For decades, cooling towers have provided an efficient means of cooling industrial process loops by providing the lowest operating temperatures available. At the same time, through the years, water-based cooling systems have evolved. Terminal units, chiller plant components, cooling towers and fluid coolers — as well as the controls that tie them together — have made strides in efficiency, controllability, heat transfer capability and water conservation. The end user, owner, installer and maintenance personnel all benefit.
Most, if not all, of these advancements reduce the maintenance required by cooling towers and closed-loop fluid coolers — and the associated costs in terms of dollars and equipment downtime.
“Many facets of tower maintenance are still best conducted manually and with a predetermined schedule, but automation and autonomy have found a place in cooling tower and closed-loop fluid cooler systems,” says Kevin Hetman, who operates Chesapeake Systems’ Mr. GoodTower division. “Mr. GoodTower’s function is to repair cooling tower systems more than to maintain them, so we’re very familiar with the fallout of ineffective maintenance. Suffice to say, if more elements in the maintenance process are handled automatically, the healthier the system will be.”
The bulk of all cooling tower maintenance tasks are aimed at controlling scale, corrosion and microbial growth within various parts of the system. Eliminating or minimizing these concerns reduces the time and effort required to maintain nearly all the components of a cooling tower.
Advancements in cooling tower maintenance can be broken into these key subcategories:
- Routine media replacement (if applicable).
- Checking structural integrity, system function diagnostics, water treatment and automated controls.
- In some cases, reducing wet cooling operation via hybrid tower design.
Implementing smart solutions can ease maintenance headaches.
Reducing Manual Water Treatment
The level of maintenance that a specific cooling tower requires is largely determined by the quality of makeup water available at the site and the manner in which system fluids are treated.
“High hardness and alkalinity are the primary water quality concerns,” says Brandon Punt, sales manager at Jaytech Inc., a water-management solutions provider. “Microbial growth results from exposure to the atmosphere if water treatment isn’t handled correctly. Proper, site-specific water treatment addresses all these factors.”
Recent advances mean that streamlined water treatment solutions for both makeup water and recirculating water are available. For instance, one pretreatment system uses capacitive deionization technology to reduce dissolved-ion concentration, lowering the makeup water’s conductivity prior to use in an evaporative cooling system. Engineered to improve water efficiency for evaporative cooling equipment, it has an integrated control panel.
Pretreating the raw makeup water can reduce ion concentration by 50 percent. This allows cycles of concentration to be safely doubled, reducing blowdown. Pretreatment also provides water savings and reduces the amount of treatment chemicals needed for the recirculating water. The system turns on automatically when the tower control system calls for makeup water.
Technicians work on bearing replacement.
Recirculating water treatment also can be improved where water quality at the site permits the use of nonchemical treatment. For instance, some systems utilize pulsed electric fields to address three water-quality challenges — hardness, alkalinity and microbial growth — typical of water-cooled systems. With this technology, recirculating water from the evaporative cooling system passes through a pulse chamber, where it is exposed to alternating high and low frequency electric fields. This exposure affects both the surface charge of small, suspended particles and free-floating microbial organisms found in cooling water. With such a system as the sole source of water treatment, all the logistics and labor associated with chemical treatment can be eliminated.
When chemical treatment is required, a solid-chemistry treatment system can provide a reliable alternative to liquid chemical systems while simplifying chemical water treatment. By using solid chemistry, the potential for liquid spills is reduced and shipping, handling and chemical storage costs are decreased. These systems can help simplify installation and commissioning, and they minimize the floor space required in the mechanical room for water treatment. The use of solid chemistry also reduces the risk of overfeeding chemicals. Liquid chemicals such as oxidizing biocides and acids can accelerate corrosion in the unit if overfed.
Beyond controlling water treatment systems, a building automation system (BAS) can be integrated with the cooling plant to provide maintenance benefits.
With multi-cell cooling towers operated at part-load conditions, one or more of the cells in the cooling tower operates while others remain off. If the order in which these cells come online is not rotated, great disparity in the runtime hours across the cells would occur. A BAS can be programmed to rotate the lead and lag role of any number of cells. Balancing operating time among cells simplifies preventive maintenance such as greasing bearings, checking belt tension and insulation testing of motors.
An autonomous function of the BAS is to recirculate water within the tower on predetermined intervals. Throughout the shoulder seasons, it is not uncommon for cooling towers to remain idle for days at a time. This increases the risk of microbial growth. Generally, it is suggested that an idle cooling tower recirculate for about one hour per day. This exposes the fluid within the tower to the water treatment system being used.
Often, a BAS incorporates other data-collection and automation functions such as water-level metering, activation of makeup water treatment components and monitoring of water conductivity. The blowdown of the cooling system is automated around the conductivity setpoint. All of this information can be accessed remotely by the owner or service provider to conduct maintenance.
Because the maintenance requirements of a water-cooled system are determined largely by water quality and treatment, using less water will inherently mean less maintenance. That, along with water conservation, are the reasons some manufacturers have begun offering hybrid cooling solutions.
Technology advances have simplified cooling tower maintenance but unit inspection is still required.
Hybrid Cooling System Designs
If you have equipment that can provide wet and dry cooling, one of the benefits is minimizing scale buildup and reducing spray pump runtime hours. The basin can be drained over the winter when dry operation is sufficient to handle the cooling load. These hybrid systems remain in dry-cooling mode until the temperature setpoint can no longer be met. While in dry mode, no water is used. As a result, plume is eliminated as are water consumption and sewage expenses.
In some systems when operated in this mode, the process fluid enters one of two closed-loop, dry-cooler coils. These coils have a high density of fins for better heat transfer than a bare-tube coil. When the fan activates, air is drawn upward through the louvers and across the coils. As the air passes over the coil, a portion of the load is dissipated to the atmosphere via the tube walls and fins through sensible heat transfer. The warm process fluid then exits the first coil and enters the second coil, where the remaining load dissipates through sensible heat transfer.
When the temperature setpoint can no longer be met, the unit switches to evaporative (wet) mode, which utilizes both evaporative and dry cooling simultaneously to increase cooling capacity. Because the unit’s dry coil dissipates some of the load before system fluid reaches the wet coil, less water is used than in a conventional cooling tower design.
Mitigating vibration from a cooling tower’s moving parts also is a critical maintenance function that helps extend equipment life. The sources of vibration can include natural resonance, rotating components that are out of balance, faulty bearings, and misalignment of rotating components.
Rather than simply isolating the tower and components from vibration, a proper maintenance plan should seek to detect vibration, allowing maintenance personnel to fix the underlying issue before damage occurs. If left unchecked, vibration has the potential to shorten component life (bearings, fans) or cause catastrophic equipment failure. Vibration also can be transmitted through the structure.
Inspection of the water loading in the basin is a necessary task in any maintenance plan.
To detect vibration early, cut-out switches, which de-energize the fan motor at unacceptable vibration levels, should be installed. Monitoring devices can be used to measure vibration levels and report trends to the BAS. A common, effective method is to use general-purpose accelerometers secured to the fan motor and fan-shaft bearings.
In conclusion, as automation and autonomy proliferate across all sectors of the heating and cooling industry, large cooling tower and closed-loop fluid coolers have not missed the trend. Because of the advancements made to these critical pieces of equipment, cooling systems have become even more economical to own and operate. PC