The latest generation of variable speed–drive models for fans are able to maximize tower cooling performance and reduce maintenance costs in large power plants.
This is the case of Emerson´s Control Techniques Unidrive M700 variable speed–drives with real-time Ethernet capability. They have been installed in a thermal power plant located in the middle of the Atlantic, in United States waters. The plant has multiple gas and oil units and they wanted to install an automated control system for the 22 fans of its cooling towers in order to replace the previous system, dating back to 1975.
Through two towers, the fan speeds can be varied to minimize power use in low load units; eliminate the beating of the wind due to the lack of a braking system; minimizing potential freezing in winter or attenuating fan vibration without having to withdraw it from service.
In addition, the use of Emerson’s Control Techniques Unidrive M700 variable speed-drives eliminates periodic blade pitch adjustments needed to avoid engine overloads in winter and low air currents in summer. Only the blades pitch adjustments themselves result in significant savings of man-hours estimated for the plant at some $ 38,000 per year.
Efficiency and saving
The issue before was that managing the use of energy and the cooling capacity meant changing the fan starters from 200 hp intermittently from on to off at full speed. Now, Emerson says in a statement, the ability to vary fan speed and therefore the cooling rate for different loads allows all fans to operate (better tower efficiency) with much lower power requirements.
That is, the system only moves the air when necessary to extract the heat, resulting in much less energy consumption.
On the other hand, managing the vibration in individual fans required with the old system that they be turned off, even when the load demand and hence the cooling demand was high. With the new system, the ability to keep all fans running, with some at reduced speed to manage vibration, provides optimum cooling when needed for the unit generation.
Asset management and protection
The fans have a fiberglass / metal transmission shaft of almost 5 meters length that connects the gearbox and the fan engine. Without a braking system, beating of idle fans was rather a constant problem that sometimes resulted in poor starting scenarios that caused engine overload, severe torque on the drive shaft and sometimes problems with the gearbox.
With the new system, soft start and pre-start braking are inherent in the capabilities of the M700 VSD. Adding soft start and fan braking minimizes the torque peaks that the drive shaft experiences during fan starts and eliminates the concerns of wind beating. The engines never experience the high current applied by an engine through the starter line.
Also freezing and thawing were a problem in cooling towers with the old system. The installed system needed the fans to countermarch if the ice formed on the sides of the towers, in an attempt to defrost the heavy load on the tower intakes. This can have very damaging effects, including tower collapse.
But if the airflow is managed properly in a cold climate using the variable speed, as the new units do, the sub-cooling which in turn causes the formation of ice can be eliminated. This eliminates the need to make fans countermarch, except in the most adverse weather conditions, explains Emerson.