Building safe, effective health care facilities: HVAC
It’s hard to think of an engineering project with higher standards than a hospital or health care facility—successfully designed and installed systems can literally be a matter of life and death. Indoor air quality, indoor environmental quality, and HVAC systems are key for engineers.
- J. Patrick Banse, PE, LEED AP, Senior Mechanical Engineer, Smith Seckman Reid, Houston
- Daniel L. Doyle, PE, LEED AP O+M, Chairman, Grumman/Butkus Associates, Evanston, Ill.
- Robert Jones Jr., PE, LEED AP, Associate Director of Electrical, JBA Consulting Engineers, Las Vegas
- Craig Kos, PE, LEED AP, Vice President, ESD Inc., Chicago
- Essi Najafi, Senior Vice President/Principal, Global Engineering Solutions, Rockville, Md.
- Paul J. Orzewicz, PE, Mechanical Engineer, Project Manager, RMF Engineering Inc., Baltimore
- David A. Smith, PE, EDAC, Principal, National Director of Health Care, KJWW Engineering Consultants, Madison, Wis.
CSE: What unique HVAC requirements do hospitals have that you wouldn’t encounter on other structures?
Kos: Hospitals and outpatient facilities must comply with ASHRAE and other regulatory standards with respect to air change rates, outdoor air exchange rates, and pressurization. Since the patient population in hospitals and some outpatient facilities can contain some of the most vulnerable patients, the systems in these buildings also need to be reliable, redundant, and as easy to maintain as possible.
Smith: Hospitals are very unique in that they require high amounts of airflows, pressure relationships between spaces, and filtration to prevent hospital acquired infections (HAIs).
Banse: Unique HVAC requirements are not confined to hospitals, but this building type does have its share. Fully ducted supply air, return air, and exhaust air systems; positive and negative pressure relationships of rooms with required minimum pressure differences to adjacent spaces; air cleanliness through MERV 14 and HEPA filtration; laminar directional airflow in many room types; minimum and maximum velocities relative to patent positions; varying room temperature ranges and relative humidity ranges for various rooms; and increased number of control zones are a few that come to mind.
Najafi: Hospitals offer the HVAC designer a litany of space types to design, each with its own set of requirements. Space types include the mundane, such as office and lobby spaces, as well as the uniquely demanding surgical procedure rooms, MRI suites, CT scan rooms, and electrophysiology laboratories. While each space has particular requirements, none of the individual space requirements are without parallel to other spaces. MRI suites, for example, require from the HVAC designer to deal with electromagnetic radiation and shielding, which is similar to sensitive compartmented information facility (SCIF) design. The high airflow rates and HEPA filtration requirements for surgical suites are very similar to that of clean rooms, while lobbies and office spaces are spaces of a more traditional design. The unique requirement for a hospital design is incorporating all of the different space types into one methodology providing the ventilation, heating, and cooling for all from a few central locations.
Orzewicz: There are varying degrees of redundancy required that typically are not in other structures. This affects not only the central generation equipment, which often requires an N+1 design, but also the air handling unit design. Along with redundancy, operating rooms have been requiring lower temperatures while maintaining acceptable relative humidity. We have maintained spaces as low as 58 F while not exceeding 55% relative humidity. Lastly, air pressurization is critical and must be maintained at certain levels in varying spaces. The type of space will determine the amount of monitoring and alarms required in the design.
CSE: What updates in fans, variable frequency drives, and other related equipment have you experienced?
Orzewicz: Recently, we have seen a shift away from belt-driven fans to direct drive fans. Many clients prefer to avoid the maintenance requirements of a belt-driven fan. Additionally, with the continuing ease and cost effectiveness of variable frequency drives, direct drive fans have provided clients with the required flexibility they need.
Smith: The use of ultraviolet (UV) light in air handling units to kill bacteria is becoming more common. It requires the maintenance staff to realize that the UV lamps must be replaced on a schedule. The lamps will continue to give off visible light long after the UV component is gone.
Najafi: The most prevalent update to fan technology lately has been the use of electrically commutated motor (ECM) driven direct drive fans. For small horsepower applications, the ECM is able to maintain 65% to 85% efficiency at all speed settings, which significantly outperforms the conventional permanent split capacitor (PSC) motor. The use of this highly efficient motor with a direct drive fan (no belt slippage or drive loss) can provide significant energy savings. In the field of VFDs, it should be noted that in recent years a more compact design has been introduced, offering higher quality and controllability at a reduced cost compared to 5 years ago. For these reasons, the VFDs are most commonly implemented in the design of AHUs and pumps even if it is for balancing reasons.
