06-06
2.3 SUPPLY AIR REQUIREMENTS (MECHANICAL COOLING)
A. The supply air volume shall be established to meet the cooling load
requirements of the occupied space. The supply air volume shall be
altered, if required, to meet any exhaust requirement.
B. If the HVAC system selection includes an all air, variable air volume
(VAV) system, the supply air volume, during part load conditions,
shall be established to ensure stable performance of the supply, and
more so, the return air fans. The reasons for this requirement are:
1. The VA selected sequence of operation for the VAV systems is such
that a constant difference in CFM, amounting to the outdoor air
quantity for the air handling system, shall be maintained between
the supply and return air fans at all times. If the supply air fan
volume is allowed to drop to a very low value, the corresponding
return air volume shall be even lower as the return air fan has to
maintain a constant difference in CFM with respect to the supply
air volume at all times. Under this condition, the supply (and
particularly the return) air fans could fall in an unstable region
where the fan operation is not recommended by the equipment
manufacturers.
2. The supply and return air fans of the VAV systems are generally
equipped with variable speed drives. If these drives are allowed
to run at very low speeds for a long period of time, the fan
motors will tend to overheat and cause damage to the winding
insulation.
2.4 OUTSIDE AIR REQUIREMENTS
A. GENERAL: The outdoor air for ventilation shall be computed in
accordance with ASHRAE 62-1989; however, how it is actually supplied
to the occupied spaces would depend upon the type of HVAC system
selected as explained in 2.4B and 2.4C.
The outdoor air quantity shall be increased, if required, to meet the
space exhaust requirements or to meet the manufacturer's
recommendations when radiant ceiling panel cooling system is used.
B. ALL AIR SYSTEMS: With an all air system, the outside air is mixed
with the return air, and the supply air is distributed on the basis
of the cooling load requirements. Since the distribution of the
supply air is done on the basis of the cooling load requirements, the
spaces with greater cooling requirements will, naturally, receive
more outside air. It is, thus, recognized that the uniform
distribution of outdoor air is inherently difficult with an all air
system.
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