The leaves on the trees are beginning to change color and temperatures are fluctuating day to day. We have officially entered the cooling shoulder season. Temperatures this time of year result in a period of intermittent system operation followed by winter shutdown. While control of water chemistry is vital to minimizing HVAC system corrosion and preventing deposit formation during the cooling season, additional factors begin to play a critical role during the shoulder season and winter months.
Shoulder Season Cycling
Operational recommendations published by The Association of Water Technologies (AWT) stress the importance of maintaining circulation during these low-load periods where the system can be off for several hours of the day. Stagnant water can be damaging to cooling equipment and the associated piping network, regardless of treatment levels . Often, poor results when opening a chiller are from the period of stagnation (sometimes months) before the end bells are removed and system inspected rather than as a result of poor treatment during operation. To minimize the impact of the stagnant water, AWT recommends that the recirculation pumps are operated for 1 hour every 6 hours .
End of Season Disinfection and Cleaning
Once it has been determined that the cooling system is no longer required for the winter, a disinfection procedure using an oxidizing biocide and bio-dispersant should be performed on the condenser water. The goal is to reduce biological activity in the water and to help loosen any dirt and debris that may have settled on the tubes. A disinfection kit such as the AquaAnalytics DK-12000 is specifically designed for this purpose. Immediately following the disinfection, the system should be drained and the chiller opened for inspection. It is at this time that the end bells are removed and the tubes are brushed to remove any particulate that may have settled on the tubes. Your water treatment professional should be notified to inspect the tubes for any signs of scaling or pitting. Additional testing, such as Eddy Current testing can be performed at this time as well.
Dry or Wet Lay-Up?
ASHRAE Guideline 12 recommends draining chillers during extended shutdowns. Maintaining a clean and dry environment helps prevent localized corrosion, fouling, and bacterial growth. Many equipment providers also advocate dry layup [2,3], however in more and more applications, dry lay up is not possible either due to piping design, or the requirement for a redundant system to be able to start on short notice. In this case, a wet lay up will be required to minimize corrosion, fouling, and biological activity. The decision between wet or dry lay up is determined by the needs of the individual facility, and can be guided by your water treatment professional.
After the disinfection and mechanical cleaning described above, the system is ready for dry lay-up. It is important that both end bells are removed so that air can circulate freely through the tubes. Failure to remove both ends could allow water to pool and create stagnant water conditions at the sealed end. The system should be left open until the following cooling season. No additional action is required.
Once the disinfection and mechanical cleaning is complete, the end bells are re-installed on the system and the chiller is filled with fresh, clean water. Consult with the chiller manufacturer to obtain the flooded volume of the system. This information is needed for your water treatment professional to specify the proper quantities of treatment required. During a wet lay-up, we need to be aware of the following:
Since the system will be flooded with water, it is necessary to add a corrosion inhibitor to minimize the corrosion rates of steel and copper. A molybdate based closed loop treatment, such as Eldon Water’s Molyklenz, contains inhibitors for both ferrous metals and non-ferrous materials (copper/brass.) A target of at least 100 ppm sodium molybdate should be achieved in bulk water.
When the chiller is layed up wet, it provides the ideal habitat for bacteria. Since the regular biocide program will be offline for the season, it is necessary to add biocide directly to the chiller. Typically non-oxidizing biocides, such as Eldon Water's Klencide GA15 or Isoklenz KT, are added per the vendor’s instructions to minimize biological activity.
Once the system is treated with corrosion inhibitor and biocide, provisions should be made to circulate the water within the chiller. This can be done with a small recirculation pump connected between the drain lines on each end of the chiller. The pump can either run continuously or at least 1 hour every 6 hours to prevent deposition and corrosion conditions associated with stagnant water . A pot feeder and filter can be added to the recirculation loop to allow for the addition of lay up treatment and to remove any particulate from the system.
Following the above best practices for your chiller shut down will ensure that the system is protected throughout the offseason and that corrosion, deposits, and bacteria are minimized. Proper offseason conditions will reduce system upsets and maximize the service life of your equipment.
 P. Sisk et al., “Guidelines for Treatment of Systems Containing Enhanced and Super-Enhanced Tubes”, The Association of Water Technologies, www.awt.org/resources/technical_papers.cfm
 “Operation and Maintenance Instructions: For Evapco Induced Draft and Forced Draft Cooling Towers”, Bulletin 113E, Evapco, 2014.
 “Operations and Maintenance: Centrifugal Liquid Chillers”, Form 160.75-O1 (211), York by Johnson Controls.