Saving energy through pump speed control

Bruce Widell, Senior Applications Engineer,
Danfoss Electronic Drives Division, Danfoss, Inc.

Some of the most commonly used methods for controlling pump flow rely upon throttling and restrictive devices. All of these devices dissipate power by friction and heat diffusion, thus wasting energy. So the obvious question arises: How can you minimize the losses?

An adjustable-frequency drive may be the answer. Adjustable-frequency drives can be implemented in a way that optimizes system efficiency, thus offering the potential for greatly improved energy savings.

How does an adjustable-frequency drive save energy? Well, most of the electrical energy in today's industrial plants operates centrifugal loads, such as pumps, fans, blowers, and compressors. Constant-speed ac motors, sized for maximum loads under worst conditions, power most of those loads. A high percentage of pumps, fans, and blowers also have fluctuating output requirements, so they need an external means to adjust flow. That's why users employ throttling and restrictive devices-such as valves, bypass lines, orifices, and recirculation loops.

Using an adjustable-frequency drive instead of a constant-speed ac motor typically eliminates the need for the throttling and restrictive devices. As a result, power losses are minimized.

The accompanying figure shows the input power and flow for a standard centrifugal pump using a throttle valve. Factors affecting energy are the percentage of flow cutback, running time at reduced volume, and cost of electricity. The power curve for the adjustable-frequency drive is based on an average amount of static head, or lift, in the system. Total head equals the sum of the static head and the friction head.

Choose the Right Slide For Maximum Performance

Gary Murphy, Applications Engineering Manager, PHD, Inc.

Selecting the right slide can be a nightmare. With so many models to choose from, consider these variables to determine the right slide for your application:

Bushing load capacity: First, decide which type of bushing best suits your application. Linear ball bushings are the most common and give good performance in a variety of applications. Fluoropolymer-composite bushings are a more recent development. These new maintenance-free, self-lubricating bushings carry greater static loads, cost less than linear ball bushings, and are ideal for working in harsh environments.

Shaft deflection: Shaft deflection affects the accuracy of the slide's tool-plate location and, therefore, the accuracy of the slide application. This deflection is a function of the shaft's diameter, material, travel length, load, and distance between the linear-bushings. Other factors that can affect accuracy are: shaft straightness; shaft weight; and linear bushing alignment. Most slide manufacturers have deflection graphs and calculations available to ensure proper application.

Air cylinder thrust: The amount of cylinder thrust depends on the cylinder size, available air pressure, and safety factors related to horizontal or vertical applications. The cylinder must generate the force required to move the load.

Kinetic energy: You should consider unit speed and the ability to stop loads to ensure that the slide can handle the amount of kinetic energy produced. To increase ability to stop a load, some manufacturers offer to provide cushions and shock absorbers.

Once you've determined these elements, the rest is easy. Your requirements can be matched with the exact slide for your application.