by Tom Worley — An energy efficient HVAC system, combined with a well-run preventative maintenance program for heating and cooling operations, provide energy efficiency, equipment longevity and lower utility bills. A building that is too warm or cold, or an electricity bill that is too high, is bad business and creates questions from both management and tenants.
Many people overlook the role a clean cooling tower plays in the performance and energy efficiency of a building’s cooling system. In a typical building, the HVAC system accounts for about 45 percent of utility costs, with the chiller being the single largest consumer of electricity. Since chillers use about five times the energy of the cooling tower, more attention is usually paid to the chiller. However, maintaining a clean cooling tower is equally important in the energy efficiency equation.
Since cooling towers are efficient air washers, they require regular cleaning to remove dust, pollen, insects and other debris captured as the tower water falls through the air stream. These contaminants are carried through the tower fill and tend to settle in the basin where they contribute to an array of problems.
Basin deposits provide an ideal environment for the growth of microorganisms that can cause severe under-deposit corrosion and even premature basin failure. These deposits can result in serious corrosion with visible rust spots in mere months with holes appearing shortly thereafter. Basin deposits also increase water treatment costs by absorbing the treatment chemicals being used in the tower, thereby making the treatment program less effective. A dirty sump can also contribute to scale deposits, algae and/or microorganisms actively growing in the system. Some microorganisms can pose a health risk to maintenance personnel and passersby, especially if released from the tower in the form of aerosols.
Cleaner Equals Energy Efficiency
Proper chemical treatment of the tower water will keep dissolved solids, such as calcium carbonate, in solution and moving through the cooling tower and the chiller condenser bundle without attaching to the metals to create scale deposits. However, where spray of the water and air meet, the solids will dry and form deposits, normally to the outside edges of the fill, mist eliminators and/or inlet air louvers. As the deposits grow they can restrict and/or divert water and air flow and become counterproductive to the natural evaporation of the cooling tower. These deposits should be removed often to keep the tower system operating efficiently.
Cooling towers that have plugged, missing or improperly operating nozzles and/or are dirty do not cool water as efficiently as clean properly-functioning cooling towers. Proper water distribution and spray patterns are vital to cooling tower effectiveness and, ultimately, to the product being cooled.
For example, a fluid cooler, which is very common in some applications, will have a closed loop heat exchanger cooled by the water sprayed from the cooling tower water distribution system. If for any reason the distribution of air and/or water is not properly functioning, the heat exchanger temperature will rise reducing the efficiency of the equipment being cooled. Fouled air inlet louvers, fill and spray nozzles also reduce heat transfer and require tower fans to operate longer to provide the required condenser water temperature. Clogged suction strainers reduce condenser water flow and chiller capacity.
Chiller efficiency is reduced if the design condenser water temperature cannot be supplied. For every 1F the condenser water supply temperature rises above design, chiller efficiency is reduced by 2 percent. To compound the problem, accumulated tower debris delivered to the chiller with the condenser water contributes to condenser fouling, which further reduces chiller efficiency.
Designing a Maintenance Program for Cooling Towers
Routine cooling tower cleaning and chlorination is important preventative maintenance to avoid these problems. Most cooling tower manufacturers recommend that cleaning and chlorination be done at least twice a year. However, each facility engineer should design a maintenance program for his or her facility that will insure proper care of the associated equipment. If the cooling tower is properly cleaned at the correct intervals, the benefits are multi-fold, saving facility dollars in energy usage, repair costs and premature equipment replacement costs.
Proper cleaning protocol involves:
- Chlorination to achieve an effective residual
- Cleaning hot decks spray nozzles and orifices assuring proper water distribution
- Application of a cleaner to remove louver and fill deposits
- Washing down tower surfaces and basin with a high-pressure washer
- Vacuuming the tower sump to remove all deposits and sediment of any kind
- Proper disposal of debris
Cooling tower cleaning is often neglected or performed improperly. It is a labor intensive process that requires expertise and training to accomplish correctly. Specialized equipment and procedures are necessary to effectively perform the work. The chlorination procedure, which helps reduce risks from waterborne pathogens, requires careful application and control. The louver and fill cleaning procedure, which helps maintain good air flow and heat transfer in the tower, requires specialized cleaners and application equipment.
Workers must wear the proper personal protective equipment (PPE) to avoid chemical and microbiological hazards. Depending on the tower design and location, specialized equipment may be required to safely access spray nozzles, air inlet louvers and fill.
Many building owners do not have the in-house resources to properly perform tower cleanings and choose to contract with an outside company for this work. Using the services of a qualified cooling tower cleaning company provides assurance that the work is performed safely and industry recognized procedures are followed. Documentation should be provided outlining the cleaning and chlorination procedures and the results of each job. When considering a cooling tower cleaning company, keep in mind some contractors only physically clean the basin and their services do not include chlorination or louver/fill cleaning.
Routine cooling tower cleaning and chlorination is an important, cost-effective preventative maintenance practice that helps reduce energy costs by keeping cooling towers and chillers operating at peak efficiency. Clean cooling towers also last longer and operate more reliably. While often overlooked by operations managers, a maintenance routine for these systems is an investment that will pay off many times over in terms of energy efficiency, cost savings and building safety.
Tom Worley is an operations manager for Chem-Aqua Services. He can be reached tom.worley@ach.com.