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.

One 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 where the relationship is linear.

VSDs are not just about energy saving – they can also improve system reliability.

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 reduceleaks 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 46,500m 2 office development near Canary Wharf in London, saved £15,000 on electricity costs in six months following the installation of 18 VSDs. Installation was spread over three applications – hot water pumps, chilled water pumps and toilet supply and extraction fans. The pumps and fans are controlled by a building management system (BMS), which switches them on and off according to pre-selected timings.

The biggest savings came from the chilled water system. This has eight drives running eight pumps. Six of the drives are 30kW and two are 37kW. The drives reduce the speed of the pump motors by 20%. In the first six months of operation, this saved 55,000 kilowatt-hours and £8,000 in energy costs. The hotel sector is also now taking a closer look at how best to apply VSDs, as a typical HVAC system can account for between 60 to 80% of all the energy consumed by a single hotel. An ABB energy survey conducted for the InterContinental Madrid revealed that the hotel’s energy use was high, particularly in the eight pump groups that serve its HVAC systems and hot water supply for guest rooms, kitchens and other facilities.

Based on these findings, the pumping systems were equipped with 13 ABB HVAC drives and 16 electric motors with IE3 energy efficiency classification and integrated into BMSs. This upgrade provides the overall system with greater stability and higher energy efficiency, as the motor speeds are now adjusted to match the precise demand at any given time. Over a year the energy saving totalled 445,000 kilowatt-hours, cutting the hotel’s annual energy bill by around £29,000 to deliver a projected return on investment of less than two years.

If similar energy savings could be replicated at just one-in-four of the world’s estimated 187,000 hotels (according to STR Global), global CO2 emissions could be reduced by nearly 12 million tons a year – similar to shutting down three coal-fired power plants.

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