Schools-EBOM-v4 WEc3: Cooling tower water use

  • All about condenser water cycles

    Cooling tower | Photo – YR&GThis credit focuses on achieving an appropriate number of condenser water cycles in a cooling tower based on the concentrations of various water quality criteria, such as dissolved solids.

    The credit is altered substantially from its v2009 cousin. All previous requirements related to implementing a management plan, providing staff training, and having conductivity meters and controls have been removed. That doesn’t mean you won’t need those elements to achieve this credit, but they have become best practices rather than credit requirements.

    What do cycles have to do with saving water?

    The key parameter used to evaluate cooling tower operation is cycles (sometimes referred to as cycles of concentrationConcentration ratio is the ratio of the level of dissolved solids in the recirculating water to the level found in the entering makeup water. A higher concentration ratio results from a lower bleed-off rate; increasing the ratio above a certain point, however, leads to scaling, and water savings diminish after a certain level. This ratio is also called the cycles of concentration. Cycles refers to the number of times dissolved minerals in the water are concentrated compared with makeup water, not to water flow over the tower or to on-off cycles. or concentration ratioConcentration ratio is the ratio of the level of dissolved solids in the recirculating water to the level found in the entering makeup water. A higher concentration ratio results from a lower bleed-off rate; increasing the ratio above a certain point, however, leads to scaling, and water savings diminish after a certain level. This ratio is also called the cycles of concentration. Cycles refers to the number of times dissolved minerals in the water are concentrated compared with makeup water, not to water flow over the tower or to on-off cycles.).

    For a well-managed condenser water system, a cycle represents the extent to which water is used efficiently before being discharged via blowdownAlso referred to as bleed-off; the removal of makeup water from a cooling tower or evaporative condenser recirculation system to reduce concentrations of dissolved solids that can cause mineral buildup. (where water is drained from cooling equipmentThe equipment used for cooling room air in a building for human comfort. in order to remove mineral build-up). A cycle is calculated as the ratio of the concentration of dissolved solids (or conductivity) in the blowdown water compared to the make-up water.

    From a water efficiency standpoint, you want to maximize cycles as this will reduce the amount of waste blowdown water and reduce make-up water consumption. The trick, however, is that dissolved solids increase as cycles of concentration increase, which can cause scale and corrosion problems unless carefully controlled. So this LEED credit sets a cap for five control parameters that typically play the biggest role in scale and corrosion.

    Calculating the maximum cycles

    Your team’s job is to measure the level of each control parameter in the cooling tower make-up water and then achieve the maximum number of cycles without exceeding the maximum concentration level for any of the control parameters.

    The example in the Documentation Toolkit from the LEED-EBOMEBOM is an acronym for Existing Buildings: Operations & Maintenance, one of the LEED 2009 rating systems. v4 Reference Guide shows how to calculate the maximum cycles for your system.

    The maximum cycles of concentration will vary depending on your system and the concentration of solids in the makeup water serving the cooling tower. That said, it is common for cooling towers to operate in the range of 5–7 cycles.

    Leveraging your building’s current cooling tower or water treatment vendor is likely the best way to assess the control parameters and implement a plan to meet the maximum cycles of concentration. These vendors can also help determine if additional chemical treatments can be employed in order to reach the 10 cycles required to qualify for an additional LEED point.

    What does each of the control parameters mean?

    Here’s how each of the parameters can affect the performance of your cooling tower. 

    Ca (as CaCO3): Calcium (as Calcium carbonate)

    Cooling towerCooling towers chill buildings by evaporating water. A 100-ton chiller evaporates about three gallons of water per minute (11 lpm), with additional water loss from drift and blowdown. In a typical office building, HVAC equipment accounts for about a third of total water use. Photo – Advance Cooling Towers Calcium carbonate is a very common form of scale found in cooling towers. Scale reduces a cooling tower’s heat exchange efficiency by insulating equipment. Because of this, a major goal of most cooling tower chemical treatment programs is to prevent scale build-up.

    Total alkalinity

    Alkalinity is an indicator of acid neutralizing or acid buffering minerals in the water. High concentrations can lead to scale build up.

