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 one half mil per year (MPY) might have been reasonably expected 20 years ago, less effective chemicals, more corrosion susceptible steels, along with other factors, have driven that rate significantly higher. Review a summary of piping quality, operating, and design changes which have occurred.

     Age is also an important element for many of today's building properties. Even a low corrosion rate, when extended over three 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 in recent years, the negative effects of cross connecting such systems still remain.

     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 lost heat transfer efficiency - with the corresponding increase in operating costs being picked up by the property owner, tenant, or management company. See Technical Bulletin # C-4 for further discussion of interior pipe deposits.

     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 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.

     As an example, consider a 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 per each 100 linear feet. Such rates increase actually 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 1,020 pounds of iron 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.)

     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 20 to 25 times that of the original steel which was lost. In the above example, approximately 0.25 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, this loss translates into a build-up of 40 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.

     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 38 º F., which would be significantly below normal design criteria, would therefore immediately suggest an interior deposit problem. A quick fix to a different problem, lowering supply temperatures often create much greater condensation of moisture at the cold pipe surface, and saturate the insulation with water to produce an exterior corrosion condition. Often, such exterior wall loss will exceed that occurring internally. See Technical Bulletin # P-6 for more about the threat of exterior pipe corrosion under insulation.

     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 specialty basket 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 filtering elements are substituted down to a final 5-10 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 piping systems, the absence of dirty filtering elements 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. See Technical Bulletin M-10 about why clean water does not always mean clean pipe.

     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 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 offer the low particle size retention needed.

     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. See Technical Bulletin W-5 for further recommendations about removing interior deposits using bag filtration.

     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. Particulates are simply unlikely to stop their movement, 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. See Technical Bulletin # W-3 for more about filter installation suggestions.

     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.

     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 negligable 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.

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