Banse: Higher system static pressures due to components and higher efficiency filtration requirements seem to be requiring the use of plenum fans more often than not. Fan arrays are becoming more prevalent and allow for fan redundancy and a smaller air handler fan section footprint. VFDs are in use more often not only on air handling units but also on small exhaust fans that aid in balancing and reduced energy use. Selecting fan motor horsepower within the next larger size is becoming standard practice to meet ASHRAE 90.1-2010 and its update 90.1-2013.
CSE: What indoor air quality (IAQ) challenges have you recently overcome? Describe the project, and how you solved the problem.
Banse: One project involved a small bistro-type café adjacent to a conference center. It used two ventless type cooking appliances (no direct exhaust duct connections) that, while approved for use, contained neither the odors nor fumes, and the conference center pre-function area became odiferous by the end of the day. The spaces were exhausted but did not fully capture the undesirable odors within the space. Rearranging the air devices and increasing the exhaust air quantity in the immediate area improved the adjacent conference room area air quality.
Najafi: In a recent project, we were tasked with the design of an MRI surgical suite. The suite consisted of three MRI rooms served by one rail-mounted traveling MRI machine and one control room. Of the three rooms, two were intended to be used for surgical procedures. The user required that the space be designed to maintain a temperature of 68 F and roughly 50% relative humidity (RH). The room was served by an existing (chilled water) air handling unit, which also served adjacent spaces, and was designed for 55 F supply air delivery. To achieve the design requirements, each surgical suite was supplied by one temperature controlled variable air volume terminal unit and one constant volume terminal unit designed to maintain airflow velocities at the operating table and air curtain (at floor level). Downstream from each terminal unit was a high-velocity HEPA filter, a duct-mounted chilled-water cooling coil, and a duct-mounted reheat coil. The chilled water coil control valves were actuated by a space humidistat, and the hot water reheat coil control valves were actuated to maintain a supply air temperature of 55 F. Using this arrangement of equipment, we successfully maintained space cleanliness, temperature, and humidity requirements.
CSE: In your experience, have alternative HVAC systems become more relevant? This may include displacement ventilation, chilled beams, etc.
Orzewicz: Alternative HVAC systems have definitely become a more frequent topic of discussion. But by and large, engineers still design to a vast majority of traditional HVAC systems. Frequently, we are designing an expansion of an existing building, or adding a building to an existing campus and tying into its central utilities. These scenarios tend to lead us to design to match the remainder of the existing buildings for continuity of the campus or building. However, alternative HVAC systems are gaining interest and will be more common in the near future.
Smith: A lot of these systems have been prohibitive due to initial cost or the possibility of introducing mold/bacteria growth locations due to condensation. Chilled beams, for example, are not legal in many states due to the condensation potential.
Banse: I think that an alternative process for the way an HVAC system is designed, constructed, and implemented is being used, and therefore the system and its components are more relevant. This includes dedicated outdoor air systems, chilled beams, more occupied/unoccupied cycles, and related features being integrated into system designs for both comfort and energy savings.
Najafi: Chilled beam technology and air-to-air heat exchange are becoming more prevalent in hospitals as well as other building types in the United States. Chilled beam technology in particular is gaining traction as an acceptable cooling alternative to VAV systems in administration areas of hospitals as far as there is minimum humidity and inoperable windows. These systems can allow for cost and energy savings as well as reduce the size of duct risers. For patient spaces, we still recommend the use of VAV systems with steam humidification due to their superior humidity control during the winter months, as well as their free cooling capacity (for facilities without cooling towers), and their adaptability to changing outdoor air requirements. For administrative areas we see the implementation of VRV/VRF systems with ceiling cassette units in an effort to increase energy efficiency and decuple dependency to a chiller plant.
CSE: Hospital-acquired infections (HAIs) are top-of-mind of hospital owners. What can you do to help mitigate these problems?
Smith: Pressure relationships between patient rooms and corridors, antimicrobial surfaces on anything that can be touched, and locating hand-washing stations to be conducive to use.
