A 500-ton evaporative condenser may assimilate more than 1 lb of particulate matter from the air and makeup water every operating hour. This foreign material contaminates the sump water, which results in the following:
- Microbiological growth, which occurs in the warm (75 to 85°F [24 to 30°C]) nutrient-rich water supply in the form of bacteria, algae, slime and fungi.
- Corrosion of the condensing coils, condenser housing, framework and fans.
- Scale buildup on coil surfaces, which reduces system capacity.
- Sludge and debris collection in the water sump, which must be removed manually.
To control contamination, most evaporative condenser systems employ a chemical water treatment system. Unfortunately, some evaporative condensers do not have a water treatment program.
Calculating Water MakeupEvaporative condensers are effective because they rely on evaporating some water to remove heat from the ammonia vapor, causing it to condense. When evaporated, 1 lb of water will remove 1,000 BTUs from the condensing refrigerant. Typical evaporation rates for an ammonia system operating at 0 and 95°F (-18 and 35°C) are show in table 1. To maintain a suitable quantity of water in the sump, the amount of water that evaporates must be replaced.
When the water evaporates, only water vapor leaves the unit. Any contamination or minerals in the water are left behind and tend to concentrate - even as new water is added to the sump. To keep the mineral concentration under control, some water from the sump is bled off to a drain. This action flushes the minerals and contamination and will result in a consistent mineral content, depending upon the amount of water bleed or blowdown. Blowdown for evaporative condensers typically ranges from one-half to one times the evaporation rate.
In addition, most evaporative condensers will lose a small percentage of water as drift - the fine water droplets carried through the eliminators - along with the evaporated water. This amount usually is 0.1 to 0.2% of the circulated water rate. Therefore, the makeup rate to maintain a constant water level in the sump must be equal to the evaporated water plus drift plus blowdown.
The cost of the water used in evaporative condensers is based upon total makeup water. Typically, this is charged at the city water and sewage rate unless the plant has its own sewage facility. For example, assume city water and sewage charges are $5 per 1,000 gal of water. The 1,000-ton refrigeration system in table 1 uses 47,090 gal per day. At this rate, the daily expense to replace evaporated water is $235. With a blowdown of 0.8% and drift of 0.2%, total water makeup is 94,180 gal per day, at a cost of $470 per day. The yearly total water cost is nearly $170,000.
Cycles of concentration measure how much or how little blowdown has been used to control the water's mineral concentration. It is typical to have blowdown in the range of one-half to one times the evaporation rate for chemically treated evaporative condensers. The number of cycles of concentration is equal to the sum of the evaporation rate plus blowdown plus drift, divided by blowdown plus drift. Table 2 shows the cycles of concentration for various makeup water quantities and blowdown. Drift is assumed to be zero.
As table 2 clearly shows, reducing blowdown increases the cycles of concentration. If the cycles can be increased into the range of 10 or more, then a significant amount of water can be saved and operation costs reduced. The same 1,000 ton unit with cycles of concentration at 20 will use only 1.05 x 47,090 gal per day, for a yearly cost of $86,000. This represents a possible saving of $84,000 a year on water and sewage costs.
To control mineral content, it is necessary to provide sufficient blowdown to keep the concentration at a de-sired level. The higher the mineral content, the more likely minerals are to bond to the heat exchanger coils' metallic surfaces and cause scale. The primary minerals of concern are calcium and magnesium, which tend to deposit on the coil surface most often as calcium and magnesium carbonate. The obvious result of scale buildup is a reduction of the coil's heat transfer characteristics and a loss in condenser capacity.
A pH factor above 7.0 is a measure of the alkalinity of the water in the sump. Values of pH should be maintained be-tween 7.0 and 9.0 as values above 9.0 tend to increase scaling and values below 7.0 are acidic and associated with increased corrosion.
In chemically treated systems, there is a continual struggle to balance mineral content, water softness and pH to minimize corrosion and scaling effects. Maintaining this balance is difficult because adding more chemicals to optimize one reading will be detrimental to the others. This balancing act is further complicated by the introduction of chemical biocides, which also may affect corrosion and scale inhibitors.
Controlling Biological ActivityChemical biocides traditionally have been used to control biological activity within evaporative condensers. In general, treatment involves using two different biocides that are changed periodically to prevent biologic material from becoming immune to the biocide. Biocides must be provided in a solution strong enough to reduce and maintain bacteria colonies at an acceptable minimum. Bacteria reduction also inhibits the growth of sludge, slime and algae and helps maintain an acceptably clean system.
