Chapter 10: Fans and Drives
Supplemental Information

10.1. Fans. 1

10.2. Capacity control strategies. 1

10.3. Drive systems and arrangements. 4

 

Figures

Figure 10.1: Typical VFD Efficiency vs. Speed. 3

Figure 10.2: Motor and Drive Arrangement Block Access. 5

 

10.1. Fans

Although fans come in a wide variety of designs, shapes, sizes, and configurations. They generally, they fall into two categories:

·       Centrifugal fans This type of fan imparts kinetic energy to the air primarily by centrifugal force. In essence, the air is drawn into the center of the fan wheel where it is captured and contained by blades. These parcels of air are then ‘flung’ to the periphery of the wheel.[1] The wheel itself can have an inlet on one side (Single Width, Single Inlet or SWSI) or an inlet on both sides (Double Width, Double Inlet or DWDI). The design of the blades on the wheel can have a significant impact on efficiency, performance and cost. Common designs are forward curved, backward included, airfoil, and radial.[2]

·       Axial fans This type of fan uses aerodynamic effects to impart velocity to the air as it passes through the impeller. Generally, the air travels along the axis of the fan and impeller as compared the centrifugal design where the air enters the impeller by flowing parallel to the shaft, but exits the impeller radially relative to the shaft. Generally, the impeller for this type of fan will be resemblance to an airplane propeller, but with many more blades.

10.2. Capacity control strategies

Regardless of the design, the rotating nature of the fan wheel can create significant structural loads on the shaft, wheel, bearings, and housing. Issues related to these factors are accounted for in the fan class rating. A fan with a wheel that is rated Class II has a higher speed and pressure capability than the same fan with a wheel that is rated Class I. Therefore, some caution must be used when changing fan speeds in the field to be sure that the new operating point is still within the fan’s class rating.

There are a variety of techniques used to control fan capacity. The most common include:

·       Discharge dampers Dampers located on the outlet of the fan can simply throttle the fan. Basically, the discharge damper increases/makes worse, the system effect associated with the fan outlet, thereby degrading its performance. Generally, this is probably the least expensive but also the least desirable approach due to the efficiency implications of a damper on the fan discharge. It can also be quite noisy.

·       Inlet vanes Inlet vanes modify the performance of the fan by ‘pre-swirling’ the air as it enters the eye of the fan. This has the effect of changing the shape of the fan performance curve as can be seen in Figure 16 of Chapter 13 of the 2000 ASHRAE Systems and Equipment Handbook. This approach is much more desirable than a discharge damper, but not as desirable as a variable speed approach. The emergence of affordable and reliable variable speed drive technology has displaced this approach, but when VAV systems first emerged, it was a common means of achieving the required capacity control and is still found on many existing systems or on systems where the variable speed drives have been eliminated by a value engineering effort.

·       Blade pitch Varying blade pitch is a efficient but mechanically complex approach to controlling capacity on axial fans. The effect is very similar to a speed change as can be seen from Figure 17 in Chapter 13 of the 2000 ASHRAE Systems and Equipment Handbook. Most fans that use this approach require additional maintenance in the form of periodic lubrication, inspection, and over-haul of the mechanical vane pitch control system.

·       Variable speed Currently, this is probably the most common approach to controlling fan capacity due to its efficiency, mechanical simplicity, and steadily improving first cost. Commonly referred to as VSD technology (for Variable Speed Drive), it is not necessarily mean VFD technology, which is a subset. Before modern electronic technology made semi-conductor based Variable Frequency Drives a practical and affordable reality, there were a variety of more exotic approaches used including:

·       Variable speed DC motors These were complex and costly and were usually found only on industrial or very large commercial applications.

·       Hydro-mechanical clutches This technology employed hydraulics and a clutch system to vary the speed of the output shaft relative to the input shaft. They too were not common on commercial HVAC systems and tended to have relatively high mechanical losses.

·       Variable pulley systems Often termed ‘pulley pincher’ drives[3], these systems did find somewhat wide application on commercial HVAC systems. The devices functioned by moving the sides of an adjustable drive pulley towards or away from each other. This changed the effective pitch diameter of the pulley, and thus, the output speed. While capable of modulating speeds, the devices tended to be hard on belts and had relatively high mechanical losses.

