FEB 12, 2017
The Power Station is now using Control Techniques Unidrive M700 variable speed drives to control 22 cooling tower fans across two towers. Replacing a system that was first installed in 1975, the advanced new inverter drive brought many benefits. It means that the fan speeds can be varied to minimize power usage at low unit loads, eliminate wind milling due to a lack of a braking system, minimize freezing potential in winter, mitigate fan vibration without removing from service, and eliminate seasonal blade pitch adjustments required to prevent motor overloads in the winter and low air delivery in the summer. Just the blade pitch adjustment itself resulted in a significant saving in man hours, which the power station estimates to be worth $38,000 per year.
The power station, located in the mid-Atlantic region of the USA, has multiple units firing gas and oil. The power station was keen to implement an automated control system for its cooling fans, which were served by a system that was more than 40 years old. The retrofit addressed these issues directly:
Energy and cost reduction without impacting tower efficiency
Asset management and protection
“We decided to look at variable frequency drive (VFD) solutions and three manufacturers were initially considered,” says Mark Leigh, technical consultant for the power company. “However, Control Techniques was the only organization that could meet the very short project delivery times we required. They were also very competent technically in terms of recommending an optimized solution.”
Control Techniques Unidrive M700 AC drives with real-time Ethernet capability were installed in a motor control room at the power station, where it is now used to control 22 cooling tower fans. The drives deliver high performance and flexible control of induction and permanent magnet motors. Most importantly for the station, the new Unidrive M700 has more control of the process.
“In a standard across-the-line starter fan motor, the variables are fan blade pitch and air density with a fixed maximum HP at a fixed speed, which was 1750 rpm in this case,” explains Mr Leigh. “In contrast, the variables in a VFD-controlled fan motor are speed and air density with a fixed maximum HP at a fixed fan blade pitch. It therefore costs nothing in man hours to adjust the speed, as this is done automatically by the VFD based on a 200 HP limit, which is the maximum rating of the motor and starter. We estimate a saving of around four men for three weeks, or about 480 man hours, which equates to approximately $38,000 per annum.”