COOLING WATER AUDITS AND PLANNING

COOLING WATER AUDITS AND PLANNING
For an existing plant audit, the key initial question is whether the plant uses once-through cooling or is on a “closed” system with a cooling tower or hybrid tower as the primary technology.  Becoming more popular are air-cooled condensers, which I will touch upon later.  Virtually no new plants are equipped with once-through cooling because regulation 316b  of the Clean Water Act protects aquatic creatures from impingement and entrainment at water intakes.  [2]
With regard to cooling towers, the following discussion of audit items for existing systems will provide valuable information for owners and developers of new power plant and industrial facilities.

• Makeup and circulating water historical chemistry data
• History of corrosion, fouling, or scaling in:
  • Condensers
  • Cooling tower fill
  • Cooling tower basin
  • Other locations
• Current treatment program details and history:
  • Reliability
  • Upset occurrences
  • Chemical costs
  • Chemical feed system design and operation
  • System materials
• Are any changes in raw water supply being planned that will have an impact on makeup water treatment?
  • A prime example; will 316b regulations require the plant to switch from once-through cooling to another method?

Let’s explore some of these issues in more detail, starting with cooling towers and the circulating system.

Fig. 1.  Basic schematic of a counterflow, mechanical draft cooling tower.  Source:  Reference 3.
A cooling tower takes warmed water from steam condensers or other heat exchangers and cools the water for service by contacting the water with a continuous air flow.  Typically, 65 to 80 percent of cooling occurs from evaporation of about 2 percent of the incoming water.  The remaining energy exchange comes from sensible heat transfer.
Choose your fill material carefully.  Simple splash fill breaks apart water droplets to increase the surface area for heat transfer.  Nowadays, however, most towers are equipped with film fill, causing water to form a film on the material surface, further increasing air-water contact.



Fig. 2.  Illustrations of three different varieties of cooling tower fill, modern splash, vertical flute, and cross flute.  Photos courtesy of Rich Aull, Brentwood Industries.  Source:  Reference 4.
When choosing the fill, consider the fouling and scaling tendencies of the water.  For clean waters containing low concentrations of suspended and dissolved solids, the high efficiency cross flute fill could be a good choice.  It provides maximum heat transfer in the tower and could reduce tower size.  For less pristine waters, combined with cooling towers superb air scrubbing capability, a fill closer to the vertical flute or even the splash type may be necessary.
Critical to many audits at existing plants or for pre-planning of new plants is reviewing the cooling water biocide program.  Given the warm and wet nature of cooling systems, microbiological fouling of cooling tower fill, condenser tubes and other system components can occur with great rapidity.  Algae will grow in areas exposed to sunlight like cooling tower decks; fungi can cause wood rot in towers constructed of this material, and bacteria will thrive in many locations, including heat exchanger tubes.

Fig. 3.  Microbiologically fouled heat exchanger tubes.  Photo courtesy of Ray Post, ChemTreat.  Source:  Reference 3.
Bacteria attached to cooling system internals immediately begin to secrete a film (slime) to protect the organisms from toxins in the environment.  The slime layer and silt captured by it are very resistant to heat transfer; in fact, the layer is more insulating than any hard scale potentially forming in the heat exchanger.  Also, the film can initiate under-deposit corrosion by a variety of mechanisms.  This type of corrosion is very serious causing through-wall penetrations even though most of the metal still remains.  Microbiological and silt accumulations in cooling tower fill have, in numerous instances, actually caused fill collapse because of extreme weight gain.
Once microbiological colonies become established, they are very difficult to remove chemically.  Thus, an audit or pre-planning evaluation should focus upon chemical feed and its effectiveness to prevent problems before they happen.  Some important points include:

• The effectiveness of chlorine rapidly decreases as the pH rises above 7.5.  Many cooling towers operate at a pH in the low- to upper-8 range.  Alternative oxidizing biocides that perform better in the mildly alkaline environment are:
  • Bleach-activated bromine
  • Chlorine dioxide
• At some facilities, cooling water oxidant feed is limited to 2 hours per day.  This gives microbes 22 hours each day to attach to surfaces and begin to proliferate.  It may be possible, by feeding a reducing agent to the cooling tower blowdown, and per permission of the proper environmental authorities, to increase biocide injection times for longer periods, and perhaps continuously if needed.  Sodium bisulfite (NaHSO₃) is a common chemical for this application.
• Periodic feed, perhaps once per week for an hour or two, of a non-oxidizing biocide can combat microbiological fouling.  An evaluation of the micro-organisms in the cooling water is necessary for the plant’s water treatment vendor to select the correct chemicals.