The control of bacteria, algae and pH factors such as alkalinity and water hardness through the use of chemicals has several disadvantages. These include chemical costs, employee exposure to hazardous chemicals and the expenses related to satisfy Environmental Protection Agency (EPA) requirements on chemical usage and its disposal. During the last 10 years, the EPA has more stringently regulated the specific chemicals that can be used as well as the criteria of usage pertaining to the acceptable materials.
Alternative SolutionsAs an alternative to chemical biocides, an ozone-based water treatment system can be used to address cooling water contamination problems in evaporative condensers, cooling towers and closed-circuit coolers. With this approach, ozone is used as the biocide in conjunction with several nonchemical components to control scaling, corrosion and sludge factors. Intro-ducing ozone into the water as it enters the distribution header of the evaporative condenser effectively controls biological growth. The ozone is generated on-site directly adjacent to the injection point.
Ozone is a highly active oxidizing biocide that reacts with organic material and destroys all bacteria and viruses with which it comes in contact. This action may result from contact with ozone or with hydroxyl (OH) free radicals produced when the ozone contacts water. The hydroxyl radical has a higher oxidation potential and is more reactive than ozone.
When introduced into the water system, ozone is in a gaseous state. Because ozone is a reactive oxidizer, its life in the water system is less than a few minutes. There is no residue and no chemical handling required on the part of employees.
The amount of ozone produced and quantity introduced into the water is determined by the water's biologic requirement. Control is accomplished using an oxidation reduction potential (ORP) monitor located in the cooling water stream. If more bacteria or organic material must be destroyed, more ozone is introduced into the water. The ORP monitor will keep the water ozone level at the point required to maintain water quality.
Corrosion in an ozone system is less than 0.2 ml per year on copper and less than 2.0 ml per year on mild steel. At this rate, the evaporative condenser's life is extended beyond that of towers with chemical water treatment systems. Ozone aids in establishing a thin metal oxide film on clean tube surfaces. This oxide film is water insoluble and prevents further corrosion.
Existing scale is removed by a magnetic water conditioner, which is part of the ozone water treatment system. When passed through the magnetic field, the water acts to dissolve and remove built up scale from the coils. The magnetic water conditioner also helps prevent additional scale once the coils are clean.
Scale forms on coil surfaces in one of two methods. It is either plated directly to the bare metal surface or, if the biological kill is insufficient, to the biological secretions adhered to the coil surfaces. These secretions are an organic glue (mucilage) that help bind the mineral deposits and insulate the coil surface, further reducing condenser capacity. Ozone prevents the mucilage buildup by keeping bacteria to a minimum and destroying existing mucilage. Because scaling is controlled, there is no longer a need to blowdown as much water as with chemical systems. Reducing blowdown and increasing the number of cycles of concentration will increase the water's mineral content beyond the saturation point. When this occurs, the excess mineral content will tend to crystallize out of the water solution. The ozone treatment system's separating device will remove it from the system.
An ozone-based water treatment system continuously removes sediment and sludge using a centrifugal separator. The separator removes mineral granules as well as other dead organic materials and flushes them to the drain. This automatic sediment removal keeps the sump clean and increases the time required between scheduled maintenance procedures. An alternative to liquid/particle separation is regularly scheduled evaporative condenser shutdown and manual removal of the accumulated sediment and sludge.
Ozone Water Treatment System ComponentsAs a biocide, ozone will provide bacteria-free water and eliminate slime and algae. It is a strong, active, organic material oxidizer that can readily kill bacteria and viruses and destroy previously deposited organic materials. It is used extensively as a disinfectant in bottled water, as a bacterial disinfectant in drinking water supplies and as a disinfectant in large swimming pools.
However, treating evaporative condenser and cooling tower water involves more than just removing bacteria, viruses and organic material. Scale, corrosion, sediment and sludge, and the hazards associated with chemical handling must be addressed. To accomplish these tasks, a complete ozone water treatment system should incorporate:
- Magnetic water conditioner to prevent scale.
- Ozone-generating equipment to kill bacteria, algae and slime and to help control scale and corrosion.
- Separation device to remove silt, sludge and granular mineral particles.
- Control system to automatically regulate the rate of ozone introduction.
Ozone water treatment systems provide an environmentally friendly method that can lengthen condenser life, reduce maintenance and cleaning costs, and permit the condenser to operate at optimum conditions year round.