Optimising Performance Of Airside Systems
The difference between AHU and FCU is that the latter is usually used for small product categories handling smaller air volume (up to 3,500CMH). FCUs are usually powered by a single phase permanent split capacitor motor and the inefficiency of the motor is often the limiting factor for maximum airflow that can be achieved...
Airside systems distribute air for heating, ventilating or cooling a building. By driving the efficient flow of air through the entire building, such systems help deliver healthy, comfortable and visually appealing environments that increase productivity and comfort. To increase the performance of Heating, Ventilating, and Air Conditioning (HVAC) systems, a more complete and holistic approach to designing the complete air path is required. Fixtures along the air path upstream or downstream of Air Handling Units (AHU) or Fan Coil Units (FCU) - which are part of the HVAC system – play an important role in determining the performance of the entire setup.
The difference between AHU and FCU is that the latter is usually used for small product categories handling smaller air volume (up to 3,500 CMH). FCUs are usually powered by a single phase permanent split capacitor motor and the inefficiency of the motor is often the limiting factor for maximum airflow that can be achieved. However, the emergence of higher efficiency motors and fans for FCU have redrawn the boundaries which will be covered in this article.
The discussion here will focus on how to achieve further energy savings in AHU and FCU by optimising the performance of the motor and fan. Some areas we will be touching on include:
• Forward Curved (FC) Fans vs. Backward Curved (BC) Fans
• Double Inlet Double Width (DIDW) Fan vs. Plug Fan
• Electronic Commutated (EC)Plug Fan
• Brushless DC (BLDC) Electric Motors / Electronic Commutated (EC) Motors
FC Fans vs. BC Fans
The common assumption propagated within the industry is that BC fans always deliver higher efficiency than FC Fans. However, this claim is debatable.
There are two key parameters that affect the performance of airside systems are airflow and static pressure. In general, there is a direct correlation between the two. When the required AHU or FCU is to deliver small air volume, the demand of static pressure is likely to be low as well.
The underlying reason for this is that small air volume serves small areas and as such does not need to go through great lengths of conduit to distribute the air.
In such a situation, FC centrifugal fans are the better choice as they are made of large quantities of machine stamping fan blades best suited to the application and delivers the best efficiency at low air pressure demand. It is for this reason that most FCUs use FC centrifugal fans as their primary workhorse.
In addition, it is imperative to note that FC fans can deliver airflow far more efficiently than BC fans if the application is for low static pressure. The chart above, plotted on a blower size of 560mm fan wheel diameter of DIDW fan normally used in AHU, demonstrates this point.
FIGURE 1: PERFORMANCE OF AN FC FAN MEASURED AGAINST A BC FAN...
As shown in the Figure 1, the FC fan outperforms the BC fan below 600 Pascal total static pressure, with the margin increasing when total static pressure decreases.
This brings up an interesting question: could a whole air system be designed in such a way that the overall static pressure demand is lower for the AHU?
If this is achieved, a cheaper FC fan that delivers better efficiency, and also benefits from increased efficiency from lower air resistance design, could be used.
Beyond that, the adoption of the latest low air resistant electronic filter to replace the traditional high air resistant filter media in AHU would help greatly in reducing the overall static pressure demand.
This opens up the possibility of using cheaper FC fans to reduce the cost of AHU, in addition to energy savingsfor the air system design.
Double Inlet Double Width (DIDW) Fan vs. Plug Fan
Another misconception is with regards to using plug fans in AHU applications for better efficiency. By meticulously running through the AHU rating software on the various airflow at various static pressure, we can see that a traditional DIDW BC fan outperforms the plug fan by almost 30% in term of efficiency.
FIGURE 2:PERFORMANCE OF DIDW BC FAN IN AHU APPLICATION MEASURED AGAINST A PLUG FAN...
Figure 2 shows that the AHU with the DIDW BC delivers approximately 30% better efficiency from the baseline compared to an AHU unit with the fan plug.
Some may argue that direct driven plug fans allow for the avoidance of the cumbersome belt and pulley drive for hygiene reasons, and others while varying fan speed via Variable Frequency Drive (VFD). A simple way to resolve these would be to set it up such that the DIDW BC fan is directly driven via direct shaft coupling.
Furthermore, the DIDW BC fan is more suitable for AHU applications as it allows the overall AHU unit design length to be kept shorter while keeping the overall AHU casing design in negative air pressure. This is in contrast to the need to generate positive pressure in the fan chamber when using a plug fan which is no easy feat.
