Adjustable Pitch Fans - Why Should I Specify Them?

Before computerized system design tools were commonly used, designers of duct systems would add up all the pressure drops of the system components, determine the highest pressure drop required, and add some ‘cushion’ to make sure the specified fan could provide the airflow and pressure required. In many cases, the installed fan then provides too much airflow, and can cause many problems. One common solution is to damper or throttle the airflow to the required value.

On systems that include components with variable pressure drops (filters, scrubbers, etc.), and on systems that are not completely installed (only part of a building may be operational, for example), the pressure drop can vary over a wide range during operation. In applications like these, adjustable pitch fans are a good solution. Another would be to use a Variable Frequency Drive (VFD).

The appropriate solution depends on a number of factors. These include: allowable variability in the system airflow and/or pressure, length of time between system changes, energy conservation, and whether it is easy to access the fan.

With the push toward reducing energy, proper selection and application of fans is becoming more important. In many parts of the world, almost all fans are direct drive, saving the losses inherent in a belt drive or gear drive system.

Let’s discuss adjustable pitch props and their features. It is a common perception that an adjustable pitch prop can be selected to exactly match your system requirements. In reality, many of them can get close, but may not be infinitely adjustable.

There are several adjustment techniques available in the marketplace. Some blades can be accessed through an access door in the housing, and the end of the blade can be turned using some type of tool. This design relies on a friction device in the hub to maintain the blade angle, and may not be suitable for some applications. The advantage is that you do not need to access the hub, or remove the prop from the fan. Another design would require the customer to access the hub, and loosen a series of clamping bolts. The blades could then be adjusted individually, either using some type of indicator mark or an angle setting tool. Care must be taken to get all the blades set to the same angle, or you will induce vibration, increase the noise level, or stress the blades and hub. A final design may require removing the prop from the fan, dis-assembling the hub, and placing pins or other locking devices in an alternate location on each blade shank. This latter type does not allow infinite adjustment.

There are also adjustable pitch prop designs that include the ability to adjust the blade angle while the fan is operating. These designs include either a torque arm extending outside the housing, or some type of pneumatic operation of the hub and blade shanks. These units adjust all the blades at the same time, but typically require much more maintenance, due to the large number of moving parts and the high actuation forces required.

The interaction between the fan and system also needs to be considered. Many applications do not require precise flow or pressure control, and can operate satisfactorily over a range of performances. In those cases, any or all of the devices described above can be used, and the decision on which type to use includes first cost, ease of maintenance, energy usage, and other factors. As an example, let’s consider the case where the system specification is 20,000 cfm at 1.0” s.p., and the customer wants to use a 36” direct drive duct fan. A typical curve is shown below, and requires 29.3 degrees of blade angle using a 6-bladed prop.

 adjustable pitch_curve1

                       

Curve 1

This curve represents an infinitely adjustable design. Typically, a select number of blade angles are tested, and the data is interpolated between the angles, as was done for this example. The data for the blade angles on either side of this selection happen to be 28 degrees and 31 degrees. The curve for the same selection showing the data at these blade angles is shown in Curve 2, below. You can see that the requested operating point (20,000 cfm at 1.0” static pressure) is between the two test curves.

adjustable pitch_curve2

Curve 2

This curve represents a prop design with fixed increments in blade angles.

Selection programs and fan manufacturers will treat this type of fan in different ways. Some will decide to build this fan at 28 degrees, and some will decide to build this fan at 31 degrees. Generally, the decision would be made based on whether the selected motor would have enough power to cover the higher curve. In the case of the fan shown on Curve 2, the two power levels are 5.9 bhp and 7.2 bhp, so the higher one would get selected, with a 7 ½ horsepower motor included.

The way the selection information is presented may also be different. Some selections programs would present this fan at 31 degrees, with the performance at 20,525 cfm at 1.05” static pressure (the higher of the two markers on the curve). Some would present it as 20,000 cfm at 1.1” static pressure (locking in the flow and providing a solution at the actual pressure point). Some may give you the option of how you want it selected.

When you compare different selection software outputs, just make sure that you are looking at the same selection values. It is very easy to miss these differences in performance.

In summary, unless your system has exacting requirements for process conditions, many types of adjustable pitch fans will perform satisfactorily. You may be slightly over or slightly under your desired operating point, but system design, system installation, fan laboratory testing, and field performance testing all have their associated tolerances and effects. An adjustable pitch fan is a good way to address all these issues.