A 400 MW lignite fired power plant has two 6000 hp, constant speed ID Fans that were controlled by dampers on the inlet sides of the fans. Extensive testing, including gas flows and pressures, as well as electrical testing using power factor meters, provided a good “baseline” of the electrical power consumed for the fan operation at different flows and boiler conditions. Using the “test block” head – capacity plots and established fan laws, TRI made calculations as to the electrical power that would be consumed at the various load points of the test data. These calculations indicated that considerable power would be saved across the entire operating range of the fans, particularly at high loads and at low loads. Another part of the justification was that the heat exchanger for cooling the circuit oil in the fluid drives was to be located inside the Forced Draft Fan Room, thereby recovering all of the heat generated by the fluid drives in the slip process.
With this background, a project was undertaken wherein TRI designed and built two fluid drives, one for each fan, along with a single oil conditioning system for both fans.
This is a back to back fan arrangement, and the reinforced concrete foundations for the motors were extended to connect to each other, making the space to move the motors back so that the fluid drives could be installed between the motors and the fans. These fluid drives were made with impellers and runners with 70 inch working diameters. TRI is not aware of any fluid drive impeller/ runner size on the North American Continent that is as large or larger than these are. Due to the short space permitted, the fluid drives were built with rolling element bearings and with compact flexible couplings.
A unique characteristic of these fluid drives is that the flow of circuit oil into the element of each fluid drive is controlled in order to maintain a constant Circuit Oil Discharge (COD) Temperature. This maintains the entire fluid drive at a constant temperature of approximately 190° F and minimizes the circuit oil flow, especially at the upper end of the speed range and at the lower end of the speed range.
An unanticipated benefit was found to be this: This plant is located in a colder portion of the United States, and the ID fans are located outside. When they have been sitting for a period of time, they develop a bow and the rotors are cold. In the original configuration, when they were started, they went to full speed and had very high vibrations until the rotors warmed up and the bow relaxed. With the fluid drives, the motors and input portion of the fluid drives start and are up to speed within about 2 seconds, and then the output section of the fluid drive and the fan gradually roll up in speed. The fans can be held at low speed, so they can warm up permitting the bow to relax. Then, the fans can be brought up to speed in the operating speed range and put into service.
Before the fluid drives were installed, the fan rotors would be brought up to max speed, and if the rotor were bowed, the vibrations would be very intense. They could be felt in offices some 200 feet away. Now, with the fluid drives, the fan rotors remain balanced as they go up in speed, and the vibrations are low, even during the start-up mode. This helps the balancing process greatly, because the bow is removed at low speeds and any vibration at high speed is due to either actual mass unbalance of the fan or deposits on the fan wheels which need to be removed.