Selecting Propeller Fans
Generally, fans are used to move air across some form of heat transfer media. The quantity of air (cfm) and resistance to air movement (static pressure) of that heat transfer media is assumed to be well documented and accurate.
Other factors affect fan operation, but these areas are more difficult to quantify and sometimes less accurate. For example, the space and geometry between the fan and heat transfer media can affect static pressure. Bug or trash screens, louvers, different ring configurations and air filters external to the cooling unit inlet or outlet also will increase the static pressure against which the fan operates. The inclination might be to estimate this pressure and then apply a generous safety factor, or rationalize this as not being a significant factor and ignore it. In either case, the fan designer will try to reach a balance between cfm and static pressure.
If the designer overstates the static pressure, the fan likely will move more air than required at a higher power consumption level than necessary. The higher velocity through the heat transfer media may alter the cooling characteristics, reducing overall heat transfer in some cases. By contrast, if the designer understates the static pressure, less airflow than desired will be produced. This can result in fan stall, where airflow drops significantly and vibration or noise increases dramatically.
Once accurate requirements are established, the fan must deliver reliably the necessary performance for the cooling equipment. Visit your fan supplier in person if possible. If a visit is not possible, at least inquire about the supplier's facilities for testing fan performance and methods for applying performance data to rating predictions. Does your supplier try to measure fan flow and pressure at the operating fan, or does he use the more accurate method of using a test facility incorporating nozzles for airflow measurement? Does he use model fan testing for development of performance predictions? If so, is the supplier able to accurately test some full-size fans to verify the extrapolation of model performance data?
It is not always practical for fan suppliers to build test facilities that can test all diameters and combinations of fans. Many companies, especially those supplying larger diameter fans, utilize model fans and some derivative of the fan laws to predict performance of the full-size fans. True fan laws work on the principle that the model fan is a geometrically scaled representation of the full-size fan. Scaling this data is reasonably straightforward and generally accurate. Because customers often desire fan sizes other than those representing scaled-up geometrically similar fans, the fan laws may need to be modified before they can be used to predict performance. As airfoils become more complex (twisted, tapered blending of multiple airfoils from blade root to blade tip), modifying the basic fan laws becomes more difficult to perform accurately. Check with your fan supplier to see that its testing included full-size fans with reduced length blades as well as full-size basic blades. This will help verify the accuracy of its performance predictions.
Many people have a hard time visualizing fan performance because it deals with airflow, which is not readily visible to the eye. Fan performance is similar to pump performance except that the fluid pumped by a fan (air) can change density and is compressible. Otherwise, the fan's cfm is similar to the gal/min on pump curves, and static pressure is similar to the pump head on pump curves. Fan curves are much like pump curves in that peak efficiency usually is not at maximum flow capability and, when plotted, will follow bands out from the peak point.
Environmental FactorsAs equipment is installed closer to populated areas and people become more environmentally conscious, sound becomes more of an issue. While fans are not the only sound-generating components on cooling equipment, they can be major contributors to the overall sound generated by the unit. Fortunately, there are ways to reduce the sound of propeller fans.
If the project is a new piece of equipment, select a fan with thick, wide blades and run the fan at lower speeds to reduce the sound generated by the fan.
Once a fan type has been selected and major modifications to fan geometry are no longer possible, fan speed is the most significant feature that can be changed to reduce generated sound. If you have an existing piece of equipment and a new fan is cost prohibitive, you still may be able to reduce fan speed by increasing the blade pitch. This will compensate for lower fan speed by delivering the same airflow at reduced sound levels. When slowing down a fan and increasing blade pitch, avoid going so far in the reductions that the fan goes into stall. Not only does fan stall fail to provide the desired airflow, operating in a stall condition can significantly increase fan noise and shorten fan life.
Further reductions in sound level can be realized by se-lecting fans that run both at low speeds and low blade pitch. Again, to accomplish this, select a fan with thick airfoils than can deliver the performance with fewer blades at low blade pitch. Be conscious of the impact that reducing the number of blades can have on the fan ring or cylinder (see sidebar).
For applications that are particularly sensitive to sound, look for fans that have few obstructions on the blade surface and tip. Obstructions generate turbulence in the airflow, resulting in higher sound levels. For example, some fans use fabricated or extruded airfoils with open ends at the blade's tip. Adding a cap on the end of the blade will reduce turbulence generated by the open blade end and reduce sound level. Also, a simple blade cleaning sometimes can make minor improvements on existing, operating fans.
