Many industrial sectors have discovered the benefits of using variable speed drive (VSD) control in a wide variety of applications. Businesses in the commercial and public sectors can also utilise the benefits of VSDs, largely in applications involving pumps and fans, such as ventilation and air conditioning systems, combustion air for boiler systems, chillers and water–pumping systems.
Now… let’s talk about the three applications where variable speed drives really rock in energy saving:
Although dampers are often used to regulate the output of fans, reducing the speed of the fan is a much more energy efficient way of achieving the same effect. This is illustrated by the graph below (Figure 1).
Note: Because of the losses inherent in VSDs, there is a point where they will be less efficient than dampers. This is around 92%-95% and is not shown on this simple graph.
With damper control, the input power reduces as the flow rate decreases. However, under variable speed drive control, the variable torque characteristic of the fan means that the relationship between flow and the speed of the fan is such that the input power reduces in a cube law relationship with the speed reduction.
This type of fan application could be used in industrial cooling, commercial ventilation systems and combustion–air control systems for boilers.
As well as being a more efficient use of power, using VSDs in fan applications can also result in reduced noise in heating and ventilation air–duct systems due to the elimination of dampers.
Making flow rate changes in a damper system can lead to unwanted vortexes appearing in the air flow, which create noise and vibration in addition to the mechanical noise generated from the damper changing position. In a VSD system, making flow rate changes generally only results in slight changes to the noise levels, which are normally undetectable to the ear. Figures 2 and 3, overleaf, demonstrates this.
Flow rate changes using dampers compared with using VSDs
Typical fan and motor with flow controlled by a damper
In the figure 2, flow is controlled by a damper, which is informed by a sensor (in some applications this could be a flow meter). The motor continues to operate at a constant speed regardless of flow required. In addition, some kinds of dampers, such as butterfly valves, can cause adverse vortexes in the flow.
Typical fan and motor with flow controlled by a variable speed drive
In the figure 3, the pressure sensor is connected directly to the variable speed drive which slows the motor when required. As well as leading to better automatic control of the motor’s speed, the adverse flow conditions are also improved.
In a similar way to using damper control in fan applications, using throttle control for pumping applications results in a poor efficiency as the pump is not being run at its design point. This is particularly true for applications with a steep flow/head characteristic, where there is a high static head such as in underfloor heating circuits, geothermal pumps and for small pumps.
Further details on this will be available from equipment suppliers.
This is illustrated in the graph, right, where the broken line indicates the power input to a fixed speed device and the solid line indicates the power input to a VSD. The shaded area represents the power saved by using a VSD for a given flow.
Note: To be strictly accurate, there is a crossover point where a VSD can use more energy than a fixed speed power input. This is because of the losses inherent in VSDs which are not balanced by savings when incorrectly used.
It is important to note that on systems with a high static head (for example, boiler feedwater pumps), where the pump must overcome the inherent resistance of the system before any flow starts, the benefits of using VSDs will be slightly reduced.
This is because the additional resistance affects the relationship between the speed of the pump and the flow. Factor this into any calculations and consult an equipment supplier for further information if needed. Typical examples of pump applications for VSDs include moving hot water in HVAC systems, boiler feedwater pumps and product flow pumps, such as chemicals or inks.
The potential for energy savings from using VSDs for air compressors will depend on the control system being replaced. The following diagram (Figure 5) illustrates the energy savings generated from fitting a VSD as compared to other methods of flow control.
Most air compressors are a constant torque load and, therefore, have less scope for energy savings than fan and pump applications, which are variable torque loads. Therefore, it is less likely to be economic to retrofit a VSD to a compressor unless it is very lightly loaded for long periods of time.
Air compressors tend to be used for industrial rather than commercial applications!
IMPORTANT! You can check if a compressor is lightly loaded by listening to the amount of time the compressor is on–load compared to the amount of time it is off–load at various times throughout the day.
You will be able to hear the pitch change when the compressor is idling in off–load!
If your compressor is off–load more than it is on–load, it may be appropriate to consult the compressor manufacturer or a VSD supplier to assess its suitability for either retrofitting a VSD, or buying a new, suitably sized VSD compressor.
Reference // Variable speed drives Introducing energy saving opportunities for business by Carbon Trust