Water Filter Replacement Scheduling Strategies 1

[rawelk]

We use a four cartridge stainless steel filter cartridge housing equipped with Parker T23R10A, 5 micron, wound polypropylene filters. These filter cartridges are 10" long x 2-1/2" OD x 1.0" ID, and rated for 4 GPM max. at 140°F max.

Chilled water at 55°F nominal comes into the filter from the process pump manifold (pump outlet pressure approximately 85 PSI), and returns to the chilled water tank (that is, in a side stream configuration) where return water flow can be observed. Our chilled water is used to cool rolls used in a plastics extrusion process, and also feeds several temperature control units used for mold temperature control.

The only unusual plumbing is I've tapped off from the filter outlet, and side-stream part of this flow over to an electrical enclosure cabinet cooler (a Kooltronics model KNHE28 heat exchanger installed late last summer), and then dump that back into the tank.

This housing has a pressure gauge mounted on the lid to show filter housing back pressure, and ranges from approximately 36 to 38 PSI after filter replacement, and reaches a maximum of approximately 10 to 12 PSI below pump pressure when fully blocked.

My question isn't one about whether these filters are a good match for the application (although I wouldn't mind any opinions in this area), but rather what are regarded as good strategies for determining when to schedule element replacement.

My opinion is filter replacement should be done once back pressure has reached approximately 90% of the "fully blocked" back pressure value (in this case, "fully blocked" ranges from 75 to 77 PSI, so I would change at 70 to 72 PSI) because not much more filter capacity remains, and flow is starting to drop off.

Another opinion holds that replacement is only necessary once return flow is visibly quite a bit less forceful.

It seems to me that replacement at the 90% blocked point serves, over time, to remove more solids from the water system than waiting for nearly complete blockage.

Another point - for one span of time over these daily observations I allowed filters to remain totally blocked for more than a week, and noticed back pressure increasing and decreasing out of step with pump pressure (for instance, 69, 74, 72, 69, 77, and 75 PSI with pump pressure ranging from 84 to 88 PSI over this time, and not necessarily higher on days with high back pressure). I am not certain, but speculate that filtered debris was breaking free from the filter ID, and re-entering the tank.

Since I'm using this flow for cabinet cooling, waiting for severe flow reduction causes cabinet temperatures to rise. This isn't a big deal now because ambient temperatures have been consistently below 80°F since mid-September, 2008, but will become more important as summer 2009 comes upon us.

I'm wondering if there are any "rules-of-thumb", or accepted practices, and will appreciate any commentary on this question.

 

[bimr]

It looks like Parker recommends "Change Out ?P: 30 psi (2.1 bar)".

If you operate past this pressure, you may force material through the filter or risk the collapse of the filter.

http://www.parker.com/literature/Pr...ature/Single sheets/HFT Stringwound C1000.pdf

Change with Parker to confirm.

One strategy that is commonly used with cartridge filters is to multiple filters in series. The initial filter would use larger micron sizes. The last filter would be the 5 micron final filter.

Another option is to use a backwashable multi-media filter as a roughing filter and then to filter effluent with a 5 micron final cartridge filter.

Pressure drop is the decrease in pressure that occurs as
the liquid flows through the cartridge. Every filter cartridge has a pressure drop vs. flow rate curve, which is dependent on the permeability of the media.

Dirt holding capacity determines the useful life of a filter
cartridge. This is determined by adding a specified amount of contaminant upstream at regular intervals and recording the differential pressure across the filter. The dirt holding capacity is then established by a limiting value of pressure drop (differential pressure) across the filter. The higher the amount of dirt held by the filter upon reaching the cut-off differential pressure, the
longer the useful life of the filter.

Depth filter cartridges of fibrous media have a variable pore size, and thus no absolute rating. Instead they are given a nominal filter rating or a nominal efficiency figure expressed in terms of percentage retention by weight of a specifiedcontaminant of given size. This applies over a range of particle sizes considered, down to the absolute cut-off size. At this point the number of emergent particles will be nil and the efficiency 100%.

As a natural phenomena, the efficiency of a depth cartridge
increases with the passage of time due to partial clogging of voids. When a new filter is put to use, initial efficiency is an important consideration to determine filtration suitability as the most amount of contaminants would pass through at this stage. A final efficiency curve shows the amount of finer particles being filtered out before reaching the cut-off pressure.

http://www.parker.com/literature/Pr...ature/Single sheets/SWC Stringwound C1060.pdf

 

[rawelk]

Thanks, bimr. That answers my question, and the idea of adding a higher micron sized roughing filter upstream gives me something to chew over.

I had performed a small experiment to establish dirt holding capacity, but in reverse of what you describe - weighing a set of clean filters, tracking how long they were installed, (and start/ending pressure differentials), then re-weighing them after they had dried.

Abandoned the effort after three sampling periods because it took a lot of time to do this, but preliminary results suggested filter replacement before they had reached their maximum differential pressure (and hence increasing replacement frequency) will remove more particulates.