Health Care Air Handling Units - Bigger Is Better
June 4, 2007
This article examines the use of air handling units for health care applications. Health care applications place special demands on the features and construction of air handling equipment. Hospitals are also some of the most expensive buildings to operate because they never close and they have very high air volume and ventilation rates.
In this article, we investigate a way to reduce air handler operating costs by as much as 11 percent with an excellent simple payback for any capital investment required. Assuming that a typical health care application will involve many air handlers, these savings can have a significant impact on the bottom line of a health care facility.
Modern health care facility design typically uses central air handling units as an integral part of the HVAC system. The units can be constant volume or variable air volume (VAV), although the minimum turndown for VAV units is kept much higher versus other building applications.
Central air handling systems are popular because most health care facilities are designed using the requirements set out in Guidelines For Design and Construction of Health Care Facilities published by the American Institute of Architects (AIA). Minimum air changes and ventilation rates for most health care area designations are provided in Table 2.1-2 of the guidelines. For example, an Intensive Care Unit (ICU) requires a minimum of six air changes per hour, two of which must be outdoor air. That works out to 1 cfm/ft2 supply air and 33 percent outdoor air. These requirements make central air handling systems an excellent choice.
AIR HANDLER DESIGN FOR HEALTH CARE APPLICATIONSThe strict requirements for health care applications put special demands on air handler design. Figure 1 shows a typical air handler configured for a health care application. Table 1 describes the components in direction of airflow.
Air handlers used in health care facilities typically operate at 5-inches static pressure or more. At this pressure, a properly designed and assembled unit is a must to avoid cabinet damage and leakage. Foam panel construction is becoming popular because it offers lightweight panels that are extremely rigid and have higher insulation values. Units shipped in sections should be sleeved so that a positive seal is formed when the sections are joined.
Many air handlers use steam for humidifiers and heating coils. For these applications, the proper trapping height must be established by using housekeeping pads, base rails, or trapping the unit below the floor. Variable height base rails offer the advantages of giving the designer flexibility in providing the proper trapping height and contributing to lower structure weight.
REGIONAL HEALTH CARE APPLICATIONSSmaller, regional health care centers often cannot justify using central chiller and heating plants, but these facilities must meet the requirements of the AIA guideline. One possibility is using rooftop air handlers, such as Skyline™ units from McQuay, with air-cooled chillers. The chillers can be provided with factory-installed pumping packages.
Another solution is to use applied rooftop units. Figure 2 shows a McQuay model RPS unit configured for health care applications. The difference between an RPS and a commercial packaged rooftop system is that it can be ordered with the required air stream components. In addition, the RPS offers flexibility in its DX cooling system (coil rows, fins, and compressor horsepower) to meet the demanding ventilation load requirements. The RPS also offers streamlined commissioning as units are factory assembled, tested, and shipped to site with complete controls. The controls can then be easily integrated into the building automation system (BAS) using LONWORKS® or BACnet® communication.
THE â€˜BIG' OPPORTUNITYMany air handlers are designed for 500 fpm through the coils and filters. This meets the filter requirement and allows for flat filter banks. 500 fpm is also proven as an acceptable velocity to avoid condensate blow off from the cooling coil.
However, there is an opportunity to increase the air handler size and reduce the air velocity. Table 2 shows the component air pressure drops in an air handler at 500 fpm and 400 fpm.
Reducing the air velocity through the air handler will reduce the air pressure drop at several of the components. This, in turn, reduces the total static pressure required of the fan. This is very important in health care design because hospitals never close (8,760 hour operation) and they are often constant volume with high static pressures. Figure 3 shows the annual energy use by components in a typical hospital in Minnesota. Fans are a dominant energy user.
Lowering the total static pressure by increasing the air handler size reduces the fan work and results in annual operating cost savings. However, this requires a capital investment for the larger air handler. Table 3 shows the operating cost savings, capital cost, and the simple payback for air handlers with an air velocity ranging from 500 down to 400 fpm in Minneapolis and Dallas.
As shown in Table 3 and Figure 4, approximately 450 fpm offers the best financial return for an additional $551 investment in a 20,000 cfm air handler. In fact, in both Minneapolis and Dallas, the operating cost savings resulted in a very favorable payback for the entire range of velocity reductions presented in Table 3. Thereafter, depending upon the size of the facility and the number of air handlers used, these savings can add up dramatically to improve the bottom line of the hospital.
In addition to the operating savings, the lower face velocity allows more filter area, which helps reduce the frequency of filter changes. Coils with fewer rows may also be used, which can help reduce the initial capital investment and make them easier to clean.
CONCLUSIONHealth care applications place demands on air handlers that require flexible design and low leakage. Options for smaller hospitals include rooftop air handlers and chillers with pump packages, or applied rooftop units.
Once the equipment type is chosen, consider designing around a 400 to 450 fpm air velocity to reduce operating costs. The payback is in the one-year range because hospitals never close. The savings can add up dramatically year over year depending upon the number of air handlers used.
To order a copy of the Guidelines for Design and Construction of Health Care Facilities, visit the AIA online bookstore at www.aia.org/books or the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) online bookstore at www.ashrae.org. For more information on HVAC system design for health care, visit www.mcquay.com.
Publication date: 06/04/2007