The impact of air distribution

People in Europe spend around 90% of their time inside and that makes high indoor air quality (IAQ) critical to their health and wellbeing. Good IAQ and effective air distribution are intrinsically linked and that is why it is no use having one without the other.

Indoor air quality (IAQ) has a major impact on the health, comfort and wellbeing of building occupants. According to the Building Research Establishment (BRE), poor air quality has been linked to problems of Sick Building Syndrome and shown to reduce productivity in offices and learning in schools. It has also been associated with asthma and other health-related problems such as rhinitis.

The primary cause of poor IAQ in offices is the presence of pollution sources such as organics from building materials, carpet, and other furnishings; cleaning materials; air fresheners; paint; adhesives; photocopying machines, and biological contaminants from dirty ventilation systems. All of these release gases or particles into the air.

Inadequate ventilation can increase indoor pollutant levels by not bringing in enough outdoor air to dilute emissions from indoor sources and by not carrying indoor air pollutants out of the space.

Getting it right

The BRE says: “Getting building ventilation correct is important for air quality – too high a ventilation results in higher heating bills, and ingress of outside pollution. Too low a ventilation rate can result in the build up of chemicals emitted from the building and furniture, stale air and dampness.”

So, good IAQ depends on two main factors – eliminating presence of chemicals in the air and adequate ventilation. Both are influenced by the effectiveness of the air distribution system employed by the building.

Poor air distribution can also have a ruinous effect on the quality of the indoor environment by creating draughts or high temperature gradients and causing the occupants to complain about comfort levels.

Effective air distribution, on the other hand, creates a draught-free environment and includes good mixing of air so an even temperature is produced throughout the space. It should also include low air velocities so that the entire space is neutral to the occupants.

This requires the designer to consider each individual element for the air distribution system from ducting right through to the air terminal devices. It pays to get the selection of each element right and it is also important to avoid ‘spoiling the ship for a ha’porth of tar’. After all, a simple diffuser costing less than £30 can condemn an entire air conditioning system worth millions of pounds if it is not fit for purpose.

On smaller projects, the contractor may be tempted to choose the cheapest products available which are mostly imports, and poorly supported by design advice. They are sold on the basis of cheap commodity products off-the-shelf and are usually simply filling a hole in the ceiling where the ducting terminates. This is hardly air distribution engineering at its best.

I believe it is important to make the distinction between grille and diffuser manufacturers and air distribution engineers. As you can probably guess, Trox falls into the second category since our air distribution performance figures are backed by our own test laboratories and supported by design advice. 

Creating the right environment

Air distribution engineering is a powerful ally in the battle to create good IAQ. It can involve the use of computational fluid dynamics (CFD), a method of analysis that uses algorithms to solve problems that involve fluid or air flows whereby computers perform the calculation required to simulate the flow of air within a room.

However, although CFD is a useful design tool, it still usually only offers approximate solutions. That is why there is no substitute for room testing. Indeed, the ability of manufacturers to demonstrate room air conditions via a mock-up is becoming increasingly important as designers and occupiers demand greater IAQ and higher comfort levels.

Clearly, not every job is big enough to warrant this type of testing, but, even in relatively standard applications, diffuser performance data supported by such test facilities is a vital consideration.

Trox, for example, can conduct mock up testing at the three well-equipped laboratories based in its Thetford factory in Norfolk. The company plans to extend these facilities further because demand for them is so great.

With the increasing use of VAV fan coil systems where both the airflow and water flow are varied, special attention has to be paid to the diffuser selection and air distribution within the room. 

Air volume

The air volume is normally varied from 100% to 60% where a significant saving in motor energy and carbon emissions can be achieved.  Recent measurements and calculations from our own laboratories are suggesting savings of over 75% in motor power consumption.

However at 60% air volume there is a risk of the air dumping which would have a significant detrimental effect on comfort.  Because of the ‘hysteresis effect’ the air volume may have to be increased to over 80% before the diffuser stops dumping and the air reattaches to the ceiling making the need for some form of anti-dumping device imperative.

The Trox solution is The Varyset which is a simple, but clever, piece of technology that is ideal for deployment in systems with variable volume flows. It comprises a simple segmented plenum with an air operated flap that covers two thirds of the spigot.

The Varyset damper is adjusted to the desired volume flow range by means of a counterweight. When the air volume is at 100% the flap is open and the Varyset operates in the same way as a standard plenum. As the air volume reduces, so the flap progressively closes until it reaches a point where the damper is closed. The air can still reach a small section of the diffuser and this constant velocity ensures no dumping and stable discharge behaviour, down to 25% volume flow.

Because of the importance of air distribution there is a move to provide single point responsibility with terminal air conditioning systems. 

With multi service chilled beams (MSCB) the manufacturer is responsible for the proper design of the heating, cooling and air distribution.  In fact MSCBs include many other areas such as lighting, sensors, controls, PA – the list goes on.

With fan coil systems the manufacturer is now taking more and more responsibility for both the fan coil unit and the air distribution through the ceiling diffusers.  It is not unusual for the client to underpin the specified parameters by getting the specialist manufacturer to provide guarantees covering aerodynamics, thermodynamics and acoustics.

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