A well-designed HVAC system keeps temperature and moisture within a range that is considered comfortable for humans, while constantly renewing the air in a building and filtering out pollutants. Mechanical engineers are, in large part, responsible for ensuring that an HVAC system is operating as it should. The system must also provide an adequate airflow, since stagnant air and draftiness are both detrimental for performance.
In most residential and commercial settings, deviations from the ideal operating conditions are allowable if they are transitory, and there is generally a broad range of acceptable temperature and humidity values. However, there are sensitive environments such as healthcare facilities, where optimal conditions must be kept at all times. Many HVAC systems in less demanding environments are controlled based on temperature only, and humidity is controlled indirectly. However, sensitive environments require that each variable be monitored and controlled independently, and specialized high-performance filters may be required by codes.
Humidity Control in Sensitive Environments
Precise humidity control is typically required for environments with sensitive electronic equipment, healthcare facilities and other similar locations were human life or important systems are at stake. For example, the relative humidity levels for healthcare typically range from 40 to 60 percent:
- Bacteria and viruses thrive with both low and high humidity levels.
- Patients who suffer from asthma or allergic rhinitis also experience symptoms in response to humidity extremes.
- Dry air absorbs moisture from mucous membranes, reducing the body’s ability to fight off infections.
- Low humidity also increases static electricity accumulation, and discharges can damage modern medical equipment, which is important for medical procedures and generally expensive
- Dust has a higher tendency to become airborne at low humidity levels, further increasing the chance of triggering allergic reactions.
- High humidity creates the ideal conditions for mold and dust mites.
Depending on weather conditions, an HVAC system may be required to operate in humidification or drying mode at different times of the year. Some areas of a sensitive environment may have more stringent requirements than others; surgery rooms in hospitals are an example of this. It is the responsibility of qualified mechanical engineers to understand what is needed across various projects.
There are two main approaches for controlling air humidity independently: the HVAC system can use cooling and heating coils in series, or a desiccant wheel can be deployed.
- Cooling and heating coils: With this approach, air is cooled and dehumidified by the cooling coil until the desired relative humidity is reached. Since this normally results in overcooling, air then flows through a heating coil to raise its temperature back to an acceptable level. This way, both temperature and humidity requirements are met.
- Desiccant wheel: This device captures air humidity downstream from the cooling coil, and releases it upstream for it to be condensed and gathered. At design conditions, this system does not require any heating input, although a preheating coil is added in case extra dehumidification is required.
Desiccant wheels typically save energy because they eliminate the need for overcooling and reheating. There may be exceptions, however, so it is important to assess each installation independently.
Healthcare humidification systems are often based on steam, since heating water to high temperatures ensures the destruction of bacteria, especially Legionella. When steam is injected into an airstream, both humidification and heating are accomplished in the same step.
In the most sensitive environments, such as surgery rooms, steam-based humidification is normally required by law to ensure that the system is free from airborne bacteria. Adiabatic humidification is accepted in some sensitive applications, and it provides considerable savings compared with steam systems, although it is necessary to ensure it can be used legally.
Vapor Diffusion Retarders
Vapor diffusion retarders, also known as vapor barriers, complement air drying and humidification systems by providing a barrier against the diffusion of moisture through walls or other elements of the building envelope. Vapor diffusion retarders are classified into three main categories, depending on their rated permeance value:
- Class I vapor barriers are rated for 0.1 perms or less. Some examples are glass, sheet metal and polyethylene.
- Class II vapor barriers are rated for permeance values above 0.1 perms but less than or equal to 10 perms. Plywood and unfaced extruded polystyrene are two examples.
- Class III vapor barriers have permeance values above 10 perms, and some examples are gypsum board, cellulose insulation, bricks and concrete blocks.
The specification of vapor barriers is strongly dependent on weather conditions, and can be especially challenging in northern states, due to how drastically temperature and relative humidity fluctuate throughout the year. Getting in touch with a qualified design firm is highly recommended.
Ventilation for Sensitive Environments: Air Changes per Hour and Filtering
Ventilation systems for sensitive environments must meet specific requirements in terms of air changes per hour (ACH). In surgery rooms, for example, the American Institute of Architects establishes 15 ACH, where 20% must be outdoor air.
- In a surgery room with a floor area of 600 ft2 and a height of 10 ft, 15 ACH is equivalent to 90,000 ft3 per hour, or 1500 cfm. The outdoor air required would be 300 cfm to meet the 20% requirement.
Filters for sensitive applications must typically meet a minimum MERV rating, and in applications that are especially sensitive compliance with the HEPA standard may be required.
MERV stands for Minimum Efficiency Reporting Value, and it is a measurement scale for the effectiveness of filters, which was developed by ASHRAE in the 80s. The scale of MERV ratings ranges from 1 to 16, where larger numbers indicate that the filter is rated for smaller particles and has a higher average arrestance.
- MERV 1-4: 60 to 80% arrestance, particles larger than 10.0 µm.
- MERV 5-8: 80 to 95% arrestance, 3.0 to 10.0 µm.
- MERV 9-12: 90 to 98% arrestance, 1.0 to 3.0 µm.
- MERV 13-16: Over 95% arrestance, particles from 0.30 to 1.0 µm.
In healthcare applications, filters with MERV ratings of 7 or more are normally specified. In some applications, two filters in tandem are used, where the second has a higher MERV rating than the first.
HEPA stands for High-Efficiency Particulate Arrestance, and a filter must remove 99.97% of particles with a diameter of 0.3 µm to qualify as such. It is important to note that the term HEPA has been adopted to refer to any high-efficiency filter, but only those meeting the requirements set forth in the standard are real HEPA filters.
In healthcare applications, HEPA filters are widely used thanks to their ability to capture airborne bacteria and viruses. Once they have been trapped, high-power ultraviolet lights are used to kill them.
It is important to note that higher performance filters also involve an increased pressure drop, raising energy consumption. For this reason, it is important to select a filter with adequate performance for the application, but not over-specified.
General Recommendations from Mechanical Engineers for Sensitive Environment HVAC Design
The most important requirement for HVAC systems in sensitive environments is being able to control humidity and temperature simultaneously, while filtering out pollutants. Therefore, designs based on rules of thumb should be avoided:
- Sizing air conditioning equipment in tons per square foot of floor area.
- Sizing ventilation equipment in cfm per ton of HVAC capacity.
Instead, each system must be designed by mechanical engineers to meet a specific temperature and humidity range, as well as air changes per hour and percentage of outdoor air. Hiring the services of qualified engineering professionals is highly recommended to ensure that requirements are met.