Would you drive your car around town with your right foot mashing the accelerator to the floor and your left foot on the brake, and then, when it’s time to slow down or stop, just push harder with that left foot? Of course not. Driving like that would cost you a bundle in fuel - especially at today’s prices - not to mention tires and brakes, and it might even ruin the engine.
Believe it or not, when it comes to industrial cooling systems, a number of plants are running exactly like that. Many traditional systems rely on a fixed-speed centrifugal pump to circulate the cooling medium (one foot on the gas) and a control valve to throttle the flow to a required rate (one foot on the brake). This type of system is highly inefficient and not only wastes electricity, which has risen dramatically in cost for most U.S. industrial users, but it also typically requires higher initial capital costs and results in excessive maintenance.
In a fixed-speed design, the pump often is oversized to ensure that the maximum system requirements are met when the control valve is throttled near its upper control limit. For most valves, this position is at about 20 percent closed, which means that the system already has inefficiencies built in because of the required valve restriction.
This problem is compounded when systems are designed for future expansion. The possibility of higher flows requires larger pumps and control valves that might operate in the middle of their control range for extended periods of time, which increases the restriction losses. When the pump is operating, the control valve opens and closes, creating a barrier that the pump tries to push through. While this design creates the desired effect of controlling flow, it also results in excessive wear and tear on pump bearings and valve seats, leading to premature pump and valve failures. In addition, this method of controlling flow causes the pump to consume much higher levels of electricity than necessary.
Intelligent PumpsWith most plants producing wider ranges of product than ever before, the need for greater system flexibility also has increased. To operate profitably, it is vital to enhance the efficiency of your cooling systems. This goal can be achieved by using intelligent pump systems that vary their own speed to adjust flow, rather than relying on fixed-speed pumps and control valves.
Intelligent pumps use a sensor to vary the motor speed electronically to match system demand. The sensor communicates with an integrated frequency drive to match the pump speed precisely to the process demand. These next-generation systems are suitable for designs requiring a single pump as well as systems requiring multiple pumps. Engineering new intelligent-pump systems is simple, and many existing systems can be retrofitted to include intelligent-pump capabilities.
Advantages of intelligent, variable-speed pumps over fixed-speed systems include lower installation costs and reduced maintenance, but the most compelling argument for variable-speed cooling systems is energy savings. According to the U.S. Department of Energy and the Hydraulic Institute, energy accounts for more than 85 percent of the life-cycle cost of a pump system. Intelligent pumps can cut energy consumption in half, resulting in significant life-cycle savings (figure 1). While fixed-speed systems can, at best, realize maximum efficiency only part of the time, intelligent pumps can maintain maximum efficiency over their entire operating range. This capability makes them well-suited for cooling applications with changing demands associated with product, volume or seasonal variations.
Even the best fixed-speed pumps tend to operate outside of their optimal efficiency points. This drawback isn’t the pump’s fault; it’s just the way the system works. Variable-speed pumps operate at or near their best efficiency point throughout the entire process range, even if the system is designed for future expansion (figure 2).
The flow restriction caused by the control valve in throttle system designs is replaced with direct control of the system output, so a control valve is not required. This design provides even more benefits by reducing the overall maintenance required to keep the system going. Without the valve, the pump does not need to work as hard and will last longer, and the control valve maintenance is eliminated.
Many intelligent pump systems have frequency converters integrated into the pump motors to facilitate installation. Some plants prefer to have all converters mounted independently, so panel- and wall-mounted intelligent pump converters are available. Unlike pumps driven by standard variable-frequency drives, intelligent pump converters are preprogrammed to work specifically with pumps. Advanced pump knowledge is not required because they are designed to be installed using simple system setpoint data. With only a basic understanding of the process requirements, a user can customize the system to monitor and adjust to changes in temperature, pressure or flow.
The next time you see your energy bill or walk past an old cooling system, ask yourself, “Is my plant driving with one foot on the brake and one on the gas?” If the answer is yes, then intelligent pumps just might help you drive more efficiently.