What is in this article?:
The National Institutes of Health's Special Clinical Studies Unit is a highly specialized biocontainment patient-care unit. This article discusses its design criteria.
SCSU Design Features
Design features specific to the SCSU include:
- Walls (outside of the OEIR) sealed to minimize uncontrolled air movement to and from adjacent spaces, sealed piping penetrations, and openings in electrical boxes and other component device boxes sealed with caulking.
- Self-closing exit doors.
- Balanced ventilation air to maintain negative pressure (inflow of air) relative to the corridor (between public corridor and the SCSU).
- Exhaust-air grilles located low on patient-room walls.
- Pressure-monitoring system continuously indicating airflow from adjacent spaces into the SCSU.
- Displays of relative space pressurization at the corridor and vestibules.
- Remote alarms at the nurses’ station.
- Twelve-air-changes-per-hour (ACH) minimum ventilation rate of non-recirculating air (100-percent outdoor air) for the SCSU and OEIR.
- Dedicated toilet, bath, and hand-washing facilities in each patient room.
- Dedicated exhaust-air system serving the SCSU and OEIR.
The dedicated constant-volume exhaust system consists of three variable-speed exhaust fans. Each fan is designed at 50 percent of capacity to provide the required level of redundancy and reliability and 20-percent reserve-airflow capacity to account for final air-balance adjustments and future needs. The fans operate continuously, their speed controlled with variable-frequency drives based on a static-pressure sensor in the exhaust ductwork. Discharge from the fans is manifolded to ensure a high stack-discharge velocity.
One of the exhaust fans receives life-safety branch power; the other two receive general standby-equipment branch power. In the event of normal power loss, life-safety power is restored within 10 sec, and the first fan is re-engaged and ramps up to satisfy required minimum exhaust and directional airflows. Shortly thereafter, power to the other fans is restored, and the fans are re-engaged. Air distribution to individual spaces/rooms is by constant- and/or variable-volume, pressure-independent supply and exhaust air terminals to each temperature- and/or pressure-control zone. Pairs of supply and exhaust venturi-style air-terminal units track airflow to ensure the specified pressurization (directional airflow) is maintained. The only prerequisite for the air terminals to operate properly is sufficient duct static pressure. This is monitored via differential-pressure switches across each air valve. Additionally, automatic bubble-tight isolation dampers are provided in the supply duct to each zone.
All exhaust air-terminal units are low-leakage (less than 3 cfm at 1 in. water), with high-speed actuators (less-than-1-sec full response time) to stay within 5 percent of airflow set point. The air-terminal units' controllers are linked between supply and exhaust. If duct static pressure falls below levels required for accurate tracking, the supply-air bubble-tight dampers close, and the exhaust-air valves are commanded to a minimum position, maintaining directional airflow.
The control devices operate autonomously, even in the event communications with the overall building control system are disrupted. All of the controllers and bubble-tight dampers are provided uninterruptible power to ensure the air-terminal units track each other to maintain the specified airflow differential. All serviceable components, such as air terminals, exhaust valves, and reheat coils, are located in the interstitial space outside of the patient-care/containment area.
All exhaust ductwork for the SCSU and negative-pressure examination room on the room side of the HEPA filter is made of welded stainless steel (Figure 3). The supply ductwork is made of galvanized steel augmented with welded stainless steel. Bubble-tight dampers are provided on all supply ducts branching from the interstitial level to SCSU patient rooms and the OEIR. These are fast-acting pneumatic actuators that close upon a loss of exhaust airflow. Ductwork from the bubble-tight dampers to air devices is made of welded stainless steel.
Backflow preventers are installed in domestic-water piping running to the SCSU. When the SCSU is in isolation mode, oxygen, medical vacuum, and medical air are provided via local dedicated units. HEPA filtration is provided on the main plumbing vents leaving the SCSU.
Emergency power is provided for critical branch, life-safety, and standby equipment in the SCSU and for exhaust fans on the interstitial level as appropriate.
Electrical services for the OEIR are sealed airtight to minimize outside penetrations. A third inner layer of sheetrock prevents air migration that can occur as the result of required utility boxes and associated penetrations.