5.6 kW (7.5 HP), and larger, separate variable speed motor controllers shall
be used for the supply and return air fans. The variable speed motor
controller may be used for smaller systems only if it is cost effective,
saving 50 percent of the energy compared to the constant volume system.
Evaluation of this requirement shall consider hours of operation (24 hours
versus 10 hours) and part-load conditions. The variable inlet guide vanes
shall not be used for fan volumetric control.
2.3.1.5 COILS
(a) Cooling coil velocity shall not exceed 2.6 m/s (500 fpm) for constant
volume systems and 2.8 m/s (550 fpm) for variable air volume systems.
Intermediate drain trough shall be provided for each coil bank more than one
coil high. The external drain of each air handling unit coil section shall
have a deep seal trap and be extended to an open sight drain.
Deep-seal traps will necessitate raising of coil section and most likely the
entire unit above the floor or disposing of drainage on the floor below.
(b) Preheat coils shall generally be of glycol/water solution or steam type.
If a steam preheat coil is used, the designer shall evaluate the impact of
numerous parameters, such as air temperature entering the preheat coil, steam
pressure, modulating or on/off steam control valve, coil length, etc. This
will ensure uniform heat transfer over the coil surface and prevent
stratification of air, which could result in actual freeze-up conditions or
false tripping/alarms of the freezestats. Further, if a steam coil is used
for 100 percent outdoor air handling units, it shall be equipped with
integral face and by-pass damper and two-position steam control valve.
(c) Heating/Reheat Coils: The unit mounted heating coils shall preferably be
hot water type. The face velocity of these coils shall not exceed 2.8 m/s
(550 fpm).
(d) Coil Sizing: When heat recovery equipment is used, the pre-heat/heating
coil and chilled water coil shall be designed to function at full load with,
and without, energy recovery. All coil schedules shall show both entering air
conditions.
(e) Piping: Piping to coils shall be offset and shall have shut-off valves
and flanges or unions to permit removal of the coil from the side of the air
handling unit.
2.3.2 AIR FILTERS
(a) General: For the critical areas, such as surgical suites, recovery rooms,
and intensive care units, the after-filters shall be located on the
downstream of the supply air fans. A factory fabricated diffuser section
shall be installed between the fan and the filters to ensure uniform
distribution of air velocity over the face of the after-filters. For others
areas, pre-filters and after-filters shall be located on the upstream side of
the supply air fan in the same frame.
(b) Filters Efficiencies:
(1) Efficiencies shall comply with test method specified by ASHRAE standard
52.1.
HVAC DESIGN MANUAL
2-6
HVAC SYSTEMS AND EQUIPMENT
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