How to Prepare for Seasonal Chiller Shutdown and Lay-Up

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Author: Andrew Young - Business Development Manager

Seasonal Chiller Shutdown and Lay-Up

The leaves on the trees are beginning to change colour 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 [1].  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 [1].

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.
Chiller Tubes - Fouled Tube Sheet
Fouled Tube Sheet
Chiller Tubes - Clean Tube Sheet
Clean Tube Sheet
Chiller Tubes - Enhanced Tube Sheet
Enhanced Tubes

Dry Lay-Up

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.

Wet Lay-Up

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:
Corrosion

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 Klenzoid’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.
Biological Activity

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 Klenzoid’s Klencide GA15 or Isoklenz KT, are added per the vendor’s instructions to minimize biological activity.
Circulation

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 [1].  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.
Chiller Tubes Inspection

Conclusions

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.

WANT TO LEARN MORE?

[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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Andrew Young is a Professional Engineer and has a degree in Mechanical Engineering from the University of Waterloo.  He is a Business Development Manager with over 10 years of experience designing and implementing solutions for water systems.  In addition to being a water treatment expert, Andrew is also an expert in “backyard” food production - producing his own tomato sauce, sausage, and maple syrup.

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Dealkalizer Technologies

Some important design considerations for the chloride cycle dealkalizer are:

  • Feed water must be softened
    • Calcium chloride can precipitate and foul the beads
  • Minimal impact on total dissolved solids
  • Potential small decrease in blowdown requirements
  • Relatively low capital cost, reasonably effective, simple to operate

 

Some important design considerations for the WAC dealkalizer are: 

  • Additional softening required. WAC can remove as much hardness as there is available alkalinity - any residual hardness needs to be removed before the boiler.
  • Efficiency reduction with increasing flow rate, decreasing kinetics.
  • Handling of acid
    • Sulfuric acid – heat of hydration is a concern (can’t have plastic tanks, plastic piping), higher concentrations are available (up to 93%), calcium sulfate precipitation can be a concern for water sources high in sulfate levels)
    • Hydrochloric acid – fumes, plastic can be used, calcium chloride precipitation is not a concern, lower concentrations available (up to 32%)
  • Higher capital cost, very effective, easy to operate, larger footprint

Ion Exchange Explained



A quick review of ion exchange is required to understand dealkalization and we’ll use the water softening process as an example, as most boiler operators are very familiar with this.  Water softeners use strong acid cation (SAC) resin for ion exchange.  SAC resin has an affinity for divalent ions (Calcium, Magnesium) meaning that the resin wants to grab a hold of these divalent ions as they’re passing through the bed and exchange them with the sodium ions. Once resin is saturated and there are no more available free resin beads for ion exchange, a brute force wash of the SAC bead with sodium chloride (salt) brine is required.

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How to Minimize Amine Requirements



Amines should be dosed at the minimum rate required to neutralize carbonic acid, and to maintain pH levels of 8.0 to 9.0 in condensate.

In situations where incoming alkalinity levels are elevated, the concentration of amine required to neutralize the resulting elevated CO2 levels may exceed OTLs or even PELs. A number of alternatives are available to decrease alkalinity levels from incoming water:
  • Reverse osmosis (RO) Weak-acid dealkalization (WAC)
  • Chloride-cycle dealkalization
  • Demineralization (Demin)
RO, WAC and Demin units remove alkalinity from incoming water sources, and are often implemented to reduce energy and/or water consumption in steam plants because they decrease the overall mineral concentration of dissolved solids from incoming water. However, the chloride-cycle dealkalizer is a standout choice if the goal is to simply reduce incoming alkalinity on a budget. It operates much like a softener unit, and can decrease alkalinity levels by up to 95%.
Did we pique your interest on chloride-cycle dealkalizers? Click here to learn more...


Chloride-Cycle Dealkalizer Operation

Chloride cycle dealkalizers use strong base anion (SBA) ion exchange resin to swap carbonate and bicarbonate ions for chloride ions.  The footprint is similar a sodium softener, and they also use salt as the primary regenerant.  A small amount of sodium hydroxide if also often used to increase the effective capacity per regeneration.



The reduction of alkalinity in the feedwater, reduces the formation of carbonic acid in condensate, thus reducing the required amount of amines to neutralize the carbonic acid to maintain pH levels of 8.0 to 9.0 in condensate.

Implementation of a chloride-cycle dealkalizer can reduce your amine requirement by up to 90%.

PELs & OTLs



There are 2 important concentration guidelines:
  • Permissible Exposure Limits (PELs)
  • Odor Threshold Limits (OTL)
The following table describes the limits set by Occupational Safety & Health Administration (OSHA) and American Conference of Governmental Industrial Hygienists (ACGIH):


Exceeding PELs poses a health risk to occupants. These PELs should never be exceeded for any period of time. See this link for a related article from the Centers for Disease Control and Prevention (CDC).

http://www.cdc.gov/mmwr/preview/mmwrhtml/00001848.htm

It is best practice to also follow OTLs to minimize the likelihood of complaints from occupants, especially from those with sensitivities.

