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Water Filtration

One very common misconception about closed HVAC heating and cooling systems is that they are essentially corrosion and maintenance free.  Most building owners and operators feel that the absence of externally introduced dirt and debris, coupled with a traditionally low corrosion rate, eliminates concern over the type of fouling problems typically associated with condenser water piping and open water systems.

This is far from true, however, since closed piping systems are still subject to all the same corrosive forces – just to a lesser degree.  While corrosion rates of under 1/2 mil per year (MPY) might have been reasonably expected 30 years ago, less effective chemicals, more corrosion susceptible pipe steels, along with other factors, have driven that rate significantly higher.

Age is also an important element for many of today’s building properties.  Even a low corrosion rate, when extended over four decades or more, will produce a substantial volume of oxidized material if not regularly removed.  Chemical cleanouts, if they are performed at all, are often light dispersant cleanings which produce little particulate removal.

One obvious source of deposits is created where the closed system and open cooling tower loop are directly cross connected via a full flow strainer such as Strainercycle to provide “free” cooling.  Here, an abundance of particulates and microbiological growths will enter the otherwise closed system during some percentage of the cooling season to settle and create serious long term corrosion problems.  The high cost of corrosion problems often negate many of the “free” benefits of decades earlier.  Even though most Strainercycle type systems have been replaced with plate and frame heat exchanges, the negative effects of cross connecting such systems still remain decades later.

• Corrosion Problems Inevitable

In fact, for most closed systems, problems will eventually arise after a number of years of service in the form of accumulated corrosion deposits, lost heat transfer efficiency, and/or microbiological contamination.  Thread failures, pinholes, and larger more threatening leak problems inevitably follow.

Since there is no blowdown at a closed system, few building operators see the usual indications that such problems exist.  Typically performed every five years, cleaning the chiller evaporator tubes may offer some suggestion of system condition, but not always.  At the very least, the mechanical equipment will eventually suffer some degree of reduced heat transfer efficiency – with the corresponding increase in operating costs being picked up by the property owner, tenant, or management company.

Given even the lowest feasible closed system corrosion rate of 1/4 to 1/2 mils per year (MPY), it is important to remember that, unlike an open system, the resulting corrosion products have nowhere to escape, and therefore will deposit out in the lowest flow and lower floor areas of the system – eventually causing operating problems.  Few closed piping systems are chemically cleaned on any regular basis, and then with only questionable effectiveness.  For higher closed system corrosion rates of 3 MPY and above, substantial deterioration of the pipe may occur.  Horizontal areas, the bottom of the system, and the furthermost extremities of any system from the circulating pumps are those mostly affected.

The below photos from actual client archives show the degree to which a relatively minor corrosion problem can multiply given sufficient time and a lack of the proper precautions.

Interior Rust Deposits

• Additional Example

CorrView International, LLC offers a series of photo galleries taken from 25+ years of past ultrasonic piping investigations, which address the above and additional corrosion conditions.  A review of the different types of corrosion is often helpful in initially determining the likely corrosion cause.

In many cases, however, a combination of conditions will exist within the same piping system.  View our extended Corrosion Photo Galleries for different types of corrosion and failure conditions.

• Defining The Problem

As an example, consider a typical 20 year old closed chill water system having 12 in. schedule 40 steel pipe main risers, and a very low average corrosion rate of 1/2 MPY.

It is a fact that during every year of operation, the inside surface of this 12 in. pipe will lose approximately 6.4 pounds of metal into the circulating system for each 100 linear feet.  Such rates typically increase over time – since as the piping system wears and the internal pipe diameter increases, so does the internal surface area, pitting depth, and the weight of metal lost.  A table showing the pounds of steel lost per year at different corrosion rates is provided below

A typical 25 story office building having perhaps 800 linear feet of main chill water risers alone, would therefore have lost 1,024 pounds of pipe steel throughout that 20 year period from the main risers alone.  (Total metal loss would actually be far greater due to the many smaller distribution lines leading to and from the main risers, chillers, and A/C units.)