·       Solid-state variable frequency drives Typically called VFDs or invertors[4], current technology drives of this type provide a nearly ideal solution to the fan capacity control problem. In and of themselves, they tend to be more efficiency than some of the other approaches (see Figure 10.1 for a typical efficiency plat), but they also tend to maintain the fan efficiency at or near the selected efficiency as they vary its capacity by changing speed. However, this is not without its complications, but paying careful attention to design and commissioning issues can readily overcome any problems and the advantages typically outweigh the disadvantages.

Figure 10.1: Typical VFD Efficiency vs. Speed

While efficiency does decay with load, these drives will generally deliver better efficiency and less decay than some of the other alternatives like variable pulley systems.

Regardless of the technique used, capacity control systems will subject the fan and its components to a wide array of continuously varying performance conditions. The interaction of the multiple operating points with the fan components, system components, and building can lead to a number of surprising and unanticipated problems, especially for larger fans with a lot of power. Examples include:

·       On one late 1990’s project the resonance between the large air handling unit fans and the building resulted in vibration in the building’s structural system under certain operating conditions.

·       In a semiconductor facility, resonance between process exhaust fans and sensitive machinery in the process clean room caused quality control problems.

·       Over the years, there have been multiple occurrences of fan failure related to resonate frequency problems including axial fans shedding blades and centrifugal fan wheels disintegrating.

These problems can be difficult to predict and often show up as commissioning issues. Often, the most viable approach to solving them is to make sure the design incorporates features that will allow you to solve the problem if it occurs. For example, avoiding operation at the triggering condition can solve most of these types of problems. And, most current technology variable speed drives will allow you to program in multiple frequency ranges that the drive will ‘jump over’ as it is commanded through its speed range. Thus, ensuring that the drives that will be supplied for your project include this feature can give the start-up and commissioning team the tools they need to solve this type of problem when if it crops up. Another desirable feature to include in the project is vibration analysis and documentation under a variety of operating modes for large fans, especially if they will be operating at variable capacities and speeds. It is also possible to do tests on the building structure to determine it’s resonate frequency and then use that information for setting up the drive systems. The project structural engineer may also be able to predict the range of resonate frequencies anticipated for the structure and this information can be reviewed by the rest of the team in light of the anticipated operating parameters for the system to allow potential problems to be identified and addressed during design.

10.3. Drive systems and arrangements

In all but direct drive applications, some sort of sheave or pulley and belt system will typically be associated with a fan and its motor. It is not uncommon for one of these pulleys to be supplied in an adjustable configuration to allow the speed of the fan to be easily adjusted by the balancing contractor in the field. While desirable from this standpoint, there are several draw backs to adjustable pulleys or sheaves:

·       Belt service life Most V-belts will provide the best service life if they run with their outside perimeter (the flat part at the open end of the V cross section) slightly above the edge of the sheaves they are installed on. If an adjustable pitch sheave requires significant adjustment, it is not uncommon for the outside perimeter of the belt to run below the top of the sheave sides. This results in extra wear on the belt and can reduce service live significantly.

·       Loss of setting Despite its advantages, adjustability can also be the downfall of adjustable pitch sheaves. It is not uncommon for the balanced setting of the sheave to be lost inadvertently when the belts are replaced, especially if the mechanic performing the work has not been trained regarding adjustable sheaves and mistakenly thinks that the adjustability feature is a convenient way to tension the belt(s) or make the set of belts that they happen to have with them fit. As a result, the once balanced system ends up out of balance and performance suffers. If the new setting delivers less air than was intended, then capacity problems may show up at a later date when design loads show up on the system. If the settings deliver more air than was intended, energy can be wasted, especially if the system is one of the constant volume reheat systems frequently found in hospital or process environments. Both problems can lead to pressure relationship problems if the misadjusted fan happens to be an exhaust fan. If the exhaust is hazardous, a loss of airflow can create a dangerous condition in the area served by the fan that may not be immediately detected.

Fans and there prime movers come in a variety of mounting arrangements. AMCA Standard 99-86 illustrates these along with other standards related to fans and air handling units including dimensioning, motor positions, etc. This information can also be found in most manufacturers fan catalogs. Usually, one or more of the following considerations will dictate the specific arrangement:

·       The needs of the HVAC process, prime mover and drive system Some HVAC applications may be sensitive to potential by-products from the drive system and there-for, may which to place the entire drive assembly outside of the conditioned air steam. Similarly, certain HVAC process may be a hostile environment for belts or motors and installing them outside of the air stream will improve their serviceability and service life. This can be particularly important for exhaust systems handling hazardous and/or explosive or flammable materials where a motor in the air stream could be a source of ignition.

