Minimizing Cooling Tower Water Usage

Published on by in Academic

Minimizing Cooling Tower Water Usage
What are recommended ways to minimize how much water a cooling tower system uses?

Media

Taxonomy

9 Answers

  1. Run an alkaline high cycle 10 cycles) program like the one that Suez has acquired from GE.  AEC and HPS1 allow it.  Soften a portion of your water if you need more cycles than that.  Clean up blowdown with RO and recycle, but then you have the reject to deal with, which could throw off your WWTP salinity.  Lots of ways.  Thanks, Sean

  2. Instead of blowing down the tower and wasting water,  consider using an electronic water treatment system like HydroFLOW to clean the water. (www.hydroflow-se.com) The low power, high-frequency decaying signal generated by HydroFLOW is induced into the water stream and travels throughout the hot, chilled or condenser water systems. The electrical signal prevents and removes scale, biofilm, & bacteria from system surfaces while adding a film of magnetite that protects the ferrous parts of the system from corrosion. Include an air & dirt separator with SweepCLEAR to physically remove 99% of the entrained dirt (scale, rust)  and air water. www.sweepclear.com  .  Clean water does not have to be blown down.

  3. Unfortunately much of what is done to maximize water use or reuse uses an old mindset and usually have divergent plans or processes. in 1994 we made a slight but significant change to the way in which a High cycle power plant operated. It was a bold change on the surface since it went opposite traditional thought. However it was based on improving mass balance and not staying with the way things have always been done.

    The plant used cold lime/soda softening and was a zero liquid discharge plant. To minimize total solids build up (dissolved and suspended) they used a brine concentrator. This worked but they still needed large evaporation ponds. There were numerous clarifier upsets and as the previous post indicated large evaporites hung from the CT slates. These would actually get so heavy they would cause structure to fail.

    The change was to take cooling tower blowdown and run it to the lime/soda softener instead of make up water.  It was far easier to treat a constant 4,000gpm of flow and drop 160+ppm SiO2 to 10ppm, than to take 16,000gpm of water with 21ppm SiO2 to 9ppm SiO.

    As with all things, time judges success. Within weeks, the SiO2 dropped to never going above 120ppm. The sulfate dropped to under 6,000ppm (had run closer to 16,000ppm), sulfuric acid use dropped to 1/3 of previous (lots of logical reasons for that). The water in the evap ponds was able to be recovered. The BC could be run at less that maximum and its fouling decreased with longer run cycles (much lower SiO2 BC feed). This plant ran under this mode until it was decommissioned and leveled.

    This was a case of not looking at the dynamics of what was desired today but staying with "how things have always been done."  Progress only comes with change.

  4. @Des Prosser 

    I have not (unfortunately for me) ever made it to Nashville.  Perhaps it was somewhere else, such as a conference meeting in Austin, TX.

    One more thing no one has mentioned for cooling water conservation is the combination of wet/dry cooling tower, where most all of the heat rejection is by dry air, followed by smaller evaporation load.  After all (if blow-down is even marginally under control) the lion's share of the water goes to evaporation.  Another water conservation item would be to totally be rid of the Rankine cycle altogether, substituting something like the closed super-critical carbon dioxide Brayton cycle (also known as Allam cycle).  Allam cycle can be successfully applied to coal, or natural gas combustion, Closed Brayton SCC cycle can also work with nuclear fission (or someday with fusion?) as heat source, or CSP (concentrated solar power).  Since no combustion takes place in the latter cases, one may not properly refer to these as Allam cycle.

    SCC Brayton cycle is very amenable to air-cooling only, since one does not rely on phase transition from gas (vapor or steam) to liquid, but rather from relatively low pressure, warm dense fluid, to cooler dense relatively low pressure fluid.  The compressor in SCC Brayton cycle looks more like a pump than a compressor.

     

    Getting back to systems out of control, I have seen a place with a high throughput, large cooling tower operating on very hard well water with high silica content, but using cold lime softening clarifier only (and that grossly undersized), with such solids accumulation in a holding pond that islands were forming.  OMG.  One could not believe the stalagtites created by mineral precipitation in the wet-dry areas of that cooling tower!  Various suggestions have been made to provide them with ZLD system (rather than the 40 cycles of concentration they are running), with no issues, other than fair wear and tear of upstream water treatment.  I don't see anyone buying anything, but the bean counters are running the world now.

