Steel Mill

$60,000 per year energy cost savings was achieved by Variable Speed Drive and logic control on the cooling tower fans.


BASELINE
The Steel plant's Rolling Mill Cooling Tower reduces the nominal 70°F water returning from the mill to 60°F. Originally, this tower had three 75 HP single speed fans that operated around the clock to assist the evaporative cooling process. Fan control was limited to "on" or "off," so water temperatures were often cooler than necessary and energy was wasted.


RECOMMENDATION
Energy Savings can be achieved with Variable speed drives on the fans controlled by the exiting water temperature.


ENERGY SAVINGS
Taking advantage of fan affinity laws (fan speed is proportional to the cube root of the power), great energy savings is realized by reducing the speeds to what is necessary to maintain cold well water temperature. The steel mill's logic control system uses the water temperature to drive the speed of a designated "trim fan." The result is a cooling tower system that can nominally maintain the required temperature with one base load fan at 60 Hz, a second trimming around 30 Hz and a third completely off. Annualized, the energy savings realized was 602,915 kWh/year or about $60,000.


Other energy saving opportunities: At a refinery, a similar cooling tower system achieved great energy savings through the installation of oversized process water pipes that nearly eliminated internal surface friction loss. Energy cost is indirectly related to the fifth power of the pipe's diameter, so increasing the pipe's size, even by a small margin, can have a great positive effect on the system's pumping power cost. The refinery's oversized piping cooling tower system enables an energy savings of 4.1 million kWh per year, or roughly $410k ($0.10 per kWh).