Automatic balancing is the key to efficient systems

Published:  14 April, 2006

abqm
Modern components for commissioning hydronic systems combine several functions. The Danfoss ABQM is an adjustable flow-limiting valve that automatically compensates for changes in system pressure.

JOHN COOPER explains the advantages of using an automatic approach to HVAC balancing.

The desire to improve the energy performance of HVAC systems has seen a dramatic increase in the popularity of variable-flow systems for commercial and public buildings.

  This growth has been achieved at the expense of more traditional, constant-flow designs and presents a variety of engineering challenges. Not least is the need for a different approach to system balancing, which can satisfy the cost-saving, time-saving and space-limitation requirements demanded in many project specifications. An automatic approach to HVAC system balancing that can capably meet these key expectations deserves a closer look.

Until recently, constant-flow systems set the standard in hydronic HVAC system design. They allow standard designs to be applied to numerous different projects, typically incorporating fixed-speed pumps sized to match the maximum load of the system.

These systems are balanced using a proportional method with manually set, fixed-orifice, double-regulating balancing valves installed to account for and reduce the impact of pressure changes in the system.

In such constant-flow systems, the capital costs are, indeed, low. However, the energy usage is high, since these systems and the pumps driving them rarely operate at the 100% load they were designed for, calling into question the effectiveness of balancing this type of system in the first place. Further, the process of proportional balancing to commission the system is long, painstaking and expensive.

Risen in popularity

Variable-flow systems have risen in popularity primarily because they reduce a system’s energy consumption. They use variable-speed, inverter-driven pumps, the speed of which is changed to match the load. 2-port, motorised control valves are often used to control flows to terminal fan-coil units, or thermostatic radiator valves are used to regulate flow to radiators in heating-only systems.

When fitted with a suitable motor, the Danfoss AVQM valve integrates the three functions of motorised valve, differential-pressure control valve and double-regulating valve commissioning valve.

Current solutions in variable-flow systems also include a combination of differential-pressure control valves (DPCVs) similar to the Danfoss AVP available from Taconova, which can be used on branches and/or across air-handling units, with double regulating valves to proportionately balance and limit flows to all terminal valves.

DPCVs help maintain a constant pressure differential across a sub-branch, thereby protecting downstream control valves from excessive pressures and nullifying the effects of pressure variations caused by the movement of control valves in other branches.

In systems with 2-port control valves, DPCVs can perform the dual duties of maximum-volume control and differential-pressure control, thereby simplifying the commissioning process.

Once a sub-circuit has been commissioned, the DPCV prevents the flow and the balance in the sub-circuit being affected by other sub-circuits.

This approach is what I term a semi-automatic method of balancing the system.

Enhance performance

However, with both variable- and constant-flow systems, fully automatic balancing can enhance the performance of the HVAC system, eliminating any problems caused by high or excessive system pressures, including noise from the valves and, ultimately, poor control of room temperature. The way the system is balanced will also have a major impact on energy usage, by minimising pumping costs.

In variable-flow systems the use of combination valves, such as, the Danfoss ABQM and AVQM range, when fitted with a 0 to 10 V, 3 point or thermal motor, integrates the three functions of motorised control valve, differential pressure control valve and a double-regulating valve commissioning set into one product. Applications for this type of product include the control of fan-coil units, chilled beams and air-handling units in variable-volume heating and cooling systems and the control of secondary flow on plate heat exchangers.

This integrated approach significantly reduces installation and commissioning costs, since the three control functions are specifically matched to ensure optimum system performance and only one valve has to be mounted in the system instead of three. The problem of high and varying pressure drops across traditional 2-port control valves is also effectively eliminated, ensuring a control-valve authority of 100%. It also saves on space, an important concern in, for example, air handling unit installations where the available plant room space for valve installation is often severely restricted.

Applying automatic-balancing technology in constant-flow heating and cooling systems brings advantages too.

Primarily, the use of adjustable flow limiting valves, such as the Danfoss ABQM (available from Taconova), is designed to optimise water flow and can prove invaluable. Such valves provide flow limiting, shut-off and adjustment functions, automatically compensating for changes in system pressure to maximise energy efficiency.

The time and cost of balancing is also significantly reduced.

For example, in a typical installation of fan-coil units, manual balancing valves are required at all terminal units, branches, risers and pumps.

In contrast, because of their added functionality, automatic balancing valves are only needed at terminal units, reducing the number of valves used, cutting costs and creating a more flexible HVAC system that can be easily adapted or expanded without having to make any new calculations or re-commission the whole system. Indeed, parts of the system can be commissioned and completed in phases, to allow for partial occupation of the building, if required — which can be especially useful in, for example, hotels. These adjustable flow-limiting valves are easy to size and set and provide effective flow limitation at terminal units, irrespective of changes in flow and pressure conditions in other parts of the system.

Summarising

Automatic balancing generates significant time, cost and efficiency advantages to engineers working with both constant-flow and variable-flow HVAC systems, particularly when the latest combination-valve technology can be applied. Getting the balance right, at an affordable cost and with all the energy consumption, low noise, cavitations and optimum room temperature benefits still in tact, is now well within reach.

John Cooper is managing director of Taconova (UK), 20 New Forest Enterprise Centre, Rushington Business Park, Totton, Southampton SO40 9LA.



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