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Volume vs. Size

The major selling point to any water filtration unit is its particle retention size in microns.  One manufacturer will claim 0.8 micron, another 0.65 micron, and some to as low as 0.45 micron.  Filters that cannot capture 1 micron or less particles are often excluded.  Competition is intense to offer purer water than all other products, with an obvious loss of vision to the purpose of the unit and its intended application.

Any water filter installed into an open condenser water system has a huge demand upon it.  Often, water filtration is in response to a corrosion problem that may or may not be defined in terms of severity or the volume of rust deposits present – thereby leaving the choice of filter type and size to sheer speculation.  Fouled heat exchangers, clogged pump strainers, and rust covered tower sump pans all drive the interest to remove such a tangible threat through filtration, at which time further thought regarding the problem is forgotten.

Filtration experts drive the remainder of the conversation based upon their own unit offering the best alternative to all clients, and under all conditions.

• What Is 1 Micron?

Like the corrosion related term mils per year, micron size is often undefined in the minds of those using it as a measurement gauge.  A micron, or a micrometer, is a metric rather than English unit of measure defined as 1 millionth of 1 meter or 0.0000011 of a yard.  One meter is the metric equivalent to 39.37 inches.  1 millimeter contains 1,000 microns.  One inch contains 25,400 microns.

In comparison, a human hair is between 40 and 120 microns thick.  Standard weight copy paper is approximately 0.9 mm or 90 microns thick.  Printing this article on a standard inkjet printer would utilize micro droplets of ink at between 50 and 100 microns in diameter.  A red blood cell is 7 microns.

In short, 1 micron is an exceptionally small unit of measure far below the approximate 50-75 micron detection limit of the human eye.  A pool filter will capture 20-40 micron dirt particles to produce water which is crystal clear.  The drinking water we consume is rarely filtered to below 30 microns, with EPA defining limits to turbidity rather than absolute particle size.  Sub-micron water filtration below 1 micron can only be measured by microscope or advanced particle counting equipment.

• Compared To The Problem

This microscopic size definition is in dramatic contrast to the typical result of a corrosion problem that can be measured not in microns, but most often in 5 gallon buckets and 55 gallon drums.

For most moderate to high corrosion problems and even where normal corrosion rates have existed over decades, large volumes of iron oxide rust deposit are generated.  As well described on this site, those deposits then limit or stop any further benefit of the chemical water treatment program to protect the underlying steel resulting in a secondary higher pitting condition.

Rust Measured in Pounds

Just how much debris is possible to accumulate within a piping system is best illustrated after cleaning the pipe using a high pressure water jet, as it provides almost total rust removal of that rust to a container for disposal.  At a New York City hospital property under construction of a new $5 million refrigeration plant, high pressure water jetting removed a demolition size dumpster of rust from the inside of each 24 in. supply and return riser over its 35 floor height.

That’s a total of approximately 7 cubic yards of iron oxide rust, organic debris and particulates removed from approximately 950 linear ft. of 24 in. condenser water riser piping.  Or approximately 9.5 cubic inches of rust product per each linear foot of pipe which could have migrated into the new chiller equipment and piping.

In another project in Washington, DC, so much rust debris was removed by high pressure water jet from an 8 floor 12 in. condenser water piping system that it filled every available 55 gallon drum in the building.  The volume of rust was so great, in fact, that property owners and engineers decided to replace the pipe even though ultrasonic testing had defined still high wall thickness available.

In an example to the benefit of filtering more water to a lower particle retention, an older style rotating 16 cell slotted strainer at 120 micron was installed full flow to a 12 in. condenser water supply line serving a large refrigeration unit at a 50 story, 37 year old uptown New York City office property.  Large flakes of rust to near 2 in. in diameter which had been clogging the condenser tubes indicated a large source of material.

A 5 ft. diameter by 6 ft. high conical bottom settling tank was installed in order to remove solid rust deposits captured by the filter.  Once operational, sufficient rust was collected to prevent the operation of the bottom 3 in. drain valve.  The unit was periodically removed from service due to the high volume of captured particulates requiring manual shoveling to remove.

• Of Missing Importance

Filtration flow capacity at a given particle retention is rarely considered as a benchmark of performance for water filtration units, as it should.  At a retention of 0.6 micron, most moderate sized units of 2 in. inlet and outlet can only filter about 100 gal/min.  Installed at a typical condenser water piping system having thousands of gallons of capacity means that only a minute percentage of water is actually filtered – with the majority of rust particulates remaining behind.

