Not so much 'suck it and see' as 'pump it and see'
30 July 2004, News release from UMIST
News release: They say that the proof of the pudding is in the eating and having benefited from previous successful applications of Danfoss drives to boiler and cold water feed pumps, UMIST has taken a novel approach to assessing the potential savings from future drive applications under consideration.
As most of the UMIST fan & pump motors fall in the range of 4 to 7.5kW, they have constructed a mobile trials rig equipped with a 7.5kW Danfoss VLT 6000 series inverter, fitted with flying leads and a pressure transducer. This rig is easily connected up to any motor to be tested anywhere on the campus and the inverter programmed to that particular motor for optimum efficiency.
During the trials, electrical consumption is noted at full speed, with the drive's energy optimisation disabled and then again with the drive's energy optimisation enabled. Open-loop speed control tests are then conducted, manually varying the system characteristics until the optimum control point is identified. Finally the pressure transducer is connected and the whole system tuned for satisfactory performance in closed-loop mode. Energy consumption is noted at each stage and from these data, the viability of variable speed control and the potential financial payback period can be established accurately.
Over the past three years, three boiler feed pumps have returned substantial savings following conversion to variable speed control. Boilers 2 and 3 have had their discharge pressure reduced from 18 to 12.5 bar with the boiler operating at full load.
Monitored over this time, the 15kW motors on these pumps have averaged 7.8kW and 6.8kW while the 5.5kW motor on boiler 1 has averaged 4.3kW consumption.
More spectacularly, the 7.5kW cold water booster pump feeding the Academic building, has seen its 5.6kW consumption reduced to 1.3kW after retrofitting a Danfoss drive controlled by a pressure transducer. Similarly, the booster pump feeding the student residence has shown a reduction from 3.6kW to 1.3kW following closed-loop control from an inverter drive. A significant additional benefit of the improved control is that daily water consumption has been reduced by 10,000 litres per day.
The combustion fan on the steam package boiler was also modified to inverter control. Following a change to dual gas / diesel fuel operation, the savings on the fuel tariffs paid for an upgrade to Direct Digital Control with inverter control of the 22kW forced draft fan. The improvement in fan speed control enabled the turndown ratio of the boiler to be doubled to 10:1 with improved combustion control during low boiler loads at off-peak periods. Average recorded consumption has been 5.25kW over 1650 hours but at off-peak periods, fan motor consumption is reduced to as low as 1.25kW. Again, there has been a significant additional benefit in that fuel savings of around 6% - 7% have been achieved.
UMIST's experience demonstrates clearly that in many cases, fans and pumps on boilers and booster pumps are substantially oversized from the outset and can return significant savings when a speed control system is employed, even on relatively small size motors.
For further information please contact UMIST