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Things to consider when choosing room units for variable air flows

Åsa Norén-Lundh

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There are a few things to consider when choosing room units for variable air flow to save energy and to achieve a comfortable indoor climate. In this article, we will walk you through the most important ones.

To achieve the optimal balance between energy efficiency and comfortable indoor climate with variable air flows, room units must be chosen carefully with sound, discharge principle, placement and mounting and work range taken into consideration.

Sound can enter the room via the duct system

A regulating damper can generate noise. This can enter rooms via the duct system, but also to its surroundings, transmitted through its casing. In order to mitigate the sound level in the room, you choose appropriate sound attenuators according to the damper. If there is no suspended ceiling, we’re talking about a visible installation where you can choose a ready-insulated product and in doing so eliminate sound to the surroundings.

Discharge principles

All products and discharge principles are not suitable for variable flows. Usually it means working with a supply air with a temperature below the temperature in the room, and then the choice is very important in order to achieve a good indoor climate, without experiencing high air velocity draughts in the occupied zone.

Air diffusers with nozzles or perforations of a smaller type are the best choice. This is because these are generally able to maintain a good ejection functionality of the supply air into the room, thanks to the so called Coanda effect, even at low supply air temperatures. These diffuser types also give a good mix of the supply air with the room air.

Displacement diffusers are also suitable for variable flows, but you must always make sure the supply air temperature is at least 2 °C below the room temperature, in order to attain the right displacement function. Otherwise there is a risk that the diffuser’s large front surface will become heated, so that at low flows and a temperature difference (dT) close to zero the supply air goes straight up instead of ventilating the occupied zone.

The design of the supply air openings is important

Naturally, also in the case of induction units and comfort modules the design and shape of the supply air openings play a significant role. These products work to produce a good coanda effect and accordingly get the largest possible area for mixing the supply and room air, to bring about good comfort in the occupied zone. For variable air flows on comfort modules, the pressure across the inlet openings, such as nozzles or slots, is important in order to achieve a good velocity out of the product and get good adhesion of the air against the ceiling, and with that draught-free comfort. Dimensioning is then important. Of course, the comfort requirements in the event of non-occupancy are set aside. However, they become even more important at occupancy air flow and maximum air flow.

Those working on selection need to keep in mind that the design operating mode is not the boost air flow, but the occupancy air flow, so as to save energy and to ensure that comfort is at its best in the most normal operating mode. The size and design of the supply air opening is important to provide the air with the optimal ejection angle, and there are different designs depending on whether it is a suspended installation – where you ideally place the air opening close to the ceiling or slightly angled upwards – alternatively an integrated product where you discharge the air along the ceiling in two or four directions.

Placement and mounting

Operation and maintenance need to be considered when placing dampers. Dampers must be placed so that any disruptions during the regulation of measurements are avoided. This can require straight sections between the damper and nearby manifolds, bends and t-pieces.  But a sound attenuator with a middle or centre baffle placed after the damper can also affect the functionality. To simplify maintenance, a clamp section can be used between the damper and duct system to facilitate access.

Working range

The system pressure in the duct system is crucial for regulation to work. There must be sufficient pressure in order for the damper to have authority. However, the pressure must not be too high. This can result in large fluctuations in flow measurements as soon as the damper opens or closes. The installation will then be very unstable.

Spending time on project planning of variable flows is beneficial. The result is a good indoor climate combined with safe operation and easy maintenance.

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