That cube law we often bang on about in these pages makes the electric motor-driven fan one of the best candidates for variable speed control. But despite this simple relationship between fan speed and energy consumption, which offers so much potential in terms of energy savings, there are still many, many fans out there running at full speed, controlled only by mechanical damping systems. Well, the message is not completely falling on deaf ears, as this example from Dairy Crest's Severnside plant illustrates
Dairy Crest Severnside in Stonehouse, Gloucestershire is one of the most modern milk processing sites in the UK, producing fresh milk, butter, cream, buttermilk powder, skimmed milk concentrates and powder. The site is a very big consumer of steam, the majority of which is used to supply heat to the various dairy processes. Running these steam boilers contributes a major proportion to the site's £1.8m annual utilities bill.
In 2000, the company became a participant in the negotiated Climate Change Energy Agreement and, since then, has invested in improved boiler control among other energy-saving measures. One opportunity for saving energy was identified in the low pressure (100psi) boiler. Rated at 4,230kg/h of steam, the unit was supplying an average of 2,200kg/h - a little over half its rated capacity. Combustion control was effected by mechanically damping the 11kW forced draught fan, which was proving inaccurate and wasteful.
Electrical engineering manager, Craig Pepworth initially considered reducing the size of the boiler burner to match the steam load, which would have allowed a reduction in the size of the forced draught fan motor. However, it was likely that process steam demand would rise at some future point with the introduction of new products, effectively ruling out any reduction in boiler capacity.
In consultation with the boiler manufacturer, Saake, a more efficient means of control was conceived. This would take the form of an electronic fuel control system and variable speed fan drive - simple retrofits that would provide full optimisation through improved control of the fuel/air ratio. A Danfoss VLT6000 drive was chosen for this application. The drive speed is regulated by a 4-20mA control signal from the direct digital combustion controller (DDCC), and the drive, in turn, provides a feedback signal to the DDCC. In addition the electrical savings, the elimination of mechanical control linkages reduced overall maintenance, while fan noise was significantly reduced.
Payback on the variable speed control investment was initially calculated at less than one year. To substantiate this, initial measurements were taken on the fan under mechanical damper control over a full year of operation. During 264 days running, the measured power developed by the 11kW motor was reasonably constant at about 9.75kW. The motor consumed 61,776kWh over the period, representing a running cost of £2,471/year (at 4p per unit).
Energy measurements were taken following the retrofit of the new fuel system and Danfoss VLT6000 drive. Average power consumed had reduced to 2.30kW at 32Hz running frequency. Averaging this figure over a typical year of operation gives an annual energy consumption of 14,572kWh and costs of £582 - an annual saving or £1,889. The total installed cost of the drive was £1,400, giving a return on investment of just nine months. Significantly, this measure alone resulted in an equivalent annual CO2 reduction of 20,294kg. Graig Pepworth is very satisfied with the outcome:
"I installed the Danfoss drive myself and found it easy to set up. As a result, I have installed many similar drives on other plant in the factory. In addition to saving us money, the drive has made my life a little easier through reduced boiler maintenance. This modification has made a valuable contribution towards our commitment to energy savings under our Climate Change Levy negotiated agreement. We are aware that we can claim valuable tax breaks from fitting energy saving equipment and, as a company, intend take advantage of this benefit in the future."