Motor Applications and Pumps
Move Electric Motors From Conditioned Spaces
Electric motors and equipment operated by them give off heat. If they are located in an air conditioned or refrigerated space, they contribute to the cooling load. If it is possible to move this equipment to an unconditioned area, energy savings would result since there would be a lower load on the air conditioner or refrigeration unit.
Lower Pumping Costs When Using Parallel Pumps
Some pumping systems are designed with two pumps in parallel, where the second pump is a spare. If both pumps are presently used to produce the required flow rate, there may be significant energy-saving possibilities if the desired flow rate can be achieved with a single pump.
If the pumping system is designed for one pump and the operator places the spare pump in service too, he has not doubled the flow rate. Instead, each pump provides one-half of the developed system flow rate and each operates at the identical head. To understand this, assume a centrifugal pump characteristic curve as shown in Figure 1. At 100 gpm of flow, one pump produces 130 feet of head. If identical pumps are on stream, the flow is 100 + 100, or 200 gpm, at 130 feet of head. The characteristic curve for two pumps was developed this way and is also shown in Figure 1. The actual flow rate through the piping system is set by the intersection of the pump curve with the system head curve. Referring to Figure 1, the flow rate is 160 gpm with one pump operating and 172 gpm with two pumps on stream. In the latter case, each pump is handling one half the flow, or 86 gpm.
The efficiency of centrifugal pumps varies with flow rate. Pumps are selected in the design phase to operate at or near their highest efficiency. As seen in Figure 1, the pumping efficiency decreased from 46.5 percent at 160 gpm to 34 percent at 86 gpm. Assuming an electric motor efficiency of 95 percent, the energy used in both cases is determined as follows:
By increasing the flow 7.5 percent, the energy requirements increased 60 percent. As an alternative to two pumps, the size of the impellers could be increased to handle the 172 gpm of flow with one pump. Assuming an efficiency of 47 percent, the energy required is:
Thus, 17.4 - 12.6 or 4.8 hp was conserved. How long would it take to recover the expense of purchase and installation of the two impellers if the pump operated at 172 gpm with 0.98 on-stream time? Assuming the cost of electricity at 7.0 cents per kWh and the replacement expense of $1,020, the payback is:
(X) (.95) (4.8 hp) (.746 kW/hp) (.070 $/kWh) = $1,020
where X=hrs
X=4,283 hrs or 6 months
Cost-Savings Analysis
The overall system efficiency associated with the use of VFDs for process control is significantly greater than with any other method. At 70 percent of process flow, the VFD can easily maintain an overall system efficiency of 75 percent, while variable speed couplings or throttling devices rarely result in efficiencies greater than 60 percent.
Terminology
Eddy Current Clutch: A mechanical device utilizing electromagnetic forces to produce variable speed output from single speed input.
Hydraulic Coupling: A mechanical device utilizing frictional forces in a hydraulic fluid to produce variable speed output from single speed unit.
Stator: The stationary
winding in a motor.
