How To Size And Select a Filter or Strainer
Figure 1: Y strainer
Choosing the right filter ensures optimal performance and longevity while meeting the specific filtration requirements. Choosing the right filter or strainer involves considering factors such as flow rate, particle retention requirements, pressure drop limitations, and compatibility with the filtered fluid. This article serves as a guide on choosing a filter for an application based on these factors.
Table of contents
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What is a filter?
A filter removes unwanted particles or impurities from a fluid. It allows the fluid to pass through while capturing and retaining undesirable particles. Filters are commonly used in diverse systems like air conditioning units, water treatment plants, automotive engines, and industrial machinery to maintain the quality and efficiency of processes.
How to select a filter
This section discusses the important considerations and criteria for choosing the optimal filter for an application.
1) Filter (strainer) type
Y strainers and t strainers are typically used for inline applications with horizontal flow directions. Y-strainers have an inlet and outlet in opposite directions, while T-strainers have a perpendicular outlet with respect to the inlet. Suction strainers are specifically designed for applications where the flow is drawn from a reservoir or tank, typically in a vertical direction. Read our types of pipeline strainers article for more information.
2) Particle characteristics and size
Microns and mesh measure a filter screen.
- Micron rating: The micron rating indicates the size of particles the filter can effectively capture and retain. A filter with a micron rating of 10 µm can capture and retain particles that are 10 microns or larger in size.
- Mesh size: Mesh sizing refers to the number of openings per linear inch in a woven mesh screen. It represents the fineness or coarseness of the screen and indirectly indicates the particle size that can pass through the openings. The higher the mesh number, the finer the screen and the smaller the particles it can retain. A filter with a mesh size of 200 has 200 openings per linear inch. Smaller particles that can pass through the openings of a 200 mesh screen would be retained by a finer mesh screen, such as a 400 mesh screen, which has twice as many openings per linear inch.
Knowing the characteristics of the particle to be filtered helps to choose the appropriate micron/mesh size for the filter. For example, water may contain inorganic materials like sand, while ponds can contain more organic matter like leaves and algae. Inorganic elements require a smaller screen to trap the sand that would otherwise flow through a filter with a larger micron rating.
3) Connection size and type
To select a filter with the proper connection size, identify and match the existing system's connection size when choosing a filter. Also, ensure the connection type matches the system it connects to.
4) Material
The filter and seal material should be compatible with the filtered fluid. The seal ensures no leaks or bypass paths between the filter and the system. Read our chemical compatibility chart for more details on the compatibility of different materials with various media.
5) Maximum operating flow
The maximum operating flow of the system determines the filter size required. For instance, for a 100 GPM (gallons per minute) system, a 2" filter that permits 70 GPM will not be suitable. In such cases, select a bigger filter to accommodate a larger flow.
6) Operating pressure
Measuring the maximum pressure accurately is important as every filter is certified for a specific maximum operating pressure. Additionally, when using an autonomous, self-cleaning filter, the minimum operating pressure becomes vital to ensure the proper functioning of the flushing mechanism.
7) Pressure drop
Pressure drop is the decline in fluid pressure when it passes through a filter. Factors such as the impurities present in the medium, viscosity, and flow rate affect the pressure drop. Pressure drop increases as the strainer fills with filtered particles. Excessive pressure needed to address this situation can affect the filter's flow.
8) Operating costs
Consider the operating costs when selecting a filter. Self-cleaning filters have a higher initial investment than manual filters, and strainers can offer long-term cost savings. They reduce labor and downtime associated with filter replacement. Consider factors like the expenses for trash disposal.
9) Risk to the operator and the environment
Certain filters can handle highly poisonous media, while others cannot. Always assess whether the operator and environmental exposure to the medium is appropriate and take necessary precautions.
FAQ
How do I size a filter?
Consider flow rate, pressure drop, and particle retention to determine filter size. Calculate the required surface area or select appropriate dimensions for optimal filtration.