    SiO2: Silica

    Silica is one of the impurities that frequently play a big role in limiting the maximum cycles of concentration for a cooling tower. As concentrations increase past maximum levels, silica is likely to form scale deposits and insoluble sludge in the cooling tower.

    Cl: Chloride

    Chloride can be corrosive to most metals, which decreases the performance or longevity of the cooling tower.

    Conductivity

    Conductivity is a measurement of the water’s ability to conduct electricity. It’s also an indicator of the total dissolved mineral content of the water, since higher conductivity levels correlate to more dissolved salts in the water. By this logic, water with very little minerals present (think purified water) has very low conductivity.

    High conductivity levels indicate increased risk of scale build up and lower performance for the cooling tower.

    What happened to biological control?

    The cooling tower credit no longer includes a component related to biological control, but that doesn’t mean you can ignore this piece. Biological growth in cooling towers is corrosive to metals and can damage other tower components, such as film fill (the forms that direct water into thin flowing sheets so that as much water surface area is in contact with the air as possible).

    Additionally, biological growth can lead to dangerous levels of bacteria such as legionella pneumophilaLegionella pneumophila is a waterborne bacterium that causes Legionnaire's disease. It grows in slow-moving or still warm water and can be found in plumbing, showerheads, and water storage tanks. Outbreaks of Legionella pneumonia have been attributed to evaporative condensers and cooling towers., which causes Legionnaires Disease. It is common practice to apply biocides to the circulating cooling water to control the growth of microorganisms and algae.

    Preventing Legionnaires Disease

    Legionnaires Disease is caused by bacterium found in potable and nonpotable waterNonpotable water: does not meet EPA's drinking water quality standards and is not approved for human consumption by the state or local authorities having jurisdiction. Water that is unsafe or unpalatable to drink because it contains pollutants, contaminants, minerals, or infective agents. systems, such as cooling towers. Outbreaks in 2015 occurred in New York and California, serving as a reminder that proper management of biological growth should be an ongoing priority.

    When Legionnaires develops in a cooling tower, it is transmitted to people as small droplets of water that contain the bacteria are released into the air. The bacterium is spread by water vapor only; it cannot be transmitted by infected individuals. When contracted, Legionnaires is a very a serious illness that can be lethal if left untreated.

    In order to prevent contamination of your cooling tower, the Occupational Safety and Health Administration (OSHA) recommends the actions below. Note that these activities represent best practice strategies for maintaining your cooling towers; the LEED v4 credit doesn’t require that teams implement these actions.

    • Add chemical biocides to control Legionnaires growth. Obtain information on appropriate biocide selection and use from equipment manufacturers or from companies experienced with the particular system used.
    • Treat circulating water for control of microorganisms, scale, and corrosion. This should include systematic use of biocides and rust inhibitors, preferably supplied by continuous feed.
    • Clean and disinfect cooling towers quarterly or at least twice a year if the unit is not used year round. Do this before initial start-up at the beginning of the cooling season and after shutdown in the fall.
    • Any system that has been out of service for an extended period should be cleaned and disinfected.
    • New systems require cleaning and disinfecting because construction material residue can contribute to Legionnaires bacteria growth.
    • Design features that minimize the spray generated by these systems are desirable.

    In addition, we recommend locating your cooling towers away from air intakes and operable windows to minimize unnecessary exposure risks.

    How cooling tower materials affect cycles of concentration

    Though less relevant for existing buildings, owners and operators should know that the material used to construct a cooling tower can determine the maximum concentration levels a given manufacturer allows without voiding the warranty.

    Stainless steel typically costs more than galvanized steel but allows for higher concentration levels of certain parameters. Reviewing the options from this perspective is prudent when considering a new or replacement tower. Manufacturers can provide recommended concentration levels for galvanized and stainless steel. Keep in mind that recommended concentrations would be specific to the particular manufacturer and piece of equipment that is installed in your building.

    FAQs for WEc3

    What if my building doesn’t use a cooling tower?

    A pilot credit is available that outlines an alternative compliance path for buildings without cooling towers. To see if you qualify, determine whether your building falls under System 7 or System 8 as outlined in ASHRAE 90.1–2010 Appendix G Table G3.1.1. If it does—and if you don’t have a cooling tower—you can use the pilot credit to achieve this credit.

