Cooling Tower System Replacement
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Open-loop or direct-contact cooling towers are the most common type, where water is supplied to the tower after heat collection and sprayed from the top while a fan establishes an upward airflow. This causes some of the water to evaporate, lowering its overall temperature. Although this method is effective, it has several limitations:
- Extra water must be supplied to compensate for evaporation losses.
- As water is in direct contact with the atmosphere, it must be constantly treated to prevent the accumulation of particles and bacteria.
Although open-loop cooling towers are simple, they are not the most efficient option available: there are alternatives that offer greater energy efficiency and simpler maintenance. The cooling tower and associated water pump typically account for about 15% of energy consumption in chilled water systems. Therefore, any upgrades that increase your energy efficiency offer an attractive financial return.
Closed-loop cooling towers with adiabatic cooling
A closed-loop cooling tower is also based on the principle of evaporative cooling, but the water used by the processes or chillers is never in direct contact with the atmosphere; instead, it circulates through a coil inside the cooling tower and water from a separate supply is sprayed in to achieve the evaporative cooling effect. When adiabatic cooling is implemented, most of the heat rejection is achieved from the fan alone; The water sprinklers are precisely controlled so that the amount of water supplied is exactly what is needed to complement the natural cooling effect of the air.
Adiabatic cooling offers several advantages over conventional open-loop cooling towers:
- The water is kept free of particles. This allows pumps, chillers and other system components to operate more efficiently, while increasing their useful life.
- Water treatment and maintenance costs are greatly reduced. The volume of water that requires treatment is significantly lower than in open circuit cooling towers.
- Intermittent use of sprinklers saves water and energy.
In general, adiabatic cooling can reduce operating costs by up to 30% compared to conventional open-loop cooling towers.
Plate heat exchangers with open circuit cooling towers
An alternative to adiabatic cooling is to maintain the conventional cooling tower, but isolate it from the process or cooling plant with a plate heat exchanger. In simple terms, the heat exchanger separates the water circuit into two segments:
- In the first segment, water circulates between the heat exchanger and the process or chiller from which heat is removed.
- In the second segment, water moves between the cooling tower and the heat exchanger.
- In this way, heat can be released to the outside without mixing the two water streams.
As the heat exchanger already isolates the cooling tower from the rest of the system, a conventional open circuit tower can be used. In fact, combining a closed-loop cooling tower and heat exchanger is redundant and a waste of money.
Additional Improvements in Cooling Tower Systems
Cooling towers can be upgraded to higher performance systems, but individual system components can also be upgraded to further improve energy efficiency and operational flexibility.
Speed control for fans and pumps
Both conventional cooling towers and adiabatic coolers use a fan to establish the required airflow. When the system is operating at part load conditions, fan speed can be reduced to achieve significant energy savings. This is accomplished with a variable frequency drive (VFD), a device that adjusts supply voltage and frequency to reduce motor speed below the nameplate value.
In open discharge fans, such as those in cooling towers, the energy savings achieved are proportional to the fan speed cubed. This means that a cooling tower fan operating at 80% speed would consume only 51.2% of its rated power (80% x 80% x 80%).
To further improve energy savings, a VFD can also be used to control the water pump. Using a control system, the speed of both motors can be coordinated to precisely match the cooling load, minimizing overall energy consumption.
Waterside economizer: Heat exchanger between water circuits in a cooling plant
In the case of refrigeration plants, it is possible to increase overall efficiency by installing another heat exchanger, but in this case between the condenser water circuit and the chilled water circuit. This type of heat exchanger is called a waterside economizer and can allow heat to be removed from the chilled water circuit before it reaches the chiller, which should provide just the extra cooling effect that is not possible with the heat exchanger.
It is important to note that this approach is only possible under certain weather conditions, and its effectiveness decreases as relative humidity increases. For example, a waterfront saver is a much better investment in New York City than in South Florida.
Premium NEMA Motors
Fan and pump motors are responsible for the majority of energy consumption in cooling towers and their water circuits, and there are numerous applications where they operate 24 hours a day. Therefore, it is possible to obtain significant energy savings by replacing these motors with higher efficiency models.
Motors in the US have three efficiency levels, where NEMA Premium indicates the highest performance. For example, the average efficiency levels of a 460 V, 15 HP, 1,800 rpm fully enclosed fan-cooled motor are as follows, according to NEMA:
- Standard Efficiency: 86.6%
- Energy Efficiency: 91.1%
- Premium Efficiency: 92.4%
In this case, upgrading from standard efficiency to premium efficiency would represent an increase of 5.8%. Annual savings increase in proportion to the total hours of engine use each year, which is typically high for cooling towers.
Final Observations
Cooling towers play a very important role in commercial and industrial environments. In large commercial facilities, water-cooled chillers are far superior to their air-cooled counterparts for air conditioning; Although air-cooled chillers do not require a cooling tower, the energy efficiency advantage of water-cooled chillers is worth the extra cost. In industrial settings, water is a practical and highly available cooling medium, and cooling towers are an economical way to remove heat from water for recirculation.
Complementing a cooling tower with a heat exchanger simplifies maintenance by isolating the cooling water from the environment, and adiabatic coolers offer this as well as greater energy efficiency. If such an upgrade is complemented with high-efficiency motors and VFDs for speed control, maximum system efficiency can be achieved.
High performance is easier to achieve when HVAC systems are first installed and can be more expensive for existing installations. Hiring a qualified engineering firm for design, consultancy and supervision ensures that cooling towers and other building systems are specified correctly, considering energy efficiency and total cost of ownership.
Editor's Note: This post was originally published in December 2016 and has been reworked and updated for accuracy and comprehensiveness.
