System optimisation
- Introduction
- Optimising pump and fan applications
- Control technologies
- Variable speed drives - selecting the right equipment
- Variable speed drives - pumps and fan or blower applications
- Variable speed drives: typical questions and answers
Variable speed drives - pumps and fan or blower applications
Before you look in detail at variable speed drives, you need to consider the three major areas where drives and motors are applied:
- Constant torque applications - when the same amount of torque is required at low speed as at high speed. Power is directly proportional to speed. Typical applications include conveyors, mixers, screw feeders, extruders and positive displacement pumps.
- Constant power applications - when a high torque is required at low speed and low torque required at high speed. Typical applications include machine tools, traction.
- Variable torque applications - when a low torque is required at low speed and higher torque required at high speed. These applications are generally centrifugal loads such as fans, pumps and blowers. These are good targets for energy savings, both during planning/construction or for retrofitting electric variable speed drives to existing installations at a later stage.
Pumps (except positive displacement pumps*), fans and blowers share the following physical principles of operation:
- Flowrate (change in volume/change in time) varies in proportion to drive speed.
FLOW1 = RPM1
FLOW2 = RPM2
- Pressure and torque vary in proportion with the square of the flowrate and hence speed.
PRESSURE1 = (RPM1)2
PRESSURE2 = (RPM2)2
- Motor shaft output power (neglecting motor and drive losses) varies in proportion with the cube of flowrate and hence speed.
POWER1 = (RPM1)3
POWER2 = (RPM2)3
* More information on pump types is found at pump and fan characteristics and at fine tuning pump and fan applications.
Pumping applications
A control valve is typically used for flow control in centrifugal pumps. For positive displacement pumps, the traditional way to control flow is to return part of the fluid back into the pump through a bypass valve. If a variable speed drive is used with a positive displacement pump, the energy consumed is directly proportional to speed. The table below lists the energy consumed by a centrifugal pump (using a throttle valve) and positive displacement pump (using a bypass valve) as the net flow varies from 100 to 40 per cent. The figures listed are percentages of the full-load energy consumption for each pumping system.
Percentage of flow
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Energy consumed as a percentage of unrestricted full-flow energy consumption.
This table also provides the total system energy consumption where the throttle valve (bypass valve) control schemes are replaced by a variable speed drive. The two columns related to the drive control scheme also indicate that at full load, or 100 per cent flow, the drive internal losses (typically five per cent ) must also be accounted for (hence the 105 per cent energy consumption indicated in the table).
Pump appication schematic
Pump application schematic
Variable speed drives have been successfully applied to large boiler feedwater pumps in power plants, hot water circulation pumps in commercial buildings, and waste water treatment plants.
Fan and blower applications
Fans and blowers used in heating, ventilation and air-conditioning (HVAC) and boiler applications generally represent over half the total energy consumed in commercial buildings and are often oversized to account for contingencies.
HVAC system schematic
HVAC system schematic
Outlet dampers and inlet vanes are the most common methods for controlling air flow and these vary in terms of energy efficiency and costs of operation. The energy consumed when dampers are closed is only marginally lower than the energy consumed while operating with dampers fully open. Inlet vanes are somewhat more efficient as a control method.
In large buildings, or in HVAC applications in industry, there are often opportunities for retrofitting variable speed drives. While the initial capital outlay is higher, the amount of energy saved by substituting inefficient control methods can result in large energy savings and short payback periods. Additional benefits include higher comfort levels for staff because of better temperature and ventilation control and lower noise levels.
The table below lists the percentage of energy consumed by a fan system controlled using outlet dampers, inlet vanes, and a variable speed drive, when the net flow varies from 100 to 20 per cent.
Energy consumption as a percentage of unrestricted full-flow energy consumption
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Energy consumption as a percentage of unrestricted full-flow energy consumption
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