Summer is Heating Up - Top 5 Ways to Save on Cooling Tower Bleed

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Author: Pat Smith - Service Manager

Cooling towers work by taking advantage of heat lost to the atmosphere when water evaporates. As we’ve just reached the summer solstice, it’s officially peak season to find ways to improve your tower performance. Whether your cooling tower is used for HVAC cooling, cold storage, or an industrial application, reducing the amount of bleed can reduce your water consumption and chemical use.
When water evaporates it leaves as pure H2O gas, leaving behind the minerals and other impurities that may have been in the water. This results in an increase in the mineral concentration of the remaining system water. Bleed is required to periodically release this highly concentrated impure water, which is then replaced with fresh make up water.

As the mineral concentration increases, the water’s ability to conduct electricity increases linearly. This means we can use conductivity as a proxy measurement to determine the relative concentration of minerals in the system water as compared to the incoming make up water. This calculation is called the Cycles of Concentration, or COC.
Example Illustration of Water Flow Across a Cooling Tower (Courtesy US Department of Energy)
Since it is not typically possible to alter your plant’s heat rejection load, you can not reduce the amount of water used for evaporation, however, here are 5 methods we can look at for potentially reducing the amount of bleed.

1. Maximize the Target COC for your System

Determine the maximum COC your application can withstand based on your incoming make up water quality and heat exchange temperatures.  A simple sliding ruler style “Stability Index Calculator” can provide the system’s scaling tendency, corrosion characteristics, and optimal chemical application.  More advanced methods can also be used for complex systems, and your water treater should be able to assist you in this calculation.
If you find your system is operating at a lower COC than allowable, try adjusting your system to maximize the COC.  As seen in the graph below, systems moving from relatively low COC to high COC will see the greatest impact.

2. Reduce Uncontrolled Water Loss

The first step to reaching your desired COC is to eliminate areas of uncontrolled water loss.  Using a typical HVAC system as an example, here are some areas that may be contributing to water loss in a system.

a) Poor level control resulting in overflow from tower basin or sump

Cooling tower sump levels are often controlled by a simple float mechanism; allowing water to enter when the float drops, and closing a valve when the float has risen to the target level.  Your system may also be equipped with overflow protection that will send water to the drain before it spills out over the top of the sump.  While this prevents flooding, it often results in undetected water loss because there is no direct evidence this is occurring.  Float system inspection should be part of your preventative maintenance program to verify valves are closing completely and effectively maintaining the desired water level.  Similar issues may occur with leaking water inlet solenoids.

b) Preventing overflow when system cycles off

Cooling circulation will often cycle on and off when area cooling is desired.  When the tower cycles off, any water that remains in the piping above the cooling tower sump will drain back to the sump.  If there is not adequate head space between the float control level and the overflow pipe, this will result in uncontrolled water loss each time the systems cycles off.  Contact your cooling tower contractor to correct this.

c) Ensure all automatic control valves are working correctly

Verify that the bleed valve and filter backwash valves are not allowing water to drain when they are closed.  These are commonly hard plumbed to drain so leaks can often go undetected.

d) Inspect pumps to ensure the packing is not allowing water to leak from system to drain

A small leak of as little as 0.1 GPM can leak 25,000 gallons of water to drain during a 180 day cooling season.

e) Inspect tower drift eliminator operation

Poor drift elimination results in unevaporated water droplets leaving the cooling tower.

3. Use a Conductivity Based Bleed Controller

The simplest and most cost effective way to control COC is to install a conductivity controller which will automate the bleed process by measuring and maintaining the system water conductivity.  The conductivity controller and bleed meter will provide a control system that can maintain the water conductivity within a target COC range.  When conductivity reaches the max setting, the concentrated water is sent to drain and fresh make up water refills the system.  For best practices, monitoring make up and bleed meter volume is also recommended.
This type of automated system can generate substantial savings as compared to a timer based bleed system which actually results in more water going to drain when an uncontrolled water loss is occurring, compounding the loss and cost.

4. Pre-treat Make Up Water to Increase COC

There are several common pre-treatment options available to remove minerals or adjust the properties of the incoming make-up water to allow the system to operate at increased COC.  The decision regarding the best option for your specific operation should be discussed with your water treatment professional and will depend upon your incoming water quality and your organization's priorities.

The following table provides a qualitative comparison of the different considerations for each of the common pre-treatment methods for Great Lakes water quality only.
*Based on Great Lakes water quality
Each organization may weight these considerations differently, which may lead to a different optimal solution.  Note, the water droplets are grayed out for the RO option because water savings are only generated if there is an on-site user for the reject water.

5. Utilize a Water Treatment Data Management System

Online data management programs for water treatment, such as AquaAnalytics, have proven to save water by actively monitoring the system and sending out alerts when program deviations occur, allowing for swift reactions and loss prevention. Long term trends can also be identified and benchmarked, providing valuable insight into continuous improvement opportunities.

Conclusions

While some bleed reduction suggestions may seem minor, the cumulative effect of all these industry best practices can result in significant water savings.  Work with your water treatment company to ensure your cooling systems are running as efficiently as possible this summer.

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Pat Smith has an Honours Physical Science degree from the University of Guelph. With over 20 years of experience designing and implementing solutions for water systems, as Service Manager, he reviews our service promise and works with our technical representatives from all districts. Pat enjoys thinking about participating in triathlons.

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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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