Controlling influences
The effect of varying flow in hydronic systems depends the system, and control valves need to be selected accordingly. Chris Parsloe gives an insight into new BSRIA guidance.
Designers of hydronic systems strive to conserve energy and achieve good comfort control. To achieve both these goals, control valves need to be selected and sized correctly. However, this process can be problematic since it falls awkwardly between designers, installers and equipment suppliers.
The recently published BSRIA guide ‘BG 51/2014 selection of control valves in variable flow systems’ provides detailed guidance on the valve options available and how to make appropriate selections.
Flow control is important because the better control achieved over heat transfer, the better control will be achieved over system operating temperatures. Poor control of heating or cooling outputs translates to larger swings in space temperatures. Furthermore, (and often overlooked) poor control will result in uncontrolled return-water temperatures to central plant. If the aim is to maximise the system temperature differential in order to get the best performance from boilers or chillers, then a control valve that does not control is a problem.
In order for any valve to provide good modulating control, its characteristic must match that of the heat transfer that it is trying to control. For example, if there is a proportional relationship between flow rate and heat transfer then the valve’s characteristic should also be proportional — i.e. the change in flow rate through the valve for a given degree of closure should be the same as the degree of closure. A proportional characteristic is suitable for water-to-water heat exchangers such as plate heat exchangers.
For-forced convection water-to air-heat transfer such as fan-coil units, active chilled beams and air- handling units, a large reduction in flow rate is required to achieve a small reduction in heat transfer. Hence, for these applications, a linear characteristic is not ideal. Instead an equal-percentage characteristic is best. These valves are so called because the percentage change in flow rate is always the same for a given percentage change in valve opening. A valve’s characteristic is influenced by a combination of the internal valve seat and plug arrangement, combined with the electronic actuator that is fitted. It is essential that the correct valve and actuator combination is selected in order to achieve the correct characteristic.
However, the valve supplier cannot always ensure that the valve will deliver the characteristic intended. This depends on where the valve is located in the system and the relative pressure losses across the circuits in which it is controlling flow. This introduces the tricky issue of valve authority. In simple terms, a valve and actuator combination might be designed to give the specified characteristic, but if it is installed it in a circuit for which the resistance across the valve is small relative to the rest of the circuit, then it will not function as intended. Some degree of the valve’s closure will be used up just trying to catch up with the other resistances in the circuit. Only when the valve is the biggest resistance in the circuit will it start to influence flow.
For 3- and 4-port valves where the valve diverts flow through a by-pass, valve selection is relatively straightforward. The circuit in which flow is controlled is that through the coil itself, so the only pressure loss is the coil pressure loss. This means that the pressure loss through the fully open valve only needs to match the pressure loss through the coil it controls in order to have good authority. For these valves, the coil manufacturer can easily select the valves.
However, 2-port control valves are more of a challenge since the controlled circuit comprises all pipes and components back to the nearest fixed pressure point — this being either the pump or an appropriately located differential-pressure control valve (DPCV). To match all of these resistances, 2-port valves must have higher resistances. It also means that the coil manufacturer cannot size the valve. The only party that has all of the appropriate pressure-loss information is the designer. But sizing the valves is complicated and best suited to a computerised calculation.
Pressure-independent control valves (PICVs) have provided a partial solution to valve selection since they combine a 2-port control valve with a DPCV. Hence the pressure loss across the control valve is fixed, giving it a theoretical authority of one. However, PICVs have their limitations in terms of limiting minimum flow rates and control characteristics. Ultra-low flow rates can be a problem, and achieving an equal percentage characteristic can be a challenge. Also, flow-repeatability issues are more noticeable as the valves reply on the flexing of a spring.
The new BSRIA guide provides some understanding on all of these issues and should be helpful to all engineers involved in valve selection.
Chris Parsloe is a consultant to BSRIA.