An argument can be made that those particulates not captured on the first pass will circulate to be captured on the 2nd or 3rd pass across the filter inlet but that is simply wishful thinking.  In reality, those particulates not captured often settle or precipitate out in lower flow areas to never return.  This is especially true today with variable flow systems and generally lower water velocity.

It is a fact that most water filters are installed where most convenient, as opposed to where it would be most efficient – thereby further reducing its effectiveness beginning day one.

Filtering the greatest volume of water to a lesser degree, as we presented earlier, is therefore often preferred rather than reducing the filtrate to a cleaner standard than even our own drinking water.  For heavily fouled piping systems, full flow filtration of 12 in. pipe at 150 micron or greater is far preferable than 100 gal/min at 0.5 micron.  This can be accomplished using highly efficiency centrifugal separators, or older style multi element automatic strainers.  The Cross filter, which we consider one of the best water filters manufactured, can address large quantities of water at 100 micron levels and at low pressure drops.

Factors To Consider

Every consideration toward water filtration should first address the following issues in order that the correct choice is made:

• Full Piping Assessment

Absolutely mandatory to any filtration question is an ultrasonic investigation to define remaining wall thickness and the level of deterioration to date.  A higher level of wall loss defines greater rust deposits present.  Defining the volume of wall loss and therefore internal rust deposits will play an important role in choosing full or side stream filtration.

For any deposit removal program where chemical additives to remove existing rust is critical to its success, establishing that the pipe is sufficiently strong to survive such cleaning is a first priority.  Any prior failures should be reviewed.

Opening pipe sections to assess hardened deposits is important, as is establishing a spool piece of existing pipe upon which to judge the effectiveness of any filtration / cleaning procedure.

Submit actual pipe samples including interior deposits to a laboratory for assessment of chemical rust removal effectiveness.  Estimate the potential volume of internal deposits.

If possible, consider using a high pressure water jet to remove the majority of deposits.

• System Engineering Evaluation

A thorough engineering review is necessary.  Total system volume and flow rate, proposed percent of filtration needs to be defined.  Most important is proposing a take-off location to the filtration unit, identifying operating pressures, understanding water flow characteristics, locating low flow areas, etc.

Installing a side stream filter to a point perpendicular to flow dramatically reduces its potential benefit.  In some examples, installation errors will render a filter nearly worthless.

• Objective

Providing light particulate removal from a closed chill water system is a dramatic difference from resolving a severe corrosion problem at a condenser water loop.  The removal of potentially a few pounds of rust deposits can be manual, while removal of hundreds and possibly thousands of pounds requires automation.

A severe corrosion problem requires a more dramatic response in the form of a larger filtration unit, and possibly full flow filtration.

• Particulate Disposal

Full flow centrifugal separators and screen type filters can potentially capture high volumes of large particulate debris capable of clogging any floor drain if discharged – thereby requiring a solid waste disposal.

Removing captured rust deposits requires planning for those examples where a large corrosion problem exists.  Where a large volume of water is backwashed and collected to a settling tank, that water can be further filtered for safe removal and the deposits reduced in volume using basket filtration.

Very few automatic filtration units, however, route their backwash to a holding tank to avoid creating a drain obstruction – possibly due to an oversight, or due to an assumption that significant deposits would not be collected.

We strongly recommend a settling tank for all back washing filters in order to both prevent creating a waste line blockage problem, and to assess the cleaning effectiveness of the filter itself.

• Filter Options

Side stream basket filters are ideal for closed systems while automatic back washing filters are best for larger open systems.  Back washing filters to blow down water from a closed system is always a waste of expensive chemical while at the same time introducing new fresh oxygenated water.

Basket style filters are not appropriate to larger open systems due to high media replacement costs.  Cartridge style drinking water filters generate excessive maintenance costs.

• Filter Selection

Cost is a major factor in any filter decision.  A high end filter with installation can reach $200,000.  Rarely considered, however is the cost for pipe replacement if remedial efforts to a serious problem are unsuccessful or not pursued.

Not all filters are useful to all applications, and often a less complicated unit such as a centrifugal separator will produce a greater result.  The combination of centrifugal separator to capture the larger particles and high efficiency sand filter to polish what remains is an excellent combined approach to some problems.

• Reliability

A high number of the more elaborate automatic back washing filtration systems are found off line and out of service due to mechanical problems.  This means no benefit and a continuation of the problems they were intended to resolve.

This is rarely the case for centrifugal or basket style units of car less complexity.

Purchasing an extended service contract for such systems is always recommended.

© Copyright 2023 – William P. Duncan, CorrView International, LLC