Fan System Components

A typical fan system consists of a fan, an electric motor, a drive system, ducts or piping, flow control devices, and air conditioning equipment (filters, cooling coils, heat exchangers, etc.). To effectively improve the performance of fan systems, designers and operators must understand how other system components function as well. The “systems approach” requires knowing the interaction between fans, the equipment that supports fan operation, and the components that are served by fans. Prime Movers. Most industrial fans are driven by alternating current (AC) electric motors. Most are induction motors supplied with three-phase, 240- or 480-volt power. Because power supplies are typically rated at slightly higher voltages than motors because of anticipated voltage drops in the distribution system, motors are typically rated at 230 or 460 volts. In recent years, because of efforts by the National Electrical Manufacturers Association (NEMA) and motor manufacturers, the efficiency of general-purpose motors has significantly improved. These improvements are also attributable to the Energy Policy Act (EPAct), which for most motors went into effect in October 1997. To improve motor efficiency, motor manufacturers have modified motor designs and incorporated better materials, resulting in slight changes in motor operating characteristics. Although initial costs of the motors have increased 10 to 20 percent, for high run-time applications, improvements in motor efficiency create very attractive paybacks through lower operating costs.

A characteristic of induction motors is that their torque is directly related to slip, or the difference between the speed of the magnetic field and the speed of the motor shaft. Consequently, in many fans, actual operating speeds are usually around 2 percent less than their nominal speeds. For example, a theoretical four-pole induction motor with no slip would rotate at 1,800 rpm with a 60-hertz power supply; however, rated operating speeds for this motor are usually around 1,750 rpm, indicating that slip rates are a little over 2.7 percent at rated load. Fans that are driven by older motors are probably operating at much lower efficiencies and at higher levels of slip than what is available from new motors.

Upgrading to a new motor can reduce operating costs, because of improved motor efficiency, while offering slightly improved fan performance. EPAct efficiency motors operate with less slip, which means fans rotate at slightly higher speeds. For applications that can effectively use this additional output, this high efficiency can be attractive. However, if the additional output is not useful, the added power consumption increases operating costs.

Another component of the prime mover is the motor controller. The controller is the switch mechanism that receives a signal from a low power circuit, such as an on/off switch, and energizes or de-energizes the motor by connecting or disconnecting the motor windings to the power line voltage. Soft starters are electrical devices that are often installed with a motor controller to reduce the electrical stresses associated with the start-up of large motors. In conventional systems, the high in-rush and starting currents associated with most AC motors creates power quality problems, such as voltage sag. Soft starters gradually ramp up the voltage applied to the motor, reducing the magnitude of the start-up current. As industrial air blower increase the use of computer-based equipment and control systems, soft starters are becoming important parts of many motor control systems. In fact, a major advantage associated with most VFDs is that they often have built-in, soft-start capabilities.

Another common characteristic of motors used in fan applications is multiple speed capability. Because ventilation and air-moving requirements often vary significantly, the ability to adjust fan speed is useful. Motors can be built to operate at different speeds in two principal ways: as a single set of windings equipped with a switch that energizes or de-energizes an additional set of poles, or with the use of multiple windings, each of which energizes a different number of poles. The first type of motor is known as a consequent pole motor and usually allows two operating speeds, one twice that of the other. The second type of motor can have two, three, or four speeds, depending on application. In general, multiple-speed motors are more costly and less efficient than single-speed motors. However, the flow control benefit of different motor speeds makes them attractive for many fan applications.

Drive System.

The drive system often offers substantial opportunities to improve fan energy efficiency and to lower overall system operating costs. There are two principal types of drive systems: direct drive and belt drive. Gear drives are also used but are less common. In direct drive systems, the fan is attached to the motor shaft. This is a simple, efficient system but has less flexibility with respect to speed adjustments.

Because most fans are operated with induction motors, the operating rotational speeds of direct-drive fans are limited to within a few percent of the synchronous motor speeds (most commonly 1,200, 1,800, and 3,600 rpm). The sensitivity of fan output to its operating rotational speed means that errors in estimating the performance requirements can make a direct-drive system operate inefficiently (unlike belt drives, which allow fan rotational speed adjustments by altering pulley diameters). One way to add rotational speed flexibility to a direct-drive system is to use an adjustable speed drive (ASD). ASDs allow a range of shaft speeds and are quite practical for systems that have varying demand. Although ASDs are generally not a practical option for centrifugal fans that are only required to operate at one speed, ASDs can provide a highly efficient system for fans that operate over a range of conditions.

In axial fans, direct drives have some important advantages. Applications with low temperatures and clean system air are well-suited for direct drives because the motor mounts directly behind the fan and can be cooled by the airstream. This space-saving configuration allows the motor to operate at higher-than-rated loads because of added cooling. However, accessibility to the motor is somewhat restricted.

