Striking the perfect balance
A well designed hydronic system that is then perfectly balanced is one of the keys to a successful building. Les Littlewood of Albion Valves UK provides us with a handy reminder of the principles involved.
A well-balanced hydronic system starts with good system design. The designer calculates the mass of water required at each terminal unit, whether heating or cooling. Balancing valves create additional resistance in each branch to direct water in the required proportion to each terminal unit.
Design considerations will take into account the size of space to be conditioned, space usage, occupancy, construction materials and their thermal properties, heat losses and gains and general climatic conditions associated with the building.
• Fixed orifice: the aperture through which flow passes cannot be adjusted
• Variable orifice: the aperture through which flow passes is adjustable
• Double regulating valve: at the set position the valve may be fully closed and re-opened to the set position
• FODRV: fixed-orifice double regulating valve
• VODRV: variable orifice double regulating valve
• LTHW: low-temperature hot water (70 to 100ºC)
• MTHW: medium-temperature hot water (101 to 120°C)
• CHW: chilled water (typically 6 to 11°C)
• Pressure: usually in kPa (kilopascal). 100 kPa is 1 bar
• Flow: For water, 1 kg/s is 1 l/s. and 1000 l is 1 m3
• Δp: Pressure drop (kPa)
• Signal: – pressure drop measured across the orifice (kPa)
Achieving optimum flow rate is essential. Too high a flow rate may cause noise, whilst too low a flow rate may not dislodge entrapped air. Flow velocities will generally be in the range 0.75 to 1.15 m/s for pipe sizes 1/2 to 2 in and 0.75 to 3 m/s for size DN65 to DN300. Pressure drop is generally calculated at 140 to 280 kPa/m of pipe run.
Fixed-orifice regulating valves (FORV) or variable-orifice regulating valves (VORV) are an option to consider, and system specification will dictate which type is to be used. Fixed orifice will nearly always be specified in the UK, with VORV more common in continental Europe and North America.
It is generally accepted that fixed orifice will provide a more accurate flow rate, usually to a tolerance of ±5%, compared to ±10% (or greater) of design flow rate in variable-orifice systems.
Some systems will specify metering stations to be installed separately to the regulating valve. These valves are available threaded or wafer style for fitting between flanges and are suitable for chilled water, LTHW and MTHW.
Devices that combine metering and flow regulation are easy and quick to install and commission, and are suitable for chilled water, LTHW and MTHW.
A combined valve and metering station benefits from having a single-piece body with no joint between body and metering station. This reduces potential leak points, as well as being compact and cheaper than a 2-piece commissioning set.
To ensure accurate flow measurement, guidelines for the installation of the valve or metering station will specify the length of uninterrupted pipe required on either side of the valve. This is to avoid turbulence caused by other fittings within the system or change of flow direction, close to the valve that could affect the accuracy of the reading.
To avoid such problems in this example, the requirement is for five diameters of straight pipe upstream and two diameters of straight pipe downstream of the valve, with the five diameters upstream being the more important.
Because valves are often installed in ceiling voids, it is important that hand-wheels/ levers/test points and drains point down for ease of access and commissioning.
|For accuracy of flow measurement, it is important to have straight runs of pipe either side of the valve.|
Pipework and valves in chilled-water systems should be insulated and provided with effective vapour seals to minimise condensation and thus prevent damage to ceiling voids.
It is important to ensure that all air is removed from the system, as the presence of air can seriously affect commissioning results. And the final point to remember is that commissioning should only be carried out when the system is cold.
Les Littlewood is sales and marketing director with Albion Valves UK.