Pumps and Systems, June 2009
Adjustable Frequency Drive Application and Use
In the early days of adjustable frequency drive (AFD) technology, the typical application was in process control for manufacturing synthetic fiber, steel bars and aluminum foil. Because AFDs improved performance and lowered maintenance costs, they replaced motor generator sets and DC drives. When the energy crises occurred in the early 1970s, saving energy became a critical goal, and the use of AFDs quickly spread into large pump applications and eventually into HVAC fan systems.
Adjustable Frequency Devices Compared To Throttling Devices
In many flow applications, a mechanical throttling device is used to limit flow. Although this is an effective means of control, it wastes mechanical and electrical energy because of high losses. Figure 1 demonstrates that an AFD will yield a reduction in energy use in the same system.
Figure 1. A mechanical throttling device versus an AFD.
If a throttling device is employed to control flow, energy usage is as shown in the upper curve of Figure 2, while the lower curve demonstrates energy usage when using an AFD. The difference is the energy saved.
Figure 2. The amount of energy saved by using an adjustable frequency drive (versus a valve or damper) to control flow.
Adjustable Frequency Drive Theory
The Affinity Laws can determine the system performance for centrifugal devices, including theoretical load requirements and potential energy savings. The first curve in Figure 3 shows that flow varies linearly with speed. If speed decreases to 50 percent, flow decreases to 50 percent.
Figure 3. The Affinity Laws.
The second curve demonstrates that pressure or head varies as the square of speed. If the speed is decreased by 50 percent, then the flow is 50 percent flow from the first curve, but the pressure or head will be only 25 percent from the second curve. The third curve shows the power required for a particular flow requirement; energy varies as the cube of speed. If the speed is set to 50 percent, flow is 50 percent at 25 percent pressure, but at only 12.5 percent power. The potential for energy savings is available as the flow requirement is reduced.
Adjustable Frequency Drive Application in a Pump System
We will now look at something a little less theoretical.
Figure 4. The characteristics of a typical pump system.
First, static head or lift is the height the fluid must be lifted from the source to the outlet. In this example, the lift is 30 ft. The second element is the friction head-the power required to overcome the losses caused by the flow of fluid in the piping, valves, bends and any other devices in the piping. These losses are completely flow dependant and are nonlinear. Add the two heads to obtain the system curve, which describes what flow will occur given a specific pressure. Therefore, if the flow is 200 gallons, then the pump head pressure will be 180 ft. A pump manufacturer uses this information to select pumps and impeller sizes.
Figure 5. The relationship between the system and the pump selection.
For example, in Figure 5, a 9-in impeller was chosen, based on the desired operating point.
Figure 6. A combination of the system and pump curves demonstrates the relationship between the system and pump selection.
In Figure 6, the system curve and pump performance curve cross at the desired operation point of 120 ft of head pressure and 160 gpm of flow. The system will have this one operating point only, unless something else is added. A typical flow control technique is to add a throttling valve.
Figure 7. Partially closing the valve adds further restriction and raises the system losses.
In Figure 7, the flow rate will be determined by where the new system curve crosses the pump curve. In this case, about 155 ft of head results in a flow of 80 gpm. The amount of energy consumed to do this is proportional to the head pressure and flow rate (represented by the blue shaded area in Figure 7).
If an AFD is used to control the flow, some interesting things happen. Since there is no additional restriction added to the piping, the system curve remains the same. By varying the AFD's speed, it is as if a new pump with a smaller impeller is used. A new pump curve is created (see Figure 8).
Figure 8. Introducing an adjustable frequency drive makes it seems as if a new pump with a smaller impeller is employed.