Energy-efficient pumping requires a comprehensive analysis of the entire system. Pumps are some of the most energy-intensive equipment in an industrial cooling operation. Yet much of this energy is wasted. In fact, one study that evaluated 1,690 pumps at 20 process plants found that the average pumping efficiency was below 40 percent, and more than 10 percent of all pumps were running below 10 percent efficiency.
It therefore makes sense that any discussion about reducing energy consumption in a cooling process should include an evaluation of the pumping systems. However, pumps cannot be evaluated independent of the systems in which they operate. To optimize pump efficiency, a total system approach is needed -- one that considers the system flow and head requirements, load profile, system control and overall power requirements.
7 Steps for Projecting Energy Requirements
To project the total energy requirements for any system with any combination of pumping equipment, and to ensure that all answers are projected from a common base, a structured procedure for the analysis should be followed. Each item in the procedure will have a significant impact on the total energy requirement of a specified pump-equipment combination and system operating procedure.
To make comparisons of dissimilar equipment or to evaluate the impact of a change in system operation, the procedure must be followed for each specified condition. The individual steps of the procedure are as follows.
Step 1. Project System Flow and Head Requirements
Before an evaluation procedure can begin, all information relative to the system design requirements must be established and assembled. An accurate projection of the system flow and head requirements is necessary at this stage. If these parameters are variable, then a profile of the relationships should be made. This projection should represent the magnitude of change compared to the length of time the change is expected to occur. If the accuracy of these data is questionable, a band of minimum to maximum values should be established from which a total system power requirement band can be developed.
Many pumps are commonly oversized by as much as 44 percent for flow and 26 percent for head simply because design engineers tend to build in a safety margin to avoid undersizing. However, in addition to wasting energy, oversized pumps can increase maintenance costs and cause collateral damage to other equipment.
Step 2. Understand the Load Profile
The system load profile is the central component of any energy evaluation process. It expresses the measure of work executed in a system compared to a unit of time. Work performed has a direct relationship to flow; therefore, the load expressed as flow provides a common base for any projection of system energy requirements. High flow/time concentration areas should be identified, and these should be the focal point for preliminary pumping equipment selection. If expected future system modifications such as expansion will significantly alter the load profile, then these considerations should be confirmed at this time.
Step 3. Evaluate the Pump Selection
Beyond the normal mechanical and hydraulic criteria associated with pump selection for a specific application, the selection procedure should encompass the primary operating efficiency at the point of selection and the amount of efficiency deviation that any system flow variation will cause. These values then should be compared to the operating time at specified flow rates to ensure that the selection offers the maximum potential operating efficiency through the range of maximum flow/time concentration. Keep in mind that fixed-speed pumps limit head and flow rate flexibility; variable-speed systems maximize flexibility.
Step 4. Determine the Controls
Preliminary decisions must be made relative to the control means that will be used to satisfy system flow and head requirements. The decision to control the system's required head, total available head or both will be a significant factor in the total energy requirements projected for the system being evaluated. A system curve represents the head required to move fluid through a system at various flow rates. In the absence of control features, the system will operate where the pump and system intersect.
Step 5. Project Pumping Equipment Power Requirements
This stage of the evaluation requires a projection of the pumping equipment power requirements over the entire system's operating flow range. The required power of the pumps being evaluated can be organized in tabular or graphical form. In a tabular form, the power requirements of each pumping system compared to the required system flow should be of sufficient quantity to allow an analysis of all significant operating areas as dictated by the flow/time profile.
Step 6. Use the Load Profile to Calculate Energy Requirements
When compared to the power project in Step 5, the load profile will determine the total energy requirements of the system, thus concluding the evaluation in energy time units (brake horsepower hours, kilowatt hours, etc.).
Step 7. Determine the Total Energy Required
Determining the total energy required to satisfy all projected system operating conditions is the basis from which all other considerations and comparisons can be made. This is the reviewing and decision-making phase of the total system evaluation procedure. If the energy summary is adequate, the evaluation procedures are concluded; if not, the evaluation can be repeated with new or modified data to determine what effect the change will have.
A Comprehensive Approach
A total system evaluation ensures that all projections, comparisons and decisions are based on as much factual information as possible, and it allows the comparisons to be made from an equal basis -- the requirements of the system. Through this structured, fundamental process, plants can determine the pump and system combination that best suits their cost, benefit and functionality requirements.
Ultimately, energy efficiency is as much about improving an industrial system's reliability as it is about reducing energy costs. Greater efficiency reduces wear and tear from heat and vibration -- lengthening the lifespan of equipment and reducing down time for maintenance and repairs (figure 4). A continuous energy improvement program based on a holistic approach is one of the best ways to avoid "lost" energy that otherwise might create system inefficiency and destroy equipment.
Equipment Power Comparisons
To accurately compare the various kinds of pumping equipment for a given application, a common point of reference must be established. That reference point is the power that the prime mover must supply at its output shaft.
To project the total system power required for a constant speed pump, divide the pump horsepower by the prime mover efficiency. The horsepower at the output shaft of the prime mover for a pump driven by a variable speed device is derived by dividing the brake horsepower by both the prime mover efficiency and the variable speed device efficiency. This applies whether the variable speed device is between the pump and the prime mover or ahead of the prime mover.
Do not be misled by the lower net efficiency of equipment that includes a variable-speed device. Significant operating cost savings can be obtained through pump operation that is closely aligned to system needs, particularly in systems that have variable loads. The greater initial cost of the variable-speed equipment often can be amortized in a relatively short time, after which a net reduction in overall system operating costs can be obtained.