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The future is variable

Published:  07 July, 2008

The impact of variable-speed pumps — John Coppin.

In just five years the number of porjects using variable-speed pumps has shot up from 10% to 90%, posing new challenges for commissioning engineers — as we found out from John Coppin

The challenges facing engineers designing hydronic building-services systems and commissioning them is encapsulated by the growth of variable-speed pumps in just the last five years. John Coppin, an associate director with consulting engineers Arup and with over 40 years’ experience of commissioning, explains, ‘Five years ago, variable-speed pumps were installed in only 10% of projects, with 90% being fixed-speed pumps. That situation has reversed, with 90% of projects now using variable-speed pumps.’

He highlights two incentives. One is the increasing concern with the energy performance of buildings prompted by Part L of the Building Regulations and the acceptable cost of inverter drives. Advances in the technology of inverter drives now makes them more affordable.

The development of pipeline equipment to enable the benefit of variable-speed pumps followed quickly, notably differential-pressure control valves (DPCVs) to maintain constant pressure in circuits and sub-circuits so that terminal devices such as fan-coil units continue to receive their design flow, irrespective of changes in the wider system. Combination valves to control differential pressure and flow are offered by several companies, and John Coppin refers to Frese, TA, Oventrop and Samson Controls.

‘Today, we have many more pipeline components for variable-volume systems, which helps commissioning and controllability of systems,’ says John Coppin. ‘Design engineers must consider how to apply the huge amount of pipeline equipment that is available to them. However, all this equipment has to be correctly applied to do the job you want it to do — cost effectively. Combination valves assist the process, but using these alone may not give the most cost-effective solution for the design.’

The starting point for exploiting variable-speed pumps is to step back to proportional balancing, the objective of which was to ensure all flows throughout a system were at the same percentage of their design flow. If the actual flows were higher than design, say 120 to 130%, the speed of a belt-driven pump would be adjusted by a simple pulley change. If the actual flows were too low, analyse why and where required upgrade the pump — if it is a pump problem. Variations to flows in secondary circuits would have been achieved using secondary valves, keeping flows in the primary circuit constant.

There is no doubt, however, that the days of simply proportionally balanced constant-flow system are slowly coming to an end in favour of variable-volume systems controlled by DPCVs.

As a practical and cost-conscious engineer, John Coppin suggests installing DPCVs in each branch, such as a floor served by 20 fan-coil units. Within such a branch, he favours using a simple double-regulating valve on each FCU to achieve proportional balancing. With all valves on the branch open, the DPCV would control the differential pressure on that branch and maintain that differential pressure as 2-port valves on individual FCUs close down. With pressures under such control, it becomes possible to control the speed of the main pump and deliver the energy-saving benefits of variable-speed pumps.

While recently introduced pipeline components that make it possible to control the differential pressure and flow at each and every terminal unit are sometimes promoted as making commissioning unnecessary, John Coppin is not convinced. He says, ‘These combination pipeline components do not remove the need for the commissioning process, but they may speed it up and make for a more effective system. It is still essential for a good-quality commissioning engineer to commission a system.’

22 Leadenhall Street in London, a 47-storey 224 m-high building of 56 800 m2 in the City of London by British Land moves commissioning engineering into a new area with variable-volume primary pumps.

Indeed, the emphasis placed by Part L of the Building Regulations on commissioning reinforces the need for a commissioning engineer. ‘Part and parcel of the requirements of Part L is for a commissioning engineer to record just how a system has been set up,’ explains John Coppin.

Commissioning is not a task that can be left to the end of a project. ‘What you need is a good design that is commissioned by a good commissioning engineer. In the UK we are fortunate to have the Commissioning Specialists Association, whose members are skilled technicians and contribute to the overall success of the commissioning process. Getting commissioning correct at the end of the project requires that it is considered early in the design stage. However, a number of design engineers across the field of design and commissioning of systems are still in their infancy when it comes to variable-volume design and fully understanding the use of the traditional and new pipeline equipment for the commissioning of water systems.’

Times are changing, and whereas John Coppin was one of the first commissioning engineers employed directly by a firm of consulting engineers so that commissioning could be considered early in the design, the number of cpractices with commissioning expertise in house is improving.

However, he stresses that commissioning engineers in consultancy need to work with the engineer who will carry out the commissioning. ‘While design for commissioning is covered in degree courses, consulting engineers may not have the relevant site-end experience and need the coal-face experience of commissioning,’ he says.

But even while the industry responds to the challenges and opportunities of variable-speed pumps, more challenges and opportunities are emerging.‘John Coppin explains: ‘We are getting more and more primary equipment with variable-speed drives, such as centrifugal compressors on large water chillers — making COP’s of 12 at part load conditions achievable.

‘The ability of equipment to perform so well at part load must be exploited by the overall system — which is made possible by a concept developed in the USA called the Hartman Loop.’

Whatever the advances in technology and the development of new components to aid commissioning, its fundamental importance has not changed. ‘A poorly commissioned system cannot be expected to perform to the designer’s or client’s expectations,’ says John Coppin. ‘One of the key benefits of good commissioning is systems that deliver better control of the environment in buildings by being commissioned to a better standard.’



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