Belt drives offer a key advantage to fan systems by providing flexibility in fan speed selection. If the initial estimates are incorrect or if the system requirements change, belt drives allow flexibility in changing fan speed. In axial fans, belt drives keep the motor out of the airstream, which can be an advantage in high temperature applications, or in dirty or corrosive environments.

There are several different types of fan belt drives, including standard belts, V-belts, cogged V-belts, and synchronous belts. There are different cost and operating advantages to each type. In general, synchronous belts are the most efficient, while V-belts are the most commonly used. Synchronous belts are highly efficient because they use a mesh-type contact that limits slippage and can lower operating costs. However, switching to synchronous belts must be done with caution. Synchronous belts usually generate much more noise than other belts. They also transfer shock loads through the drivetrain without allowing slip. These sudden load changes can be problematic for both motors and fans. Another problem with synchronous belts is the limited availability of pulley sizes. Because the pulleys have a mesh pattern, machining them alters the pitch diameter, which interferes with engagement. Consequently, pulleys are available in discrete sizes, which precludes an important advantage of belt drives: the ability to alter operating rotational speeds by adjusting sheave diameters. Because of these factors, synchronous belts are not as widely used as V-belts in fan applications.

In contrast, V-belts are widely used because of their efficiency, flexibility, and robust operation. V-belts have a long history in industrial applications, which means there is a lot of industry knowledge about them. An important advantage to V-belts is their protection of the drivetrain during sudden load changes. Service conditions that experience sudden drivetrain accelerations cause accelerated wear or sudden failure. While synchronous belts tend to transfer these shock loads directly to the shafts and motors, V-belts can slip, affording some protection. Although they are less efficient than synchronous belts, V-belts offer many advantages such as low cost, reliable operation, and operating flexibility. In applications that use standard belts, upgrades to V-belts should be considered.

Although they are not commonly used, gear systems offer some advantages to belt systems. Gear systems tend to be much more expensive than belt drive alternatives; however, gears tend to require less frequent inspection and maintenance than belts and are preferable in applications with severely limited access. Gears also offer several motor/fan configurations, including in-line drives, parallel-offset drives, and 90-degree drives, each of which may provide an attractive advantage in some applications. Gear-system efficiency depends largely on speed ratio. In general, gear efficiencies range from 70 to 98 percent. In large horsepower (hp) applications (greater than 100 hp), gear systems tend to be designed for greater efficiency because of the costs, heat, and noise problems that result from efficiency losses. Because gears require lubrication, gearbox lubricant must be periodically inspected and changed. Also, because gears—like synchronous belts—do not allow slip, shock loads are transferred directly across the drivetrain.

Ductwork or Piping.

For most fan systems, air is directed through ducts or pipes. In general, ducts are made of sheet metal and used in low-pressure systems, while pipes are sturdier and used in higher-pressure applications. Because ducts are used for most air-moving applications, “duct” will be the common reference for this sourcebook; however, most of the same principles can be applied to pipes.

In ventilation applications in which a fan pulls directly from a ventilated space on one side and discharges directly to an external space (like a wall-mounted propeller fan), duct losses are not a significant factor. However, in most applications, ducts are used on one or both sides of a fan and have a critical impact on fan performance. Friction between the airstream and the duct surface is usually a significant portion of the overall load on a fan.

As a rule, larger ducts create lower airflow resistance than smaller ducts. Although larger ducts have higher initial costs in terms of material and installation, the reduced cost of energy because of lower friction offsets some of these costs and should be included during the initial design process and during system modification efforts. Other considerations with ducts are their shape and leakage class. Round ducts have less surface area per unit cross sectional area than rectangular ducts and, as a result, have less leakage. In hot or cool airstreams, this surface area also influences the amount of heat transferred to the environment.

Duct leakage class, typically identified by the factor CL (which has units of cfm/linear foot) is an indicator of duct integrity. Variables that determine CL include the type of joints used in construction, the number of joints per unit length of duct, and the shape of the duct. Depending on the length of the duct system, leakage can account for a significant portion of a fan’s capacity. This is especially applicable to systems with rectangular ducts that have unsealed joints. In many cases, the system designer can improve the performance of the ventilation system by specifying ducts that have low C_Ls.

Airflow Control Devices.

Flow control devices include inlet dampers on the box, inlet vanes at the inlet to the fan, and outlet dampers at the outlet of the fan. Inlet box dampers are usually parallel blade dampers. Inlet vanes adjust fan output in two principal ways: by creating a swirl in the airflow that affects the way in which the air hits the fan blades, or by throttling the air altogether, which restricts the amount of air entering the fan. The inlet vanes and dampers must be designed for proper fan rotation and are to be installed in such a way that these inlet vanes and dampers open in the same direction as the fan rotation. The pre-rotation or swirl of the air helps reduce the brake horsepower of the fan. If the inlet dampers on the inlet box are located too far away from the inlet of the fan, the effect of pre-rotation may be lost or reduced, and horsepower savings may be negligible.

