Economizer case study

Atwell and McGeddy (1989) present a case study of a VAV retrofit project in a New York hospital.

A second method of reducing the energy consumption of the operating rooms to incorporate variable air flow and the use of outside air according to occupancy and weather conditions was developed. The hospital requested and received permission from the New York State Department of Health to use the federal standard of 15 air changes per hour with 100% outside air when outside conditions made that selection more economical than the current state standard of 25 air changes per hour with 20% outside air. To enable the system to easily change back and forth from 25 to 15 air changes per hour, variable speed drives were installed on the supply and return fans. An energy management system was installed to control the heating and cooling coils, humidifiers, and variable speed drives. The system also monitors space conditions.

This system helps the HVAC systems run as efficiently as possible. The air flow rate is now determined by outside conditions; as a result, the system uses outside air with more frequency. The energy management system monitors outside air conditions during occupied hours and then determines whether the HVAC system should operate at 15 air changes per hour with 100% outside air or 25 air changes per hour with 20% outside air. When the outside temperature is between 35°F and 70°F, the system will operate according to the federal standard. When the outside temperature is below 35°F or above 70°F, the system will operate according to the New York State standard. During unoccupied hours, the systems operate at 15 air changes per hour with full return air.


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