Upping the efficiency of pumps and fans

Manufacturers once assumed that energy costs were, for the most part, beyond their control. But such attitudes became outdated with the advent of power monitoring and control devices such as energy-efficient motors, sophisticated controllers and software, and variable-speed drives (VSD). They make sure that key processes use less energy, thus turning energy into a manageable expense.

The potential savings are mind-boggling. Domestic manufacturers spend more than $33 billion on electricity every year, with motor systems consuming approximately 63% of the total. More than half of the motors run either fans or pumps — key areas with significant potential for energy savings.

As a result, manufacturers are looking for ways to reduce power consumption for these applications. In general, any application in which pump or fan speed varies can potentially benefit from a variable-speed drive. Here's how to determine if a VSD is worth the price.

Most fan applications do not require maximum air movement all of the time. So, to vary airflow, managers use methods such as:

• Cycling, typically used in residential settings and generally not applicable in industrial applications.
• Outlet dampers with control louvers to restrict outlet airflow.
• Variable inlet vanes that modify the physical characteristics of the air inlet, which changes airflow.
• Variable-speed drives, which change the fan speed.

In industrial settings, variable-speed drives are generally the most effective way to control energy use because they regulate motor speed. Inlet vanes and outlet dampers only control airflow, not the motor.

As a rule, energy consumption in fans and pumps varies by the cube of motor speed, also called centrifugal load. For example, if the fan motor speed can be cut in half, energy consumption = ( ¹ /2) ³ = ¹ /8, meaning the energy used to power the motor actually decreases by ⁷ /8. The same rule applies to pumps.

To illustrate, let's compare outlet dampers and VSDs. Changing airflow or fan speed can affect the system's natural operating point, as well as a fan's efficiency and power requirements.

Outlet dampers affect a system by increasing resistance to airflow — stated mathematically as:

P = KQ ²

where P = pressure required to produce a given flow, K = resistance to airflow, and Q = airflow. Outlet dampers affect K. Power requirements for this type of system gradually decrease as flow decreases, as shown in the Outlet damper graphic.

Variable-frequency drives take advantage of changes in fan behavior as speed changes. This is quantified by a set of equations called affinity laws. These laws, for centrifugal pumps and fans, relate how changes in speed or geometry affect capacity or power consumption.

Q2 /Q1 = N2 /N1,

P2 /P1 = (N2 /N1) ² , and

H2 /H1 = (N2 /N1) ³ ,

where N = fan speed, rpm; Q = flow, cfm; P = pressure, static in. H2O; and H = horsepower.

Reducing fan speed significantly reduces power requirements, as shown in the Variable speed graphic.