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Heat Exchangers

Excessive corrosion activity can devastate a condenser water system or open process piping in as little as a few years.  Generally co-dependent upon pipe size, a corrosion rate of 25 mils per year (MPY) can deteriorate ASTM A 53 standard 12 in. piping to its minimum safe limit in only 10 years, and its 1-1/2 in. threaded distributions lines in as little as 2 years or less.

High corrosion rates exceeding 8 MPY are typically due to under deposit or cell corrosion, microbiologically influenced corrosion (MIC), and various advanced forms other than general corrosion.  Often, such advanced corrosion activity can develop even though no obvious physical signs suggest a problem, and where years of corrosion coupon monitoring show low corrosion rates.

• Difficult To Control Once Established

Unfortunately, all aggressive forms of corrosion are difficult, if not impossible, to control and correct using standard procedures once the problem has been firmly established.  Most advanced corrosion mechanisms produce wild variations of pitting or depth penetration into the pipe, extensive iron oxide deposits, and large tubercles under which the most severe pipe loss is located.

Depending upon the precise corrosion mechanism, deposits can line the entire pipe interior, exist at only the bottom area, or develop in random locations.  Our Photo Galleries from prior case histories offers endless examples of both interior deposits and their ultimate final result, which is often failure.

It is precisely the insulating effect of such interior deposits that reduce the corrosion control benefits from any chemical water treatment program.  And since any chemical inhibitor will likely never even reach the base steel to provide protection, it becomes virtually impossible to control a high corrosion condition without first removing those deposits entirely.

While all chemical treatment providers argue to the effectiveness of their inhibitors to reach below interior deposits to control corrosion to low levels, 30+ years in this field and the evidence we have accumulated proves otherwise.  Once interior deposits are established, chemical treatment effectiveness is often reduced to zero.

• Not Acting Guarantees Problems

Many property owners and managers therefore find themselves in the difficult position of not being able to effectively control an advanced corrosion problem without first removing the existing rust deposits – yet at the same time being unable to remove those deposits without the likely failure of known weakened areas.  An ultrasonic investigation will often help in defining such vulnerabilities prior to planned corrective actions.

For many facilities, threaded pipe is the weakest area of the entire system and too week to permit any form of chemical cleaning.  Whereas a chemical cleaning may prompt some failures, not acting guarantees even more failures as the deposits increase in volume and produce even higher corrosion rates.  It’s a very difficult no-win situation.

• Corrosion Rates Vary

There are a number of reasons for the typically large difference in corrosion rates which exists between closed and open piping systems.  Aside from the greater vulnerability and threat level to an open cooling tower system due to greater particulates, aeration, and microbiological influence, open piping systems are chemically treated to a far lower concentration of corrosion inhibitor.  It is the high cost of inhibitors which are quickly blown down from open water systems that limits higher treatment concentrations.

Compared to closed chill or secondary systems, open piping systems are under significantly greater threats, such as:

Increased biological contamination – Various forms of biological growths, from simple bacteria to algae and higher multi-cell forms, exist to produce elevated corrosion levels – either directly or indirectly.  MIC is the worst form of microbiological attack.

Increased dirt loading due to the air scrubbing function of the cooling tower – All cooling towers are essentially high capacity, high efficiency air filters depositing some proportion of the captured dirt and particulates at the interior pipe wall.

An open piping system is an oxygen rich and saturated environment conducive to biological growth – Oxygen is a key component of most corrosion mechanisms.

Cooling tower systems offer optimum temperature (85-95º F) for biological growth to occur – Open systems supply a source of sun energy conductive to algae and most forms of microbiological growths.

Chemical corrosion inhibitor concentrations for open systems are generally lower than closed systems by a factor of 10 to 20 –Typically, lower recommendations of water treatment chemicals for an open piping system are not due to a lack of need by the more stressed and threatened metal components, but rather due to the economic reality of providing a reasonably effective corrosion treatment program customers are willing to pay.

• Closing The Open Piping System

Given the high heat transfer efficiency of today’s plate and frame heat exchangers, often at near 1-2° F., a very reasonable solution to a high and threatening corrosion rate problem exists by turning the major portion of the existing piping into a closed system.  For the most severely damaged piping systems, this may often be the last and only alternative to replacing the entire piping system.

