Sizing a Filter – More than Meets the Eye

Selecting the correct filter for the application should be approached from a methodical, questioning angle. If filters are application specific, meeting filtration specifications, physical and chemical conditions of the process must be considered before selecting the filter for the application. One must look at the system flow rate, system pressure, system temperature, maximum differential pressure, the type of fluid and the micron level to be filtered down to. Therefore, it is essential that a methodical process for identifying the customer’s needs is followed. To that end, we want to make sure we get all information requested on the Filter Cartridge & Housing Selection Form. Using one of these forms per application will ensure we don’t miss anything.

Let’s start by discussing differential pressure. The term "differential pressure" (Delta P) refers to fluid force per unit, measured in pounds per square inch (PSI) or a similar unit subtracted from a higher level of force per unit.

In filtration, the fluid pressure is measured both at the inlet of the filter housing and on the outlet side of the filter housing. The difference is the differential pressure. The lower the clean differential pressure, the longer the filter will last. Furthermore, pressure drop across the filter housing also needs to be understood when considering the clean pressure drop.

One of the most common questions asked is “How long will the filter last?”. The answer is always “Until it reaches the maximum differential pressure allowed by the manufacturer.” That number can vary from one filter manufacturer to another or from one filter media or design.

Figure 1.1 is an example of the filter’s life cycle. In sizing the filter, it is best to start as low as possible. The normal practice is to start with a clean pressure drop of less the 2 PSI. The ‘hockey stick’ design of the curve occurs due to the filter loading with contaminant and clogging the available area for the fluid to flow. It could be thought of as closing down on a valve or adding some other device that will restrict the flow.

Generally, as the differential pressure increases, the flow will decrease. In some applications, this can be a problem. There are applications where a minimum flow is required for the process to work. In these cases, the manufacturer’s change recommendation is not as concerning. Instead, we are concerned with maintaining process flow. To do this, we may need to change the filter more frequently and continually monitor the differential pressure.

There are several ways of measuring the differential pressure of a filter. We can install a standard pressure gauge rated for the system pressure in the pipeline before and after the filter housing. During this process, two readings need to be taken. The reading on the gauge after the filter housing is subtracted from the reading on the gauge before the filter housing. The resulting number is the differential pressure Another option is to use a differential pressure gauge.

By definition, a differential pressure gauge is a visual indicator designed to measure and illustrate the difference between two pressure points within a process system. The gauge usually has two inlet ports that are each connected to the pressure points on the filter housing being monitored. Differential pressure gauges are some of the most underutilized and misunderstood products in manufacturing. As a result, two standard pressure gauges are often used when just one differential pressure gauge would suffice. Using this type of gauge enables the operator to walk past a housing and see the condition of the filter inside.

Examples of Differential Pressure Gauges are below:

There are also systems that measure differential pressure along with flow, temperature and other important system functions. This information is collected and fed back to a central control center. These systems allow an operator to monitor the health of the process and take action if something like high differential pressure occurs.

Close monitoring of the differential pressure is important. If the differential pressure exceeds the manufacturer’s specifications, there is a possibility of a catastrophic failure of the filter. If this happens, all or a large portion of the contaminants captured will be released into the system.

Now that the differential pressure is understood, let’s return to the other criteria for the filtration needs. Asking the questions on the “Cartridge & Housing Selection Form” will help clarify what is trying to be achieved. What fluid is being filtered? What are the characteristics of that fluid? And what is the expected result?

Interestingly, the items collected by the filter will not always be considered a contaminant. In some semiconductor fab applications, the filters may be incinerated after use in order to collect the gold the filters removed from the process fluid. In a pharmaceutical or bio-technology application, the items collected in the filter may be the product the customer is producing.

Basic depth or pleated filters are generally used for water filtration unless one is making ultrapure water for the semi-conductor, pharmaceutical or bio-technology industries. The filtration process for this type of water used in these industries will involve several filtration steps. The first step uses the simple depth or pleated pre-filter as a pre-RO filter. The RO system is sometimes followed by a pleated filter that is placed prior to the DI resin canisters. Next will be post DI resin filters, and finally ending with a more advanced set of membrane final filters. The filters in this type of system are cartridges that can be a 5-micron depth or pleated pre-filter to a final filter that is a 0.03-micron membrane filter.

In a 350 GPM high purity water system the first filters would be the pre-RO filters. These filters typically have a 2.5” depth and either a string wound or a melt blown. Using the manufacturer’s data sheet will help determine the number of filters needed.
 

Pre RO depth filters rated at 1 micron are traditionally rated at 3-5 GPM per 10” equivalent through the element and the housing to maintain a less than a 2 PSID clean rating. This means one will need approximately 100 10” equivalent filters.

