NELSON ENVIRONMENTAL

TECHNOLOGIES, INC.

 

PILOT STUDY FOR ELECTRICAL COOLING TOWER WATER TREATMENT

 

Scope

This pilot study was designed to prove the efficacy of using electrolysis and ion generation in the precipitation-chemistry mode to treat cooling tower water.  The study includes a 1,500-watt heating element (representing heat exchange coils in a commercial air conditioning system) as well as an aerator (representing a mini cooling tower).  Two identical recirculation systems were built with one exception: one contains an OxionTM water conditioning unit.  Make up water from the local (McAllen, Texas) municipal supply (source: Rio Grande River) was added daily.  There was no blowdown.  Solids removal were by 5-micron depth cartridge filters only.  The conditions of the systems were monitored daily.

 

Theory

Studies have shown that when cooling tower systems are allowed to operate in the precipitation-chemistry mode (metal salts allowed to reach their saturation levels) the dissolved solids will precipitate out and not scale plate the heat exchange coils when the water chemistry is favorable (sufficient silica and metal ions, in addition to calcium ions, in the water).

 

The only difference between the two water systems in this pilot study is the inclusion of an OxionTM water treatment system in one of them.  This equipment consists of titanium electrodes to provide the electronic water conditioning, marine brass electrodes to provide the copper and zinc ions, and the controller.

 

Set-up

The pilot study was set up at the Nelson Environmental Technologies, Inc. facility located at 813 E. Fir, McAllen, Texas.  Two water systems were constructed as shown in photographs 1 and 2.  They are identical with one exception.  One of the systems (on the right side) included the OxionTM electrolysis and ion generation water treatment system.  The water systems consist of 24 gallon water storage tanks, recirculation pumps, automobile radiators with cooling fans, 5-um sediment filter, 1,500-watt heating element in a glass tube housing, a screen trickle aerator with exhaust fan (simulating a mini cooling tower), and return to storage tank.  The flow, TDS, alkalinity, and pH were measured along with daily visual observations as to the scaling of the systems.

 

 

Photograph 1.

 

Photograph 2.

 

The automobile radiators were the only element of the pilot study what are not representative of an actual cooling tower system.  They were added as the efficiency of the screen trickle aerator was thought to be not enough to reduce the temperature of the water by 10 degrees as cooling towers are designed to do.

 

The supply water was from the municipal McAllen, Texas water system which is fed from the Rio Grande River. Coffee cup type immersion heaters were added to the tanks to measure scaling ability.

 

Method

  1. The components were assembled as shown in Figure 1.  The recirculation flow was important as the flow needed to be slow enough in order to heat up the water sufficiently to simulate an actual cooling tower but not too fast as to provide turbulence around the heating coil as that would introduce the possibility that the turbulence was responsible for keeping the coil clean.  A flow in the 2 to 3 gallons per minute was chosen as being ideal.  This would be much slower than in actual cooling tower systems but be fast enough to stop any scale build-up.

  2. A log was kept for each system.  The daily entries were done at approximately the same time each day.  The log included:

    1. Amount of water added since previous day

    2. TDS of influent and recirculating water

    3. Alkalinity of the recirculating water

    4. Flow of the water in the recirculation loop

    5. The concentration of copper ions in the recirculation water

    6. The OxionTM water treatment controller settings

    7. Notes such as any changes to the systems (filter changes, etc.) and any other observations.

  3. A photograph was taken of the heating elements periodically

  4. The system data was entered into Microsoft Excel and the data was plotted.

 

Figure 1.

Results

The supply water chemistry is shown in Table 1.  It shows that there is 20 mg/L of silica.  The concentration of solids was allowed to build up in the systems for the first 29 days.  On day 30, the OxionTM water treatment system was energized. 

 

The OxionTM controller was set up to energize the titanium electrodes for 23.5 hours per day and energize the bronze electrodes for 0.5 hours per day.  However, for the first 20 days, the OxionTM system provided both the oxidation and ionization 24 hours a day.  The OxionTM system was then turned off for 20 days to observe any changes.

 

First 19 Days

After the original 20-day period, the water in both tanks looked yellowish and cloudy. 

 

Days 20 to 49

Within 5 days of starting the OxionTM system, there was a marked difference in the appearance of the tanks.  The control tank was yellowish and murky.  The OxionTM-system tank was crystal clear.  Photograph 3 was taken on day 25 .   Also, during this period, the sediment filter had to be changed almost daily on the OxionTM system as it clogged up with sediment reducing the flow below 2 gpm (the targeted minimum flow rate).  The control system had only 3 filter changes during the same time.  Observing the water tanks, much scale could be seen on the sides of the control system tank but very little on the OxionTM system tank.

 

Photograph 3.

 

The TDS of water during this period reached above 10,000 PPM for the control system and almost 7,000 PPM for the OxionTM system.  The alkalinity of the control system reached around 200 to 220 mg/L whereas the OxionTM system reached only 160 mg/L.

 

Days 50 to 69

For 20 days the OxionTM system was turned off to observe that it was actually making a difference and that something else was not making a difference in the observations.  The results were that the TDS and alkalinity rose over this period to reach similar values as observed in the control system.  The water become slightly yellowish and murky.

 

Heating Element Scaling

One of the main reasons for the pilot study was to observe to see if scale accumulated on the heating elements.  This would be of prime concern to anyone wanting to use this technology in their cooling tower system.  As can be seen in photograph 4, there is no scale on either the control or the OxionTM system heating elements.  This confirms observations reported by others when using precipitation-chemistry method of scale control.  Scale did form on the coffee cup type of immersion heaters that were placed in the water tanks as seen in photograph 5.  The difference is that the flow past the heating elements flushed away the precipitation before it could be deposited onto the element.  The minimum flow was targeted to be 2 gpm.  This means that the flow in the 3 glass tube was 0.33 feet per second.  This is significantly slower than cooling tower systems designed flows.  In reality, the flow often was only 1 gpm because the sediment filter was restricting the flow.  This proves that scale will only build up on the heating elements if the precipitation occurs faster than it can be carried away (as on the coffee cup type immersion heaters in the tanks where there was not flow past them).  The TDS reached 13,700 PPM and the alkalinity reached 60 mg/L without any scale forming on the heating elements.

 

 

Photograph 4.

 

Photograph 5.

 

Conclusions

  1. Precipitation-chemistry method of scale removal allows the removal of dissolved solids from the cooling tower water without the loss of water through blowdown.

  2. Scale will not build up on the heating coils as long as there is a minimum flow past them.

  3. OxionTM water treatment system is an effective way to remove alkalinity and aid precipitation (reduce TDS) in the water.

 

Table 1.   McAllen Municipal Water Analysis

Constituent Name

Amount

Chloride

153 mg/l

Sulfate

233 mg/l

Total Hardness (as CaC03)

267 mg/l

pH

7.6

Total Alkalinity (as CaC03)

101 mg/l

Bicarbonate

123 mg/l

Carbonate

0 mg/l

Total Dissolved Solids

675 mg/l

Silica

20 mg/l

 

 

 

 

 

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Published since April 25, 2001, copyright by Nelson Environmental Technologies, Inc.