Kurt Truniger explains the disadvantages of duct pressure control in variable-volume ventilation systems and shows how they can be replaced with forward-looking, demand controlled systems that comply with EN 15232, Class A requiring highly energy-efficient building automation and technical building management
MUCH HAS already been written about the benefits and possibilities of demand-controlled ventilation (DCV) systems. This technology measures the conditions in the room and calculates the amount of energy actually required. To do this, it uses sensors and control devices for CO2, VOC, temperature, light and so on. The required volume of air is supplied to the room by precise volumetric flow controllers - a technology known as variable air volume (VAV).

If we look at the average consumption of typical room zones Fig. 1, we can see that these are mostly operated at part load. The best efficiency point, the maximum air exchange rate, is only rarely needed.

Fan control
Efficient fan control is a vital part of a DCV system. To regulate fan output, frequency converter-controlled fans are increasingly being joined by EC fans. To adapt the fan power made available to the ventilation system, the DCV system must gauge the ventilation system's requirements and set a suitable setpoint. This is the weak point of traditional duct pressure control systems Fig. 2 and the strength of the pressure feedback Fan Optimiser system Fig. 3.

Duct pressure control - The setpoint K for duct pressure control Fig. 2. corresponds to duct pressure P1, the pressure required to move the maximum air volume V1 through the air duct system. The actual pressure is measured in the air duct, ideally at the most unsuitable point in the ductwork. Question: Where is the most unsuitable point in a variable-volume ventilation system? The answer is it moves around the duct system according to the ventilation system's current load distribution. So it is only possible for the pressure sensor to be incorrectly positioned; it is usually installed immediately downstream of the fan.

The main drawback of this method is that the fan is controlled on an open loop basis. The volumetric flow V2 required at any given moment, is not used to calculate the setpoint as there is no feedback from the volumetric flow controllers available. The duct pressure and VAV boxes are operated independently of each other. If the volumetric flow is reduced from V1 to V2, the pressure in the air duct system rises in line with the fan's characteristic curve. The pressure control system then brings the duct pressure back down to point K, the full load level; the correct, reduced setpoint R is unknown. The downstream VAV boxes are forced to eliminate the surplus duct pressure P1-P2 by throttling the dampers. In practice, systems of this type sometimes contain VAV dampers throttled by up to 10 per cent - see damper diagram Fig. 4. The result: excessive noise and unnecessarily high pressure losses in the air duct system, leading to excessive energy consumption by the fans. Another disadvantage of pressure control is that every change in use and every adjustment to volumetric flow requires a manual correction of the duct pressure setpoint parameter, which in practice usually does not happen - and this has consequences too.

Fan Optimiser - the pressure feedback volumetric flow control system In a pressure feedback volumetric flow control system Fig. 3 the damper positions of the VAV-Compact controllers are gathered via a field bus (MP-Bus, LONWORKS, Modbus, KNX etc.) and used as the trigger for energy efficient control of the fans. The damper positions are evaluated by the Fan Optimiser function and the fans are brought down to the optimum setpoint O until most of the dampers are in the optimum operating range Fig. 4.

Dampers close - falling demand / pressure too high, dampers open - rising demand / pressure too low: the fan is powered up or down accordingly following the ventilation system's characteristic curve (optimised setpoint O). The result: less noise and reduced pressure losses in the air duct system, leading to reduced energy consumption by the fans.

Damper diagram
The efficiency of both methods can be seen in the damper diagram Fig. 4. In the Fan Optimiser system, the lowered duct pressure P2 takes strain off the system and helps to extend the lifetime of the actuators through the reduced
number of part-cycles.

Further advantages:
· A Fan Optimiser system automatically finds the required operating point at any given time, so there is no need for time-consuming balancing work of the duct pressure system.
· With Fan Optimiser technology, duct pressure controlled systems require no reserve p additions.
· Changed system configurations due to a change of use etc. are detected automatically by the Fan Optimiser.
· Undersized systems will work as long as the required set volume O is less than the available volumetric flow. This is not generally the case with duct pressure controlled systems.

System design
Pressure feedback Fan Optimiser systems can be designed in two ways: a) DDC/programmable controllers: Bus master devices with custom-programmed Fan Optimiser application and b) Fan Optimiser hardware: Device with preconfigured, ready-to-use Fan Optimiser function, e.g. COU24-A-MP.

In both variants the VAVCompact controllers are integrated into the control system with a field bus (MP-Bus, LONWORKS, Modbus, KNX etc.) and the Fan Optimiser function uses the damper positions to calculate the setpoint for the air supply.

If the system is configured as a bus system, or if a bus system is already installed, there are no additional hardware costs. The duct pressure control equipment and sensor positioning are not required for solutions a) and b).

Conclusion
DCV systems are good for zone-by-zone demand-controlled energy supply. To operate the whole variable-volume ventilation system efficiently, the fans need to be fully integrated in the DCV system.

The complete integrated system solution for this is the pressure feedback Fan Optimiser. From room level to air supply, it works on the principle of only as much as necessary - not as much as possible.

//The author represents Belimo Automation AG//