When Aqua Cooling Solutions recently installed a 3MW cooling system with pumps controls based on Danfoss drives, the overall energy efficiency was already very satisfactory. However, as part of the installation, Aqua asked an engineer from Greenville Electrical to visit the site to see if there was any way efficiency could be improved even further. The engineer examined the system and, by making a small change to the drive configuration, further boosted the efficiency of the cooling system.
Aqua Cooling Solutions, the UK’s leading specialist in process cooling and associated products and services, designed, manufactured and supplied the 3 MW cooling system for a major manufacturer of food packaging products. The system includes two sets of pumps, one with two 45 kW pumps, the other with two 55 kW pumps. All of the pumps are controlled by Danfoss VLT® FC102 HVAC drives.
As originally configured, each of the pumps in a pair operated independently with closed-loop speed control and its own individual pressure transmitter. This arrangement was chosen so that if one of the pumps failed, the other could continue to operate and maintain the pressure in the system.
While this arrangement is perfectly satisfactory from an operational standpoint, it means that most of the time the pumps will be running at different speeds to maintain the required pressure.
An alternative is to operate the pumps in master-follower mode, where the drives for the two pumps are interconnected with one designated as the master and the other as the follower. With this mode of operation, the follower drive tracks the speed of the master, which means that both pumps run at the same speed and the speed of both is controlled simultaneously to deliver the required pressure.
The drive configuration suggested by the Greenville and Aqua Cooling engineers was that the master drive should, in normal circumstances, operate in closed-loop mode with its speed controlled by the signal from the pressure transmitter. Under these conditions, the follower drive operates in open-loop mode and tracks the speed of the master drive. If the master drive fails or is turned off for any reason, the follower drive automatically reverts to closed-loop operation with its speed controlled by the signal from the pressure transmitter. The modification was subsequently agreed and the drives were adjusted to the new configuration – a simple and inexpensive task that was quickly accomplished.
Energy Saving Results
The results were gratifying. Before the change, the pumps in the set with two 45 kW drives typically had pumps running at different speeds. The total load was around 58 kW. With the new configuration, both pumps typically operate at around 40 Hz, with a total load of around 45 kW. This is a saving of 13 kW, which equates to no less than 113,880 kWh per year, assuming that the pumps run continuously.
The results from pumps in the set with two 55 kW drives were equally impressive. Previously, these ran at different speeds, with a total load of 85 kW. After the change, they both now run at 42 Hz, with a total load of 65 kW. Here the saving is 20 kW, equivalent to 175,200 kWh per year.
In these days of high and ever-increasing energy costs, the savings are very significant, especially if taking into account the minimal time, effort and expenditure involved in re-configuring the drives to achieve them. As this story has amply demonstrated, fitting Danfoss drives is an excellent step toward energy efficiency; but seeking advice from Danfoss or an appointed Competence Centre about how to optimise that energy efficiency is also a wise move that can bring big benefits!