The example below shows a 60,000CMH AHU installed with a DIDW BC fan and plug fan. With the DIDW BC fan, the length of the AHU is kept at 3,842mm where else the use of a plug fan means that the length of the AHU stretches to 4,182mm. The difference is significant as it impacts plant room space as well as an increase in other associated costs.
However, using a plug fan in an AHU does allow for the supply air duct to be connected to any position of fan chamber capitalising on the positive pressure being generated.
AHU WITH DIDW FAN
AHU WITH PLUG FAN
Electronic Commutated (EC) Plug Fan
The use of EC direct driven plug fans to improve performance was boosted by the efforts of German fan manufacturers who through some ingenious design managed to integrate a BLDC motor, plug fan and VFD into a single piece, allowing for ease of installation.
'INTEGRATED' EC PLUG FAN
The EC plug fan draws its efficiency from two fronts: the motor and the fan. By using a permanent magnet in the BLDC motor, the efficiency of the motor jumps to above 95% at full load for a 3kW motor model, even higher than IE4, the highest possible efficiency class reserved for future for cage induction motor under unifying worldwide efficiency classification, IEC60034-30 standard.
By taking advantage of engineered plastic mouldable capability, the EC plug fan can further eke out efficiency by having aerodynamic plastic plug fan blade design – to enable smoother airflow while reducing air turbulence. This can be fully simulated in advanced fluid dynamic software before any production starts.
IEC MOTOR EFFICIENCY GRAPH...
AHU WITH EC PLUG FAN...
FIGURE 3: PERFORMANCE OF EC FAN IN AHU APPLICATION MEASURED AGAINST A DIDW FAN...
All in all, by applying an EC plug fan into an AHU application, a 30,000CMH airflow AHU power consumption can be plotted as shown in figure 3.
However, a point to note would be that even with all the advancements for EC fans, a 30,000CMH AHU with EC fan is only more efficient at the total static pressure below 600 Pascal; above that, a DIDW fan is more efficient for AHU.
There is limit for the economically viable size of EC plug fans to be moulded with engineering plastic and hence, multiple EC plug fans are adopted in AHU design for achieving the required airflow. This does come with a cost however; if redundancy is in play then the multiple fan array design could become the critical feature for clients such as datacenter, or for healthcare applications which eschews single point of failure.
Brushless DC Electric (BLDC)Motors/ Electronic Commutated (EC) Motors
The adoption of higher efficient FCU is slower than desired but this may soon change when more and more countries embrace Green Mark or LEED (Leadership in Energy and Environment) standard to push for building efficiency and environment responsibility. The Singapore Green Mark push is unprecedented by formally enacting it into law for buildings to comply with pre-determined standards before being certified fit for occupation. The current baseline requirement on FCU efficiency is 0.17watt/CMH fan system input power.
The adoption of BLDC motor into FCU may come sooner as we think as the industry continues to raise the bar of baseline efficiency requirement year on year. The efficiency gain of switching over to BLDC/EC motor from single phase permanent split capacitor motor is significant – as it is notoriously difficult for permanent split capacitor motor to achieve above 60% efficiency.
PERFORMANCE OF EC FAN IN FCU APPLICATION MEASURED AGAINST AC MOTOR...
FIGURE 4: ENERGY LOSS GRAPH ON DIFFERENT TYPES OF MOTORS...
Figure 4 shows that just by embracing BLDC/EC motorover the rest will enable an efficiency gain of 25 to 30%. However, the main gain of BLDC/EC motor is not purely at the full load efficiency play. The capability of not tapering too much in the motor efficiency during part load brings in more savings in spite of being capable of providing exact airflow requirement at all times via step-less speed modulation.
The further boundary push is made possible by improving on fan design.Improving airflow fluid dynamics through engineering plastic moulding capability would allow the overall FCU efficiency to reach below 0.10 watt/CMH – a level never seen before.
Taking advantage of superior EC plug fan efficiency, the maximum ampere drawn by single phase electricity power supply can be well below the range the maximum general single phase electrical wire can accommodate, while delivering airflow up to 5,000CMH at the external static pressure of 150Pascal and beyond. This whole new category of large air flow single phase power supply FCU would not be possible if not for the superior efficiency of EC plug fan.
In conclusion, improving the performance of airside systems can often be achieved by optimising the yield of components. Hence, it is critical that the industry to adopt a holistic approach at the design stage to obtain the desired efficiency and performance from the entire system.
AUTHOR CREDIT AND PHOTOGRAPH
Chai Huei The