Most fans are sized to handle the largest expected peak design condition. Because normal operating conditions are often well below these design conditions, air-moving equipment is often oversized, operating below its most efficient point and creating several types of problems. Among these problems are high energy costs, high system pressures and flow noise, and, in systems with high particulate contents, erosion of impeller and casing surfaces. Consequently, the combination of extended operating times and the tendency to oversize the air-moving equipment creates a need for efficient flow control. To accommodate demand changes, the volume of air is adjusted by four principle methods:
- Outlet dampers
Outlet or discharge air dampers are installed to add resistance at the fan. . As dampers close, they reduce the amount of flow and increase pressure on their upstream side. By increasing system resistance, dampers force fans to operate against higher backpressure, which reduces their output. As a fan works against higher backpressure, its operating point shifts to the left along its performance curve. Fans operating away from their best efficiency points suffer increased operating and maintenance costs. Sizing of discharge air dampers should be done with great care. There are many rules-of-thumb, but the recommended procedure is to size the discharge air dampers for a wide open pressure drop of from 7 to 10 percent of the system pressure.
Advantages: Damper control is a simple and low-cost means of controlling airflow
Disadvantages: The penalty is high resistance and increased fan horsepower.
- Inlet vane control
Inlet vanes often referred to as pre-rotation vanes, cause the air to swirl before it encounters the fan wheel. The fan wheel cannot “grip” the air as well and consequently, capacity is reduced more efficiently than with discharge damper control. Excess pressure is not created and wasted. The fan inlet vanes are positioned by an actuator in response to a signal received from the system static pressure receiver-controller.
These pre-rotating swirls lessen the angle of attack between the incoming air and the fan blades, which lowers the load on the fan and reduces fan pressure and airflow. By changing the severity of the inlet swirl, inlet vanes essentially change the fan curve. Because they can reduce both delivered airflow and fan load, inlet vanes can improve fan efficiency. Inlet vanes are particularly cost effective when the airflow demand varies between 80 and 100 percent of full flow; however, at lower airflow rates, inlet vanes become less efficient.
Advantages: Because variable-pitch fans maintain their normal operating speed, they avoid resonance problems that can be problematic for certain fan types. Additionally, variable-pitch blades can operate from a no-flow to a full-flow condition without stall problems. During start-up, the fan blades can be shifted to a low angle of attack, reducing the torque required to accelerate the fan to normal operating speed.
Disadvantages: Disadvantages of this flow-control option include potential fouling problems because of contaminant accumulation in the mechanical actuator that controls the blades. Also, because motor efficiency and power factor degrade significantly at loads below 50 percent of rated capacity, operating at low loads for long periods may not provide efficiency advantages and can incur a low power factor charge from the utility.
- Variable Pitch Blades
Variable pitch axial-flow fans deliver an amount of air in accordance with
the pitch of the fan blades. As more or less air is needed in the system, an actuator positions the pitch of the fan blades accordingly. The positioning of the blades is similar to the positioning of the inlet vanes. However, the fan is always spinning while inlet vanes remain stationary. The degree to which the blades are pitched determines how much air can be “gripped” and passed on into the system Variable-pitch fans allow the fan blades to tilt, changing the angle of attack between the incoming airflow and the blade.
- Fan speed control
Changing the rotational speed is the most efficient. The laws governing fan operation provide a useful clue that power varies as cube root of RPM or airflow. When fan speed decreases, the curves for fan performance and brake horsepower move toward the origin. Fan efficiency shifts to the left, providing an essential cost advantage during periods of low system demand. Reducing fan speed can significantly reduce energy consumption. For example, according to the fan laws, reducing fan rotational speed by 20 percent decreases fan power by 50 percent.
If the volume requirement is constant, it can be achieved by selecting appropriate pulley sizes. If the volume varies with the process, multiple-speed fans or variable-speed drives (VSDs) can be used. VSDs allow fan rotational speed adjustments over a continuous range, avoiding the need to jump from speed to speed as required by multiple-speed fans.
Many types of ASDs are available, including mechanical (eddy current drives, variable-ratio pulley, and hydraulic drives), direct current (DC motors), and electronic. Although mechanical drives and DC motors have been applied extensively in industrial settings, they are seldom used in commercial buildings for economic or technical reasons. The mechanical variable-ratio pulley is applicable to commercial buildings (from 5 to 125 horsepower), but space requirements and mechanical problems usually make commercial applications impractical. DC motors comprise a mature technology, but they are expensive and have a reputation for high maintenance costs. The electronic load-commutated inverter has also been used in industry, but it is not an energy-conscious choice for commercial buildings. Frequency operated adjustable speed drives commonly known as variable frequency drives (VFD’s) are most commonly used ASD, largely because of their proven effectiveness in reducing energy costs.
Advantages of VFDs: Among the primary reasons for selecting VFDs are improved flow control, ability to retrofit to existing motors, their compact space advantages, and elimination of the fouling problems associated with mechanical control devices. The benefits include:
- VFDs decrease energy losses and offer substantial savings with respect to the cost-per-unit volume of air moved.
- VFDs eliminate the reliance on mechanical components, providing an attractive operational advantage, especially in “dirty” airstreams.
- VFDs decrease airflow noise during low system demand, they can improve worker comfort.FDs offer operating improvements by allowing higher fan operating efficiency and by increasing system efficiency as well.
- VFDs provide soft-start and allow the motor to be started with a lower start-up current (usually about 1.5 times the normal operating current in contrast to 5 to 6 times higher than normal operating currents in case of normal motors). This reduces wear on the motor windings and the controller. Soft starting a fan motor also provides benefits to the electrical distribution system reduces voltage sags and wear on the motor windings.
Disadvantages of VFDs: VFDs are not appropriate for all applications.
- Decreasing the rotational speed of a fan too much often risks unstable operation, especially with axial fans and some centrifugal fans, such as backward inclined airfoil and forward-curved types. With these fans, careful review of the performance curves should precede the selection of a VFD.
- Fans, like most rotating machinery, are susceptible to resonance problems. Resonance is an operating condition in which the natural frequency of some component coincides with the frequency set up by the rotation. Fans are usually designed so that their normal operating speeds are not near one of these resonant speeds. However, decreasing the rotational speed of a fan increases the chances of hitting a resonant speed. The effects of operating at resonant speeds can be damaging. Shafts, bearings, and foundations are particularly susceptible to problems with resonance.
- When a fan’s rotational speed is reduced, the fan generates less pressure, and some fans, like many types of turbo-machinery, operate poorly against shut-off conditions. If a VFD slows the fan, the static pressure requirement may exceed the pressure generated by the fan and no airflow will be generated.
- In some VFD applications, power quality can also be a concern. VFDs operate by varying the frequency of the electric power supplied to the motor. The solid-state switching that accompanies inverter operation can create voltage spikes that increase motor winding temperatures, accelerating the rate of insulation degradation. To account for the added winding heat, conventional motors usually must be de-rated by 5 to 10 percent when used with VFDs. A classification of motors known as “inverter-duty” has been developed to improve the matching of VFDs to motors.
- VFDs can also generate electrical noise that interferes with the power quality of the supporting electrical supply. These problems are typically correctable with the installation of coils or electrical filters.
- Energy Comparison
Figure below shows the approximate power savings that can be obtained by reducing air quantities for the four methods of capacity control. From power consumption standpoint, variable speed motors and blade pitch control are the most efficient. Inlet vanes save some power, while discharge dampers throttling at the fan save little. From a first-cost standpoint, dampers are the least costly. Inlet vanes and blade pitch control follow, with variable speed motors being the most expensive.
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