Are 'winglets' a help or hindrance to HVLS fan performance?
Does a winglet actually help the performance of an HVLS fan?
In response to the growing popularity of high-volume, low-speed (HVLS) fans, manufacturers are offering add-ons or upgrades aimed at enhancing fan performance. One of the most common features is the “winglet” or curved end cap, which some manufacturers claim increases downward air velocities, stabilizes air movement, eliminates turbulence, and captures air that would slip off the end of the blade.
Despite these claims, does a winglet actually help the performance of an HVLS fan? Some engineers say that it does not, quite simply because air has weight.
In fact, one cubic foot of air weighs 0.08 lb, and a large, 24-ft HVLS fan can move about 375,000 ft3 of air every minute. That equates to about 30,000 lb of air moving through the fan blades every minute.
A good indication of how much air a fan can move is the thrust, which shows how much force the fan is exerting on the air around it. This is calculated by subtracting the operational weight of the fan from the dead weight of the motionless fan. For example, a large, 24-ft HVLS fan can produce about 100 lb of thrust.
HVLS fans intake and then push a large column of air downward to the floor. When that column of air hits the floor, it changes direction and becomes a floor jet moving outward in all directions until it hits the walls, at which time it travels back up to the ceiling, returning to the area of the fan in a horizontal direction, and is pushed back down to the floor. This is a natural, circular path created by the fan’s huge airfoil blades.
The design and shape of the blades help move air in one direction—down. Once the air is moving in one direction (remember, at 30,000 lb per minute), it wants to continue moving in the same direction. That’s Newton's First Law of Motion. The clean lines of the airfoil shaped blades, without winglets, support air movement in the same direction; nothing is trying to change the direction of all that moving air too quickly.
When a winglet is built into the design of the blade or affixed to its tip, the winglet tries to change the direction of the air too quickly. This increases the torque, making the fan work harder. The winglet does slightly increase the thrust of the fan but decreases the efficiency.
To illustrate this point, MacroAir has measured the thrust and torque of five different 24-ft HVLS fans—with and without winglets—at 25% to 100% of the fans’ maximum speeds. The different fan configurations include:
- 10-blade fan without winglets
- 10-blade fan with winglets
- 6-blade fan without winglets
- 6-blade fan with winglets
- 6-blade extended fan (blade slightly longer to match the overall diameter of a blade with a winglet).
Figures 1 and 2 show the results of the thrust and torque readings.
The results show that the 10-blade fan increases in thrust when fitted with a winglet, but the torque is also increased. The six-blade fan has a similar result but with a less dramatic increase in torque. When using a 6-blade fan with blades extended to the same length that the winglet extends to, the thrust and torque increases are similar to the winglet results.
One good way to look at the overall efficiency of a fan is to find the ratio of the thrust to the torque (see Figure 3). For the 10- and 6-blade fans, the addition of a winglet provides slightly better efficiencies at some speeds but worse efficiencies at others.
Table 1 shows the average increased thrust, torque and efficiency for the three different types of fans. While the winglet slightly increases the efficiency, the extended blade length is more efficient than the fans with the winglets on the blades.
Because there are winglets on airplane wings, it’s tempting to think that we can gain similar advantages by putting them on the ends of fan blades. However, the circular motion of the fan and the fact that each successive fan blade enters the disturbed air of the previous blade results in a minimal performance gain.
The motion of a helicopter rotor is much more analogous to the motion of the fan blades. The British Experimental Rotor Programme spent many years researching various rotor tips and winglets. The program developed a rotor blade with a paddle shaped end, which came to be known as the BERP blade (shown in Figure 4).
NASA and the U.S. Army Research Laboratory performed various tests on the BERP blade compared to a standard blade of equal length. They came to a similar conclusion with the helicopter rotors: “When compared with the baseline rotor, the BERP-type rotor offers no performance improvements in either hover or forward flight.”
Similar to what NASA found with rotor blades, MacroAir found with the addition of specialized winglets or blade ends: it offers little or no significant improvement to the overall fan performance.
If you’re considering adding an HVLS fan to your HVAC arsenal, here are additional things to consider when comparing HVLS fans and manufacturers to ensure you’re getting the best performance for your application and need:
- Performance needs: Take a good look at your facility space and existing HVAC systems to see which HVLS fan best meets your need. Can you take advantage of natural ventilation or do you need to supplement a forced-air system?
- Installation: Building parameters, support beams or structures, clearance requirements, and built-in control features will all affect installation and, ultimately, the fan's performance. To maintain continuous air exchange, MacroAir recommends installing one HVLS fan per 20,000 sq ft of space.
- Customer service and warranties: The HVLS fan manufacturer you select should be just as available as the representative group selling you the fan, and help you fully understand the warranty coverage.
Michael Danielsson is engineering manager of MacroAir, which manufacturers HVLS fans for warehouses, manufacturing plants, airplane hangars, agricultural arenas, and retail facilities.