Another topic of great importance for cooling systems, and especially recirculating systems with cooling towers, is scale control.  Decades ago, treatment for many cooling systems was straightforward, and was based on two primary chemicals, sulfuric acid and sodium chromate.  The fatal flaw in this cost-effective and straightforward program was chromate-use generates toxic hexavalent chromium (Cr⁶⁺).  The program was banned in basically all applications where the water could be released to the environment.  
The popular replacement uses inorganic and organic phosphates (the latter are commonly known as phosphonates) as the core chemicals.  These compounds maintain a mildly alkaline pH range, 8.2 to 8.8 or so minimizing general corrosion. They also exhibit other corrosion and scale prevention properties, with the aid sometimes of a small zinc residual.  A typical additional supplement is a polymer to inhibit calcium phosphate scaling.
But now, many bodies of water in the U.S. have been designated as “phosphorus-impaired.”  Any additional discharge of phosphorus into such waters may not be allowed, which for new plants could eliminate phosphate/phosphonate treatment if the cooling tower blowdown will be discharged.  The major water treatment companies are diligently developing non-phosphorus (non-P) technologies, which are based on polymer chemistry.  Co- and ter-polymers containing the active groups shown below are the outcome of these efforts.

Fig. 4.  Common active groups on the polymers being developed for non-P cooling water treatment programs.  Source:  Reference 3.
The polymers serve as crystal modifiers and sequestering agents to inhibit scale formation.  Evidence suggests the polymers form a thin coating on metal surfaces to inhibit corrosion.  A common dosage concentration is 2 to 10 ppm active in the cooling water. [5] In some cases, an all-polymer program may be less expensive than an equivalent phosphate/phosphonate program.
Some Thoughts on Air-Cooled Condensers
At times, I’ve seen suggestions that if plant personnel are worried about cooling tower issues, air-cooled condensers (ACC) are an easy cure.


 Fig. 5.  General schematic of a large ACC.
Yes, ACCs may be the only choice where water is scarce, but consider these factors before choosing an ACC:

• ACCs are enormously large, especially in comparison to water-cooled condensers, because of the much lower density of air compared to water.
• ACCs will only cool the turbine exhaust steam to a temperature approaching dry bulb.  Cooling water temperatures approach wet bulb.  In warm climates, dry bulb can represent a significant efficiency penalty.  For example, consider a wet tower operating on a summer day at 90^oF with 30 percent relative humidity.  The dry bulb temperature is obviously 90^oF, but the wet bulb temperature is 66^o F.  So a cooling tower that provides an approach within 10^o or so of the wet bulb offers significantly better condenser efficiency than an ACC.
• Typically, finned tubes are the choice for ACCs to improve heat transfer but  the fins clog with  blown debris from the fans.  Fouling of course affects heat transfer, and the material is difficult to remove.
• ACCs often suffer from two-phase flow-accelerated corrosion (FACwhere steam, as it condenses, initiates a chemical-mechanical attack on carbon steel surfaces impacted by the two-phase fluid.  FAC not only reduces tube life but introduces many iron oxide particulates to the condensate Any steam generator equipped with an ACC needs a condensate particulate filter to prevent transport of the iron oxides to the boiler.

