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

Author: Pat Smith - Service Manager

Cooling towers work by taking advantage of heat lost to the atmosphere when water evaporates. 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.

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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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