Technical Bulletin # P-08


Plate And Frame Heat Exchangers --- The Ultimate Safeguard
For Critical Or Problem Condenser Water Applications



THE PROBLEM:

     Excessive corrosion activity can doom a condenser water piping system in as little as a few years. Generally co-dependant upon pipe size, a corrosion rate of 20 mils per year (MPY) can deteriorate Standard type 12 in. piping to its minimum safe limit in 10 years, and its 1-1/2 in. threaded distributions lines in as little as 2 years or less.

     High corrosion rates of over 8 MPY are typically due to under deposit or cell corrosion, microbiologically influenced corrosion (MIC), and various advanced forms other than general corrosion. In many cases, such advanced corrosion develops even though no obvious physical signs suggest a problem, and where years of corrosion coupon monitoring show low corrosion rates. Read Technical Bulletin # C-01 about the limitations of corrosion coupons.

     Unfortunately, all such forms of advanced corrosion are difficult, if not impossible, to control and correct using standard procedures once 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.

     It is precisely the insulating effect of such interior deposits that prevent the corrosion control benefits of any chemical water treatment program from even reaching the base steel - therefore making it virtually impossible to reduce a high corrosion rate without first removing the deposits entirely.

     Since a high corrosion rate virtually defines a piping system which has already been heavily damaged, typical deposit removal measures such as on-line cleaning using an acid, chelating, or other chemical agent cannot be safely performed. High pressure water jet, while a safer cleaning process, is strictly limited by the physical layout and operation of a piping system. Read Technical Bulletin # C-02 for more information about high pressure water jet cleaning.

     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, and yet being unable to remove those deposits without the likely failure of any weakened areas. Such a threat is greatest where threaded joints exist.


THE SOLUTION:

     There are a number of reasons for the typically large difference in corrosion rate which exists between closed and open piping systems. Compared to closed chill or secondary systems, open piping systems are under significantly greater threats, such as:

  • 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.

  • 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.

  • An oxygen rich and saturated environment conducive to biological growth. Oxygen is also a key component of most corrosion mechanisms.

  • Cooling tower systems offer optimum temperature (85-95 Fahrenheit) for biological growth to occur.

  • Chemical corrosion inhibitor concentrations for open systems are generally lower than closed systems by a factor of 10. 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 treatment program customers are willing to pay for.

     Given the high heat transfer efficiency of today's plate and frame heat exchangers, 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 is often be the 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 similar in shape to the above examples installed. Pumps move the condenser water from the tower to the heat exchanger on the open or primary side, and on the isolated or secondary side, a second set of pumps move water of a slightly higher temperature to and from its destination - package units or main chiller units. A reasonable temperature loss of only a few degrees generally exists.

     While requiring available space for the heat exchanger itself, the addition of one or more additional condenser water pumps, an added power requirement, and some piping modifications, the installation of a heat exchanger immediately offers significant benefits.

     With outside influences effectively eliminated, it becomes possible to capture and remove any suspended particulates using an inexpensive side stream bag filter. Adding dispersing agents 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. Significantly higher dosages of chemicals not economically feasible for an open system, 500 PPM of molybdate as opposed to a typical maintenance level of 8 PPM for open systems, will offer greater penetration through the existing deposits to the metal sub-surface. 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.

     Even more advanced corrosion control products outside of typical water treatment company inventories are available which will 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 once a piping system is closed. Read Technical Bulletin # P-05 about VCI advanced corrosion inhibitors.

     Of course, the existing piping from the cooling tower to the plate and frame heat exchanger, if not replaced, 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 is designed for a normal amount of particulates, but will clog and require frequent cleaning if presented with excessive dirt loading.

     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 10, fewer operating problems overall, and a limit to the water damage which might occur during a major piping failure.



Request Further Information On A Specific
Application Or Operating Problem


Turn Back One Page




Main Page        Testing Services        Technical Bulletins        Case Histories
Business Opportunities        Specialty Products        Recommended Links
Request Information        Mailing List        About ECI        Contact Us

© Copyright 2000