How is this accomplished? 2.5” filter cartridges are available in lengths up to 40”. If one uses a 40” cartridge, someone will need to change out those 40” cartridges. They will need at least 40” of head clearance to take the filter out after they get up to the top of the housing, which is going to be much more than 40” off the floor due to plumbing and other considerations for mounting.

The question to the customer is, “What are the space constraints?”. This needs to be understood so a decision can be made to use a multi-round 40”, 30” or even a 20” housing. One may also be able to use a vertical housing instead of a horizontal housing. Another consideration is to go to a high flow housing. This can solve a multitude of issues.

The math works out nice for a housing that holds the 40” though. One will need a vessel that will hold 25 40” filters. If the operator wants to run without stopping the flow to change the filters, they should use a duplex system that enables them to change from one set of filters to the alternate when the differential pressure gauge indicates it is time to change the filters.

There is another option the customer may want to look at. As each housing can handle the full flow, they may opt to flow through both housings during normal operation and one housing during change-out after the suggested differential pressure is reached.
 

Doing this allows the customer to take advantage of the “if you double the surface area of a filter you triple its life” scenario. This means, if the flow and everything stays the same, filters that last one month will go three months by simply doubling the surface area. This happens because of the reduced clean pressure drop and reduced velocities in the filter. By tripling the life, there are fewer changeouts required, and overall costs of operation are reduced, including a reduction of waste that needs disposal.

Following the RO, there is a series of DI resin beds followed by resin trap filters or the post DI filter cartridges. Post DI filters are going to have a different construction and possibly a different clean flow rate vs DP than the pre-RO filters. These filters will usually be 0.2 pleated filters. I have seen everything from pleated polypropylene to pleated polyether sulfone (PES) and in very rare instances PTFE (Teflon).
 

The amount of media (surface area) and the filter rating has a direct impact on the flow vs pressure drop (DP) of the filter. For this application, the filters selected are polypropylene membrane with polypropylene support material and 9.8 square ft of surface area, giving a flow of 3 GPM per PSI differential (PSID) per 10” element. This means one will need 58 10” equivalents to manage the flow of 350 GPM at a maximum 2PSID on a clean filter.

If the system is running well, the DP will never increase because at this point, it is using Ultrapure water (UPW). More than likely, the post DI filters and the final filters will be changed out on a Planned Maintenance (PM) Schedule.
 

The “Final Filters” are the key to UPW. These filters are forever getting tighter and tighter. Today, there are membrane filters rated at 0.02 micron. To achieve this micron rating, it is necessary to pack as much media per 10” filter as possible to maintain a respectful clean filter flow vs DP. But only so much fits in a 2.5” X 10’ package. I have seen PES and PTFE used in different facilities. For this application, a Polyether Sulfone membrane is used with all polypropylene support media and cage material. These filters will have a media pack that is 8.8 square ft per 10” equivalent. This will produce a flow rate of 1 GPM per PSID or 2 GPM at a clean 2 PSID per 10” equivalent. This means 175 10” equivalent filters are required to do 350 GPM at the desired clean pressure drop of 2 PSID or less. Older systems may not have the filter vessels or real estate for this many filters. If they want to get to the lower micron rated filters, they will need to increase the DP they are willing to live with.

“How long do we need to rinse the filters before we can bring them online?” is always a question asked. All new filters will have a certain amount of manufacturing debris remaining. In the UPW system, this debris is undesirable. Filters can be ordered pre-rinsed to minimize the need for rinsing. Alternatively, the filters can be rinsed at 1 GPM for approximately 20 gal per 10” equivalent to achieve a resistivity rinse-up to background minus 0.2 megohm-cm of feed needed for UPW.

If filters aren’t being changed on DP, they are going to need to be changed on a PM basis. How often do we need to change the filters? Most generally, one can expect filters in UPW water systems to run for a year, seeing the DP to increase marginally during that time. I have observed customers that have allowed their filters to be in longer. My advice along with the filter manufacturers is to not leave the filters in for longer than 18 months. The issue is erosion of the media. With the fluid velocities through the media, eventually it will degrade and start shedding particles. This can be worse than if the filter had a catastrophic failure. One starts shedding filter media downstream, and the DP never changes. Nobody knows until the particle analyzer signals that there is an issue. By that time, the system is contaminated, yields are affected, and production is lost.

I have always considered filters as one of the few things you can put into a customer’s process that has the potential of completely changing the customer’s product, shut down the process completely, or in some cases, cause a catastrophic failure. Therefore, we need to understand the application and as much of the process as we can before recommending a filter.

Once we have the right filter in the application, we council the customer to make sure the right filter is changed at the correct time.

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