·       The arrangement of the fan By their nature, the arrangement of some fans precludes some of the drive arrangements. In addition, physical constraints of the fan installed location may place limitations on the type of drive arrangement that might be used.

Figure 10.3: Motor and Drive Arrangement Block Access

On this new construction project, access to the inlet side of this SWSI fan, which was difficult to begin with, will be totally blocked by the belts and belt guard between the motor and shaft (red circles). It was too late to solve the problem on this project but a different arrangement may have prevented it. On this project, the maintenance staff will need to remove the belt and drives to inspect the fan wheel.

·       Service requirements Some arrangements may make service of the motor or fan wheel impossible in the installed location or may block access to some other component in the fan room (see Figure 10.3)

·       Balancing A belt and pulley system provides a convenient way to adjust fan speed for balancing purposes. Direct drive fans do not have this option and require other methods to adjust for final balance such as adjustable blade pitch or a variable speed drive. Adjustable blades to not have to be automated but are labor intensive to set as compared to a sheave change. Variable speed drives are attractive from an ease of use standpoint, but add unnecessary cost, complexity, and failure modes to a constant volume system.

·       Heat gains Because they are doing work on the air stream and air is compressible, all fans will show a temperature rise across them, even if the motor is not in the air stream. This temperature rise is called fan heat and can be calculated by converting the fan brake horsepower into btu’s per hour and then solving the following equation for the fan temperature rise:

If the motor is located in the air stream, then the motor efficiency losses will also show up as a part of the temperature rise.[5] For large fans with large motors, this can be a significant load on the system that could be avoided by locating the motor outside of the air stream. These advantages have to be weighed against the complications this can introduce for some arrangements in terms of sealing the drive shaft where it penetrates the casing and vibration isolation.

·       Vibration and sound isolation The method by which vibration and sound isolation will be accomplished can also affect arrangement selection. Mounting the entire fan and drive on an isolation mount will allow the assembly to be further soundproofed by locating it inside an acoustically treated fan casing at the cost of placing the motor in the air stream. By their nature, direct drive fan usually have their vibration isolation problems addressed by the motor mounting arrangement. A hidden but sometimes significant aspect of the vibration isolation technique relates to how the equipment will be seismically restrained (see Functional Testing Basics: Supplemental Information for details).

It is becoming increasingly common for manufactures to provide two parallel fans in packaged equipment. Usually space constraints, redundancy requirements, or both drive this design. When employed, there are several issues that need to be considered.

·       Backdraft Even if the intent of the design is to always run two fans, it is quite likely that at some point in time a failure in the power system, drive system or fan itself will result in one fan needing to operate while the other sits idle. Backdraft dampers are commonly employed to prevent air from the active fan from re-circulating into the inactive fan. However, if not carefully applied, there can be some operational difficulties that will show up during the commissioning process.

·       Surge When two identical fans are operated in parallel, there is a potential for surge to occur between the two fans.[6] This is because it is very difficult to create two fans that are exactly identical and then get them operating at exactly the same point on their performance curve. Since the fans are coupled to the same system and but that system places them at slightly different points on their operating curves, pressure fluctuations can occur as the fans shift around and interact, trying to find a mutually agreeable operating point. The effects from this can range from unnoticeable to noise to (in rare cases) fan damage.

Chapter 18 of the 2000 ASHRAE Systems and Equipment Handbook, AMCA publications 99-86, 200, 201, 202-88, and 203, and the Trane Fan Engineering Handbook are all excellent resources for additional detailed information regarding the topics outlined above.



[1]   It’s the same effect you experienced as a child on the merry-go-round at the playground.

[2]   While less common than the other designs, radial blade fans are sometimes found in exhaust systems, especially exhaust systems that handle materials like dust or other particulate matter or where high pressures are required.

[3]   For those who are wood workers, this is basically the same approach as is used for varying speed on a Shopsmith® multipurpose tool.

[4]   This is a reference to the electronic process going on in most drives; basically the drives take alternating current, rectify it to direct current perform their ‘magic’, and then invert the direct current to create an alternating current out put with the desired frequency and other electrical characteristics necessary to control the motor.

[5]   This temperature rise can be calculated in the same manner as the fan heat but the motor horsepower (vs. fan brake horsepower) at the current operating condition is used.

[6]   This should not be confused with the surge that can occur in a single fan if it is operated at a point on its curve where the pressure difference across it fights with the fans ability to generate that pressure difference causing sporadic flow reversals through the impeller.