A healthy cooling tower
Cooling towers are essential to a building’s operation; however, there are many that suffer from years of neglect. It’s a facility manager’s responsibility to maintain the health of their cooling tower, and ARIAN BAHRAMSARI tells us how.
What are cooling towers and how do they work? Essentially, cooling towers are heat rejection devices that circulate water through mechanical equipment to collect a facility’s heat. The hot water is then pumped into the cooling tower to reduce its temperature.
Basically, the purpose of a cooling tower is to lower the temperature of the water pumped from a heat source. The hot water is sprayed down the tower over a material called ‘fill’ to increase the time it takes for the water and the air to cross paths. At the same time, air is drawn to the top of the cooling tower by a fan and travels upward through the wet deck fill opposite the water flow. The cooled water drains to the basin at the bottom of the cooling tower and is recirculated to the heat source.
Cooling towers can serve different purposes depending on the site requirements, the available space, the climate and the environment. Two of the most common types of cooling towers in HVAC (heating, ventilation and air-conditioning) are ‘induced draft’ towers (air is pulled) and ‘forced draft evaporative’ towers (air is pushed). Where water is a more valuable resource than energy, dry cooling towers or air-cooled systems come into play. In dusty or dirty environments, cooling towers with closed loop heat exchangers are used (although less efficient) to avoid contaminating the water.
COOLING TOWER CHECKS
Cooling towers are a significant part of each facility and must be checked on a regular basis. Apart from mandatory requirements on cooling towers to be controlled, a consistent maintenance plan can minimise downtime and increase the efficiency in cooling towers.
LEGIONELLA OUTBREAK CHECKS
Typically, untreated cooling towers represent the perfect breeding ground for legionella bacteria, which can be a serious public
health concern as the virus can cause severe respiratory infections and illnesses in humans.
The solution is to treat the cooling tower continuously with chemicals and other agents to minimise scale formation, corrosion and fouling with a bio-dispersant. It’s worth mentioning that bio-dispersants need to be non-corrosive and compatible with other chemicals used.
Algae and biological growths flourish when exposed to sunlight and moisture. A physical check of the cooling tower should confirm that its sides and basin are protected against direct sunlight.
The solution to this is to install sunlight protection to the structure of the tower.
SHUTDOWN TIME RISK EVALUATION
Generally, cooling towers that are shut down during winter or intermittently used have a higher potential for health risks, as idle systems can create stagnant water and encourage legionella growth.
The solution is to implement a recirculating pump that can be equipped with a timer. This pump can automatically run water through the system at regular intervals.
REMOVING AND ACTIVATING ‘DEAD LEGS’
‘Dead legs’ are pipes filled with water that have no or low flow through them and are longer than the internal diameter of the main pipe. Dead pipes are associated with legionella and stagnant water problems. The solution is either to remove or to activate the dead pipes, as they need to be eliminated from the pipe network in the cooling tower. In cases where removal is not an option, they can be activated by connecting them out to another pipe or pump.
CORROSION CONTROL PLAN
Corrosion is one of the main potential threats to cooling towers. In addition to its effect on a cooling tower’s efficiency, the iron released from corrosion can encourage legionella to grow.
The solution is to add an anti-corrosion plan to the cooling tower maintenance plan. This can be from adding an extra coating
on key parts of the cooling tower to choosing the right bio-dispersant with minimum corrosion effects.
FLOAT VALVES AND OVERFLOW
Float valves need to be checked regularly to ensure they are not set too high. This adjustment is required to balance a cooling tower’s operation with its make-up line and ensure that no water is wasted through overflow during a pump shutdown.
AUTOMATED WATER TREATMENT CONTROLLER Remote controllers enable facility managers to monitor their cooling tower’s parameters and they can also be fitted with automated dosing devices. By installing this device, the quality of the cooling tower water can be continually monitored and a facility manager can be dispatched when on-site resolution is required.
EVALUATING VIBRATION, ALIGNMENT AND NOISE LEVEL Cooling towers can be subject to a lot of vibration and misalignment, and can create
a community noise problem. There are a few checks that can be performed on the vibration and sound levels via laser alignment equipment. According to Herbert W Stanford’s HVAC Water Chillers and Cooling Towers:
Fundamentals, Application and Operation, cooling towers normally have an anticipated noise level of 65 to 85 decibels at 50 feet (15 metres) away from the tower. Depending on the facility’s requirements, this noise level can be controlled and minimised; for example, replacing the fans with centrifugal fans or adding vibration isolators to support the tower are some measures that can reduce the noise level.
COOLING TOWERS’ STRUCTURE ACCESS
In addition to the aforementioned checks, cooling towers need to be checked for easy and safe access, as well as having clear labels and signs on their structures.
THE BOTTOM LINE
The Victorian requirements for cooling towers are defined within the Public Health and Well- being Act 2008 and the Public Health and Well-being Regulations 2009.
Facility managers need to ensure that regular compliance is met and there is a developed and comprehensive risk management plan for a cooling tower’s cleaning and maintenance. Periodic maintenance can prevent bacterial build-up and keep the tower running at peak performance at all times. ●
Arian Bahramsari is a facility manager at Facility Management Victoria, based in Docklands, Melbourne.
This article also appears in the August/September issue of Facility Management magazine.
Lead image: 123RFs Korn Vitthayanukarun © 123RF