Increasing Cycles of Concentration thru Side Stream Softening

Reducing Cooling Tower Water Treatment Cost

The number of cycles of concentration (CoC) a cooling tower operates at will greatly impact the overall cost of the water treatment program.  Higher CoC results in reduced water, chemical, and sewer cost.  The goal of any sound water treatment program is to operate CoC as high as possible without causing system problems related to mineral scale.  The water treatment industry utilizes an index called the Langelier Saturation Index (LSI) to predict when problems will occur.  Generically, a well designed system can operate at an LSI of up to 2.5.  By utilizing a softener to remove significant portions of the make water Calcium content higher CoC can be achieved before the LSI threshold is reached.  This can result in substantial operational savings.  Consider the simple model below.  

	Make Up	3.0 CoC
TDS	800	2400
pH	8.0	8.8
Calcium	160	480
Alkalinity	140	420
°C		45
LSI		2.45
Silica	9	27
Chloride	90	270
Predicted Value @ 88% Ca Hardness Removal
	Make Up	5.0 CoC
TDS	800	4000
pH	8.0	9.2
Calcium	19	95
Alkalinity	140	700
°C		45
LSI		2.34
Silica	9	45
Chloride	90	450
Assumptions: 1,000 Ton tower @ 70% load, 16 hours/day, 5 days/week, 52 weeks/year
Annual Blowdown Savings: 4,960,404 liters/1,310,400 gallons

Water Analysis: Las Vegas, Nevada USA

                                    Make Up                3.2 CoC
Conductivity                904                         2892
pH                                 8.2                           8.7
Ca Hardness                188                          601
Alkalinity                      140                          448
Chloride                        65                            208
Total Hardness           376                          1203
Silica                              8                              25
Predicted LSI                                               2.4

 
This make up water sample from Las Vegas, NV USA was sampled and analyzed for use in a cooling tower application. By using the Langelier Saturation Index (LSI) we can predict the scale forming tendency of the water. Modern water treatment chemistry can prevent scale in waters with a LSI of 2.3-2.5. In waters with a pH  greater then 8.5 that contain silica and magnesium hardness a mineral scale problem could occur. Under the right conditions Magnesium Silicate will form. This scale is a barrier to heat transfer and is one of the most difficult scales to remove. Silica levels are typically not a concern below 150ppm and can be handled as high as 200ppm with proper chemistry.


We can see from the above calculation that this tower can be operated safely at 3.2 Cycles of Concentration (CoC). At 3.2 CoC the water will have an LSI of 2.4 with a silica level of 25ppm. Both levels are within the acceptable range of modern water treatment chemistry. It is important to note that at a LSI of 2.4 the water is extremely scale forming. If the proper amount of water treatment chemical is not maintained at ALL times the system will rapidly develop scale. Las Vegas routinely has extremely high heat loads. For this reason the cooling system will experience high load.  The conditions exist to form enough mineral scale in 24 hours to result in high enough head pressure to shut a chiller down.  Professional, correct, and consistant water treatment is absolutely necessary in this environment.

Why does this information matter? Higher CoC results in lower water and chemical consumption. Consider an average casino with 8,000 tons (RT) of cooling operating 24 hours a day, 365 days per year at full load. By increasing tower cycles from 1.0 to 3.2 cooling tower make up is reduced by 12,487,730 liters (3,298,909 gallons) per day! This significant savings can easily be realized with a sound water treatment program and routine monitoring.

Water Analysis: Hongqiao, China

                                 Make up          8.0 CoC
Conductivity             304                   2432
pH                              7.6                     8.7
Ca Hardness             55.5                  444
Alkalinity                  73                      584
Chloride                    28                      224
Total Hardness        111                    888
Silica                          5.5                    44
Predicted LSI                                    2.4

This make up water sample from Hongqiao China was sampled and analyzed for use in a cooling tower application. By using the Langelier Saturation Index (LSI) we can predict the scale forming tendency of the water. Modern water treatment chemistry can prevent scale in waters with a LSI of 2.3-2.5. In waters with a pH greater than 8.5 that contain silica and magnesium hardness a mineral scale problem could occur. Under the right conditions Magnesium Silicate will form. This scale is a barrier to heat transfer and is one of the most difficult scales to remove. Silica levels are typically not a concern below 150ppm and can be handled as high as 200ppm with proper chemistry.




