Finding the right balance
Published: 08 January, 2015
Steve Tomlinson of Siemens Building Technologies explains how pressure-independent control valves (PICVs) offer energy efficiency and consistent comfort for the occupants of buildings.
The sizing of valves for HVAC systems in buildings can be challenging and requires accurate data about flow that may not necessarily be readily available, particularly in new buildings. However, the latest valve technology means that the selection, sizing and commissioning of valves is now much easier.
Pressure-independent controls valves (PICVs) are sometimes known as combi-valves. This is because they incorporate the functions of a number of different products that have to be specified and installed separately using ‘traditional’ valves. These functions usually include the following.
• A control valve
• A differential-pressure controller to shield against pressure fluctuations
The integrated differential-pressure controller manages the pressure across the internal control valve to maintain a constant volumetric flow, regardless of any pressure fluctuations in the building’s hydraulic network. Hence the name, pressure-independent control valves.
Valve selection is more straightforward than for other valves, as the only information necessary to size the valve is the required flow rate through it. This saves time for the specifier.
The combi-valve automatically shields against pressure fluctuations in the network, providing constant volumetric flow. This allows for exact control of room temperature, ensuring greater occupant comfort as well as lower energy consumption.
Versions are available with the inclusion of pressure-measuring points, which is also useful for commissioning, and ongoing maintenance. They are there to check you have the required differential pressure. With increasing emphasis on continuous commissioning of buildings, this sort of easy-access testing is invaluable for building managers.
Combi-valves are so flexible that a building can be commissioned in sections, or late changes to the design can be more easily incorporated. And if parts of a building are not in use, then it is easy to shut down that section of the HVAC system — saving energy and running costs. This is particularly useful for hotels or hospitals. It also means that this type of valve is good for refurbishment projects.
An additional benefit of combi-valves is that because their control is independent of differential pressure, they ensure more accurate temperature control. They ensure that return temperatures remain at the correct level under all operating conditions, ensuring high efficiency in the generation of heating and cooling energy, which in turn reduces heating and cooling costs for the building.
The benefits of PICVs are particularly clear when considering a demand-controlled heating and cooling system. These are increasingly popular as they support energy-efficient operation of the building. But such systems call for valves which can support the energy-efficient benefits without causing discomfort in occupied spaces.
Using a combi-valve on this type of system ensures there is never any under-supply to the system because of the integrated differential pressure controller in the PICV. The desired volumetric flow is always ensured, so the demand-based heating system works smoothly, effectively and efficiently. The system is energy efficient and also ensures comfort for occupants.
By having a number of functions, the PICV removes the requirements for 2-port control valves, differential pressure controls and the respective partner valves that are seen in ‘conventional’ HVAC systems. There are potential cost savings due to the reduction in balancing time required on site, as this process is simplified.
The PICV can be used on a broad range of systems, including heating, cooling and chilled-water applications. They are flexible and highly precise, giving guaranteed system performance and optimised energy use. In today’s new-build and refurbishment projects, this sort of technology is making the work of consulting engineers, installers and commissioning engineers much easier.
Perhaps more importantly, the impact on occupants and building managers is tangible, in terms of comfort and time saved trying to adjust the system so that it is in balance. The combi-valve provides those benefits automatically and efficiently.
Steve Tomlinson is field-devices promoter for Siemens Building Technologies
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