     
  • WE Credit 3: Cooling tower water use

    Intent

    To conserve water used for cooling tower makeup while controlling microbes, corrosion, and scale in the condenser water system.

    Requirements

    Establishment

    For cooling towers and evaporative condensers, conduct a potable waterWater that meets or exceeds U.S. Environmental Protection Agency drinking water quality standards (or a local equivalent outside the U.S.) and is approved for human consumption by the state or local authorities having jurisdiction; it may be supplied from wells or municipal water systems. analysis within five years of submission for certification, measuring at least the five control parameters listed in Table 1.

    Table 1. Maximum concentrations for parameters in condenser water

    Parameter Maximum level
    Ca (as CaCO3) 1000 ppmParts per million.
    Total alkalinity 1000 ppm
    SiO2 100 ppm
    Cl- 250 ppm
    Conductivity 2000 µS/cm





    ppm = parts per million

    µS/cm = micro siemens per centimeter

    Calculate the number of cooling tower cycles by dividing the maximum allowed concentration level of each parameter by the actual concentration level of each parameter found in the potable makeup water. Limit cooling tower cycles to avoid exceeding maximum values for any of these parameters.

    Table 2. Points for cooling tower cycles

    Cooling tower cycles Points (except data centers) Points (data centers)
    Maximum number of cycles achieved without exceeding any filtration levels or affecting operation of condenser water system (up to maximum of 10 cycles) 2 2
    Achieve a minimum 10 cycles by increasing the level of treatment in condenser or make-up water

    OR Achieve the number of cycles for 1 point and use a minimum 20% recycled nonpotable waterNonpotable water: does not meet EPA's drinking water quality standards and is not approved for human consumption by the state or local authorities having jurisdiction. Water that is unsafe or unpalatable to drink because it contains pollutants, contaminants, minerals, or infective agents.
    3 4



    Performance

    None

    Pilot Alternative Compliance Path available

    A pilot alternative compliance path is available for this credit to certain project types without cooling towers. For more information, please visit the Pilot Credit Library

    Pilot ACPs:
    No Cooling Tower - alternative compliance path (BD+C)
    No Cooling Tower - alternative compliance path (O+M)
    No Cooling Tower - alternative compliance path (O+M: Data Centers)




Publications

ASHRAE Journal, Maintaining Cooling Towers

This article discusses routine preventive maintenance practices and operational measures to optimize cooling tower performance.

Web Tools

U.S. Department of Energy, Federal Energy Management Program, Cooling Tower Operations and Maintenance

This website provides a cooling tower maintenance checklist to ensure optimal performance of cooling towers. 


U.S. DOE, FEMP, Cooling Tower BMPs

Provides an overview of cooling tower best management practices to improve water efficiency.

Potable Water Analysis Report

Here's an example potable waterWater that meets or exceeds U.S. Environmental Protection Agency drinking water quality standards (or a local equivalent outside the U.S.) and is approved for human consumption by the state or local authorities having jurisdiction; it may be supplied from wells or municipal water systems. analysis report, which indicates the level of each control parameter present the cooling tower makeup water for the example building. You'll need to get an analysis like this in order to complete the table on the credit form. The highlighted values correspond to the parameters tracked by this credit.

Analysis of Makeup Water

This sample calcuation from the LEED Reference Guide demonstrates how to calculate the maximum cycles of concentrationConcentration ratio is the ratio of the level of dissolved solids in the recirculating water to the level found in the entering makeup water. A higher concentration ratio results from a lower bleed-off rate; increasing the ratio above a certain point, however, leads to scaling, and water savings diminish after a certain level. This ratio is also called the cycles of concentration. Cycles refers to the number of times dissolved minerals in the water are concentrated compared with makeup water, not to water flow over the tower or to on-off cycles. for makeup water.

0 Comments

Start a new LEED comment thread

Feb 25 2017
Type the characters you see in this picture. (verify using audio)
Type the characters you see in the picture above; if you can't read them, submit the form and a new image will be generated. Not case sensitive.

Copyright 2017 – BuildingGreen, Inc.