Pumps & Systems, June 2008
While driving a car with the gas pedal to the floor and then controlling your speed using the brake is obviously inefficient, many facilities use the approach for pump control. Flow control with throttling or restrictive devices sacrifices energy efficiency and results in unnecessary costs. However, with an understanding of basic principles, an analysis of the specific application, information about available control solutions and evaluation of technologically advanced equipment, facilities can make a quantum leap in improving the efficiency and economy of pumping operations.
Energy efficiency starts with motor speed control. Sixty-five percent of all electrical energy used in the United States operates flow loads such as pumps, fans, blowers and compressors, mostly powered by constant speed induction motors. When output flow requirements fluctuate in such systems, an external means of adjustment is needed.
Commonly used methods for flow control include throttling or restrictive devices such as valves, outlet dampers, inlet vanes and diffusers. Mechanical speed changers and recirculating systems are also sometimes used. However, all these devices waste energy, dissipate power by friction and diffuse heat.
Fixed-speed pumps draw nearly full horsepower and consume nearly maximum energy full time, regardless of demand. Power requirements for throttled systems drop only slightly even when flow or volume is reduced significantly.
Variable speed devices such as belts, gears, magnetic clutches and hydraulic drives accomplish this function mechanically, but they are costly, bulky, waste power and require high maintenance.
DC adjustable speed drives can provide speed variation. However, DC motors are two to three times the cost of an equivalent-rated AC motor. DC motors are also larger, heavier, require more maintenance and are more difficult to operate in challenging environments.
Variable frequency control of AC induction motors provides an economically sound and operationally effective solution for speed control and reduced power consumption. It can also be made responsive to signals from flow sensors, programmable controllers and other control systems. Microprocessor-based AC motor control affords users options that can provide short- and long-term productivity and profitability improvements.
Figure 1. Variable frequency drives permit users to consume the least amount of power to obtain desired pressure and flow.
Curves Determine Centrifugal Pump Efficiency
In-line valves are often used to regulate flow or pressure in liquid pumping systems. The valve can be a significant source of energy loss by causing a restriction in the flow path, thus increasing the pressure. An AC drive provides more efficient flow control by varying the pump motor speed. By comparing the energy requirements and costs when a throttling device is used for flow control on a centrifugal pump with the power used when an variable frequency drive (VFD) is used to control the same flow, the potential savings become evident.
The first step is to determine the theoretical load requirements and potential energy savings for the specific application using three interrelated Affinity Laws.
Adding the two heads together creates the system curve, which describes what flow will occur given a specific pressure. Knowing the system curve, the pump manufacturer can select an impeller size to meet the flow requirements specified.
The point where the Pump Curve and the system curve cross determines the operating point of the system. This system will have only one operating point, so if variable flow is required, something needs to be added.
Not all options are created equal. The typical technique for flow control is the use of a throttling valve. Partially closing the valve adds another restriction, raising the system losses and the system curve. The flow rate will now be determined by the point where the new system curve crosses the Pump Curve. The amount of energy the system consumes to do this is proportional to the head pressure and the flow rate.
Using a variable frequency drive to control the flow means no additional restriction is added to the piping. Therefore, the system curve remains the same. Varying the speed with a variable frequency drive has the same effect as installing a different-size impeller on the pump: a new Pump Curve results.
Each flow control method has different levels of energy efficiency: