Aside from baseball's Twins and the Twin Cities, Minnesota also has a twin-tower government complex in Minneapolis. A unique feature - and air-handling challenge - is its 150-ft, 24-story atrium.

With extensive facilities and many buildings involved in some county governments, it is necessary to keep a watchful eye on energy expenditures while maintaining an acceptable level of comfort for the buildings' occupants. In Minnesota's Hennepin County, this challenge is the responsibility of Roy Earl, senior mechanical engineer for Property Services.

"Today there is a general trend to update existing buildings to be more energy efficient through various technology upgrades. Therefore, our upgrade program started primarily as a utility cost avoidance measure, which included the application of adjustable-frequency drives (afd's), the retrofitting of light fixtures, the use of electronic ballasts along with the installation of other energy reduction projects."

A Tale Of Twin Towers

Earl points out the 25-story, twin-tower downtown Minneapolis Government Center that in essence is the headquarters building for the entire county government system. This downtown facility also has an atrium located between the towers. With over 3,000 occupants, it has been one of the key projects involved in the overall retrofitting program.

"We looked at the amount of constant-speed air handlers that we had in the Government Center," recalls Earl. "As a result, we felt that there were initially twenty-eight air handlers totaling 2,510 horsepower that would be good matches for energy-saving adjustable-frequency drives.

"There, we have installed twenty-eight Danfoss Graham (Milwaukee) adjustable-speed drives to control the building's air handlers. The energy management system we selected for the building is an Automated Logic system which, among other requirements, controls the starting and stopping of our air handlers." Earl points out, "These afd's are used mainly on the supply side of our hvac units. However, there are instances in some of our larger hvac applications where we have also incorporated drives on the return fans. Also, as part of the energy conservation mandate, we have, in certain applications used afd's on variable-flow pumping and chilled water systems."

According to Earl, the installation of the afd's was accomplished quite easily. "There was no disruption of the building's occupants since nothing was physically changed in any of the occupied floors throughout the building. The afd's were installed on the fan motors in two separate mechanical rooms. One of the mechanical rooms is located four levels below the street, and the other being located in a 25th floor penthouse. The fans below the street level force air up through the 13th floor, and the installation in the penthouse mechanical room forces air down from the 25th floor down through the 14th floor."

Earl further explains the scenario. "The atrium is twenty-four stories of open space which includes a sky walk. In Minneapolis there are a number of buildings that are part of a skywalk system, and our Government Center is part of the system. The skywalk matches up with our second level which is equivalent to our second floor. From the skywalk, the ceiling of the atrium is twenty-four stories above you, with a distance between the towers of 150 feet."

"From this second level you can use a down escalator to reach the street level. This street level allows people to physically open doors to the outside, which includes two sets of revolving doors. This opening and closing of doors has created problems since this opening and closing of the doors interferes with our ability to maintain a positive airflow in relationship to the outside air. As a future consideration, we do plan on adding return fans in the atrium system and this should help with the pressurization problem."

Earl continues, "The adjustable-frequency drives are controlled by static pressure sensors in the towers and in the atrium. These sensors are designed to signal the building automation systems in order to call for increasing or decreasing the heating or cooling."

Playing Hardball

According to Earl, an interesting side note to the contracting for the drives was the fact that they were very insistent about taking delivery prior to Northern States' rebate reduction date.

"It was advantageous for us to begin the energy reduction program because of the rebates offered by Northern States Power," Earl reports. "The rebate was $70 per horsepower prior to their deadline date, and it was reduced to $40 after that date. In order to take advantage of the higher rebate rate, we placed a firm delivery deadline on the afd's 15 days prior to the rebate reduction date. In order to achieve this, we contracted for the drives through competitive bidding and took possession of the afd's prior to going out for bids and selecting an electrical contractor to do the installation.

"As one of the bid requirements, we asked that a completion bond of $85,000 be provided." Earl says, "This was required in the event the vendor missed our December first cutoff date for delivery to us so that we would have the time to supply the necessary paperwork to qualify for the maximum rebate. The figure of $85,000 was arrived at as being the difference between the $40 and $70 rebate reduction being offered by Northern States. Therefore, we asked for a certified check for that amount in the event the supplier could not meet the deadline.

A Ripple Effect

"In addition to energy savings we have also experienced better airflow throughout the Government Center," Earl continues. He points out that although the drives were actually put in for an energy cost reduction, they did provide the ability to maintain a more comfortable working environment.

"As an example, in the past, when we had a constant-speed system, we did have complaints about airflow at different times. Since we've put in the afd's, we've had fewer air movement complaints. Also, the retrofitting of the afd's has made it easier for the building engineers to operate the building. They have made it easier to maintain a positive static pressure in the Government Center atrium, which in the past has been hard, if not impossible to maintain."

He states, "I do know that the system has made it easier for the engineers to maintain the building's static pressure. Our heating and cooling is made available from our district's central plant. The steam and chilled water we receive from the district has to be pumped from four levels below ground, up to the 25th floor for use in the heating and cooling coils, which are in these penthouse air-handling units. They temper the air coming out of the penthouse because, in the winter, we want the discharge from the air-handling units to be a certain set amount of degrees, and in the summertime we want to control the chilled water for air conditioning the same way. So in order to control the flow, we also put vsd's on those water pumps."

The Bottom Line

In summing up Earl states, "The utilization of adjustable-frequency drives in the building's hvac system has been part of an overall energy conservation program. At the same time that we installed the afd's, we also relamped the Government Center and Medical Center. Since that time, we have done spot checks of both facilities, and in doing so, it appears we have achieved a substantial reduction in energy usage. The savings from the installation of the 55 afd's in our hvac system has provided an overall annual savings of 2,587,375 kWh. This provides us with an approximate annual savings of $142,306. Our total purchase price for the drives was $239,061, with a Northern States rebate of $205,660."