Moving beyond proportional balancing
By delivering a preset flow, regardless of changes in differential pressure Frese’s Optima pressure-independent control valve (right) and automatic balancing valve (left) provide an effective approach to setting up systems with incorporating variable-speed pumps.
Proportional balancing and variable flow do not go together, but it is taking time for a more effective approach to setting up waterborne systems to be widely adopted. While it is perfectly easy to specify the design flow required for hydronic HVAC terminal units such as fan-coil units, heat emitters, underfloor heating and chilled beams, delivering those design flows in actual projects has, traditionally, presented many challenges. The heating or cooling capacity of a terminal unit is simply determined by the flowrate and the change in temperature (Dt) of the water in the circuit. Even when all the circuits and sub-circuits in a complex building have been proportionally balanced and the pump set to deliver the desired flow, any change in the flow in any part of a system will affect the flow elsewhere. Such actions as a thermostatic radiator valve or a 2-port valve closing will immediately throw out of balance a system that has been proportionally balanced for design conditions. As Matthew Dunk, managing director of Frese in the UK, points out, ‘Proportional balancing sets up a system at full design condition and is only a snapshot in time in a dynamic system. As soon as a system is started up, it is immediately out of balance.’ Variable-speed pumps
Matthew Dunk is also critical of the use of variable-speed pumps in systems with static balancing valves, explaining that the variable-speed capability is merely being used to commission rather than to achieve energy-efficient operation at part load. The problem with static balancing valves in a variable-flow system is that as valves open and close, the differential pressure (DP) across other valves in the system changes. The effect is readily made clear by a simple equation. Q=Kv√DP Q is the flow. Kv is the characteristic of the balancing valve, which is determined by the orifice plate and is, therefore, a constant. DP is the differential pressure, and the effect of the square root is for a doubling of differential pressure to increase the flow by 40%. Dynamic
That is why Frese, and other manufacturers, have developed dynamic approaches to commissioning waterborne systems. Not only does dynamic balancing enable systems to perform better, but installation and commissioning time are both dramatically reduced. BSRIA drew attention to the problem in an application guide on variable-flow water systems published in 2002. That guide observed ‘the hydraulic stability of the water distribution system is not improved by the use of variable-speed pumping systems’ and explained the role of differential-pressure-control valves. Frese in Denmark has been developing automatic balancing valves over the last 20 years, but it is only recently that they have started appearing in the UK. Matthew Dunk tells us that the number of reference projects is approaching a hundred, and that some equipment manufacturers are incorporating this valve technology in their products. Notable examples are Trox and TEV. Trox chilled-beam modules, for example, include a Frese dynamic balancing valve selected and set to deliver the required flow — independently of changes in differential pressure. TEV uses these valves in its Quartz Solutions range of ‘plug-and-play’ fan-coil units launched last Autumn. Such has been the success of Frese’s approach to flow control, that it grew to become 63% of a business that also included domestic hot water (13%) and sanitary equipment (24%) — and is an even higher proportion of turnover since the sanitary business was sold last year. Simple sizing
Jens Johansen, product and marketing manager at the company’s headquarters at Slagelse about a hundred kilometres south west of Copenhagen, explains, ‘The sizing and selection of Frese dynamic balancing valves and pressure-independent control valves is very simple; all that is required is the design flow. From this design flow rate, we can correctly select the cartridge for a DBV [dynamic balancing valve] or set the valve position for a PICV [pressure-independent control valve] to ensure that the required design flow rate is achieved. Using such valves on each terminal unit or group of terminal units eliminates the expense of double-regulating valves on sub-circuits, main distribution circuits and risers — everywhere else in the system, in fact. Isolation valves will still be required, but these are relatively cheap compared with DRVs. Not only do they deliver accurate flow without the requirement for a straight pipe run upstream and downstream, as required by a DRV, but control is accurate to ±5% — which should be compared with the -0/+10% required by the BSRIA guide for the proportional balancing of static systems. Subject to the requirement minimum differential pressure being provided by the pump, the Frese Alpha DBV ensures that the design flow is not exceeded at fluctuating pressure conditions up to 600 kPa. Likewise, when the design flow has been set on a Frese Optima PICV, that flow will not be exceeded up to 400 kPa differential pressure. These valves can be controlled using a 0 to 10 V signal with, because excess differential pressure is taken care of, full-stroke modulation. New perspective
Once the decision has been taken to use valves that can be set to deliver the required flow rate, commissioning takes on a whole new perspective. The basic requirement is to ensure that there is adequate differential pressure at the least-favoured valve (probably the furthest from the pump on the index circuit). For example, an Optima PICV requires a minimum of 18 kPa to be in its control range and deliver the required flow rate, regardless of pressure fluctuations.
Look, no double-regulating valves — using valves that can be preset to deliver the required flow to every terminal unit eliminates the cost of double-regulating valves.
Jens Johansen explains that only a differential-pressure reading is required on the index circuit. If adequate pressure is available at this point, there will be sufficient pressure in the rest of the system closer to the pump for all PICVs and DBVs to be in their control range and, thus, provide the required design flow. Matthew Dunk adds to the UK perspective by explaining that there is then no need for proportional balancing — saving significant time and cost at what tends to be a critical stage of the project. He suggests that each static balancing valve needs adjusting three times, on average, before the required design flow is achieved — which would take a very long time on a large system. The benefits of setting the required flow at a valve carry over when a system is extended or modified. As long as the pump remains adequately sized, the various DBVs and PICVs will continue to deliver the required flow — avoiding the system having to be rebalanced. Learning curve
The shortcomings of proportional balancing, especially in systems with variable-speed pumps are increasingly being recognised, and it is recognised that many design engineers involved in design and commissioning still have a steep learning curve in front of them in the application of variable-speed design. Matthew Dunk concludes, ‘The way we have designed systems traditionally is not necessarily the most efficient way, and quite a lot of established engineers are getting round to that way of thinking.
For more information on this story, click here: April 08, 86