We can see from the above calculation that this tower can be operated safely at 8.0 Cycles of Concentration (CoC). At 8.0 CoC the water will have an LSI of 2.4 with a silica level of 44ppm. Both levels are within the acceptable range of modern water treatment chemistry. It is important to note that at a LSI of 2.4 the water is extremely scale forming. If the proper amount of water treatment chemical is not maintained at ALL times the system will rapidly develop scale. Many facilities in Asia do not utilize modern chemical feed and control equipment. Instead they manually dose chemical and control bleed valves. It is not possible to maintain tight enough control with manual methods to achieve the water savings possible at 8.0 CoC without eventually forming scale. For this reason an automated chemical feed and control system must be installed.



Why does this information matter? Higher CoC results in lower water and chemical consumption. Consider a 1,000 ton (RT) tower operating 24 hours a day, 365 days per year at full load. By increasing tower cycles from 1.0 to 8.0 cooling tower make up is reduced by 1,611,938 liters (425,829 gallons) per day! This significant savings can easily be realized with a sound water treatment program and routine monitoring.

Water Analysis: Bali, Indonesia

                              Make up            3.5 CoC
Conductivity          1033                   3615
pH                            7.9                      8.8
Ca Hardness           102.5                 358.7
Alkalinity                176                    616
Chloride                  173                    605.5
Total Hardness      205                    717.5
Silica                         26                       91
Predicted LSI                                     2.4


This make up water sample from Bali Indonesia was sampled and analyzed for use in a cooling tower application. By using the Langelier Saturation Index (LSI) we can predict the scale forming tendency of the water. Modern water treatment chemistry can prevent scale in waters with a LSI of 2.3-2.5. In waters with a pH greater than 8.5 that contain silica and magnesium hardness a mineral scale problem could occur. Under the right conditions Magnesium Silicate will form. This scale is a barrier to heat transfer and is one of the most difficult scales to remove. Silica levels are typically not a concern below 150ppm and can be handled as high as 200ppm with proper chemistry.


We can see from the above calculation that this tower can be operated safely at 3.5 Cycles of Concentration (CoC). At 3.5 CoC the water will have an LSI of 2.4 with a silica level of 91ppm. Both levels are within the acceptable range of modern water treatment chemistry. It is important to note that at a LSI of 2.4 the water is extremely scale forming. If the proper amount of water treatment chemical is not maintained at ALL times the system will rapidly develop scale. Many facilities in Indonesia do not utilize modern chemical feed and control equipment. Instead they manually dose chemical and control bleed valves. It is not possible to maintain tight enough control with manual methods to achieve the water savings possible at 3.5 CoC without eventually forming scale. For this reason an automated chemical feed and control system must be installed.


Why does this information matter? Higher CoC results in lower water and chemical consumption. Consider a 1,000 ton (RT) tower operating 24 hours a day, 365 days per year at full load. By increasing tower cycles from 1.0 to 3.5 cooling tower make up is reduced by 573,008,400 liters (151,372,800 gallons) per year! This significant savings can easily be realized with a sound water treatment program and routine monitoring.

Water Analysis: Chennai, India

                                Make up              3.0 CoC
Conductivity            1192                    3576
pH                            7.4                        8.6
Ca Hardness            138.5                   415.5
Alkalinity                 182                      546
Chloride                   178                      534
Total Hardness        277                      1108
Silica                        28.9                     86.7
Predicted LSI                                        2.3

This make up water sample from Chennai India was sampled and analyzed for use in a cooling tower application. By using the Langelier Saturation Index (LSI) we can predict the scale forming tendency of the water. Modern water treatment chemistry can prevent scale in waters with a LSI of 2.3-2.5. In waters with a pH greater than 8.5 that contain silica and magnesium hardness a mineral scale problem could occur. Under the right conditions Magnesium Silicate will form. This scale is a barrier to heat transfer and is one of the most difficult scales to remove. Silica levels are typically not a concern below 150ppm and can be handled as high as 200ppm with proper chemistry.

