Case study: Hospital surgical suite HVAC system
The following case study examines unoccupied setback of an air handling unit dedicated to a surgery suite
Unoccupied setback of an air handling unit dedicated to a surgery suite was examined in a hospital. In this particular case, the hospital wanted the entire surgery suite system to be in the same mode (occupied or unoccupied) at once, with a single override switch located at the control desk. Such a system can easily be set up to allow only a portion of the rooms to be overridden, provided that override buttons are installed for each room.
The local authority, the Texas Health and Human Services Commission, would not permit the outside air quantity to be reduced during unoccupied modes. The additional preheat required during unoccupied mode in the winter months to maintain an acceptable mixed air temperature, is included in the overall energy numbers.
The system consists of a single 20,000 cubic feet per minute AHU dedicated to serving eight operating rooms and the sterile core. Outside air is constant at 4,000 cfm (20%). During unoccupied mode, the airflow to the ORs is reduced from 20 to 5 air changes per hour. Pressure-independent air valves on the supply and return are set for a constant offset to help ensure positive room pressure is always maintained.
The ORs are generally unoccupied 7 p.m. to 5 a.m. Room temperature setpoint was maintained at 62°F at all times.
The energy consumption specific to the OR heating, ventilation and air conditioning can be broken down into five main components:
- Supply fan energy kilowatt.
- Return fan energy kilowatt.
- Cooling plant energy kilowatt.
- Preheat energy 1,000 Btu/hour (aka MBH).
- Terminal reheat energy 1,000 Btu/hour.
Note that there is also energy consumed for humidification. However, the difference between occupied and unoccupied modes is negligible because outside air is constant and is not included in this analysis.
The analysis included a heating and cooling load analysis on an hourly basis for an entire year (“8,760 hourly analysis”). Table 3 illustrates the annual sum of energy consumption of each of these components, with the base assuming 24/7 operation with no airflow setback, as well as the energy savings associated with the setback sequence.