BSEE-MAR22-PG28-29.qxp_Layout 1 22/02/2022 11:24 Page 29
FOCUS: HEATING & VENTILATION
BSEE
One of the first choices you should make when planning a ventilation system is which principle to use. The two most common principles are: mixing and moving air (also called thermal control).
For air mixing, the air diffuser is placed high in the room, near the ceiling. The blown air enters at a relatively high speed. The air is distributed throughout the room and is, at least in theory, mixed homogeneously with the ambient air. By carefully choosing the supply air diffusers, it is possible to deliver completely draft-free ventilation this way. Specifiers could also consider displacement ventilation, where the air diffuser is placed low in the room, at floor level. The cool supply air enters at a relatively low rate and spreads over the room surface. As the cool air meets warmer heat sources, as it rises it displaces the ambient air upwards which is removed via the exhaust in the ceiling.
Comfort
Which of these techniques to choose depends on many factors, including the
room size, the ceiling height, and furnishings. Designers also need to know how the space will be used. One of the most important aspects to keep in mind is that ventilation should provide good comfort in the occupied zone, usually from 0.1 to 2.0 metres above the ground.
Mixing ventilation is well suited for rooms with low or medium ceilings that have high demands on comfort, for example classrooms, small restaurants, and offices. It can also provide a high cooling effect where required. On the other hand, it is less suitable for large premises with high ceiling heights where the blown air may have difficulty reaching the occupied area. It is also less suitable for activities where there is a need to exhaust ambient air instead of mixing it with the blown air, for example in operating theatres. Displacement ventilation, on the other hand, is often a better choice for premises with medium to high ceiling heights, such as industrial premises, airports, and large cafeterias. As the blown air enters at ground level, the occupied area can be ventilated more quickly, and the existing ambient air pushed upward.
Displacement ventilation is best suited to rooms with limited cooling needs and where there are large local heat sources, for example, in restaurant kitchens. It is less suitable for use in heavily furnished rooms, where the air flow is easily blocked, and for rooms in need of heating, because the supply air is colder than the ambient air.
These are just some examples of how we can apply first engineering principles to ensure we achieve the right ventilation for the specific needs of the premises and occupants in question.
Yes, most places will find they need greater airflow to provide the improved IAQ everyone is clamouring for in the wake of the pandemic, which has hugely improved awareness of how ventilation reduces the risk of viral transmission. However, it is not always a case of simply ramping up fan speeds or installing larger capacity systems. In some cases, ventilation rates might be perfectly adequate but the method of introducing the air to the space and
distributing it around the occupied areas is not. This can lead to unventilated ‘dead zones’ and other potentially health threatening problems.
Ventilation has never had a higher profile and our industry has, therefore, never had a better opportunity to emphasise the importance of high-quality planning, design, and implementation – and to explain the subtleties of air movement.
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BUILDING SERVICES & ENVIRONMENTAL ENGINEER MARCH 2022 29
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