Essentially, the condenser water piping system is separated in two, preferably at the point nearest the cooling tower itself, and a plate and frame heat exchanger installed similar in shape to this example.  A wide range of units exist to accommodate almost any need.

Providing twin units in parallel is advised where a severe corrosion problem exists, and the need to frequently clean and service the heat exchanger may be anticipated – thereby requiring a standby unit.

Pumps move the condenser water from the tower to the heat exchanger on the open or primary side.  On the isolated or secondary side, a second set of pumps move water to and from its destination – to either package units or main refrigeration chillers.  A reasonable temperature loss of only a few degrees generally exists.

• Piping Modifications Required

Such an addition to the condenser water system requires a significant amount of work and preparation.  Available space for the heat exchanger is required – as is also the addition of one or more additional condenser water pumps, controllers, an added power requirement, and some piping layout modifications.  Typical costs can easily exceed $250,000 for smaller installations.

This is a generally reasonable expense when compared to the alternative of pipe replacement.  The installation of a heat exchanger immediately offers many significant benefits – the most important being the isolation of the system.

• Clean The Newly Closed System

With outside influences effectively eliminated, it becomes possible to capture and remove any suspended particulates using an inexpensive side stream bag filter.  Automatic backwashing and blow down units should be excluded in favor or basket style manual units.  Adding dispersing agents or other neutral chemicals to gently remove and suspend existing deposits offers the option of gradually cleaning the system without the inherent threat which exists in the use of strong acid cleaners.  Microbiological growths, often the underlying cause of an advanced corrosion problem, can be significantly suppressed, if not virtually eliminated.

Most importantly, chemical corrosion inhibitors, which would normally be added in relatively weak concentrations, and which would be continuously blown down in an open cooling tower loop, will now remain within the closed side of the system.  Significantly higher dosages of chemical inhibitors not economically feasible to maintain for an open system, will offer greater penetration through the existing deposits to the metal sub-surface.  For example, instead of a typical maintenance level of 8-10 PPM of molybdate common for an open cooling system, a far more effective 2,500 PPM of nitrite, or 500 PPM of molybdate or phosphonate can be maintained.

High concentrations of dual corrosion inhibitors can also be employed to protect both the anodic and cathodic areas of the metal – thereby further improving corrosion control.  Once the continuous volume of blowdown water is eliminated by closing the system, any combination of chemicals suddenly becomes not only more effective, but far less costly to use.

Far more advanced corrosion control products outside the inventory of most water treatment companies are available to virtually stop most corrosion mechanisms – even those existing under heavy deposits.  While economically prohibitive and technically questionable to succeed under open system conditions, such options become available to the building owner or plant operator once a piping system is closed.

• Minor Threat Remains

Of course, the existing piping from the cooling tower to the plate and frame heat exchanger, if not replaced or tested and found suitable for further service, will continue to present a threat if the fundamental corrosion problem is not corrected – but to a far lesser degree.  The small separation between plates are designed for a normal volume of particulates, but will clog and require frequent cleaning if presented with excessive dirt loading.  An effective chemical water treatment program is still required.

Given that the main supply and return lines between cooling tower and heat exchanger are typically of larger size and heavier wall thickness, an existing corrosion problem will have less effect than at the small distribution lines within most facilities.  More aggressive cleaning agents also present far less threat to the larger main lines in comparison to the smaller threaded pipe which may exist in any condenser water or open piping system.

• Recommended For Threaded Pipe Installations

Many critical condenser water applications are, in fact, designed from the beginning having a closed secondary system serving the actual A/C units.  Such a system offers an inherently lower corrosion rate by as much as a factor of ten times, fewer operating problems overall, and a limit to the water damage which might occur during a major piping failure.

We highly recommend plate and frame heat exchangers for any package unit service loop, and consider it virtually mandatory where multiple small diameter threaded risers are installed serving individual HVAC or heat pump units – common to residential or condominium properties.  It is a low cost alternative to tearing open the building from top to bottom to replace 20 sets of column risers over 25 stories after only a decade or two of service.

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