• Rust Deposits The Real Threat

In addition to the loss of metal, the various iron oxide products produced by the corrosion process are far less dense, and therefore occupy a volume approximately 12 times that of the original steel which was lost.  In the above example, approximately 0.15 cubic feet of iron oxide rust products are produced per 100 linear feet of pipe per year as a result of this low 1/2 MPY corrosion rate.  At typically higher corrosion rates, substantially more deposits are formed.  A table of iron oxide deposits produced for different size pipe and at different corrosion rates is shown below:

Over a period of 20 years in our example, this loss translates into a build-up of 25 cubic feet of rust and corrosion products from the main chill water risers alone.  Again, total accumulations, including that originating from the smaller distribution piping, would be significantly greater.

It should be noted that most building properties have significantly higher closed system corrosion rates than 1/2 MPY – which dramatically increases metal loss, accumulated corrosion products, and loss of heat transfer efficiency.  At the higher corrosion rates common today of between 2 and 4 MPY for closed systems, such deposit buildup would expand this problem proportionately.  Additional factors such as periodic drain downs, lower quality pipe, low flow areas, schedule 40 threaded pipe, or winterization of HVAC coils using antifreeze, simply amplify the problem.

• Secondary Effects

For chill water systems, a secondary problem is produced as the deposits slowly reduce the heat transfer efficiency and engineering staff or building automation computers respond by lowering the chill water supply temperature.

A chill water supply temperature of 38º F., which would be significantly below normal design criteria of 45º F., would therefore immediately suggest an interior deposit problem.  A quick fix to insufficient cooling capacity by lowering supply temperatures often creates much greater condensation of moisture at the cold pipe surface – thereby saturating the insulation with water to produce an exterior corrosion condition.  For most chill water systems, such exterior wall loss will exceed that occurring internally.

• Filtration The Best Option

Gradually removing the iron oxide deposits is the key to preventing an operating problem, or to clean up years of accumulated rust and debris.  By continuously by-passing 5% or 10% of the total system circulation through a side stream filtering system, it is possible to gradually remove any iron oxide deposits over time – thereby greatly improving system heat transfer efficiency and lowering operating costs.  Side stream filtration, therefore, is one maintenance expenditure which will pay for itself in many ways.

This low cost and simple solution to eliminating closed system deposits requires the installation of a basket style filter to the system in question, and can typically be installed in-house using 2 in. to 4 in. threaded black pipe or type L copper.  A pressure differential gauge across the filter housing inlet/outlet, or an in-line flow meter, will provide quick visual indication of when filter elements should be cleaned or replaced – a simple maintenance operation.

A coarse filter bag element should always be used during the start-up months in order to first remove the larger suspended particulates.  Magnetic inserts improve filtering efficiency by capturing small iron filings and debris.  As the system is gradually cleaned, progressively finer filter bags are substituted down to a final 1-5 micron size.  After the heaviest suspended particulates are removed, we recommend the addition of a non-acid chemical dispersant in order to penetrate existing iron oxide deposits on the pipe wall and re-suspend them in solution for eventual removal by the filter.

With hundreds if not thousands of pounds of iron oxide and particulates attached to the interior walls of most larger and older piping systems, the absence of dirty filter bags should not be viewed as a signal of a clean system.  Rather, it suggests that only the suspended particulates have been removed, and that a more effective chemical agent is needed to loosen, break down, and resuspend the more hardened and well adhered deposits.  It may also suggest a problem with either the installation of the filter, the chemical treatment and cleaning program, or both.

Cleaning any closed system by this method is gradual and safe.  There are typically no acids or aggressive chemicals used which may potentially cause a shutdown due to a sudden increase in dislocated solids, blocked strainers, or leaks.  Due to the low maintenance cost once the system has been cleaned, most building or plant operators choose to leave the filtration system on-line as protection against future deposit problems.

We strongly recommend against the use of sand filters or any other automatic backwashing filter for this application simply due to the volume of fresh water introduced into a normally closed system.  Centrifugal separators are also not recommended since they simply do not provide the low particle size retention needed.

• Installation Recommendations

The below flow schematic illustrates the most common installation for side stream filtration, utilizing the pressure differential across the pumps.  Actual effectiveness depends upon filter size and capacity, the effectiveness of the chemical dispersant, piping size and layout, filter element material and micron retention.  Most important, however, is the location of the filter inlet – with most filtrations units installed incorrectly to waste much of their potential benefit.

