What are duplex filters and why should I consider using them?

Because they are so convenient! You can accomplish filter change-out without shutting down the system. You just divert the flow from filter A to filter B, change the filter A, then reconnect the flow. Much easier! Our HMK series DURAMAXÒ filters are particularly well-suited to duplexing because they are spin-ons yet handle high pressures. Typically, you need a valve for duplexing two filters, and a manifold plus two valves for three or more filters. Many of our fluid power distributors have the capability to combine DURAMAXÒ filters with the proper valves and manifolds.

One example of a duplex filter circuit


Should I use synthetic or cellulose filter media?

While cellulose is appropriate for some applications, synthetic media typically offers the lower differential pressure, higher temperature resistance, and longer shelf life. Synthetic filters can also be used with a greater variety of fluids and are more chemically stable. Cellulose elements are usually less expensive, but will probably have to be changed out more often.

What’s the difference between “static” pressure and “working” pressure?

Static pressure is the maximum rated pressure that the filter assembly can handle before it bursts apart. Working pressure is the reasonable pressure the filter can handle for a sustained length of time under controlled cycle fatigue conditions.

How do I know what size filter I need?

You need to know flow rates, pressures, and type of oil. Then call a Donaldson authorized distributor to help you choose the right filter.