Retrofit System, Variable-Frequency Drive Cut Energy Costs and Increase Comfort

Aug. 1, 2011
Overcome shortcomings of single-zone rooftop system

The most common type of HVAC control, a single-zone system uses one thermostat or control point per HVAC unit. A supply fan moves air from the HVAC-system intake to distribution points throughout the facility. Typically, this fan runs constantly at full speed when the facility is occupied. This mode of operation is inefficient, tough on equipment, and only moderately successful in providing comfort.

Electric Motor Sales & Supply Co. Inc. (EMS) had just such a system at its 70,000-sq-ft industrial supply and distribution facility in Chattanooga, Tenn. Looking to reduce energy costs, EMS engaged Chattanooga-based Electronic Controls Inc. (ECI) to improve the operation of a 20-ton rooftop system providing heating and air conditioning to a 5,000-sq-ft space devoted to sales. EMS also was looking to improve comfort and cut maintenance costs, as the existing system caused too much wear on the belts of a 7.5-hp supply fan.

Because the motor of the fan was started across the line at full power, the belts would stretch, requiring frequent tightening. The fan ran at maximum speed when the unit was operating, creating a high noise level and resulting in excessive airflow in the sales office. This caused a drafty and low-humidity environment during winter. During summer, high humidity made the space uncomfortable, even when the temperature was maintained in the low 70s.

EMS needed a solution that could be implemented at low cost with minimal downtime. The answer was to improve system automation through a strategic retrofit.

Automation Is the Answer
Designed and built by ECI, Enerfit is a HVAC-retrofit system that can be applied to any brand of single-zone HVAC system. It varies airflow across a direct-expansion coil without the need for hot-gas bypass, reducing upfront and operating costs.

Enerfit's controller senses HVAC parameters to determine the optimum speed of a supply-fan motor. To vary motor speed and, consequently, airflow, a variable-frequency drive (VFD) is needed. For the EMS project, a VFD supplied by Yaskawa America Inc. was installed in the motor power circuit. The VFD input receives full power, while the VFD output supplies power to the motor in such a way as to control its speed from 25 percent to 100 percent. The VFD also ramps up motor speed gradually, eliminating high starting torque.

An optional Web server is connected to the Internet and accessible through any Web browser. A graphical view of the entire HVAC system, along with specific operating parameters, alarm settings, and control set points, is provided. Through the Web browser, HVAC operation can be viewed and controlled.

A quick payback analysis showed a return on investment of 24 months or less. That, combined with anticipated improvements in comfort, was enough to compel EMS to go with Enerfit and VFD retrofits.

Money Saved, Maintenance Cut
From January 2010 to November 2010, the HVAC system was run in Enerfit and pre-Enerfit modes to compare performance. Total electrical costs were reduced by approximately 53 percent. Based on kilowatt-hour charges in the Tennessee Valley area, EMS recognized more than $2,500 in energy-cost savings over the first 12 months.

Fixing air leakage around the roof curb and installing the retrofit components helped to raise relative humidity considerably during winter, greatly increasing occupant comfort. The effect of this repair was not considered in the energy comparison.

EMS is able to monitor and control the set points and schedules of the HVAC system via Enerfit’s Web browser. Changing heating and cooling set points during periods when the facility is unoccupied saves energy and results in less mechanical wear and tear. Valuable data for the two compressors and the VFD are monitored for maintenance and troubleshooting purposes.

EMS later added Enerfit to another unit at the facility.

Enerfit can be applied to any type of building with single-zone HVAC systems. The Enerfit/VFD retrofit works with systems of any size, but is most cost-effective for multiple-compressor systems of 8 tons or larger. The retrofit is effective in any climate and is especially useful for reducing interior-space relative humidity during cooling season.

Information and image courtesy of Yaskawa America Inc.

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About the Author

Scott Arnold | Executive Editor

Described by a colleague as "a cyborg ... requir(ing) virtually no sleep, no time off, and bland nourishment that can be consumed while at his desk" who was sent "back from the future not to terminate anyone, but with the prime directive 'to edit dry technical copy' in order to save the world at a later date," Scott Arnold joined the editorial staff of HPAC Engineering in 1999. Prior to that, he worked as an editor for daily newspapers and a specialty-publications company. He has a bachelor's degree in journalism from Kent State University.