ASHRAE Standard 62 sets minimum ventilation rates and defines acceptable indoor air quality (IAQ) to "avoid adverse health effects." It also presents general system and equipment requirements for outdoor airflow, air distribution, design documentation and microbial growth control. In short, Standard 62 dramatically impacts the design, layout and arrangement of the air handler and its components.

Provide Adequate Ventilation Airflow

ASHRAE Standard 62-1989 increased the per-person outdoor air requirement from the 1981 version’s 5 cfm to 20 cfm in commercial office spaces. (Other types of occupied spaces such as operating rooms require even higher ventilation rates.) It also mandates provision for measuring the amount of outdoor air brought into the building as proof of compliance. Consequently, it’s important to include a means of monitoring airflow in your Modular Climate Changer designs.

Direct measurement of airflow — i.e., either a factory-mounted Traq ™ damper assembly, or a duct-mounted hot-wire- anemometer or pitot-tube monitoring station — provides greater accuracy and reliability than indirect monitoring methods such as mixed-temperature or trace gas calculations, fan-inlet monitors, fixed-position dampers or fixed-differential pressure plates. See "AHU Components: Mixing Box Module" later in this section for a brief summary of the Traq damper’s operating characteristics and application considerations.

Protect Hydronic Coils from Freezing

Bringing more outdoor air into the air handler to satisfy Standard 62’s ventilation requirement increases the likelihood of air stratification. If a layer of freezing air moves through the air handler, it can damage unprotected hydronic cooling and heating coils. Traditionally, a low-limit thermostat (or "freeze-stat") installed on the entering- air side of the cooling coil trips when it detects a dangerously low air temperature. That stops the supply fan, closes the outdoor air damper . . . and ultimately degrades the building’s indoor air quality.

It’s important to design the air handler so that it effectively treats the required amount of outdoor air — regardless of temperature — without risking coil damage, tripping the low-limit thermostat or compromising indoor air quality. Following are several means of providing coil protection. Choose the technique that best suits the application’s requirements.

• Drain the coils. This approach necessitates vent and drain connections on every coil, plus shutoff valves to isolate them from the chiller(s).
  
• Add glycol to the cooling system water to lower its freezing point, as well as an inhibitor to resist corrosion.
  
• Preheat the outdoor air stream with a steam coil, hot-water coil or energy- recovery device to raise its temperature above freezing and eliminate any entrained snow before it reaches the cooling coil.
  
• Use an air blender to improve mixing of the outdoor and recirculated air streams.
  
• Introduce ventilation air downstream of the cooling coil with dual-path or bypass techniques.

Recover Energy to Reduce Operating Costs

The increased ventilation requirement imposed by Standard 62 often increases the air handler’s heating and cooling loads which, in turn, leads to higher operating costs. Air-side energy recovery techniques offer an excellent means for alleviating this added cost burden by preconditioning entering outdoor air with energy salvaged from the exhaust air stream (i.e., warming and humidifying entering outdoor air during the heating season, or precooling and dehumidifying entering outdoor air during the cooling season). Reducing energy use with recovery can offset the increased first cost of the heat exchanger. It also minimizes air stratification, might permit downsizing of mechanical equipment, and may be eligible for utility rebates.

As described later in this section (see "AHU Components: Energy Recovery Module), one of the Modular Climate Changer’s "building blocks" is a custom module that makes energy recovery an integral part of the air handler’s design; see Figure A-2. This approach eliminates the need for a discrete outdoor-air energy-recovery system, reducing installed cost.

Control Particulate and Gaseous Contaminants

ASHRAE Standard 62-1989 emphasizes the importance of including appropriate filters in the air handling system to effectively control particulate (e.g., dust, fibers) and gaseous (e.g., oxidants, formaldehyde) contaminants.

Controlling Particulates.

To provide good indoor air quality, both the Environmental Protection Agency (EPA) and ASHRAE recommend that the concentration of particulates in the air not exceed 0.05 to 0.07 mg/m3. While Standard 62 doesn’t set specific particulate filtration guidelines, other design manuals recommend filtration levels of 90-percent arrestance and 40- to 60-percent dust spot efficiency for new air handlers. Filters with dust spot efficiencies greater than 50 percent remove most microorganisms from the passing air stream. (ASHRAE Standard 52 specifies the internationally recognized procedure for testing particulate filters used in HVAC systems, and defines "arrestance and "dust spot efficiency" as performance measures.)