Hospitals contain many different types of environments, including public areas, sterilization facilities, soiled-laundry rooms, operating rooms, intensive-care units, and neonatal-care units. Certainly, there are many biological and chemical contaminants in various areas of a hospital that impact the safety of patients and staff members in the rest of the building. An HVAC system and its associated control system is the primary means of keeping contaminants in one part of a building from entering another. For example:
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Operating rooms, intensive-care units, nurseries, and protective-environment rooms must be kept at a positive pressure with respect to all surrounding spaces.
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Airborne-infection-isolation environments (e.g., tuberculosis isolation) must be kept at a negative pressure with respect to surrounding spaces.
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Autopsy, sterilization, and soiled-laundry rooms must have all air vented to the outdoors.
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Air-pressure relationships must be maintained in all operating conditions.
In addition to temperature and air pressure, HVAC systems also play the primary role in humidity control, another factor critical to maintaining consistent therapeutic environments. Consider:
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Nursery-suite temperatures typically are maintained between 75°F and 80°F.
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Burn-patient treatment rooms sometimes are maintained with a relative humidity between 90 and 96 percent.
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The temperature of pediatric-surgery units may need to be kept as high as 86°F.
As with any modern structure, it is important that hospitals and other health-care facilities are built with a tight envelope so that they can be as resource-efficient as possible and prevent uncontrolled outdoor air from being drawn into patient spaces. HVAC insulations play a key role in this.
HVAC insulations include equipment insulation as well as insulation for air-handling applications. This article will examine the functions and benefits of the two primary HVAC air-handling insulations—duct wrap and duct liner—as well as acoustical silencers, a common noise-control product.
ENERGY EFFICIENCY
Not surprisingly, one of the primary functions of HVAC insulations is to control the amount of heat loss/gain through equipment housings, ducts, and pipes. HVAC insulations enable air to be delivered to a point of use at an intended temperature. Maintaining consistent air temperatures from a source unit to a final destination often can result in reduced equipment loads.
Duct wraps are applied to the exterior of sheet-metal ducts and can be made of a variety of materials, including fiberglass, cotton, and foams. Wraps commonly are used in hospital applications, especially with systems carrying cold air, such as those servicing intensive-care units and other critical-care areas. Duct liners are used on the interiors of sheet-metal ducts and commonly are manufactured from the same materials as duct wraps.
HVAC systems in hospitals and other health-care facilities transport a huge amount of air—as much as 15 to 30 cfm per person or six to 25 air changes per hour—compared with the HVAC system in an average commercial building, which typically delivers 5 to 15 cfm per person. Even with high ventilation and associated heating and cooling loads, HVAC insulations can make an impact on energy efficiency. With proper HVAC insulations, a facility may be able to reduce the size and cost of its HVAC equipment.
Non-insulated pipes often are the most prone to heat loss, as water traveling large distances through copper piping quickly can lose thermal energy. The farther air or water travels within an HVAC system, the more likely its intended temperature will change.
While HVAC insulations can improve the overall energy efficiency of a hospital or other health-care facility, it should be noted that because of the nature of these facilities—such as 24-hr operation and significant electronic equipment—the average health-care facility uses drastically more energy than other similarly sized commercial buildings. Despite the potentially smaller overall energy-efficiency benefit, the energy savings and other benefits associated with HVAC insulations far outweigh their costs.
CONDENSATION CONTROL
Keeping cold equipment, such as duct and pipe surfaces, insulated and/or isolated from humid ambient air prevents condensation that, in a health-care facility, can lead to dangerous microbial and infection-control issues. As anyone in hospital design and maintenance knows, controlling microbial contamination is a never-ending effort, and, as in commercial buildings, microbial problems almost always are moisture-control problems. While rain and plumbing leaks are the main source of moisture problems, controlling condensation through an HVAC system can contribute greatly to the solution.
Many interior environments, such as an operating room, in which temperatures are maintained between 68°F and 75°F or colder with 30- to 60-percent relative humidity, can pose condensation risks on uninsulated ducts carrying 55°F chilled air. ASHRAE Standard 62.1, Ventilation for Acceptable Indoor Air Quality, Section 5.15.2, “Condensation on Interior Surfaces,” requires insulation on pipes, ducts, and other surfaces that can be expected to be below the surrounding dew point.
NOISE CONTROL
Patients in a hospital environment may have their sleep compromised by pain, anxiety, and adjustment to a new environment. Excess noise can exacerbate these factors and contribute to insomnia, stress, and elevated heart rates and blood pressure. Studies have shown that hospital noise is a factor in staff job satisfaction and stress and can contribute to staff burnout.
The continuous operation and high ventilation rates demanded of a health-care HVAC system increase noise issues. Common design elements, such as centralized equipment plants, low-velocity airflow, and large duct sections, can be effective in addressing the problems, but such components may not always be feasible or cost-effective to implement.
Addressing acoustical issues in an HVAC system is one of the best ways to reduce the overall noise of a health-care environment. Duct liners are the primary means of reducing HVAC noise because insulation materials—such as wraps, which are used on the outside of HVAC-system components—do little to reduce the transmission of sound waves.