The Force Behind the ChangesCustomers are demanding better performing cooling equipment, which is forcing cooling equipment manufacturers and users to take a closer look at the fans they select. There are a number of quality fan suppliers in the marketplace. The key to success is understanding your requirements and matching them with a fan that best suits your application. Generally, the more a customer knows about the fan and the more the fan supplier knows about the application, the better the odds are of a happy marriage between cooling equipment and fan.
SIDEBAR: What Should Your Fan Be made Of? Considering Materials of ConstructionFans are constructed from many different materials, and like everything else, there are pros and cons to any option. Each pro and con must be weighed before a final decision is made. Blades can be constructed from a variety of materials, including fiberglass and metals. Hollow construction fan blades offer thick airfoils that move large volumes of air, yet the blades are lightweight and relatively stiff. Fiberglass construction provides corrosion resistance for applications in chemically aggressive environments.
In the past, following conservative engineering practices, a number of applications required blades with fiberglass construction. However, in today's market, blades are being constructed from aluminum alternatives. For example, many heat exchangers and cooling towers operating near the ocean now use aluminum fans. Several manufacturers offer fan blades constructed of marine-grade aluminum, which provides good corrosion resistance. For more aggressive environments, special coatings can be applied to aluminum blades, offering acceptable fan life at a lower initial cost compared to fiberglass blade construction.
Fan hubs also are constructed from a number of materials ranging from aluminum to all stainless steel. For most applications, an aluminum fan hub may be acceptable. The user should consider upgrading hardware to stainless steel as it is the most likely part of a fan hub to experience corrosion-related problems. Employing this option provides an easy-to-handle, lightweight aluminum hub with extended life expectancy over the typical standard materials.
Many hubs are designed to minimize stress in critical areas. For example, a two-plate hub puts compressive forces into clamp blocks around the blade shank, reducing bending loads in the hardware and blade-to-hub joint area. Older designs using a single-plane structural support for the hub with blades clamped on top can experience significant bending stress from both centrifugal force and aerodynamic loads on the fan blade.
Wet cooling towers can have airborne water droplets that are surprisingly abrasive to a fiberglass fan blade's leading edge. Most molded blades have a parting line at the leading edge. This area is especially susceptible to erosion damage due to trimming material that may be caught in the mold's parting line during blade fabrication. Blades constructed with epoxy resins have more natural erosion resistance than blades using vinylester or polyester resins, but epoxy resins are less desirable to work with from a manufacturing perspective. Make sure fiberglass fan blades with vinylester or polyester resin materials have a barrier material - either external or internal to the fan blade - to protect against erosion at the leading edge.
A high operating temperature generally is not a limiting factor for wet cooling towers, but it can be a factor for induced-draft air-cooled heat exchangers and other equipment. Fiberglass fan blades begin to experience reduced mechanical properties at much lower temperatures than aluminum fan blades. A recommended maximum temperature for fiberglass construction might be 180°F (82°C) while aluminum blades can perform satisfactorily at 300°F (149°C). Be sure to consider the maximum temperature the fan can experience when the process is operating but the fan is off.
A fan's mass moment of inertia varies as much as the materials of construction. Older fan designs may have solid-cast aluminum or other heavy construction methods for the blades, resulting in high moments of inertia. With increasing market pressures, fans generally are applied with less of a safety factor between motor size and fan requirements. Heavy solid-blade fans may experience extended starting times due to the high inertia load that the motor must accelerate to speed. It is not unusual to have electric motor overloads trip from extended starting times when applied on high inertia propeller fans.
Most propeller fans are driven by NEMA design B electric motors. However, other high torque design electric motors or engines sometimes are used as the fan driver. Engine drives can have high acceleration and deceleration loads that must be taken into account during fan design to produce ac-ceptable fan life. Fans with higher moments of inertia may put the drive components (bearings, couplings, gears and belt drives) under additional stress as well.
The need for higher performance fans has driven manufacturers to produce fans with bigger airfoils and fewer blades. Newer design fans may use half as many blades or fewer when compared to older fans. Most fans run inside a ring or cylinder to improve performance. As each blade rotates in the ring, it imposes a force on the cylinder that is related to the fan operating pressures and number of blades. While a three-blade fan may move the same desired volume of air that used to require a six- or seven-blade fan, a careful review of the cylinder should be made to ensure it is structurally capable of the rotating loads generated by three-blade fans. A four- or five-blade fan may be necessary to keep cylinder vibration at acceptable levels.