Pumps are either positive displacement or centrifugal. Positive displacement pumps generate pressure by displacing liquid with a mechanical rotary device or a reciprocating action such as a moving piston. Flow is created by forcing or drawing the liquid through mechanical motion. A positive displacement pump will produce the same flow at a given RPM, regardless of the discharge pressure. Applications in common use are pneumatic pressure systems, feeding small boilers, condensate return, gasoline and light oil pumping, irrigation, liquid transfer systems for soaps, tars, paints and varnishes.
In systems that are pumping fluid with high viscosities (greater than 4.3cst), positive displacement pumps can be more efficient and have lower maintenance than a centrifugal pump. Because the output of this type of pump is constant, reduction in flow is achieved through discharge throttling, suction throttling or recirculation systems. These are mechanical methods that cause friction and increased wear on system components. A variable-speed drive can reduce maintenance in the system and save the waste energy associated with mechanical flow regulation.
Centrifugal pumps use a rotating impeller to move liquid in a circular path. Liquid is drawn in at the inlet due to the low pressure this rotary motion creates. It is accelerated to velocity with the rotors; creating a centrifugal force against the pump wall. Liquid is then expelled through the outlet of the pump at a pressure equal to the centrifugal force. Because the force required for pumping fluid increases exponentially with velocity, these types of pumps are not used for thicker liquids such as in a positive displacement application. The most common uses are for applications such as general water supply, mill, plant, mine, irrigation and sewage.
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Pumps are normally sized so they are the most efficient at their maximum flow rate. However, in reality, systems do not operate at these levels except for short periods of time. Centrifugal pumps offer the greatest energy savings potential when less than 100% flow or pressure conditions are required due to the affinity laws shown below. On average, 80% of the time pumps operate at 60% of their full capacity.
Pumps with a fixed speed motor control their flow by using a mechanical valve. The valve restricts the output of the pump system and, therefore, the flow volume is reduced. Controlling the flow with a throttling valve is like changing the speed of a car by only using the brake pedal. The accelerator is at a fixed point and applying the brake restricts the output by increasing resistance. The car consumes a fixed amount of fuel and the engine must work to overcome friction losses in the system. This would be an inefficient way to drive a car, and yet most pump systems with valves operate in much the same fashion. A better approach is to simply change the operating speed using the gas pedal; or in the case of our pump, by using a variable-speed drive (VFD).
Affinity laws in action
Energy savings are possible because of the affinity laws that govern the operation of centrifugal pumps. The basic principle is that the faster you try to move a liquid, the more force it requires. This is the same principle as walking versus running through water. A slow walk requires little effort, but try to run through water and you will find it takes a considerable more amount of exertion. This is why speed reduction provides significant energy savings at partial load over using mechanical valves. Running a pump at 60% of its operating volume requires only 22% input power.
It is obvious from this chart (left) that operating a pump at reduced speeds decreases the horsepower required. To calculate the actual cost savings, the brake horsepower must be converted to watts and then multiplied by hours of operation:
Kilowatts = HP x 0.746 [1 / SYSEFF]
This value is then multiplied by energy cost per kilowatt hour:
Energy Cost = Kilowatts x Hrs/Yr x $/kWh
At 100 HP, a variable-speed drive operating a pump at 60% speed will result in more than $20,000 a year in energy savings when compared to running at 100% flow volume and using a valve. This can pay for the installation of a drive in less than six months.
Fixed Speed Motor:
(100 HP)x(1/95% eff.)x(.746 kW/Hp)x(.08 $/kWh)x(12 H/Day)x(360 D/Year) = $27,139 per year
VFD Run Motor:
(100 HP)x(0.22)x(1/95% eff.)x(.746 kW/Hp)x(.08 $/kWh)x(12 H/Day)x(360 D/Year) = $5,979 per year
Where VFDs would be of most benefit:
- All friction system (No static head)
- System where control valve is constantly modulated
- Pumps in parallel or series operation
- Pumping system with multiple design points
- System with modulating bypass valve
- Cooling towers that start and stop frequently
Mark Gmitro is a drives specialist for Baldor Electric Co., Fort Smith, Ark., Contact him at (864) 281-2354 and firstname.lastname@example.org.