Prognosis is good
In the fall of 1998, BJC, the largest health care delivery network in the St. Louis metropolitan area, targeted increased energy efficiency and optimized control of its hvac equipment as areas that promised to provide significant savings.
Steadily escalating energy costs provided a sufficient incentive for BJC to initiate studies aimed at achieving an effective hvac system upgrade. Goals were to achieve maximum utility cost savings through efficient energy use and lower operating costs through installation of a state-of-the-art energy management system.
As the first step in its program, BJC solicited proposals for an hvac system upgrade for its Christian Northeast Hospital. In response, Siemens Building Technologies (SBT), formerly Landis-Staefa, was able to provide a turnkey solution, which in the first year of use has saved more than $17,000 in BJC’s Christian Northeast facility.
SBT’s solution was based on a state-of-the-art Siemens “Apogee” building automation system and centered on installation of two dozen MagneTek GPD Series ac adjustable-speed drives on the facility’s air-handling units.
Drives produce substantial savingsChristian Hospital’s hvac system, like many installed in the 1970s and 80s, used a constant-air-volume approach.
In a constant-air-volume system, fans and motors are sized to meet maximum load demand. Both run at constant maximum speed while air volume is modified by a variety of mechanical devices, including dampers and vanes, which throttle flow to various portions of the facility.
In a typical building, however, maximum capacity is required about 10% of the time; 90% of fan operating time requires only 40% to 70% of maximum volume.
At Christian Northeast, consistently applying 30% to 60% more energy to fan operation than was necessary was producing a significant waste of electricity and inflated utility bills. Adjustable-speed drives, on the other hand, regulate fan speed and air volume to match flow requirements, significantly reducing fan and motor energy consumption.
The adjustable-speed drive delivers its savings by controlling the motor and spinning the fan only fast enough to maintain the desired air volume. The drive’s ability to adjust system air volume by changing fan and motor speed and, therefore, power consumption, produces significant energy savings (typically 10% to 75%).
The savings are produced as a function of universally accepted centrifugal fan laws: System volume varies directly with fan speed. System pressure varies with the square of fan speed. Power consumption, however, varies with the cube of fan speed. Therefore, a small reduction in speed produces a significant reduction in power consumption.
In other words, the same system that requires 100% of full power to produce 100% of volume needs only 28% of full power to deliver 60% of volume. Obviously, significant savings are achieved by the drive’s ability to reduce fan speed during that 90% of the day when full system capability is not required.
As in most facilities, at Christian Northeast the vast majority of fan units are required to operate at only 60% to 80% of maximum speed most of the time, producing substantial energy savings over the constant-air-volume system previously used.
Based on results compiled from installation of the drives one year ago, Jim Nelson, Siemens project manager, has reported more than $17,000 a year in energy savings from the drive installations alone. (These savings are based on the local electrical cost basis of $0.05/kWh and do not factor in any potential impact of reduced utility demand charges.)
Adjustable-speed drive installationThe 24 GPD Series ac drives ranged in size from 7.5 to 75 hp. All were configured for fan-motor operation and installed to control power to both supply and return air fans.
All of the drives were built with integrally mounted bypass options to switch directly to “line” power in emergency situations and integrally mounted three-phase input line reactors to smooth line current fluctuations.
All drives also incorporated advanced transistor technology to reduce system noise and harmonics, and lower motor and blower noise levels for a quiet hospital environment.
Installation of the turnkey system required that all energy-saving components be compatible with the Siemens high-speed “Floor Level Network (FLN)” serial communication system. MagneTek achieved this by incorporating an FLN serial communication option card into each drive, so that the maintenance staff could access all drives and other intelligent system devices from any location to modify and monitor settings.
This high-speed network control system uses two-way communication via a PC and provides the ability to monitor, change, and record drive parameters including current, kW, speed reference, fault-alarm codes, and other settings.
Drives integral part of upgradeAccording to Nelson, “Installation of the drives as part of our upgrade was a given. The drives were easy to justify through projected energy savings and short payback cycles alone.
“The drives have provided other advantages, including more accurate temperature control and the ability to easily reduce peak demand and usage charges,” Nelson said.
“We’ve also realized significantly reduced maintenance costs because of the drives’ ability to accelerate and decelerate the motors and mechanical equipment smoothly. This soft-start capability cuts down on mechanical stress that occurs when you have to simply apply line power. Additional savings are also realized since maintenance formerly required on vane and damper installations is completely eliminated.
“The drives themselves have proven to be virtually trouble-free,” Nelson said. “Our next step is to explore the advantages of installations in the rest of our St. Louis-area facilities.”