Open-circuit or direct contact cooling towers are the most common type, where water is supplied to the tower after gathering heat, and sprinkled on the upper side while a fan establishes an upward airflow. This causes part of the water to evaporate, lowering its overall temperature. Although this method is effective, it comes with several limitations:
Extra water must be supplied to compensate for evaporation losses.
Since water is in direct contact with the atmosphere, it must be constantly treated to avoid the accumulation of particles and bacteria.
Closed-Circuit Cooling Towers with Adiabatic Cooling
A closed-circuit cooling tower is also based on the principle of evaporative cooling, but the water used by 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 sprinkled to achieve the evaporative cooling effect. When adiabatic cooling is deployed, most of the heat rejecting is achieved with only the fan; water sprinklers are controlled precisely so the amount of water supplied is exactly as needed to complement the natural cooling effect of air.
Adiabatic cooling offers various advantages over conventional open-circuit cooling towers:
Water is kept free from particles. This allows pumps, chillers and other system components to operate more efficiently, while increasing their service life.
Water treatment and maintenance costs are greatly reduced. The water volume that requires treatment is significantly lower than in open-circuit cooling towers.
In general, adiabatic cooling can reduce operating costs by up to 30 percent, compared with conventional open-circuit cooling towers.
Plate Heat Exchangers with Open-Circuit Cooling Towers
An alternative to adiabatic cooling is to keep the conventional cooling tower, but isolating it from the process or chiller plant with a plate heat exchanger. In simple terms, the heat exchanger separates the water circuit in 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.
This way, heat can be released outdoors without mixing both streams of water.
Both conventional cooling towers and adiabatic coolers use a fan to establish the required airflow. When the system is operating under 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 the supply voltage and frequency to reduce motor speed below its nameplate value.
In open discharge fans, like those of cooling towers, the power savings achieved are proportional to fan speed cubed. This means that a cooling tower fan operating at 80% speed would only be consuming 51.2% of its rated power (80% x 80% x 80%).
To improve energy savings further, a VFD can be used to control the water pump as well. Using a control system, the speed of both motors can be coordinated to match the cooling load precisely, minimizing overall energy consumption.
Waterside Economizer: Heat Exchanger Between Water Circuits in a Chiller Plant
In the case of chiller plants, it is possible to increase overall efficiency by installing another heat exchanger, but in this case between the condenser water circuit and chilled water circuit. This type of heat exchanger is called a waterside economizer, and it may allow removal of heat from the chilled water circuit before it reaches the chiller, which must only provide the extra cooling effect that isn’t possible with the heat exchanger.
It is important to note that this approach is only possible under certain weather conditions, and its effectiveness is diminished as relative humidity increases. For example, a waterside economizer is a much better investment in New York City than in South Florida.
NEMA Premium Motors
Fan and pump motors are responsible for most of the energy consumption in cooling towers and their respective water circuits, and there are plenty of applications where they operate for 24 hours a day. Therefore, it is possible to achieve significant energy savings by replacing these motors with higher-efficiency models.
Motors in the USA have three efficiency levels, where NEMA Premium indicates the highest performance. For example, the average efficiency levels of a 460 V, 15 HP, 1800 rpm, totally-enclosed fan-cooled motor are be the following, according to NEMA:
Standard Efficiency: 86.6%
Energy Efficient: 91.1%
Premium Efficiency: 92.4%
In this case, upgrading from standard to premium efficiency would represent an increase of 5.8 percent. Yearly savings increase in proportion to the total hours a motor is used each year, which is typically high in the case of cooling towers.
Cooling towers play a very important role in both commercial and industrial settings. In large commercial facilities, water-cooled chillers are far superior to their air-cooled counterparts for air conditioning; although air-cooled chillers don’t need a cooling tower, the energy efficiency advantage of water-cooled chillers compensates the extra cost. In industrial settings, water is a practical and highly available cooling medium, and cooling towers are a cost-effective way of removing heat from water for recirculation.
Complementing a cooling tower with a heat exchanger simplifies maintenance by isolating cooling water from the environment, and adiabatic coolers offer this plus enhanced 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, consulting and supervision ensures that cooling towers and other building system are specified correctly, while considering energy efficiency and total cost of ownership.
Editors Note: This post was originally published in December 2016 and has been revamped and updated for accuracy and comprehensiveness.
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