More than ever, commercial building owners and industrial process users need to reduce their maintenance costs and achieve energy savings. Recent innovations in motor and drive technology are making this a reality for commercial and industrial cooling tower applications. A new technology uses a variable frequency drive (VFD) and a low-profile, permanent magnet (PM), direct-drive motor that replaces all the mechanical components—such as gearboxes, drive shafts, disc couplings and existing motors.
The removal of these mechanical components also eliminates mechanical energy losses that decrease overall system energy demands. Higher motor efficiency gains are achieved with the PM technology compared to standard induction motor efficiencies found in a traditional system. It is also environmentally friendly because it eliminates the gearbox, which may eventually leak oil into the water, contaminating the environment.
Cooling Tower Technology
To fully appreciate the benefits of this technology, understanding cooling tower applications is necessary. The two types of cooling towers are typically cross-flow towers and counter-flow towers. These are defined by the direction of air passed over the wastewater.
A cooling tower is a structure that extracts waste heat from a process and distributes it to the atmosphere. The most common method is to let heated water fall through a moving air stream created by a fan that is located at the top of the tower. This evaporation takes a large amount of heat from the process. The heated water is distributed over a “fill” material and is cooled as it descends through the fill. The cooled water is then collected in a cold water basin below the fill and pumped back through the process to absorb more heat.
The size of a tower is commonly identified by the diameter of the fan. Fan sizes range from 6 to 40 feet, with the most common applications in the 10- to 26-foot range. The speed of the fan is limited by industry standards for stressing, which are typically rated as a maximum fan tip speed of 12,000 feet per minute. This maximum tip speed generates a fan speed in the range of 147 to 382 rpm. The most common solution for driving the fan in current cooling tower designs uses a standard National Electrical Manufacturers Association induction motor, drive shaft, disc couplings and gearbox arrangement.
Energy Savings Potential
Cooling tower applications follow fan affinity laws which state that horsepower varies by the cube of the fan speed. To put this in perspective, if an application requires a 40 horsepower load at full speed, but the speed could be reduced to 50 percent because of lower heat load requirements, the application needs would only be 5 horsepower or only 12.5 percent of rated full load requirements. This reflects a great reduction of energy consumption.
The use of VFDs on new construction has become much more commonplace in recent years because of the energy savings associated with the fan affinity laws. Additionally, most towers being upgraded or refurbished are also being equipped with VFDs. These drives have the advantage of a soft mechanical start, which means no large starting current draw, and they offer the ability to run the fan at any desired speed from zero to the maximum design speed for the application. The energy savings realized by using a VFD are well recognized and documented and have been proven to save between 37 percent and 47 percent in energy costs when compared to applications without VFDs.
Historically, the mechanical components of the fan drive system—specifically the right angle gearbox—have been the largest maintenance issue for cooling tower installations. Gearbox failures, oil leaks, oil contamination, failed drive shafts, misaligned drive shafts and excessive vibration are all significant problems related to this type fan drive system.
The features and benefits of the direct-drive cooling tower technology have been field proven and deliver the following advantages: