When designing a modern building HVAC system, three areas are of critical importance; the indoor air quality (IAQ), how much energy will be consumed and the capital cost of the equipment.
Fläkt Woods has designed new products which tackle these concerns head-on, and by linking the equipment together, such as chilled beams, twin wheels, and chillers, the benefits are compounded to a point where overall energy consumption can be reduced by as much as 50%.
Chilled Beams
Chilled beams create a stable and comfortable environment by supplying draught free cooling. By using Active Chilled Beams, up to 75% of the total cooling capacity will be handled by the water. Apart from the energy saving it also means less ductwork for air, smaller air handling units and lower noise.
Chilled beams provide savings by replacing fan energy with pump energy. They use pumped chilled water instead of blowing cold air. Water has much higher heat capacity, both by mass and volume. In typical pump and fan arrangements, this has been shown to translate into a reduction in fan energy by a factor of seven.
These systems offer exceptional comfort and IAQ, however to avoid the issue of condensation on the chilled beam’s cold surfaces (cooling coil), the dew point temperature of the air needs to be controlled at a level below that of the cold surface in question. The dew point temperature of outdoor air in the summer can often be well above the surface temperature of the coil in a chilled beam.
This means that we must remove moisture in the primary air before supplying it to the room. The traditional method is to use a cooling coil in the AHU to condense out the water and then a re-heater to warm the air to a suitable supply-air temperature. This method demands a large cooling plant & boiler and can be less efficient to run.
Now, with the development of twin-wheel it is possible to utilise the advantages of the chilled beam system, and provide fresh air controlled on its dew point without the inherent energy inefficiencies of traditional air handling units with dew point control.
Research is on-going and it is possible that in the near future the active MSCB can meet cooling loads up to 120 W/m² with total airflows much reduced and around the fresh air volume requirement.
A further development recently launched by Flakt Woods is demand control ventilation in chilled beams which again improves the efficiency over the traditional fan coil unit.
Explains Linden Shuttleworth, Fläkt Woods UK Sales Manager, Fläkt System Products, said, “using our knowledge of twin wheel technology and active chilled beam systems, engineers have created a more efficient method of providing dehumidification to the primary fresh air, in order to avoid problems with condensation.”
How twin wheels work
As the name implies, the system uses two thermal wheels (rotary heat exchangers). In winter, the heat of the exhaust air is absorbed by an aluminium rotor, which then delivers the heat to the supply air; and vice versa, in summer. Rotary heat exchangers are used when the supply and exhaust air ducts converge at one point. Low velocities through the rotor can ensure optimum heat exchange efficiencies typically between 70 -85% and pressure loss of 60 Pa. What’s more, owing to the reheat wheel, a separate re-heater is no longer required. This is ideal for a chilled beam installation that avoids any moisture problems.
In winter operation, the benefits are substantial as well. Not only do you get high levels of heat recovery but also most of your humidification load is supplied by the hygroscopic wheel further reducing the energy demand.
Free Cooling Chillers
Chilled beam systems don’t run on low chilled water temperatures in the same way as fan coil systems do. Due to the nature of the system, higher chilled water temperatures are necessary and this leads to another potential energy saving by utilising free cooling chillers.
Because there are long periods of time where the ambient air condition is cooler than the return water temperature, water can now be cooled without running the compressors. This not only reduces wear and tear on them, prolonging the life of the chiller, but offers significant annual energy savings. The chart below shows savings of around 35% for London based projects.
Air handling
Linking all the equipment together, includes the role of the air handler. To achieve the optimal energy savings, work focuses on reducing energy consumption and creating factory complete air handling system solutions with controls. Everything from single supply air or exhaust air units to complete units with heat recovery, energy efficient components and built-in control and regulation systems are available.
Concludes Linden Shuttleworth: “By combining chilled beams into an integrated system, utilising smart control, customers can achieve the best energy solutions, with overall savings of up to 50%. Chilled Beams are energy efficient due to many factors including the removal of localised fan power and decreased usage of very low temperature chilled water, but has been further enhanced by the use of free cooling chillers, removal of heating within the AHU and reduction of the total cooling required due to the latent wheel.