The most common installation of the filter across the pump suction and discharge headers is also the least effective configuration.  Filter take-off points perpendicular to the quick flow rate of a water stream make the capture of any but the smallest particulates almost impossible.  Rust and other particulates are simply unlikely to stop their movement during full flow, turn 90 degrees, and then navigate into the smaller filter inlet.  In fact, the most effective installation places the filter at the very bottom of the down feed riser, and in a straight line of travel to the filter inlet.  In this way, any particulates are directed through inertia into capture by the filter.  A supplemental pump is often necessary in such cases.

Having the filter inlet take-off in straight line with the direction of flow, while forcing the water to turn an elbow, will boost the efficiency of any supplemental filtering system tremendously.

• Filter Types And Configurations

A wide variety of filter housings exist offering different shapes and sizes, inlet/outlet configurations, flow rates, materials, particle retention, and pressure ratings, etc.  Priority importance should be placed upon sizing the filter proportionately to accommodate the greatest flow, and selecting the best location in order to utilize the filter to its maximum effectiveness.  Shown below are a few common examples.

Filter Configurations

Multiple Bag Unit – A typical bag filtration unit having multiple internal filtering elements.  For smaller piping systems, such a large unit may be suitable for providing full flow filtering.  For larger systems, it would provide capacity to filter 10% of total flow.	Various Types And Sizes – Bag filters are available in a wide variety of sizes from small single units, to large multicell units offering thousands of gallons per hour in flow rate, and supporting inlet and outlet pipe dimensions of 12 in or larger.

Magnetic Capture – Given that most of the iron or steel based particulates within a system are magnetic, the addition of high strength internal magnets to each filter element will greatly improve removal efficiency.  Magnetic inserts are especially helpful in removing smaller diameter particulates common to most chill water systems.  A larger basket strainer, where the flow velocity is lower, improves capture efficiency.

Particulates Captured – This is the end result of a properly sized and installed bag filter unit.  For an older and larger piping system, thousands of pounds of material likely exist – meaning an extended cleaning cycle that may last years.  Many factors determine the volume of material removed – including particulate retention, pressure drop, water flow and particulate composition and volume.

Piping Take-Off – The best location for cleaning a deposit laden system is always from the bottom of any risers or dead legs.  In most cases, this will require the addition of available ports for connection to the filter.  The further from the pump the lower the pressure drop generally, and a supplemental pump may be necessary in such cases.	Mobile Filtration – Smaller units, mounted on movable platforms, offer an excellent alternative if the cleanup operation is expected to be small or temporary.  It is also a more cost effective alternative if many smaller and independent closed piping systems exist – which do not require constant filtration coverage.

Control Package – Controller packages are available to any degree of complexity desired – from signaling the need for filter changes, to switching over the unit into by-pass based upon certain operating criteria.  Totally fabricated and skid mounted units provide minimum installation expense – usually requiring no more than running inlet and out let lines, a drain line, and power.	Pump And Filter – The addition of a circulation pump offers significant advantages.  Most importantly, it allows placement of the filter at the most ideal location, rather than where suitable pressures exist.  The higher pressure from the pump also extends the filtering cycle and therefore media life.  In areas of lowest pressure differential, a transfer pump becomes mandatory.

Easy Maintenance – The standard flow is through the top of the filter housing, through the bags from inside out, and to discharge.  This makes replacement of the bag elements a question of simply unbolting the swing away lid, removing the old elements and replacing them with new.	Single Filtering Units – A wide range in filter sizes exists – from small 6 in. x 18 in. single cells, to multiple units holding 12 or more 8 in. x 32 in. elements.  Where the space necessary for a multi cell unit as above doesn’t exist, multiple single units connected to a header is a perfect alternative.

• Filtration No Longer A choice

Side stream filtration offers perhaps the easiest and most cost effective step any property manager or plant operating engineer can take to ensure trouble free service from any closed circulating system.

Installation and operation costs are almost negligible in comparison to the energy savings, lower corrosion rates, and other benefits involved.  Interior deposits should be view as an inevitability that can be easily addressed sooner, or be faced as a significantly greater problem later.

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