Healthy hospital success: HVAC
With hospital projects, it is imperative that engineers get everything exactly right—after all, the lives of patients treated in the health care building may depend on it. HVAC systems are top-of-mind for engineers.
Richard Heim, PE, LEED AP, Mechanical Project Engineer, RMF Engineering Inc., Baltimore
Tim Koch, PE, LEED AP, Electrical Engineer, HDR Inc., Omaha, Neb.
Nolan Rome, PE, LEED AP, Senior Vice President, ccrd, a WSP | Parsons Brinckerhoff Co., Phoenix
Raymond Schultz, PE, Project Engineer, CannonDesign, Grand Island, N.Y.
Kunal G. Shah, PE, LEED AP, RCDD, President, PBS Engineers, Glendora, Calif.
Tommy Spears, PE, Vice President of Design Solutions, TME, Little Rock, Ark.
CSE: What unique HVAC requirements do hospitals have that you wouldn't encounter in other buildings?
Shah: The hospital HVAC system's primary function is to improve indoor air quality (IAQ) and mitigate airborne transmission of diseases. To deal with this, the hospital HVAC system design will have to consider filtration, dry-bulb temperature, wet-bulb temperature/humidity (space dewpoint control), air changes, cross-contamination, pressurization, ventilation (outdoor air) rates, and exhaust. This makes it much different from a typical commercial building HVAC system, where comfort is the main objective and there is a much more lenient set of code requirements to be dealt with.
Rome: Serving a patient-care area requires pressurization, additional filtration levels, and required air changes that other building types don't have. These requirements minimize infection risks, promote patient healing, and are a significant responsibility to manage alongside minimizing energy use.
Spears: HVAC systems have to meet infection-control standards that include directed airflow paths from more to less clean areas, higher-efficiency air filtration, and tighter humidity and temperature control. Also, a more robust and elaborate BAS is required to both control and monitor performance of HVAC systems and to help hospital owners trend and track energy cost. Automatic fault detection and daily measurement and verification (M&V) of utility costs, often through the HVAC BAS, are becoming much more common tools to help hospital facilities personnel quickly catch anything that is operating improperly to make sure they maintain a high-quality patient-care environment and to minimize any wasted energy.
Heim: Redundancy is a requirement not often found in other buildings. Additionally, hospitals require a "high quality of air" for the building. That varies depending on the space being served, but it frequently means high levels of filtration, humidification for humidity control, and special diffusers for specific environments. Hospitals often have 24/7 requirements, so an "unoccupied mode" does not exist and the equipment must be designed to run through the coldest part of the evening and the hottest day during the summer. Additionally, many hospital systems require emergency power, which has to be factored into the design.
Koch: The biggest difference in HVAC requirements for hospitals versus other buildings is the air-change requirement. Hospital rooms or spaces require a certain amount of ventilation air changes per hour as listed in ASHRAE 170. Certain spaces are also required to maintain certain pressure relationships to surrounding spaces. For example, infectious isolation rooms are required to maintain a negative pressure to surrounding areas to protect staff and other patients from the infectious patient. ORs are required to be positive with respect to surrounding spaces to protect the patient in surgery from contaminants outside the clean environment of the OR.
CSE: What changes in fans, variable frequency drives (VFD), and other related equipment have you experienced?
Schultz: Many health care facilities understand the benefits of fan arrays in air-handling systems. Multiple fans provide a degree of inherent redundancy for system operation beneath peak. If one fan were to shut down, it is likely that its absence would not be realized from an airflow perspective. Additionally, all fans in the array can be controlled with a single VFD. In lieu of providing a bypass on the drive, a second redundant drive can be installed in an adjacent control panel. If the primary drive fails, the secondary redundant drive can be engaged to operate the fan array until the primary drive can be replaced or repaired.
Heim: VFDs are everywhere. Even if the system will not vary flow, due to the relatively low cost and the benefit of balancing the fan, the majority of fans we design have a VFD. In an effort to provide more levels of redundancy, we have seen a move away from one big fan to multiple smaller fans. In many ways, this is like a fan-wall approach. Frequently, we have designed air-handling units with two supply fans, so that in the event of a fan failure one fan could operate and provide partial capacity. When coupled with a VFD, this allows the fan to ramp up to its maximum capacity during this fan-failure mode.
Koch: The advent of the fan array system in AHUs has changed AHU design as well as VFD technology. The fan array system has brought about a fan system that has replaced the large, single-supply fan system with a smaller array of fans. This has decreased the amount of space required for fan sections, thus minimizing the amount of space required for the AHUs. The use of smaller fans has also virtually eliminated the need for sound attenuators that were previously used to attenuate noise from the large, single-fan system. Multiple manufacturers are utilizing fan array systems in AHUs. These systems have also led to changes in fan horsepower ratings of AHUs. Manufacturers used to list motors at 5, 7.5, and 10 hp. They have begun to list motors at 6, 7, 8, or 9 hp. These are the same motors as before, just relisted with a different horsepower. This allows fan array manufacturers to minimize connected horsepower, thus minimizing electrical requirements. These newer motor ratings have also changed the VFDs that control them. The standard VFD used to operate at 60 Hz for 100% airflow. The newer motors have been listed to operate at 60 to 120 Hz. VFD technology also has evolved with the implementation of fan array systems. In lieu of controlling all fans with a single drive, newer technology allows a micro drive for each fan, thus allowing greater redundancy and individual control of fans in the array.
Rome: Fan arrays and VFDs have revolutionized how we can manage control of the demands of a building while maintaining temperature and humidity performance of specific areas, and it has also been key to energy-reduction strategies for static pressure reset and system performance.