Managing the quality of fluids handled in commercial and industrial applications is a world upon itself. Fluid quality treatment solutions on the market today span a huge array of different technologies and products, with the single most common piece of equipment found across all of these applications being a filter. Because filters are so universal, we should all understand a clever method of comparing filters and their performance using the measurement of Filtration Rate, as we'll explain below!
Starting with a few definitions. filtration is the process of passing a fluid through another porous substance or device, where unwanted components within the fluid are physically separated out. For example, water can flow through a cartridge filter having pores so small that good water is allowed to pass through, but larger dirt and mineral particulates are not. Filtration rate then is the amount of fluid that can flow through the surface area of the filter media within a given amount of time, usually described in Gallons per Minute per Square Foot (or GPM/sq ft).
Why is Filtration Rate Important? Three reasons:
1. Filter Sizing
Filtering out undesired media from fluids is a balancing act between cost and efficacy. Too large of a filter, and you're wasting money. Too small of a filter, and you're creating a bottleneck of ineffective filtration. This balancing act is compounded by the characteristics of your application as well - temperature, soils load, viscosity, and other physical properties impact ideal filter sizing. So, filtration rate gives us a singular measurement value to use in comparing filter sizes against established standards, regulatory requirements, and published best practices.
2. Filter Performance
We measure filter performance in two ways: beta ratio and loading percentage. The beta ratio compares the number of particulates of a given size sampled before and after the filter, which tells us how efficient the filter is at capturing those particulates. Loading percentage refers to how much of the filter area can be clogged (or loaded) before we fall under the performance requirements of the system (such as dropping too low in flowrate). Both values can be approximated using filtration rate - that is, hitting a desirable filtration rate directly solves for good beta ratios and loading percentages together, providing ideal filter performance.
3. Process Performance
For filters where a secondary reaction or process is occurring, filtration rate is used to ensure that this process will occur correctly. For instance, with an activated carbon tower, the carbon needs a certain amount of time exposed to the incoming water volume to react and adsorb particular constituents. In this way, targeting a specific filtration rate assures that this reaction occurs as intended and can hit the desired outbound water properties.
Calculating Filtration Rate
Historically, filtration rate has been most used to describe fixed sand filter basins in water and wastewater treatment, so we'll use this application as our calculation example.
Imagine a square concrete vault dug into the earth, filled with sand media. Water flows in at the top of the vault, falls by gravity through the sand volume from top to bottom, and then flows out of the vault at the bottom. Incoming water has large particulate soils such as minerals and organic matter, which the sand filters out as the water cascades down the vault's depth. Water flowing out of the bottom is much cleaner, now filtered of this unwanted material.
Our objective is to determine how fast water flows through the sand filter in a given amount of time. So:
Step 1 - Gather Physical Data
We need surface area and volumetric flow rate.
The vault measures 5' wide x 5' long x 3' deep.
Water flows in at 7,500 gallons per hour continuously.
Step 2 - Calculate Surface Area
Since we're only concerned with area and not volume of the vault, we set the depth measurement aside. Using only the length and width, we calculate:
Surface Area = Length x Width
Surface Area = 5' x 5'
Surface area = 25 Square Feet
Step 3 - Calculate Flow Rate
We were given an incoming water rate of 7,500 gallons per hour.
We would like to arrive at a flowrate in gallons per minute, as this is the format of most published filtration rates, so we will convert:
Flow Rate = Gallons Per Hour / 60 Minutes Per Hour
Flow Rate = 7,500 GPH / 60
Flow Rate = 125 GPM (Gallons Per Minute)
Step 4 - Calculate Filtration Rate
Now we can combine our data above to calculate filtration rate.
Filtration Rate = Flow Rate / Surface Area
Filtration Rate = 125 GPM / 25 sq ft
Filtration Rate = 5 GPM / sq ft
The above result says that for every square foot of surface area in our filter, water passes at a rate of five gallons per minute. So, we flow 5 gallons per minute per square foot of filter surface area, otherwise stated as a filtration rate of 5 GPM / sq ft.
Key Considerations in Industrial Filtration Sizing
A few quick additional considerations for applying filtration rate and selecting filters:
The above calculation explains the relationship between surface area and flow in a gravity vault, and this same principle applies to any type of filter where we can measure a surface area, fed by gravity or under pressure. In the industrial world, we see many more cylindrical, basket, filter bag, membrane, and plate filters - all of which can be described by filtration rate in the same manner. For such applications, readers can adjust 'Step 2-Calculate Surface Area' with the most applicable area formula, such as to calculate the surface area of a cylinder. Worst case, contact the filter's manufacturer to have them provide a surface area for you (especially when using pleated or membrane filters, where surface area is not clear or may even be proprietary to that manufacturer).
Hydraulic and thermal parameters of an application can have big impacts on filtration rate requirements, such as temperature, viscosity, pressure, and particulate size distribution, to name a few. Review potential impacts carefully.
Filter loading and cleaning is a huge part of filter selection. How long a filter can build up particulates; cleaning method selection such as backwashing or regeneration; and hydraulic performance losses under increasing load are all important parameters to be reviewed thoroughly.
Filtration rates that are too low or high can have many adverse effects that might not be obvious. One example is having a correctly sized filter, but over time, portions of the filter area become restricted, which increases the relative flow over the limit of the remaining area. This can lead to tunneling, media damage, or worse issues. Be sure to thoroughly discuss operational nuances and risks before selection, which will likely drive design tweaks beyond just filtration rate.
Lastly, filters rarely see continuous flow. Spikes and lulls in flow rate impact filtration performance. Our calculations above treat the flow rate value as continuous, but in reality, this is not often the case. For systems where rates vary, users should calculate minimum, maximum, and average flow rates, and consider all expected rates (as well as the amount of time each exact condition occurs) during filter selection.