Pumps and fan or blower applications

  1. Introduction
  2. Optimising pump and fan applications 
  3. Control technologies 
    1. Selecting the right equipment
    2. Pumps and fan or blower applications
    3. Typical questions and answers

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
Percentage of flowThrottle valve Bypass valve VSD replacing throttle valveVSD replacing bypass valve
100 100 100 105 105
95 96 100 90 98
90 94 100 78 93
85 93 100 66 87
80 89 100 57 82
75 86 100 48 76
70 83 100 41 71
65 81 100 35 66
60 79 100 30 61
55 76 100 25 56
50 74 100 21 51
45 72 100 17 46
40 71 100 14 41

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

A pipe runs from water tank to a pump, through an pressure transmitter, to users. An adjustable speed control and PI control is attached to the pump motor and pressure trasmitter.

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 schematic showing an adjustable speed drive connected to the fan motor.

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
Percentage of flowOutlet dampersInlet vanesVSD
100 111 109 105
95 109 100 86
90 107 93 73
85 106 86 64
80 104 82 57
75 102 78 50
70 99 75 44
65 96 72 38
60 94 69 32
55 91 66 26
50 87 65 21
45 84 64 17
40 80 63 14
35 76 62 11
30 72 60 8
25 68 56 6
20 64 51 5

Energy consumption as a percentage of unrestricted full-flow energy consumption