  5. A big one I see is that people will set a conductivity target for cycles of concentration and then never validate that they are achieving that either through mass balance or cycled up ionic comparison. Too many people employ "set it and forget it" and subsequently under cycle a tower. 

  6. There are ways to treat the water so that blow-down is not required!  Extreme softening of ground or surface fresh water, or even seawater (with at least some level of desalination) will leave behind one key ingredient - silica (both the reactive silica, and "polymerized" silica that is un-reactive to molybdate). It turns out that when silica approaches and exceeds a critical residual concentration of above 100 ppm, nearer 200 ppm, this becomes the ultimate corrosion inhibitor known to mankind.  Corrosion rates such as 0.01-0.001 mpy (mils per year) were reported in NACE for this.

    There are a number of ways to arrive at the water condition:

    (1) high efficiency softeners (sodium ion exchange, counter current, special bed geometry

    (2) Nanofiltration with back addition of small amounts of silicates

    (3) membrane capacitive deionization (perhaps this will be best especially for seawater source water, since it removes salt along with hardness, is tunable as to deionization extent, and to recovery fraction, is very robust (tolerant of chlorine), is very easily scaled up to vast arrays, and has the lowest specific energy input of any water purification system.  mCapDI does not remove silica.

  7. Hi James - we met a few years ago at Nashville

    your ideas for minimal water use are music to my ears.

    However we have to be careful when we look to automated control of cycles. It's not as good as people suspect.

    modelling a dynamic process such as a cooling tower where we are expecting to achieve the best control with a batch operation which is the usual conductivity control method (more on PID control later).

    When we look at water level control with probes , blowdown cuts in and out on predetermined conductivity levels. Blowdown (purge) will start at there high limit and continue until the lower limit is reached. This is a function of continued evaporation and system concentration  - balanced by solids removal via purge (teaching how to suck eggs - sorry). 

    If we now consider water levels in the system sump make up will be controlled on the upper and lower probes. De-concentration of there system water will only occur when the make up valve is operational. During the purge operational situation we have to move from high water to low water in order for the make up top activate. this volume of water removed as purge could equate to almost 20% of the system volume.

    excess water use over theoretical is the biggest uncontrolled loss in a cooling system

    PID control will overcome this - also float valve control of make up negates the above.

    Do the maths!!!

     

    regards

     

    Des Prosser

  8. I'll add a trivial aspect related to design and maintenance of evaporative factory-assembled cooling tower.

    • Check and possibly test the drift losses. Some manufacturers guarantee a maximum drift loss. Relative numbers might appear of little impact (e.g. 0.001%), but proper installation and maintenance of drift eliminators can save tons of water in mid-size and large installations.

    1 Comment

    1. Drift is easily miscalculated if one only follows the EPA-42 (or is it 142) formula.  That formula really is wrong as TDS gets into higher levels, as at the higher residual TDS, there is no way a particle of 10 microns will be formed.  Thus the PM10 calculation is completely erroneous.  Drift will be represented by the limiting cycles of concentration of the tower.

  9. I'll throw out a few ideas for minimizing cooling tower water usage here.

    • Run at maximum cycles of concentration
      • Know what the maximum cycles are based upon water chemistry, control, etc.
      • Use the appropriate chemistry to achieve this (e.g., scale inhibitors, polymers, acid feed)
      • Use the appropriate pretreatment to achieve this (e.g., water softener, blended soft/hard water, reuse/recycled water)
      • Use automated control equipment to maintain cycles
      • Fix any water leaks or "uncontrolled blowdown" sources preventing the system from running at maximum cycles

    2 Comments

    1. I would add: Control the biological factors to reduce their contribution to scale formation and the resulting heat transfer inhibition. In a pilot we did in a petrochemical plant in China, we reduced water use by 35% with significant reduction in energy use as well. 

    2. I totally agree.  I also am aware that some radically new ideas have surfaced within the last ten years, that allow really high cycles of concentration with no effect on heat transfer (other than likely improvement over time).  Water leaks really are a problem issue when high cycles are sought, and are a clear mode of failure on any closed loop system of coolant, since make-up rates will exacerbate corrosion issues.