Commissioning to deliver the full benefits of variable-speed pumps
All the essential components for commissioning and heating and chilled-water systems with variable-speed pumps can be contained in commissioning modules designed to enable a number of terminal units from one point. Vital components include a differential-pressure control valve (top left corner) and an integral bypass to ensure that flow will never fall below 10%.
Lars Fabricius highlights some of the commissioning issues raised by the switch to variable volume systems.Consultants and mechanical contractors are taking a keener interest in commissioning, because of the drive to save energy. Using variable-speed pumps in line with CIBSE Knowledge Series Guide KS7 means that interest will be keener than ever! There is no doubt that variable-speed pumps can make a significant contribution in the drive to save energy. Systems require new considerations, however, if the savings made by modulating pumping power are not to be swallowed up by losses arising from poor control. Locating large differential-pressure control valves (DPCVs) on each floor may seem convenient, but if the branches are of any real size — as in a typical office block — the performance of the modulating 2-port control valves may suffer. This problem is recognised in CIBSE Guide KS7 ‘Variable flow pipework systems’. The authors recommend that DPCVs should be located on straight branches feeding no more than 12 terminals such as fan coils, active chilled ceilings etc. This arrangement makes it easier to select 2-port control valves with good authority. KS7 also recommends that valves under normal design conditions should have an authority of 0.3 or better. That means that the pressure loss through the valve should be at least 30% of the loss in the entire terminal branch in which it sits. Meeting the requirements of CIBSE GuideKS7 needs a new approach to system design and to balancing and commissioning.
Elegant solution SAV’s modular approach provides an elegant solution to the variable-speed issue. Balancing and adjustment for up to six fan coils can be carried out at a single point (i.e. the module), which makes achieving the right flow and pressure conditions over, for example, a long hotel corridor much easier. The incorporation of individual DPCV's into these modules is also a boon. It means that they are close to the terminals, so that once the design condition for the longest pipework run and the highest resistance have been met, the rest will just need trimming. This modular approach to commissioning also offers major advantages to system designers. A huge range of SAV options can be used with electronic balancing monitoring equipment to promote a far better understanding of how the system is actually performing. With such help, the commissioning engineer’s work on site is made much easier and can be carried out far more quickly. Using the latest balancing and monitoring equipment, with very accurate readings up to 250 kPa, the commissioning engineer can perform a range of tasks — including balancing flow through each valve, measuring and adjusting the pressure drop across each DPCV, measuring the pressure drop on the flow and return mains and the losses through the actual terminals. All hydraulic parameters can be measured and recorded. Pump speeds can be set in the light of this data, and real energy savings can be achieved with no compromise to environmental conditions. That, surely, is what KS7 is all about. SAV pioneered the modular concept for distribution of hot and chilled water in heating and air-conditioning systems, and the SAV FloCon range of commissioning modules is now a standard solution for 2- and 4-pipe fan coil systems and chilled beams. FloCon commissioning modules used with flexible multi-layer pipework make it possible to sub-divide the system into smaller, self-contained sub-systems, each served by a single distribution module with an integral DPCV. The distance from DPCV to control valve is minimised, and accurate modulating control of the pumping system is maximised. The right 2-port valves can be selected. Modules also reduce the need for additional sub-branch rigid pipework, which might otherwise be required to comply with KS7. Such commissioning modules enable one compact DPCV to be shared by up to six terminal units with a maximum loading of 11.6 kW heating and 6.3 kW cooling per terminal — a very cost-efficient ratio. If the pressure available from the pump should increase, the DPCV will close in order to take out the excess pressure. If the pressure available should reduce, the DPCV will open so that more pressure becomes available downstream. Moreover, because the DPCVs are close to the terminals, the pressures they control are relatively low, typically from 0.2 to 0.5 bar, so that crucial good valve authority can be achieved every time.
Integral bypass In future all SAV FloCon commissioning modules for variable-volume applications will be supplied ready fitted with a bypass to ensure that system flow will never fall below 10%. This offers a simple, but highly effective, solution to the Achilles’ heel of the variable-volume approach to energy saving by providing automatic protection for expensive variable-speed pumping systems, which cannot be allowed to run dry. This feature will also improve system response time by ensuring that some hot/chilled water circulates at all times — even when valves are closed. The bypass addresses another frequent concern to consultants by ensuring good local circulation of chemicals added to the system water to combat pseudomonades. These slime-forming organisms can be a problem in closed-circuit lower temperature systems, especially those with nil or low levels of oxygen. Bypasses will be an integral feature of all SAV FloCon variable-volume modules, sized and selected to match individual application requirements. Pump selection is unlikely to be affected in most cases. Lars Fabricius is managing director of SAV Modules.
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