Handbook of air conditioning and refrigeration / Shan K

23.4 CHAPTER TWENTY-THREE
The new Standard 62-1999 contains the entire 1989 version, unchanged, along with the following
four new addenda:
● Addenda 62e removes the statement that ventilation rates in Table 2 accommodate a moderate
amount of smoking because of the dangerousness of second-hand tobacco smoke.
● Addenda 62c removes the requirement that the temperature and humidity conditions specified in
Standard 55 be maintained in all ventilated space, such as even in garages.
● Addenda 62d rewords the standard scope to state that compliance with the standard will not ensure
acceptable indoor air quality.
● Addenda 62f clarifies that carbon dioxide (CO 2) is simply a useful indicator of the concentration
of human bioeffluents.
For details, refer to ASHRAE Standard 62-1999. Standard 62-1999 is under continuous maintenance
by a Standing Standard Project Committee for which the Standards Committee has established
a documented program for regular publication of addenda or revisions.
Recirculation of Unused Outdoor Air in Multizone Systems
For a multizone VAV system, the ratio of outdoor air to supply air required to meet the ventilation
and thermal load requirements is often different from zone to zone. The system ventilation air required,
in most cases, is actually the amount of outdoor air intake at the AHU or PU. The problem
is how the system-fixed ratio of outdoor air to supply air can satisfy the different ratios of outdoor
air to supply air in various control zones.
In a specific control zone, if the ratio of outdoor air to supply air is higher than is needed in this
zone, the result is an increase in unused outdoor air in the recirculating air (more truly, the concentration
of air contaminants is lower) which raises the dilution power of the supply air (including
outdoor and recirculating air) for other control zones.
Methods of calculating ventilation requirements using the unused outdoor air contained in recirculating
air were developed in Australia and are included in ASHRAE Standard 62-1999. The required
corrected fraction of outdoor air supply in system supply air Y, in cfm (L/s), considering the
outdoor requirement of the critical zone and the unused outdoor air contained in the recirculating
air can be calculated as
and
Y �
X
1 � X � Z
Y � V˙ o,cor / V˙ s
X � V˙ o,sys / V˙ s
Z � V˙ o,cr / V˙ s,cr
where V˙
s � supply volume flow rate of air system, cfm (L/s)
V˙ o,cor � corrected outdoor air supply volume flow rate, considering critical zone and unused
outdoor air in the recirculating air, cfm (L/s)
V˙ o,sys � calculated system outdoor air volume flow rate, cfm (L/s)
V˙ o,cr � outdoor air volume flow rate required in critical zone, cfm (L/s)
V˙ s,cr � critical zone supply volume flow rate, cfm (L/s)
X � uncorrected fraction of outdoor air supply
Y � corrected fraction of outdoor air supply
Z � fraction of outdoor air in supply air in critical zone
(23.1)
(23.2)