counts. If classification is the objective, determination of the particle distribution and size are of primary concern. At the critical particle size, the particle removal efficiency is stated as a maximum. For larger diameter particles, the removal efficiency should be as high as possible. This means that the filter media needs to have a relatively sharp cutoff in its particle-size selectivity.Step 8: Other Contaminants
Fluid systems can contain more than one contaminant. For example, in addition to particulate contaminants, a system may contain gels or agglomerates. For each type of contaminant in the system, Steps 5, 6, and 7 should be repeated. Step 9: Potential Housings and Filter Media
Specifiers usually select a housing or media rating that is just above the minimum requirement calculated in Step 7. These ratings appear in most manufacturers published data. This step is the initial step in the selection of potential vendors for the filter or separator. The choices are further narrowed in subsequent steps. Step 10: Sizing the Housing
The key issues in this step are minimal pressure-drop contribution, element life, pressure rating, and housing style. Housing style considerations include physical shape, dimensions, inlet and outlet configurations, mounting methods, element removal clearance, and method of access to filter elements.
Housings meeting style and system pressure criteria can be chosen from flow/pressure graphs or charts for manufacturers published data or by consulting with the manufacturer.
Cost per gram of dirt removed reveals the true value of the element. However, intentionally oversizing a housing can provide additional life that is more desirable than the lowest possible housing and element cost.Steps 11 & 12: Cost Comparisons
Selecting a housing or filter media for a given application involves tradeoffs between removal efficiency, pressure drop, housing size, service life, cost , and other variables specific to the fluid system. In addition to housing and media costs, other direct costs of owning and operating a separator include the costs of routine housing maintenance, spent media disposal ,process interruption, and media replacement labor. All direct costs should be combined to determine the cost per gram of contaminant removed.
Valid comparisons of features and prices of various manufacturers products can be made after subjecting tradeoffs to "what-if" analyses to discover the impact of changing performance, cost, and benefit variables.Step 13: Accessories and Options
Certain accessories and options can add convenience and reduce housing maintenance costs. These include differential-pressure gauges or indicators, housing drains, captive covers, media scaling devices, and mounting brackets or legs.
The final phase of the selection process is to specify the manufacturers model number or part number. Most manufacturers provide ordering guides to aid in specifying products.
Important Considerations of a Process Filtration PlanProcess priorities Filtration objectives Economic factors associated with filtration Process flow characteristics (batch vs. continuous flow) Fluid type and operating parameters Contaminant type and concentration Fluid system design and component features
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