Health Care Air Handling Units - Bigger Is Better
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.
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.
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.
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