While the acoustical benefits of duct liners are clear, it should be noted that several groups, including the American Institute of Architects (AIA), have made recommendations regarding porous duct-liner materials in hospital applications. Because of concerns about the ability of porous materials to absorb dirt and other particulate, HVAC design professionals must review these recommendations and balance acoustical needs against other health-care indoor-air-quality (IAQ) demands.
Acoustical silencers are another HVAC-system component that can help control noise. Silencers are mechanical muffler-type devices placed between sections of ducts. Acoustical silencers may or may not include internal filler or baffles. Packless silencers employ a honeycomb backing behind perforated metal. Because they do not use these porous materials, packless silencers perhaps are the most common mechanical silencers in hospital applications.
THERMAL COMFORT
HVAC insulations help to ensure that heated or cooled air conditions the occupied spaces of a building, rather than the mechanical, plenum, or interstitial spaces. Given that many patients in a health-care facility wear little clothing, often have little or no control over the clothing they wear, and frequently are unable to adequately regulate their body temperature, delivering thermal comfort is not just a question of warmth, it is a matter of patient health. Further, health-care staff members often engage in physically strenuous activities and may require much cooler environments in which to work or rest comfortably. HVAC insulations help to maintain separate temperatures in patient, staff, and public areas.
When evaluating duct liners and wraps, several material choices are available. Liners and wraps should be evaluated based on a number of factors, including thermal, moisture, noise, and IAQ performance. Following is an overview of the most common HVAC duct-insulation materials.
Fiberglass. From a thermal and acoustical perspective, fiberglass typically is the most cost-effective solution. Fiberglass HVAC insulations mitigate moisture concerns because mold does not grow on inorganic materials. As a result, fiberglass also commonly is used for pipe-insulation products.
For hospital HVAC applications, several organizations and building codes state that fiberglass must be used with a facing material to improve condensation control. Also, because of the porous nature of fiberglass, specifying a fiberglass duct liner may not be feasible for ducts delivering air to certain patient-critical areas. However, using fiberglass duct wraps in hospital HVAC systems generally is accepted under most codes and recommendations.
For more information on fiberglass and other porous duct liners, a notable resource is the 2006 edition of the “Guidelines for Design and Construction of Health Care Facilities,” published by the AIA.
Cotton. Cotton has many of the performance characteristics of fiberglass, including thermal and acoustical benefits. However, because cotton also is a porous material, the same recommendations regarding the use of cotton duct liner in hospital applications apply.
Cotton's primary problem is moisture because the material readily absorbs water.
Elastomeric foam. Elastomeric foam scores high in the thermal-performance category. Because it is a closed-cell product, it is impermeable to water vapor, reducing concerns about moisture condensation. Because the surface of elastomeric foam is largely non-porous, the material does not accumulate dirt and particulate easily, making it a good choice for duct liner in hospital applications. Similar to fiberglass, elastomeric foam is another material commonly used to insulate pipe.
The non-porous nature of elastomeric foam greatly decreases the material's ability to absorb sound waves; thus, it does not perform as well from an acoustical perspective when compared with more porous materials. Additionally, when used as a duct liner, foam is still susceptible to damage from cleaning.
Closed-cell foam. Similar to elastomeric foam, closed-cell foam (such as polyurethane) is a thermally efficient material that handles moisture well and does not require a facer in HVAC applications. Closed-cell foams are less likely to accumulate dirt and particulate than porous materials, but, as with elastomeric foam, the tradeoff is reduced acoustical performance.
Before specifying any foam product, especially products for air-handling systems, design professionals should consult local Life Safety Code requirements and choose materials that meet those specifications.
Open-cell foam. In contrast to closed-cell foam, open-cell foam (such as melamine) performs similarly to fiberglass or cotton because it is porous. In this regard, open-cell foam can accumulate dirt and particulate, so it may not be an appropriate duct-liner material. Similar to fiberglass, open-cell foams are effective in controlling noise.
CONCLUSION
Whether designing a modern Leadership in Energy and Environmental Design-certified hospital or retrofitting a small community health-care facility, spending the extra time to properly evaluate and specify a building's HVAC insulation will improve the system's ability to deliver thermal comfort, reduce noise, and address IAQ and moisture issues. Efficient and smart HVAC systems offer hospital occupants a safe and comfortable environment in which health professionals can be their most productive and patients can recover and recuperate in the best-possible conditions.
The principle scientist for acoustics and noise control, Francis (J.R.) Babineau is a researcher in Johns Manville's building-science platform, which is responsible for technology and application development related to building energy efficiency, comfort, and health and safety. He holds a bachelor's degree in physics and a master's degree in engineering from the University of Colorado. He is a member of the Acoustical Society of America; the American Society of Heating, Refrigerating and Air-Conditioning Engineers; the American Society of Healthcare Engineers; ASTM International; and the Institute of Noise Control Engineering. A frequent lecturer on acoustics, noise control and noise-control materials, and building science, he has six patents and has been published in several technical journals.