Other alternatives I see are wet-surface air coolers (WSACs) and hybrid cooling towers, in which the equipment approaches wet bulb but does not consume as much water as standard cooling towers.  Additionally, some designs address plume abatement issues. Space limitations prevent a full discussion now, but perhaps the features of these technologies can be addressed in a future article. The technologies certainly can be addressed in an audit or new plant design.
Wastewater Issues
Existing plants and those to come face increasingly stringent discharge requirements,   and water recovery and recycle is mandated with growing frequency.  In some cases, zero liquid discharge (ZLD) may be required.  ZLD is not a decision to be made lightly.
For many years, the primary impurities that had to be controlled in plant discharge were suspended solids, residual oxidant (from biocide feed), oil, grease, and pH.  Excluding coal plants, new regulations, may limit discharge concentrations of TDS, sulfate, copper, phosphorus, and ammonia.  Also, the USEPA is proposing cooling tower blowdown limits of 1.0 ppm for zinc and 0.2 ppm for chromium per pending effluent limitation guidelines (ELG).  [6]  An audit and/or comprehensive planning is necessary.
Consider the increasing number of facilities whose makeup source is or will be the effluent from a municipal wastewater treatment plant (WWTP).  The effluent often contains significant quantities of ammonia, phosphorus, organics, and suspended solids.  All of these impurities can influence microbiological fouling in the cooling system, so removing them in the makeup stream rather than in the industrial plant discharge may be beneficial.  Advanced biological processes are available for this treatment. [1, 7]  WWTP effluent can exhibit widely variable chemistry, so treatment must be planned very carefully.
Depending upon the extent of wastewater treatment required at industrial facilities, disposal methods may range from straightforward to exceedingly complex. Some plants are permitted to discharge spent water to a local WWTP, provided the industrial wastewater does not contain excessive concentrations of harmful impurities such as heavy metals.  At plants in arid regions of the country, evaporation ponds may serve the purpose of final wastewater disposal.  However, these ponds must be permitted and installed in a proper manner.  Lined ponds are de rigueur in today’s strident environmental climate.  In some cases, deep-well injection may be an option, but permitting issues are again important.  Also, practical experience has shown that wastewater injected into wells may develop severe scaling tendencies as the water rises in temperature during its passage to the deep geologic formation.  Scale formation can close off the injection piping.
If none of the above options are available, mechanical-thermal evaporation of the waste stream may be the only choice.  Accurate analysis of influent water chemistry is vital for design and selection of such systems, as hardness, alkalinity, and silica can cause severe scaling problems in evaporator/crystallizers.  Maintenance costs for periodic cleaning and other repairs on these units can be time consuming and expensive.  In addition, due to the concentration of salts in evaporators and especially crystallizers, exotic metals are typically required as the materials of construction greatly increasing the capital cost of such equipment.  Also, energy requirements for conventional evaporator/crystallizers are quite large.  A modification to the process has been successfully applied in the salt production industry--evaporation-crystallization with a mechanical vacuum applied to the evaporation chamber lowering the temperature at which the liquid boils, and overcoming the boiling point rise that occurs in conventional units. Considerable energy savings appear possible with the vacuum systems.
Becoming popular are treatment methods to reduce the volume of the plant waste stream before final treatment.  Most notable is high-recovery reverse osmosis. 

Fig. 6.  Generic outline of an emerging wastewater treatment and recycling technology.
The schematic illustrates main features of a high-recovery reverse osmosis process.  Often included upstream of the equipment shown above may be a forced-draft decarbonator with upstream acid feed and/or a high-rate clarifier perhaps with lime softening.  Keys to the process outlined in Figure 6 are:

• Microfiltration (MF) or ultrafiltration (UF) to remove suspended solids in the waste stream is a critical procedure to prevent suspended solids from fouling reverse osmosis (RO) membranes.
• Sodium bisulfite (NaHSO₃) feed to remove residual oxidizing biocides is also critical to remove oxidizers degrading softener resin and RO membranes.
• A weak-acid cation (WAC) exchanger to remove calcium and magnesium or the downstream equipment would suffer from calcium carbonate and magnesium silicate scaling.
• Sodium hydroxide injection to elevate the pH above 10. The combination of hardness removal and pH elevation keeps silica in solution.
• Two-pass reverse osmosis (RO) treatment.

Under proper conditions, the RO recovery may reach 90 percent.  The RO permeate recycles to the plant high-purity makeup water system or other locations.  While the process appears straightforward, a number of lessons-learned have emerged regarding this technology in actual application, including:

• For MF or UF membranes, an outside-in normal flow path (my preference) with inside-out backwash flow, better cleans the membranes during backwash cycles.  The flow path also allows air scrubbing of the membranes during the backwash cycle.
• Modern systems include periodic chemically-enhanced backwashes (CEB), where a caustic/bleach step is followed by an acid backwash.  These chemical washes help to remove organics/microbes and iron particulates from the membranes, respectively.  However, impurities in the backwash water can cause scale formation if not properly considered.  In this application and others where MF or UF, and also RO, are downstream of a clarifier or multi-media filter, carryover of cationic polymers from the upstream pre-treatment devices has caused severe and irreversible fouling of membranes.

Some Quick Thoughts About Industrial Steam Generation 
I have also worked with and observed industrial boilers operating at lower pressures, with less stringent makeup water requirements.  However some effort is still necessary to protect the steam generator from chemistry upsets.  This is where audits can be quite valuable.  For example, many industrial boilers do not require demineralized water for makeup, but they do require softened water with hardness removed.
The deposits reduce heat transfer and can cause tube overheating.  Likewise, excessive silica in the makeup water may form magnesium silicate deposits in the steam generator.  Silica deposits and organic introduction of any kind are very difficult to remove
Thanks for reading