Pressure controlled DCV
Pressure-Controlled DCV (PC-DCV) correspond to the traditional DCV systems (Figure 3). The purpose of static pressure-control is to indirectly control the airflow rates by controlling the pressure in a strategic duct position. PC-DCV requires installation of active DCV units controlling supply and exhaust airflow rates to each DCV room/zone. Controlling fan speed to maintain a constant static fan pressure rise, will result in unnecessary throttling along the critical path during most of the AHU’s operating time, and therefore unnecessary fan energy use. The duct path with the greatest flow resistance from the AHU to any terminal is called the ‘critical path’, and is used to dimension the necessary fan pressure rise.
One unfortunate experience concerning PC-DCV systems, is that minor changes in room demand just redistribute the airflow rate in the duct system, while the airflow rate in the AHU remains more or less constant. The consequence is that no energy saving is achieved, or that the amount of supply air is insufficient. This is normally caused by inadequate precision or wrong placement of the pressure sensor, for example a placement to close to the AHU or next to a branch. A rule of thumb is to place it ¾ out in the main duct.
The main purposes when balancing a PC-DCV system are:
- controlling the placement of the pressure sensor
- setting the right pressure set point
In addition, the balancing will reveal connection and communication errors.
Source from article on “Commissioning for energy-optimal Demand-Controlled Ventilation” by Mads Mysen, Peter G Schild, Axel Cablé and John Woollett