| Application Considerations
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.) |
