Filtration sizing is a great example of an engineering exercise that is one part science, one part art. On one hand, making an initial filter selection is a straightforward matter of calculating hydraulic and contaminant separation performance within specific application parameters. On the other hand, however, this initial filter sizing must be interpreted based on an engineer’s experience, gauging how well the selection will perform under varying conditions well into the future. For these reasons, filters can be difficult to size correctly, especially when the art of predicting future variables falls short. When a filter is sized incorrectly, the issue may go unnoticed for some time until gradual changes in the system’s flow rate are detected. Luckily, this correlation between filter sizing and flow rate can provide buyers with unique ways to understand how to correctly size their next industrial fluid filter, as we’ll walk through below.
Let’s cover a few definitions to get us started:
- Filter Sizing - the process of selecting a filter based on a combination of physical and performance-based criteria including surface area, soil holding capacity, filtration porosity size, flow rate, and pressure drop. In short, determining the ideal filter size is a function of selecting the minimum physical area and maximum porosity size that will achieve the required flow rate, pressure drop, and soil holding volume necessary to protect downstream processes.
- Flow Rate - the amount of fluid that flows through a system in a given period of time. Flow rates are typically expressed in volumetric units such as Gallons per Minute (GPM), Liters per Minute (LPM), Cubic Meter per Minute (M3/M), Cubic Feet per Minute (F3/M or CFM), or Standard Cubic Feet per Minute (SCFM).
Connecting Industrial Filtration Sizing and Flow Rate
With these definitions established, let's now spell out the connection between filter sizing and flow rate:
- Generally speaking, as a filter's physical size decreases, the achievable flow rate that passes through that filter also decreases.
- In addition, as a filter's porosity size decreases even when the physical surface area size remains constant, the flow rate decreases as it takes more energy to push fluids through smaller gaps in the filter’s surface.
- As a filter loads with soils over time, the filter’s available open surface area decreases, causing a decrease in flow rate proportionally.
- Putting these points together, properly sizing a filter should target a forecast future point in time when the filter will be moderately loaded, worn, and derated through normal use. Even better, a safety factor should be applied to account for system flow and soil peaks, as well as delays in maintenance.
It is important to recognize that the three main variables in play - surface area, porosity size, and flow rate - can be changed independently and blur the lines between the above simple principles. There are applications where decreasing filter physical size while increasing porosity size can achieve a net increase in flow rate capacity. In this case, even though the filter's surface area (or physical size) decreased, the increase in porosity provides more open area through the filter's surface. Readers should remember that "filter size" is often used generically to refer to both surface area and porosity size, making sure to address both independently.
Technical Considerations Regarding Filter Flow Rates
From the above, readers will have a sense of the general relationship between filter size and its flow rate. Next, let's dig deeper into more technical factors that influence flow rate.
Friction & Abrasion
A filter’s materials and design both affect how much friction occurs as fluids pass through. Higher friction restricts flow, resulting in higher upstream pumping energy demands and lower flow rates. Additionally, a high flow rate can result in excess fluid velocity, causing abrasion and wear on the filter element.
Pressure
A fluid filter can be visualized as a porous surface that fluid must be forcefully pushed through, capturing unwanted soils and allowing clean fluid to emerge. The force pushing fluids forward is known as pressure, resulting from upstream pumping action, fluid head, fluid weight, and gravity. Filter flow rate capacities are based on a required upstream pressure value, such as "200 GPM at 50 PSI." If pressure falls below this value, flow rate will decrease accordingly.
Viscosity
Fluid viscosity (how "thick" or resistant to flow it is) directly impacts filtration flow rate. Fluids like air and solvents have low viscosities and flow through filters with less motive force, whereas fluids like heavy oil and epoxies have high viscosities and take much more force to flow. In this way, filter sizing must consider a fluid's viscosity to ensure that it can meet the system's flow requirements. Most filter sizing charts are based on ambient temperature water and must be adjusted to non-water viscosities accordingly.
Pressure Drop
Just as filter sizing depends on upstream pressure, it also depends on downstream pressure. When fluids are pushed through a filter, friction consumes some of the fluid’s motive pressure while the remaining pressure carries through with the fluid. For example, a filter sized for 200 GPM at 50 PSI input may have a 10 PSI pressure drop, leaving 40 PSI at the filter’s outlet. This discharge pressure must then be checked against the required force needed to move fluid to its destination, and if insufficient, can lead to low flow rates and even complete flow stalls.
Energy Consumption
Short of static head pressure and gravity, generating the pressure needed to push fluids through a filter takes some form of input energy - most often seen as electric pump power. When sizing a filter, system designers usually seek the lowest viable input pressure and pressure drop so that they can also utilize lower pump sizes and horsepower upstream.
Advanced Application Parameters
Advanced applications present even more considerations for filter sizing including material compatibility and reactivity with the fluids being processed, compression factors of the soils captured in the filter's body, flow-induced degradation of captured soils, thermal derating of filter capacity, and much more.
The Economics of Filter Sizing - Too Large vs Too Small, Cost, and Time
While we've mainly used examples of a filter's flow rate being too low so far, we'll now point out the equally possible issue of a filter's flow rate being too high. In practice, oversizing a filter is just as problematic as undersizing a filter. At face value, selecting a filter that is larger than an application calls for may seem to only be a waste of upfront money, but there are many more knockdown effects. A few examples: wasteful energy consumption, onerous maintenance frequencies, uneven filter saturation leading to premature failure, large system hydraulic swings, and insufficient filtration protection. As these problems mount, system processing times drag out, operating costs increase, and production throughputs drop (or at the least, are not improved in any meaningful way relative to the extra filtration capacity).
Don't get us wrong - there is certainly value in stepping up to the next size filter, but there can definitely be too much of a good thing. With that, we encourage readers to round out their filtration sizing efforts from one last vantage point, this time weighing investment cost versus long-run operating cost. If a prospective filter provides a good balance of these costs, buyers can be confident in their sizing. Whether readers think the cost equation is more science or more art, we’ll leave that to you!