20 FOCUS ON building services
improve pump efficiency and reliability with a vsd It is 50 years since the first variable speed drive (VSD) was applied to electronically vary the speed of an electric motor. Since then the market has grown steadily. However, according to Carl Turbitt, HVAC Drives UK Sales Manager at ABB, it is estimated that only 5% of the world’s installed motors are fitted with a VSD.
O
ne the biggest markets for VSDs are in buildings – for a multitude of heating, ventilation and air conditioning (HVAC) applications. In particular, they benefit all types of pumps including circulating water in HVAC systems, booster pumps, dosing pumps, fountain pumps, utility pumps, boiler feedwater pumps, industrial process cooling pumps and chilled water pumps. Because pumps represent the largest single use of motive power across industry and commerce and because they are variable torque, the addition of speed control will provide energy saving benefits. The most commonly-used methods for controlling the flow of a pump that transfers a liquid from a location of lower pressure to one of higher pressure are throttling, bypassing, on/off control and VSD control. While throttling is the least efficient of these methods, it is the one that is most frequently used. Meanwhile, VSD control is a more efficient method but is used less frequently. The potential for improving energy efficiency will depend on the type of pumps used and specifically on the relationships between the pumps’ performance variables. These relationships, for pumps and other applications, are shown in Table 1. Applications with a cubic relation of power versus speed (i.e. centrifugal pumps, fans, blowers and compressors) offer the greatest scope for energy saving with variable speed control. When the process requires a lower rate of flow and the VSD reduces the speed of the application, this has a much greater impact on the amount of power consumed than in applications
Application
Flow-v-speed
where the relationship is linear. However, VSDs are not just about energy saving – they can also improve system reliability; simplify pipe systems by eliminating the need for control valves and by-pass lines; provide controlled start and stop, thereby reducing wear and tear on the motors and water hammer; and reduce leaks caused by pressure surges. All this leads to lower maintenance, lower lifecycle costs and increased plant availability. Office equipment manufacturer Ricoh, for instance, installed VSDs on its air handling units, water circulation pumps and compressors. While this resulted in a saving of over £100,000 per annum in energy costs, the VSDs also enabled the company to gain better control over several processes including a supervised relay output function, which allows the drive to control motors dependant on the status of connected equipment, and Modbus compatibility, which allows control and monitoring of applications over a PLC network. Meanwhile, Harbour Exchange Tower, a 46,500m2 office development near Canary Wharf in London, saved £15,000 on electricity costs in six months following the installation of 18 VSDs. Installation
“VSDs are not just about energy saving – they can also improve system reliability.”
Pressure or torque-v-speed
Power-v-speed
Positive displacement pumps*, positive displacement blowers, screw and piston compressors
Linear
Constant
Linear
Centrifugal pumps, fans, blowers and compressors
Linear
Quadratic
Cubic
*Including piston, screw, gear and progressive cavity types Table 1: Relationship between performance variables for different motor-driven applications Quarter 2 2019
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