Handbook of air conditioning and refrigeration / Shan K

24.2 CHAPTER TWENTY-FOUR
● IAQ is closely related to the health of the occupants inside a building, whether a building is a
healthy building or a sick building.
● IAQ and thermal control (zone temperature and relative humidity control) indicate primarily the
quality of the indoor environment in a building.
● IAQ and thermal control represent mainly the functional performance of an air conditioning
(HVAC&R) system.
As described in Sec. 4.10, the indoor air contaminants are the basic pollutants that affect IAQ.
According to the results of many field investigations and recent IAQ conferences, the primary IAQ
problems and the appropriate mitigations and improvements are based on the three basic strategies:
contaminated source control, removing contaminants by air cleaner, and ventilation air dilution,
which are summarized and discussed in the following sections.
24.2 VENTILATION, DILUTION OF
CONCENTRATION OF CONTAMINANTS
Outdoor Air Requirements for Ventilation
Persily (1989) took field measurements of the outdoor ventilation rates of 14 office buildings for approximately
one year in 1983 and found that they are typically between 0.6 and 1.2 ach, with a mean
value of 0.94 ach. Among these 14 office buildings 52 percent had a minimum level of outdoor air intake
that was lower than the design level, 45 percent had less than 20 cfm/person (10 L/s�person), 8
percent had less than 10 cfm/person (5 L/s�person), and 1 percent had less than 5 cfm/person (2.5
L/s�person). ASHRAE Standard 62-1999 specified outdoor air requirements for ventilation for each
occupant in various commercial facilities. Some of these requirements are listed in Table 4.5. Outdoor
air requirements for ventilation per person V˙ o,req, such as 20 cfm/person (10 L/s�person) in an office
building, must be provided at both design flow and part-load reduced volume flow in VAV systems.
From Eqs. (23.2) and (23.3), the calculated system outdoor air volume flow rate V˙ o,sys � NocV˙ o,req, in
cfm (L/s). Here Noc indicates number of persons, and V˙ o,sys is the minimum outdoor air intake at design
flow in an AHU or a PU for the dilution of the concentrations of indoor air contaminants. For a
typical office building with a ceiling height of 9 ft (2.7 m) V˙ o,sys is 0.93 ach, as listed in Sec. 23.2.
According to Mendell (1993), a statistically significant correlation between outdoor ventilation
rate in offices and sick building syndrome complaints was found. Ventilation has been proved to be
the most important factor to improve IAQ.
Importance of Time of Operation and Ventilation Rate
An often discussed topic is whether the type of HVAC&R system is of any significance that affects
the aspect of ventilation and thus the SBS and IAQ. Sundell (1996), based on a study of 210 office
buildings in northern Sweden, noted that the type of air system—whether it was a modern new
HVAC&R system or an old simple system—had no significance on the aspect of ventilation and
IAQ. Instead, the important aspects of ventilation were the time of operation and the ventilation
rate. If the time of operation of the air system to provide ventilation was less than 10 h/day, the risk
of SBS was increased. As the ventilation rate cfm/person (L/s� person) was reduced, the odds of
SBS symptoms were raised accordingly.
Adopting an Air Economizer and Purge Operation
Select an AHU or a PU with an air economizer and purge mode control. As discussed in Sec. 23.11,
if an air economizer is used, the amount of outdoor air intake in the AHU or PU during 100 percent