Handbook of air conditioning and refrigeration / Shan K

ASHRAE Standard 62-1999
control, a volume flow of outdoor air that exceeds the amount of minimum outdoor ventilation air is
always supplied to the conditioned space. Ventilation problems often occur during minimum outdoor
air recirculating mode, i.e., minimum ventilation controls. Currently used minimum ventilation
controls in VAV systems include the following:
CO 2-based demand-controlled ventilation
Mixed plenum pressure controls
Outdoor air injection fan control
Fan tracking control
AIR SYSTEMS: MINIMUM VENTILATION AND VAV SYSTEM CONTROLS 23.3
As discussed in Sec. 4.10, ASHRAE Standard 62-1999, Ventilation for Acceptable Indoor Air Quality,
specifies two alternative procedures to obtain acceptable IAQ:
1. Ventilation rate procedure. In the ventilation rate procedure, acceptable indoor air quality is
achieved by providing ventilation air of specified quality and quantity to the conditioned space.
This procedure prescribes the following:
● The quality of the outdoor air shall be acceptable for ventilation, i.e., contaminants in outdoor air
do not exceed the concentrations listed in the Table of National Primary Ambient-Air Quality
Standards by EPA in Sec. 4.10.
● Outdoor air treatments are required when necessary.
● Ventilation rates for typical spaces are listed in Table 4.5. For details, refer to ASHRAE Standard
62-1999.
● Ventilation systems for spaces with intermittent or variable occupancy may have their outdoor air
quantity adjusted by damper or cycling of fan operation to provide sufficient dilution to maintain
contaminant concentrations within acceptable levels at all times.
● For intermittent occupancy, when contaminants are associated only with occupants or occupant
activities, do not result a health hazard because contaminants are dissipated during unoccupied
periods.
● When contaminants are independent of occupants or occupant acivities, the supply of outdoor air
should lead occupancy lag or lead time depending on the ventilation rate and air capacity per person
in the space.
● Where peak occupancies are less than 3 h in duration, the outdoor airflow rate may be determined
according to average occupancy for the duration of operation of the system provided the average
occupancy used is not less than half of the maximum.
2. Indoor air quality procedure. In the indoor air quality procedure, acceptable air quality is
achieved within the conditioned space by restricting the concentration of all known contaminants of
concern to some specified acceptable levels. The acceptable concentration level in Table of National
Primary Ambient-Air Quality Standards, as listed in Sec. 4.10, also applies indoors for the same exposure
time. The following limits apply for four other indoor contaminants according to Standard
62-1999:
Contaminant Concentration ppm Exposure time
Carbon dioxide CO2 1.8 g/m 3 1000 Continuous
Chlordane 5 �g/m3 0.0003 Continuous
Ozone 100 �g/m3 0.05 Continuous
Radon 4 pCi/L Annual average