The change of season often is a sign that it is time to prepare for seasonal chiller shutdown. These helpful tips will give you peace of mind knowing that your system will be protected all year long.
When the temperatures start to fluctuate from day to day, this is your sign that shoulder season is upon us.
Shoulder season is a term used in the HVAC industry to refer to the spring and fall, in between the height of winter heating and summer cooling. The mild outdoor temperatures leads to a reduced need for constant utility demands and lower energy consumption.
Temperatures this time of year result in a period of intermittent system operation followed by winter shutdown. Control of water chemistry is vital to minimizing HVAC system corrosion and preventing deposit formation during the cooling season. However, additional factors begin to play a critical role during the shoulder season and winter months.
Operational recommendations published by The Association of Water Technologies (AWT) stress the importance of maintaining circulation during these low-load periods where the water treatment 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 [1]. Often, poor results when opening a chiller are from the period of stagnation before the end bells are removed.
KEY INSIGHT: To minimize the impact of stagnant water, AWT recommends that the recirculation pumps operate for 1 hour every 6 hours [1].
Determine that the cooling water treatment system is no longer required for the winter. Then, perform a disinfection procedure on the condenser water using an oxidizing biocide and bio-dispersant.
KEY INSIGHT: The goal is to reduce biological activity in the water and to loosen any dirt and debris that may have settled on the tubes.
A disinfection kit is specifically designed for this purpose.
Steps to Take After Disinfection:
Step 1: Drain the system.
Step 2: Open the chiller for inspection.
Step 3: Remove the end bells.
Step 4: Brush the tubes to remove any particulate that may have settled.
Step 5: Notify your water treatment professional to inspect the tubes for signs of scaling or putting.
Step 6: Additional testing, including the Eddy Current testing, may be necessary.
Dry or Wet Layup?
Many equipment providers also advocate dry layup [2], [3], but in increasingly more applications, dry layup is not possible. This is due to either piping design or the requirement for a redundant system to be able to start on short notice.
In this case, a wet layup will be required to minimize corrosion, fouling, and biological activity. The needs of the individual facility decides whether wet or dry layup. It can be guided by your water treatment professional.
Dry Layup
After the disinfection and mechanical cleaning, the system is ready for dry layup. It is important that both end bells are removed so that air can circulate freely through the tubes.
KEY INSIGHT: 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.
Wet Layup
Once the disinfection and mechanical cleaning is complete, the end bells are reinstalled 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 layup, it is important to be aware of the following:
Corrosion
Since the system will be flooded with water, add a corrosion inhibitor for water to minimize the corrosion rates of steel and copper. A molybdate-based closed loop treatment contains inhibitors for both ferrous metals and non-ferrous materials including copper and brass.) A target of at least 100 ppm sodium molybdate should be achieved in bulk water.
KEY INSIGHT: A molybdate is a compound containing molybdenum in its highest (+6) oxidation state within an oxyanion.
Biological Activity
When the chiller is laid up wet, it provides the ideal habitat for bacteria. Since the regular biocide program will be offline for the season, add biocide directly to the chiller. Typically, non-oxidizing biocides are added according to the vendor’s instructions to minimize biological activity.
Once the system is treated with corrosion inhibitor and biocide, make provisions 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 for at least 1 hour every 6 hours to prevent deposition and corrosion conditions associated with stagnant water [1]. A pot feeder and filter can be added to the recirculation loop to allow for the addition of layup 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.
Looking to learn more how to care for your chiller in cooler weather?
Sources
[1] 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
[2] “Operation and Maintenance Instructions: For Evapco Induced Draft and Forced Draft Cooling Towers”, Bulletin 113E, Evapco, 2014.
[3] “Operations and Maintenance: Centrifugal Liquid Chillers”, Form 160.75-O1 (211), York by Johnson Controls.
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