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5.9 HVAC Systems And Components
HVAC System Components
Air-Handling Units (AHU’s). Air-handling units shall be sized to
not exceed 11,800 l/s (25,000 cfm). Smaller units are encouraged to
facilitate flexible zone control, particularly for spaces that involve
off-hour or high-load operating conditions. To the extent possible,
“plug-n-play” AHU configurations should be considered, facilitating easy
future adaptations to space-load changes. Psychrometric analyses (complete
with chart diagrams) shall be prepared for each air-handling unit
application, characterizing full and part load operating conditions.
Air-handling unit/coil designs shall assure that conditioned space
temperatures and humidity levels are within an acceptable range, per
programmed requirements, and ASHRAE Standards 55 and 62.
Depending on sensible heat ratio characteristics, effective moisture
control may require cooling coil air discharge dew point temperatures as low
as 10°C (50°F). As required, provide face-by-pass or heat recovery features
to re-heat cooling coil discharge temperatures for acceptable space entry.
Provide a direct form of re-heat and/or humidification only if space
conditions require tight environmental control, or if recurring day-long
periods of unacceptable humidity levels would otherwise result.
Supply, Return and Relief Air Fans: Centrifugal doublewidth double-inlet
forward curved and airfoil fans are preferable for VAV systems. All fans
shall bear the AMCA seal and performance shall be based on tests made in
accordance with AMCA Standard 210. Fans should be selected on the basis of
required horsepower as well as sound power level ratings at full load and at
part load conditions. Fan motors shall be sized so they do not run at
overload anywhere on their operating curve. Fan operating characteristics
must be checked for the entire range of flow conditions, particularly for
fans. Fan drives shall be selected for a 1.5 service factor and fan shafts
should be selected to operate below the first critical speed. Thrust
arresters should be designed for horizontal discharge fans operating at high
Coils: Individual finned tube coils should generally be between six and
eight rows with at least 2.1 mm between fins (12 fins per inch) to ensure
that the coils can be effectively and efficiently cleaned. Dehumidifying
coils shall be selected for no more than negligible water droplet carryover
beyond the drain pan at design conditions. All hot water heating and chilled
water cooling coils shall be copper tube and copper finned materials.
Equipment and other obstructions in the air stream shall be located
sufficiently downstream of the coil so that it will not come in contact with
the water droplet carryover. Cooling coils shall be selected at or below 2.5
m/s face velocity (500 fpm) to minimize moisture carryover. Heating coils
shall be selected at or below 3.8 m/s face velocity (750 fpm).
Drains and Drain Pans: Drain pans shall be made of stainless steel,
insulated and adequately sloped and trapped to assure drainage. Drains in
draw-through configurations shall have traps with a depth and height
differential between inlet and outlet equal to the design static pressure
plus 2.54 mm (1 inch) minimum. Filter Sections: Air filtration shall be
provided in every airhandling
system. Air-handling units shall have a disposable pre-filter and a final
filter. The filter media shall be rated in accordance with ASHRAE Standard
52. Pre-filters shall be 30 percent to 35 percent efficient. Final filters
shall be filters with 85 percent efficiency capable of filtering down to 3.0
microns per ASHRAE 52. Filter racks shall be designed to minimize the bypass
of air around the filter media with a maximum bypass leakage of 0.5 percent.
Filters shall be sized at 2.5 m/s (500 FPM) maximum face velocity. Filter
media shall be fabricated so that fibrous shedding does not exceed levels
prescribed by ASHRAE 52. The filter housing and all air- andling components
downstream shall not be internally lined with fibrous insulation.
Double-wall construction or an externally insulated sheet metal housing is
acceptable. The filter change-out pressure drop, not the initial clean
filter rating, must be used in determining fan pressure requirements.
Differential pressure gauges and sensors shall be placed across each filter
bank to allow quick and accurate assessment of filter dust loading as
reflected by air-pressure loss through the filter and sensors shall be
connected to building automation system.
Ultraviolet light (C band) emitters/lamps shall
be incorporated downstream of all cooling coils and above all drain pans to
control airborne and surface microbial growth and transfer. Applied
lamps must be specifically manufactured for this purpose. Safety
interlocks/features shall be provided to limit hazard to operating staff.
Access Doors: Access Doors shall be provided at air handling units
downstream of each coil, upstream of each filter section and adjacent to
each drain pan and fan section. Access doors shall be of sufficient size to
allow personnel to enter the unit to inspect and service all portions of the
Plenum Boxes: Air-handling units shall be provided with plenum boxes
where relief air is discharged from the airhandling unit. Plenum boxes may
also be used on the return side of the unit in lieu of a mixing box.
Air-flow control dampers shall be mounted on the ductwork connecting to the
Mixing Boxes: Air-handling units shall be provided with mixing boxes
where relief air is discharged from the airhandling unit. Mixing boxes may
also be used on the return side of the unit in lieu of a plenum box. Air
flow control dampers shall be mounted within the mixing box or on the
ductwork connecting to the mixing box.
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