Increasing Pump Energy Efficiency for Industrial Cooling Systems
Intelligent pumps provide benefits and efficiencies not possible with fixed-speed models and control valves.
When it comes to industrial process heating and cooling systems, a number of plants are running with systems that rely on a fixed-speed centrifugal pump to circulate the medium and a control valve to throttle the flow to a required rate. This type of system is highly inefficient, wastes electricity (which has generally risen for most U.S. industrial users and remains a volatile market), typically requires higher initial capital investment, and results in excessive maintenance costs.
In a fixed-speed design, the pump often is oversized to ensure that the maximum system requirements are met when the control valve is positioned to accommodate 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 typically operate in the middle of their total range, which increases the restriction losses. When the pump is operating, the control valve opens and closes, creating a flow restriction that the pump tries to push through. While this design creates the desired effect of controlling flow, it also results in excessive wear on pump bearings and valve seats, leading to premature pump failure. In addition, this method of controlling flow causes the pump to consume much higher levels of electricity than necessary.
Intelligent Pumps for Process Use
While most plants are producing a wider range of product than ever before, the need for greater system flexibility has increased. To operate profitably, it is vital to enhance the efficiency of cooling systems. This goal can be achieved by using intelligent pump systems, which vary their own speed to adjust flow rather than relying on fixed-speed pumps and control valves.
Also referred to as E-pumps, intelligent pumps use a sensor to electronically vary motor speed to match system demand. The sensor communicates with an integrated variable-frequency drive (VFD) to match the pump speed precisely to the process demand. These systems are suitable for designs requiring a single pump as well as those requiring multiple pumps. Engineering intelligent pump systems is relatively simple. Many existing systems can be retrofitted easily to include intelligent-pump capabilities.
Advantages of intelligent, variable-speed pumps over fixed-speed systems include lower installation costs and reduced maintenance. The most compelling argument for variable-speed cooling systems, however, is energy savings.
According to the U.S. Department of Energy and the Hydraulic Institute, energy accounts for more than 85 percent of the lifecycle cost of a pump system. Intelligent pumps can cut energy consumption in half. While fixed-speed systems — at best — can realize only maximum efficiency part of the time, intelligent pumps can maintain maximum efficiency over their entire operating range.
This around-the-clock capability makes them well suited for cooling applications with fluctuating 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 is not the pump’s fault; it is 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.
The flow restriction caused by the control valve in throttle-system designs is replaced with direct control of the system output, so there is no need for a control valve. This design also reduces 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.
Variable-Frequency Drives Deliver Smart Solution
Many intelligent pump systems have VFDs integrated into the pump motors to facilitate installation. Some plants prefer to have all VFDs mounted independently, so panel- and wall-mounted intelligent pump VFDs are available. Unlike pumps driven by generic VFDs, intelligent pump VFDs are preprogrammed to work specifically with pumps, so they do not require an extensive knowledge of programming languages. They do not even require advanced pump knowledge because they are designed to be installed using simple system setpoint data. With only a basic understanding of a particular system’s requirements, a system can be customized to monitor and adjust to changes in temperature, pressure or flow.
Intelligent Pumps and Three-Way Valves. In cooling systems that have fan coil units or similar devices with three-way valves — and therefore almost constant system characteristics — adjusting the pump performance based on the differential pressure will be impossible. In such systems, another control parameter must be used. The best choice is to control the circulator pump according to temperature. The return pipe temperature of the system signals changes in the system load. A decrease in the return pipe temperature (i.e., the temperature approaches the supply pipe temperature) indicates that a large amount of the circulated flow is bypassed in the control valves due to a reduced cooling demand in the system. In such situations, the circulated flow in the system should be reduced to lower the power consumption of the pump.
A differential temperature control could be used as an alternative to the constant temperature control of the return pipe temperature. The temperature control of the pump offers the same operational advantages as that of differential pressure control.
Controlling Flow in Cooling Towers. In cooling systems that use cooling towers, the relatively hot water (typically 80 to 90°F [27 to 32°C]) is circulated from the condenser/heat exchanger to the tower, where the water is cooled by direct means and by evaporation. The tower’s cooling capacity depends on the amount of both air and water flowing through the tower.
Airflow usually is controlled with a fan. When the cooling demand is low, the fan often is stopped, and cooling is achieved only by evaporation as the water circulates. When the cooling demand increases, the fan restarts to provide additional cooling. In some cases, the fan might be speed-controlled.
The water flow through the cooling tower usually is controlled with a single- or three-way valve and a pump (non-controlled). The valve controls the flow through the cooling tower (and, thus, the cooling power) according to the temperature of the return water.
In the case of three-way valves, the pump operates with an almost constant flow, regardless of the load in the tower. However, if two-way valves are used, the pump will be throttled when the cooling demand is reduced. A more energy-efficient solution would be to use an intelligent pump, which would reduce power consumption when the system is only partially loaded and would make the control valve superfluous.
An intelligent pump that has a built-in PID controller can be connected directly to a temperature transmitter, thereby allowing the pump to provide closed-loop control of the cooling system. (Note: To ensure a sufficient function of the nozzles in the cooling tower, a minimum flow is required. If the flow is too low, the nozzles cannot atomize the water satisfactorily. Therefore, the minimum pump speed must be set to ensure that this requirement is always fulfilled.)
Intelligent pump systems provide greater system flexibility and enable a cooling system to operate at optimum efficiency continuously.