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Seminar highlights airflow design failures

A packed seminar organised by the CIBSE Young Engineers Network (YEN) in London recently heard that air conditioning and ventilation systems were being undermined by poor product selection and a lack of understanding of the detail of the mechanics of air distribution.

Young building services engineers are keen to get to grips with many of the problems that hamper the successful design of air distribution systems, evidenced by the strong turnout.

CIBSE/ASHRAE Group chairman Tim Dwyer explained to the YEN members that proper selection of air terminal devices (ATDs) was key to the success of the whole system, but was often overlooked in the rush to complete the main air conditioning and ventilation design.

In his presentation to the meeting, which was supported by Ruskin Air Management and held at the offices of Max Fordham, Professor Dwyer looked at how the selection and location of air supply diffusers and return air grilles could improve comfort and reduce noise problems.

He explained that this was an area where building services engineers were frequently able to improve as the impact of poor air distribution on the lifetime effectiveness of commercial buildings was financially significant.

“Temperature, air velocity, humidity and noise all play a role and the design of the air distribution system is critical to getting the right balance,” said Professor Dwyer. “Variations in velocity and temperature differentials can lead to occupant dissatisfaction.”

Comfort
He added that, for human comfort in a mechanically ventilated space  air speed should normally be maintained within 0.15m/s and 0.25m/s – outside this range can lead to unwanted local cooling effects and draughts or ‘stuffiness’. The design and position of ATDs is central to getting that right, he explained.

“The perceived success or failure of an installation can depend as much on acoustic performance as on achieving good room air distribution,” said Professor Dwyer.

Poorly installed or manufactured ATDS can lead to unwanted turbulence, unpredictable throw and spread of the air, as well as noise disturbance, the young engineers heard.

“This is a hugely significant part of an engineer’s work because productivity is frequently the real value of a building,” he said. The ratio of cost is 1 to 5 to 200 i.e. the initial capital cost of a building being one; the running cost being five, but the value of the business carried out in that office over 20 years being 200.

“This is why it is so critical that we get design of air distribution systems right to keep office workers healthy, safe and productive,” said Professor Dwyer.

David Fitzpatrick, Ruskin’s group sales and marketing director, said the company was delighted with the level of attendance and interest shown in the subject by young engineers.

“There is clearly a lack of deep understanding of the physics of air distribution, which we are keen to address because it can have such a massive impact on the success or failure of a building,” he said. “It is really heartening that so many young engineers recognise there is a gap in their knowledge and are keen to get to the heart of this very important topic.”