Najafi: We apply infection control for hospitals at the air handling unit. To that end, we apply MERV 14 (or better depending on the application) filtration (with pre-filters) at the air handling unit, as well as UV disinfection systems. Eliminating air recirculation also assures no redistribution of infectious diseases through the air conditioning duct supply system. Where appropriate, we will supply greater than the code minimum outdoor air (up to and including 100% outdoor air) and localized HEPA filtration. In surgical areas in particular, we provide HEPA filtered air, which is delivered with laminar diffusers over the patient location, and a linear diffuser air curtain around the operating table. Airflow is designed to achieve low velocities of 40 to 60 ft/min at the operating table, as well as at the floor level (air curtain). For the plumbing systems we design a fully recirculated domestic hot water distribution system that generates and stores the hot water at a minimum temperature of 140 F. We also specify open, non-aerator faucet outlets and recommend a protocol for regular domestic water sampling and testing for Legionella.
Banse: Directional airflow, tight duct construction standards, maintaining air change rates, and proper filtration are all features of good design. Proper equipment maintenance access is important as well. Many HAI are produced by human contact with equipment and instruments rather than air circulation according to the Centers for Disease Control (CDC); however, working with a facility Infection control representative will allow HAI concerns to be addressed in the HVAC system design. The Affordable Care Act puts great pressure on institutions to improve clinical outcomes and reduce readmissions.
Orzewicz: The best way to try and control infections is to understand where the air is moving in a building. Since a vast majority of infections are transmitted through the air, understanding airflow throughout a building is critical. Typically, we analyze the air pressurization of every room to determine if it is negative, neutral or positive. This allows us to verify if air is moving from clean, to normal, to dirty spaces.
CSE: When working in high-rise hospitals and medical structures, what tips/tricks can you offer? Define any points engineers should consider in these vertical structures.
Banse: Building pressurization with appropriate BAS controls is a key element to minimize infiltration and control air migration. Reviewing the floor plans with both egress and departmental restricted access doors located and discussing staff flow with users and the architect can help identify how air migration might occur, especially with proximity to loading docks, lobby access, and food service areas with high exhaust volumes and areas of clean access such clean linen routes.
Orzewicz: Understanding the life safety plans is important in determining smoke and fire damper locations. This always requires significant coordination, and if it is missed, it can be magnified due to the frequent repetition of each level. With this in mind, engineers should be sure to calculate fan statics early and include all dampers. These long vertical risers can add static to a fan quickly and should be sized conservatively to avoid a lack of flow.
CSE: How do you deal with stack effect in larger hospital buildings, or those with central open atriums?
Najafi: We certainly need to be mindful of the smoke evacuation requirements for areas such as open atriums where large airflows are introduced at the floor level and exhausted from the high points of the atrium during a fire event. In normal conditions we are mainly conditioning the occupied zones of the open atrium and the perimeter interconnecting corridors while we return air at low points to assure air circulation around the breathing zone.
CSE: In densely packed facilities, how do you achieve optimal HVAC and air quality?
Orzewicz: In densely packed facilities, it is critical to understand the quantity of people, the peak hours of operation, and how the building occupancy varies throughout the day in order to ensure optimal HVAC and air quality. Once this is understood, the system can be designed properly. One major component to contemplate is the number of air changes and the amount of outside air. Frequently, both of these values will be greater than typical, due the increased quantity of people.
Banse: From a facility location, congested medical centers and campuses offer unique challenges to HVAC design for air exhaust and air intake requirements. The location of emergency generator exhaust, cooling tower discharge, contaminated exhaust air streams, and helipad locations can be sources of odors and contaminants. Proper discharge locations and equipment type selections are critical. Wind studies are very helpful in modeling building exhaust and air intake locations with adjacent structures and prevailing winds.
Najafi: When density of occupancy is a factor, then demand ventilation is the preferred approach. A 100% outdoor air unit of the variable flow type is coupled with VAV reheat boxes controlled by a combination of thermostat and CO2 sensors per zone. As the facility zones are populated, the CO2 sensor modulates the VAV boxes open or closed, to maintain a predetermined CO2 ppm concentration in a given space. The thermostat keeps the space at a comfort setting by modulating supply air temperature. This optimizes the ventilation airflow and minimizes the costly conditioning of outside air. An alternative system that may be used is a VRV system with a dedicated outdoor air handling unit. This system uses local temperature controlled fan coil units (cassettes) and a variable volume CO2 controlled outdoor air handling unit which provides tempered (neutral/72 F) outdoor air to the space.