A More Detailed Look at the Components of Steam



Oxygen

Liquid water always contains some concentration of oxygen (O2). The solubility of oxygen is primarily determined by the temperature of the water. Higher temperatures reduce the solubility of oxygen in water (see graph).
Because oxygen is extremely corrosive in high temperature water, steam boiler treatment programs use chemical and/or mechanical means of eliminating dissolved oxygen in water. An effectively treated steam boiler, and the steam it produces, will have near-zero dissolved oxygen concentrations.

Carbon Dioxide

Carbon dioxide (CO2) is released by the heating of carbonate (CO32-) and bicarbonate (HCO3-) in boiler water. These ions are naturally present in water from lakes, rivers and underground wells, and their concentration determines the alkalinity of the water source. The amount of carbonate alkalinity entering the boiler is proportional to the volume of carbon dioxide gas that will be in the generated steam. Carbon dioxide eventually forms carbonic acid in condensate. Higher alkalinity values result in greater carbonic acid concentrations.
Are you a chemistry nerd? Click here to see the chemistry behind the release of carbon dioxide...


The Release of Carbon Dioxide



The above reactions describe the release of carbon dioxide gas from sodium bicarbonate (1) and sodium carbonate (2).

The heat energy in boiler water is sufficient for the first reaction to proceed to 100% completion.  The completion of the second reaction is dependent on increasing pressure and temperature.

Higher carbonate and bicarbonate levels in boiler feedwater will lead to proportionally higher concentrations of CO2 in steam.

Amines

The amine compounds used in boiler water treatment are selected based on their boiling point, and their distribution ratio. The distribution ratio is a measure of how far the amine will travel before condensing. An optimal blend of amines will protect the entire condensate piping network (near and far). Amines are considered volatile organic compounds, and their concentration must be monitored to prevent exposure to levels beyond permissible limits.
Click here if you want to impress your water treatment professional with your knowledge of amines...


Lesson about Amines to Impress Your Water Treatment Professional



Amines are a functional group in organic chemistry, and are derivatives of ammonia. They are separated into three main groups, primary, secondary and tertiary amines. These groups are defined by the number of hydrogen atoms replaced by organic substituents.

The most commonly used amines for neutralizing carbonic acid in condensate are:
  • cyclohexylamine (CHA)
  • diethylaminoethanol (DEAE)
  • morpholine
These amines are selected for their availability, basicity (ability to neutralize acids), boiling points, and most importantly, distribution ratios.

Distribution ratios (DR) are a measure of the how far amines will travel with steam before condensing. A proper blend of amines will include low DRs to protect condensate piping closest to the boiler, and high DRs to protect piping in longer and more complex condensate networks. Below is a table with the properties of the amines discussed above.

Other Types of Humidification Systems

Pan Humidifiers:

Pan humidifiers are essentially small shallow basins filled with water. The basins are heated with electric elements or steam, with the intent of evaporating water.

Pan humidifiers are found in smaller HVAC systems, and are susceptible to biological and corrosion fouling.

Water Spray Humidifiers:

This design uses an array of nozzles to atomize liquid water directly into the air stream. The phase change from liquid to vapour causes a noticeable drop in air temperature.

This type of system is most susceptible to biological and corrosion fouling. Facilities with year-long continuous cooling loads requiring high RH are best suited for this technology.

Steam to Steam or Clean Steam Generators:

These systems are small steam boilers, specifically designed to produce steam from high purity water sources, such as demineralization, or reverse osmosis. The energy input comes from steam raised elsewhere in the facility by a traditional steam boiler.

This design is typically more costly, and adds complexity, but produces steam with no boiler water treatment compounds.
Interested in this option? Click here to learn more about clean steam generators specifically.


Clean steam generators can only produce steam at low pressures.  The packaged heat exchangers rely on the higher energy content of higher pressure steam.

Water purity is critical for clean steam generators.
  • Low hardness levels (>3ppm of calcium, magnesium, or iron) will lead to fouling of heat exchange surfaces.
  • Water with even moderate alkalinity levels will release CO2 gas which will corrode any condensate piping components.
  • Moderate levels of total dissolved solids (TDS) will lead to priming or carry over, which may damage the steam control valves and/or contaminate the steam.
Therefore, Reverse Osmosis (RO) systems are ideal for humidifier makeup.  These units are designed to remove nearly all of the minerals from incoming water sources, and produce water with TDS concentrations of 0-5 ppm.

Steam to steam generators do cycle up.  Despite high purity makeup, there are always some dissolved solids.  If the generators do not purge some volume of water regularly, the bulk water will concentrate beyond acceptable levels, causing water discolouration and may lead to fouling and/or corrosion to system components depending on materials of construction.

Effects of Humidification on Occupant Comfort and Building Materials

RH levels have a direct impact on the health of patrons in a facility.

When humidity is too low occupants will get dry skin, irritated sinus, throats and eyes.

When humidity is too high mold/mildew problems can occur in the building, thus increasing the risk of illness to occupants. These health impacts are of increased concern with health care facilities who treat immunocompromised patients.

RH levels also have an impact on building materials.

The amount of moisture the material can hold will determine the extent to which it shrinks and swells with fluctuations in humidity. The effect is especially pronounced in wood and drywall, where gaps and cracks will form over time.

Windows are also prone to condensation in cold climates because they generally have little insulation value. The likelihood of condensation on windows increases as the indoor relative humidity rises, and the outdoor temperature decreases.

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