Sorting Out Filters – About Re-Circulation

For any filter to be effective, the liquid being filtered must circulate or re-circulate through the filter in an efficient way.  In a spray system, the cleaning or rinsing liquid usually returns from the filter directly to the spray head(s).  This means that the liquid spray used to clean or rinse is as good as it can get with the filter being used.  Immersion cleaning tanks with external filtration loops as described in an earlier blog present a little different story.

Illustration showing tank with filter loop
The liquid exiting the tank is a mixture of previously filtered liquid and liquid that has not been previously filtered.

In a immersion system using an external filtration loop, as shown in the illustration, the liquid in the tank is not filtered sequentially.  Instead, some of the liquid exits the tank through the intake to the pump, flows through the filter and is returned to the tank.  The returning liquid mixes with the rest of the liquid in the tank.  The influx of freshly filtered liquid reduces the overall concentration of particles in the tank but, since it is not isolated from the overall volume of liquid, mixing with liquid still ladened with particles is inevitable.  Ideally, as little of the previously filtered liquid as possible should be drawn into the intake to the pump to assure efficient tank filtration.  Placement of intakes and returns can help in increasing the efficiency of filtration and should be considered in the design of a cleaning system.  A “cascading” arrangement as shown in the blog “From Rags to Rinses” can help increase the efficiency of a multi-station wash or rinse.

What Are the Numbers?

The chart below shows the effect of filtration on a re-circultaing tank.  The chart assumes that the filter is 100% efficient and that the returning liquid is perfectly and instantaneously mixed with the liquid remaining in the tank on returning.

Illustration showing the relationhip between filtration effectiveness vs. time depending on re-circulation rate

The result is quite enlightening and has an important impact on the development of cleaning processes and machine programming.  Obviously, filtration in a recirculating tank takes time.  Even if the re-circulation rate is the entire volume of the tank every minute, achieving nearly total particle removal requires 5 minutes.  With a re-circulation rate of 25% of the tank volume every minute (which is more typical in most systems) near total particle removal requires more than 15 minutes.  Clearly, it is best to maximize filter time whenever possible.  As we have said before, it is not possible in my experience to provide too much filtration in a cleaning system.