Successful Cooling Tower Start-Ups in 5 Simple Steps

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Author: Mark Bertulli - Business Development Manager

Many HVAC evaporative cooling systems are idle or off throughout the winter months, and are often drained to prevent freezing. These extended shutdowns provide excellent conditions for deposits to form and bacteria to grow. When starting up the system for cooling operation, some basic steps should be completed to ensure peak mechanical performance for the duration of the cooling season, and to verify that best practices for Legionella prevention are in place. In addition to the recommendations below, each cooling tower manufacturer may also have seasonal preventative maintenance requirements for mechanical components such as fans and controls.  These will be specific to each cooling tower and should be followed in conjunction with your regular start-up procedure.

Here are 5 steps to get the cooling season started properly:

1. Physical Cleaning

Visually inspect all wet areas of the cooling towers.  This includes the water basin, water sumps, fill materials, spray nozzles, wet decks, etc.  Each of these areas should be physically cleaned where accessible.  Physical cleaning may be done by site maintenance staff or contracted out to a specialized company.  Whether the physical cleaning is subcontracted or completed in-house, it is important to keep written records of all work performed prior to start-up.  At this time, also inspect any permanent filter housings to clean or replace media if required.

2. Fill the System

Fill the cooling water system and initiate pumps for circulation.  Ensure the water circulates through all piping and heat exchangers in the system.  Place any filtration units online.  Re-inspect the spray nozzles and wet decks to remove any debris that may have deposited after initiating circulation.  Repeat as necessary over the initial operation period; frequency will vary system to system.

3. Water Quality

Consult your water treatment specialist to ensure the cooling water treatment program is properly commissioned for the season.  Water treatment control equipment such as automated pumps, meters, sensors and valves should be inspected, calibrated and functional.  Perform a sanitization of the cooling tower(s) utilizing an oxidizing biocide in conjunction with a bio-dispersant.  This may be followed in accordance with a written procedure from your building water management plan, or by use of a sanitization kit such as the AquaAnalytics DK-12000.  Upon completion of the sanitization procedure, system fans can be turned on once water quality has been confirmed within normal operating levels.  Completion of this procedure should again be documented in your log books or water management plan.

4. Begin Operation

The system can now be placed into normal operation mode.  Ensure that systems utilizing multiple cooling towers, chillers, or heat exchangers are rotated frequently.  The frequency of rotation will depend on your system design and should be documented in the building water management plan.  This will ensure the active biocide program contacts all wetted areas regularly to minimize biological growth.  Depending on local weather patterns, your system may only operate intermittently after initial start-up.  Steps should be taken to prevent equipment from sitting idle for long periods of time as stagnation can lead to deposits, fouling, and bacterial growth.  Control programming or manual operation of water recirculation pumps may be required to allow the water treatment program to maintain appropriate control parameters.

5. Validation

As part of a best practice approach, both the ASHRAE Standard 188 philosophy and steps outlined in ASHRAE Guideline 12 recommend that a validation should be performed to confirm the efficacy of the actions taken as part of Legionella prevention.  A sample of water should be taken from the water basin or flow to the spray nozzles for Legionella culture analysis.  Results of the culture testing should be documented in your log books or building water management plan.  Throughout the season, detailed records of water quality should be kept.  To confirm the system is operating properly, additional Legionella culture samples may be completed depending on the requirements of your building Water Safety Plan, or in accordance with local laws.

Conclusions

Following best practices during your cooling water system start-up will set the tone for a successful season of operation.  These five simple steps will give you the best chances to operate safely and efficiently throughout the season.  Klenzoid representatives are available to discuss your site specific needs.

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Mark Bertulli has a Degree in Specialized Honours Biochemistry from The University of Guelph. As a Business Development Manager, he has over 15 years of experience designing and implementing solutions for water systems. At the young age of 38, Mark learned to skate and play hockey, so he could join a beer league.

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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.

Legionnaires’ Disease Guide for Employers and Building Owners Download

Aqua Analytics DK-12000 Download

Checklist for Minimizing Legionella Risk Download

Purchaser Checklist for Setting Base Case Scenario Download

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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