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Within the HVAC system the grille is the final piece of the puzzle. As an all-encompassing phrase I will henceforth use ‘ATD’ (air terminal device) to describe grilles, diffusers and air valves. ATD’s are the component that directly connects the room with the rest of the system. As such, a poorly sized or selected ATD can negate all the design work and careful selection of all other components that precede it. In this post we will look at why it’s important to select the correct type of ATD and the implications if selection is conducted incorrectly.
Within the ATD selection procedure there are 3 common important parameters to consider with respect to performance. These are throw, acoustics and pressure loss. Failure to select any of these parameters correctly will present its own unique problem.
Throw (Airflow distribution) - The throw of an ATD is the distance from the ATD at which the jet velocity has decayed to a given terminal velocity. Details around velocity decay, temperate differentials, jet discharge principles and correct ATD location could be an entire post in itself, so for the purpose of this post I will keep it to the point of ‘why’ the throw selection is important. Correct selection of the throw will eliminate occupants feeling draughts. If a throw is too high, the velocity of the air will enter the occupied zone at too high of a level and occupants will become uncomfortable. For comfort conditions, the occupants should not consciously feel the air. As a rule, the air velocity should have decayed to average room velocities of between 0.1-0.25m/s in the occupied zone. This becomes even more important within cooling applications where discharged air is cooler than room air which can be observed by a draft rating in ISO 7730.
However, if throw distance is too low, the air jet can ‘dump’ in the space. This is the principle of when the air jet separates from the ceiling and falls directly into the occupied zone before it’s effectively mixed with room air. This would lead to pockets of stagnant air in the space and areas not being effectively conditioned. Air that is unmixed and therefore too cool will enter the occupied zone and cause draughts and discomfort. To summarise, a throw selection which is either too high or too low, could cause uncomfortable draughts for occupants, poorly conditioned spaces and poorly ventilated spaces.
Acoustics - The acoustic performance is the sound characteristics associated with the ATD. Depending on the manufacturer and the country in question the acoustic performance is displayed in different ways. Common acoustic performance parameters used are NR, NC and dB(A). As with throw, an entire blog post and more could be dedicated to what each of these means and how they are calculated, but for the purpose of this post, specifically the issues occurring due to not selecting correctly, will be focused on. Acoustic performance is linked to the velocity of air through the ATD. The higher the velocity, the higher the sound level. Excessive noise generated by this air can create an unpleasant acoustic environment. Whilst this is different to the aforementioned draughts, it’s still an important consideration for occupant comfort. The type of space might dictate the acceptable sound level. Using the ‘NR’ (Noise Rating) unit as an example, a bedroom may allow for a maximum NR25 rating, whereas in a classroom, NR35 would be acceptable. That same level of noise from an ATD being produced in a classroom, which isn’t considered uncomfortable, might be too much in a bedroom, where a person is sleeping.
If an ATD is sized too small, or the incorrect product type is selected, then the velocity is at risk of being too high due to insufficient discharge area, which in turn leads to noise. Unlike throw, there are less issues with having too low of an acoustic performance. However, in some applications, acousticians will aim to achieve a specific sound level to give a subtle background sound, as perceived silence can be deemed non-desirable.
Pressure loss - The pressure loss is the amount of resistance created by the ATD to the airflow. The pressure loss varies with the square of the velocity (fan law 2). The fan moving the air must overcome this resistance in order to get the air into the room. When a fan is selected, it’s done so on its capability of moving a certain volume of air in a specific timeframe, this is known as the volumetric flowrate. The fan will be able to achieve this volumetric flowrate whilst operating against a certain amount of pressure. If the pressure created by the ATD (or the system) exceeds the amount the fan is capable of operating against, it will achieve insufficient volumetric flowrates to that which it has been designed to achieve. Depending on the application of the ATD, this could mean the following:
Air conditioning- Insufficient volumetric flowrate will mean the airflow cannot adequately cool or heat the space to the degree intended. Which in turn leads to incorrect temperature and incorrect humidity, causing thermal discomfort for occupants.
Ventilation- Insufficient volumetric flowrate will mean insufficient amounts of fresh air is entering the space. Approved document F and the CIBSE guidelines set out how many air changes per hour should occur depending on the space. This means a specific volumetric flowrate of fresh air must be achieved based on the volume of the room. If the pressure of the system is too high the fan will not be able to achieve this flowrate. Poor ventilation could lead to mould growth, bad odours and health problems for its occupants if left unchecked.
Velocity - I have not listed velocity as one of the 3 selection criteria above however it is worth a mention. Whilst the velocity of air in itself is often not a parameter found on selection tables or technical submittals, it is linked to each of the above. As such, it’s often useful to keep an eye on. If the velocity of air approaching an ATD increases, so does each of the above parameters and vice versa. It is important to maintain ATD terminal duct and spigot velocities to less tan 3m/s to support an appropriate selection.
In conclusion, selection of ATD’s can sometimes be very simple, but equally, is sometimes a rigorous fine-tuning exercise to balance various criteria. Maybe a long throw is needed, but in order to achieve this the sound and pressure loss becomes excessive and the designers must become creative with which product is used, what size it is and where it is to be located. Failure to select correctly can lead to draughts, noise and possibly even health problems if left unchecked. At Swegon we have years of technical expertise to help in selecting the correct ATD, as well as selection nomograms and tables for each of our products. We also have a selection tool for our most prominent products in our range to further aid in the selection of our products in a quick and convenient way which is available upon request.
