FANS
Precision positioning perfects performance
Ian Thomas, product technical manager – air flow controls, TROX UK, discusses effective fan speed optimisation using damper blade positioning.
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ariable air volume capability has increased the efficiency of HVAC systems dramatically, in comparison to constant air volume systems, through the reactive control of fan speeds. However, there is an opportunity to achieve even greater improvements in energy efficiency. By relying on the measurement of duct pressure to dictate fan speeds, systems miss out on a chance to drive down energy consumption still further. This article will explore an alternative methodology based on fan speed optimisation using damper blade positioning which can achieve an additional 45% reduction in energy consumption. Conventional VAV control
In a conventional constant duct pressure system, pressure transducers are utilised to measure duct pressure, and the results determine the speed of the fan. As Figure 1 illustrates, the damper closes to reduce volume flow as a result of demand reducing, which in turn causes system pressure to rise, following the fan curve. In constant duct pressure systems, the controller addresses this by reducing fan speed, and thus the system pressure drops back to the set point (275 Pa).
The problem with this methodology, however, is that at a lower volume flow, the 275 Pa set point, which is being maintained by the controller, is no longer appropriate in terms of the characteristic fan curve. The more appropriate pressure would be 125 Pa, to follow the curve. There are three significant disadvantages to this method of operation.
Firstly, because of the forced pressure drop required to achieve the set point of 275 Pa, the VAV units are creating pressure which is not actually needed, which results in a degree of energy wastage, as shown in Figure 2. Secondly, the system is forced to operate in its least effective working area (damper position below 40% open) as a result of the deviation from the characteristic curve (as a set point of
10 January 2021
275 Pa is being maintained rather than 125 Pa which better reflects the curve). This brings us to the third disadvantage, which is that, as well as requiring higher energy consumption to meet the same load, this damper position also elevates operational noise.
Fan speed control via damper positioning The latest generation VAV technology makes it possible to rethink fan speed control by utilising damper blade positioning data rather than pressure. This avoids the pitfalls of constant duct pressure systems, whilst retaining and improving upon the benefits of the variable air volume. In the age of the smart building, VAV units are readily available with BMS protocols including BACnet ms/tp, Modbus RTU, KNX and LON. Most projects today, however, install VAV controllers with analogue 1-10v communication, fitting a local digital BMS controller to take command of the VAV with analogue signals. This limits the amount of data that can be communicated.
The best practice is to use a digital VAV controller, which makes all data available over the BMS network. This provides the opportunity to increase levels of real-time data management in contrast to an analogue alternative. The position of the damper blade itself can be used to indicate the appropriate fan speed to meet demand. This cuts out the need for fan speed to be controlled by a pressure transducer, as well as the need for the measurement of duct pressure and the maintenance of unnecessarily high pressure set points. Instead, as demand falls, both fan speed and pressure fall in tandem, following the characteristic curves of the devices themselves, as shown in Figure 3.
This brings about a number of direct benefits. Firstly, enabling the fan speed and pressure to reduce uniformly along the device’s characteristic curve makes it possible for energy consumption to be reduced by harnessing the
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