The outlet damper, when used for controlling airflow, is usually of opposed-blade design for better flow distribution on the discharge side of the fan. If the outlet damper is going to be used for open/ close service or for isolating the fan, a parallel-blade discharge damper may be used. Typically, fans with inlet vanes provide better power savings while operating the fan at part load conditions, as opposed to fans with inlet box dampers operating in a similar situation. Inlet vanes provide better controllability with optimum power savings compared to other dampers. Outlet dampers adjust resistance to airflow and move the operating point along the fan’s performance curve. Because they do not change air entry conditions, outlet dampers do not offer energy savings other than shifting the operating point along the fan horsepower curve.

Dampers can be used to throttle the air entering or leaving a fan and to control airflow in branches of a system or at points of delivery. Dampers control airflow by changing the amount of restriction in an airstream. Increasing the restriction creates a larger pressure drop across the damper and dissipates some flow energy, while decreasing the restriction reduces the pressure differential and allows more airflow.

From a system perspective, proper use of dampers can improve energy efficiency over traditional system designs, especially in HVAC systems. In variable-air volume (VAV) systems, dampers are effective at rerouting airflow and at controlling the amount of air delivered to a particular workspace. Because VAV systems are much more energy efficient than their precursors (constant-volume or dual-supply systems), dampers can be used to lower system operating costs.

However, in many applications, dampers can decrease fan efficiency. Dampers decrease total fan output by increasing backpressure, which forces the operating point of a fan to shift to the left along its performance curve. Often, as the fan operating point moves to the left along its curve, it operates less efficiently and, in some cases, may perform in an unstable manner. Unstable fan operation is the result of an aerodynamic phenomenon in which there is insufficient air moving across the fan blades. The airflow rate surges back and forth resulting in inefficient performance, annoying noise characteristics, and accelerated wear on the fan drive system.

Another airflow control method that is available for axial fan applications is the use of variable pitch blades. Variable pitch fans control fan output by adjusting the fan blade angle of attack with respect to the incoming airstream. This allows the fan to increase or decrease its load in response to system demand. In effect, this method is similar to that provided by inlet vanes, which adjust the angle of attack of the entering airstream by creating a swirl in the airflow pattern. Variable pitch fans provide a highly efficient means of matching fan output to system demand.

Another method of airflow control is fan speed adjustment. Recalling the fan laws, speed has a linear relationship with airflow, a second-order relationship with pressure, and a third-order relationship with power. By slowing or speeding up a fan, its output can be adjusted to match system demand. In general, fan speed adjustment is the most efficient method of airflow control.

There are two primary speed control options: multiple-speed motors and ASDs. Multiple-speed motors have discrete speeds, such as “high,” “medium,” and “low.” Although these motors tend to be somewhat less efficient than single speed motors, they offer simplicity, operating flexibility, a relatively compact space envelope, and significant energy savings for fan systems with highly variable loads. ASDs include several different types of mechanical and electrical equipment. The most common type of ASD is a VFD. VFDs control the frequency of the power supplied to a motor to establish its operating speed. Unlike multiple speed motors that operate at discrete speeds, VFDs allow motors to operate over a continuous range of speed. This flexibility provides accurate matching between fan output and the flow and pressure requirements of the system.

Air Conditioning and Process Equipment (Filters, Heat Exchangers, etc.).

Other equipment commonly found in air-moving systems includes devices used to condition the airstream to obtain certain properties. Heat exchangers are used to heat or cool an airstream to achieve a particular temperature or to remove moisture. Filters are used to remove unwanted particles or gases. Conditioning equipment influences fan performance by providing flow resistance and, in some cases, by changing air density. Filters, including cyclone types or mesh types, inherently create pressure drops, which are often significant components of the overall system pressure drop. Mesh-type filters create increasingly large pressure drops as they accumulate particles. In many systems, poor performance is a direct result of inadequate attention to filter cleanliness.

Cyclone filters remove particulates by rapidly altering the direction of the airflow so that heavy particulates, unable to change direction quickly, get trapped. Although cyclone filters are less effective than mesh filters, they tend to require less maintenance and have more stable pressure-drop characteristics.

The effects of heating and cooling coils on fan system performance depend largely on where in the system the heat exchangers are located, the extent of the temperature change, and how the heat exchangers are constructed. Where there are large changes in airstream temperature, fan performance can change as the air density changes. Heat exchangers that have closely spaced fins can accumulate particulates and moisture that not only impact heat transfer properties, but also increase pressure losses.

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