We can see from the above calculation that this tower can be operated safely at 3.0 Cycles of Concentration (CoC). At 3.0 CoC the water will have an LSI of 2.3 with a silica level of 86ppm. Both levels are within the acceptable range of modern water treatment chemistry. It is important to note that at a LSI of 2.3 the water is extremely scale forming. If the proper amount of water treatment chemical is not maintained at ALL times the system will rapidly develop scale. Many facilities in India do not utilize modern chemical feed and control equipment. Instead they manually dose chemical and control bleed valves. It is not possible to maintain tight enough control with manual methods to achieve the water savings possible at 3.0 CoC without eventually forming scale. For this reason an automated chemical feed and control system must be installed.

Why does this information matter? Higher CoC results in lower water and chemical consumption. Consider a 1,000 ton (RT) tower operating 24 hours a day, 365 days per year at full load. By increasing tower cycles from 1.0 to 3.0 cooling tower make up is reduced by 567,039,500 liters (149,796,000 gallons) per year! This significant savings can easily be realized with a sound water treatment program and routine monitoring.

Corrosion Cell

Corrosion is a never ending battle in the water treatment industry. It is an electrochemical process in which a difference in electrical potential develops between two metals or between differeing parts of a single metal. Electrons flow from the anode to the cathode. Metal ions go into solution at the anode causing metal loss (corrosion). The speed at which the ions go into solution can be increased with higher conductivity waters (sea water, cooling water, boiler water). There are many types of corrosion and many more ways to control it. Having a basic understanding of the process will enable you to better choose a treatment option best suited for the application.

ASHRAE Project Number 1361-RP

The topic of Non Chemical Device (NCD) water treatment seems to always be popular. ASHRAE Project Number 1361-RP focused on Biological Control in Cooling Water Systems using Non-Chemical Treatment Devices. Five different NCD devices were analyzed in the study from the following categories: Magnetic, Pulsed Electric Field, Electrostatic, Ultrasonic, and Hydrodynamic Cavitation.

The experiment concluded, "...no statistically significant difference...in planktonic or sessile microbial concentrations (HPC) was observed between the control tower and a tower treated by any of the five NCDs evaluated in this study...", (i-ii).

You can download the entire report here: http://rp.ashrae.biz/page/ASHRAE-D-1361-RP-20101005.pdf

Water Treatment Case Study: Low Cycles of Concentration (CoC)

Size: 800 Ton Tower
Load: 90% Summer; 30% Winter
Location: Tokyo, Japan
Problem Description: The tower achieved target operating cycles of concentration (CoC) of 7.0 during the summer months. During the winter it was hard to achieve a CoC of >5.0.
Assumption: Cooler temperatures did not enable enough evaporation for tower to “Cycle Up”.
Finding: The calculated summer blowdown rate is 3.6 gallons per minute (GPM) @ 90% load. The calculated winter blowdown rate is 1.2 GPM @ 30% load. Upon inspection a condenser water pump was found with a badly leaking seal (see photo). An impromptu field test was conducted to measure the amount of water being lost through the seal leak. Using a sample bottle to capture the water loss from the pump and a watch to calculate the time it took to fill the bottle it was estimated that just under 2.0 GPM were being lost at the seal.
Conclusion: Water loss through a seal leak has the same effect on a tower that excessive blowdown does. Since the leak rate of 2.0 GPM exceeds the winter blowdown rate of 1.2 GPM it is not possible to achieve the results desired. This problem was not noticed in the summer because the summer blowdown rate of 3.6 GPM exceeds the leak rate of 2.0 GPM.

Recommendation: Repair seal

Chiller Inspection

When is the best time of year for selling cooling tower water treatment? It is during the winter months. During the cooler season many clients will begin to do their preventative maintenance on Chillers, Cooling Towers, and Heat Exchangers. It is during this maintenance period that the effects of water treatment will become apparent. When a chiller is opened up for an inspection you want to be there to give your professional opinion. Having a boroscope, digital camera, and a flashlight will assit you in making a quality inspection.