Handbook of air conditioning and refrigeration / Shan K

23.6 CHAPTER TWENTY- THREE
cfm/person (L/s-person). Time-based constant-volume ventilation control intends to supply
constant-volume conditioned outdoor air to the conditioned space for dilution during the occupied
period even if the occupant density drops.
Compared with the time-based constant-volume ventilation control, demand-controlled ventilation
has the following advantages:
.DCV supplies the demanded amount of outdoor air to the conditioned space to maintain an
acceptable air quality and, at the same time, saves heating and cooling energy to condition the
excessive amount of supplied outdoor air in the time-based constant-volume ventilation
control. According to the simulation results in Carpenter (1996), compared to a system without
DCV, l000-ppm CO2-based DCV reduces 25 to 30 percent of space heating energy use. With
1000-ppm CO2-based demand-controlled ventilation, the average ventilation rate is 0.1 to 0.3
ach (air changes per hour), 10 to 30 percent lower than that of the constant-volume ventilation
control-
.DCV actually has taken into account the dilution power of the infiltrated outdoor air, which resuIts
in a greater energy saving.
.DCV often uses CO2 or a group of mixed gaseous contaminants as an indicator (controlled variable).
The space concentration of CO2 itself provides feedback of the control system, whereas in a
constant-volume ventilation control system, the volume flow rate of the outdoor air is difficult to
measure exactly.
In a VAV system, DCV is nonnally activated only in minimum outdoor air recirculating mode.
During the air economizer cycle, the amount of outdoor air intake in the AHU or PU is always
greater than the system outdoor air volume flow rate ~.sys. The amount of outdoor air intake is determined
by the free-cooling requirement of the V A V system.