Questions From our Clients

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Question: What is the difference between the double ended filter and single ended filter?

Reply: Thank you for your question!

Double-open-end (DOE) filters generally use a “knife edge seal” which means ridges in the filter housing press into the filter. Both ends of the filter are open when they come out of the box, but of course the system is properly sealed when the filter is installed.

Single-open-end (SOE) filters have extra end-fittings on them. One of the ends will be some type of cap to close off that end. The other end typically has a fitting on it with two O-rings. This adapter with O-rings will push down into a receptacle in the filter housing.

A DOE filter’s “filter-to-housing seal” can be very slightly leaky – there can be a tiny bit of fluid bypass around the ends. However, many applications do not require 100% removal efficiency type filters, so the DOE style is often sufficient. On a string wound filter with “plain ends” the “knife edges” of the filter housing push directly against the filter medium to create a seal. Some customers using string wound filters feel that they obtain better performance by adding a flat gasket seal, especially when the removal rating of the filter is around 5 microns or below. When flat gaskets are added to the DOE filter, it is still a DOE filter, but it has gaskets and no longer has “plain ends.” Delta Pure offers a variety of flat gasket seal materials: EPDM, BUNA, VITON and polyethylene are the most common. Delta Pure also offers string wound filters with a variety of DOE and SOE end fittings. Plus we offer melt blown, carbon and pleated type filters with a variety of end fittings.

SOE are better for critical applications with “absolute rated” membrane type filters because the filter-to-housing seal is very leak tight. SOE end fittings are often specified for more critical pre-filtration applications using melt blown, string-wound, pleated and even carbon filters. Some people use “rules of thumb” for when a SOE vs. DOE filter should be used. According to one “rule”, any filter rated finer than around 5 microns should use SOE (the exact cutoff for the “rule” would be a matter of opinion). As an example, if a customer were selecting a sub-micron rated membrane filter, I would encourage her to use a SOE type. There are always exceptions to any rule of thumb – it all depends on the requirements of the application!

Another layer of complication is “temperature changes” – SOE is often better where the temperature of a process can fluctuate significantly (example: processing at 70 deg F followed by hot water flushing at 176 deg F.) This is because a filter can grow and shrink significantly when temperature changes, and a SOE filter will typically do a better job because one end is anchored in place. Polymeric membrane filters that are steamed in place (SIP) are almost always SOE type and generally never DOE, however, metallic filters for filtering steam may often be double-open-end type – so again, the selection of filter type depends on the application.

DOE filters will often be at a much lower price point than SOE filters.

One potential strategy is to use a filter housing that can accept both DOE and SOE types. For example, our SHU housing (a stainless steel industrial housing which accepts one 2.5″ diameter filter) can accommodate both DOE and SOE styles. We can also provide variety of multi-round housings that provide flexibility like this. With a housing that offers flexibility, one can start off with either the DOE or SOE format and switch to the other depending on evolving requirements.