Armstrong takes a fresh look at variable-speed pumps

Published:  01 November, 2012

Armstrong, pump, variable speed
Variable-speed pumping that is less complex and less costly — Armstrong’s Design Envelope.

Not only has Armstrong made it easier to realise the energy-saving benefits of variable-speed pumps, but it has also brought down the installed cost substantially. Ken Sharpe has been finding out more.

For a goodly number of years pumps with variable-speed drives have held out the promise of much reduced energy consumption and enabling systems to respond better to changes in demand. However, such pumps are generally rather more expensive than their fixed-speed counterparts and also require information from remote sensors so that the speed can be adjusted to meet the required flow conditions as the heating/cooling load changes.

With the power consumed by the pump motor varying as the cube of the speed, slowing down the pump gives large savings very quickly. Halving the speed, for example, reduces power consumption to just an eighth of full power.

However, the cost and complexity of setting up systems to fully exploit the benefits of variable-speed pumps means that a lot of pumps out there use the variable-speed capability to set the pump at the design point — and then operate at that fixed speed all the time. In that respect, little progress has been made since the era of belt-driven pumps with pulleys that were sized to achieve the required operating speed.

To Peter Wolff, Armstrong’s UK business development director, running pumps having variable-speed capability at constant speed loses the opportunity to save huge amounts of energy. He says, ‘Delivering HVAC services is a part-load industry, wherever you are in the world.’ He elaborates by explaining that for 90% of the time pumps operate at 10 to 60% of their design load.

Armstrong’s response to such a pressing need and opportunity has been to develop a range of intelligent variable-speed pumps with built-in inverters and embedded software that obviates the need for external devices such as differential pressure sensors.

At the same time, Armstrong has implemented a different approach to pump selection and operating characteristics to further reduce energy consumption. More below.

The Design Envelope IVS pump range is available as integrated sensorless variable-speed pumps from 1.1 up to 250 kW. Larger pumps from 315 to 1000 kW come with standalone inverter and control units.

The extra capital cost of Design Envelope pumps is more than offset by eliminating the cost an external sensor and its installation. Installed at a remote point in the system and cabled back to the pump, such a sensor could, according to Peter Wolff cost around £1000. He explains, ‘Design Envelope gives all the savings that can be achieved with a remote differential-pressure sensor, but without the expense of the device and linking it back to the inverter and BMS.’

The information used to control the pump is the current drawn and its speed. The embedded software uses this information to maximise operating efficiency by choosing the operating point where pump performance, system resistance and control curves converge.

Driving down the cost of pumping — Armstrong’s Design Envelope has much lower costs over the first three years than any other system in common use.

How pumps are selected also reduces energy consumption.

Peter Wolff explains that the peak efficiency for a pump is about two-thirds of the way along the pump curve, for both fixed-speed and variable-speed pumps. Traditionally, pumps are selected so that the design point is at or near the pump's best efficiency point. This makes sure that if a pump operates at this point all the time, power consumption is minimised.

However, variable-speed pumps rarely operate at full load, so Design Envelope pumps are selected with the design point to the right of best efficiency point — i.e. less efficient at the design point.

The result is that it is likely that a Design Envelope pump may have a smaller nominal bore size than a conventionally sized pump. comparing an 80 mm Design Envelope pump with a 100 mm conventional selection might see the Design Envelop pump being 4% less efficient at the deign point. However at 50% of design, the 80 mm pump is 6% more efficient. Given the typical load profile that variable speed pumps live though, the 80 mm selection will consume less power through its operating cycle. Put succinctly, selecting a smaller pump with its duty point to the right-of the best-efficiency point makes for more efficient operation most of the time.

Another contribution to reducing overall costs is that Design Envelope pumps are vertical inline units that are installed in the pipework, avoiding the need for a concrete plinth and anti-vibration mounts. Adding in suction guides and Flo-Trex valves reduces the requirement for pipe and fittings. Flo-Trex combination valves combine the features of a check valve, throttling valve and a shut-off valve; they are designed for installation on the discharge side of centrifugal pumps. Armstrong’s literature suggests that the installation cost of a 5.5 kW Design Envelope pump is less than half that of an end-suction base-mounted installation with traditional piping.

The hard figures are a 20%-plus reduction in first cost and over 30% on life-cycle costs. Those figures are based on a 150 mm Design Envelope IVS pump compared to an end-suction pump with its invertor drive on the wall. These figures do not include pipe savings.

Figures are also available to compare total costs for the first three years compared with other systems using fixed-speed and variable-speed pumps. Those total costs are made up of the operating cost and first cost. Design Envelope achieves a saving of over 60% compared with a fixed-speed pump and throttling valves to control flow. And savings of over 35% are achieved compared with a variable-speed system having a sensor in the plant room.

Very seldom, no matter where in the world they are installed, do pumps in HVAC systems operate at or even near to their full capacity. For 90% of the time, pumps operate at 10 to 60% of their design load.

Already Design Envelope pumps have lower installed costs and energy costs. But Armstrong recognises the benefits of acknowledging that operating requirements will change during the life of a building. Common possibilities include changes in occupancy, alterations of building design or modifications in shade conditions caused by new buildings erected nearby.

Such changing needs are addressed by a different way of presenting pump curves. They display the complete operating envelope of the system, showing where the pump performs efficiently, at full and part load.

Designers can look at a range of performance envelopes and decide which envelope is best likely to suit present and future needs. This approach reduces risk, helps avoid budget over-runs and can make future duty changes cost free. It’s all about future proofing.

And while we are looking to the future, it is appropriate to draw attention to a design feature that makes it much quicker and easier to replace the mechanical seal in these split coupled pumps.

These mechanical seals are the most commonly replaced items on such pumps. They typically require replacing every five to seven years, amounting to about four times in a 20-year life. The design of these pumps enables the mechanical seal to be replaced by one person in about half an hour, compared to about two hours for a base-mounted pump.

The pump industry generally is well ahead of EU legislation, and this range of pumps is no exception. Design Envelope IVS pumps meet 2017 IE3 legislation and comply with 2015 legislation for minimum efficiency index.

And in this era of exploiting renewable energy, the capabilities of Design Envelope can facilitate the integration of such complementary systems without requiring a redesign of the base plant.

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