Around the perimeter

We’re used to designing air conditioning schemes with equipment located in ceilings or floors but, particularly in other parts of Europe, other viable options are gaining in popularity. Iskender Gençer of Trox UK looks at some of the latest façade-based solutions and their pros and cons.

The latest generation of mechanical perimeter systems for example take the external fabric of the building as their starting point. These systems (typically positioned near windows, or integrated into exterior walls, often at sill level) can deliver significant energy savings by working with, rather than against, the effects of the external environment.

There are also air conditioning solutions particularly suitable for installation adjacent to the building façade, which help HVAC system designers tackle the challenges presented by the architect’s choice of material for the building’s façade, for example those with extensive use of glazing.

Solar gain

Glass façades continue to be popular with architects throughout the world, but there has traditionally been a trade-off between the aesthetic impact of extensive glazing and the heating/cooling challenges it presents. New developments by glass manufacturers are helping to limit the impact of solar gain on building occupants through new technology for the glazing materials themselves. But where solar gain remains an issue, perhaps in buildings pre-dating the availability of these materials, then the building services engineer is charged with creating a balance between energy consumption and optimum thermal conditions.

One solution is to install passive chilled beams near to glazing at the perimeter (see Figure 1). This can help to achieve attractive BREEAM ratings and provide optimum comfort conditions for room occupants by harnessing the inherent advantages of the technology.

A traditional active chilled beam would have to work harder than usual near to the glazing to tackle solar gain on warm days, risking the creation of draughts. In contrast however, passive chilled beams use natural convection, meaning that they can be operated at higher outputs at the perimeter, without causing discomfort in the occupied zone. This enables comfort conditions to be maintained in these internal occupied areas with downsized components and lower air change rates.

Hot air, from the vicinity of the glazing, rises and hits the coil of the passive chilled beam and moves downwards through the coil to provide cooled air. A cavity barrier prevents warm air ‘overshooting’ the beam. The result is the delivery of cooled air into the space without causing discomfort in the space inside the area affected by the perimeter beam.

This leads us to discussion of the various façade-based systems that actually locate the cooling/ventilation device as an integral part of the external wall itself. Most notable are the latest generation of mechanical and non-powered façade ventilation systems. The range of products varies significantly, reflecting the different levels of ambition of system designers.

Hot spots

At the less ambitious level this might involve retrofit installation of just a few units to improve ‘problem spots’, perhaps in a hotel or office. This would involve equipment such as the Trox FSL-B-60 façade ventilation unit. In effect this is a high performance acoustic trickle vent, suitable for providing supply or extract air for individual rooms, or for complete buildings. It is integrated into the external wall and provides non fan-powered ventilation into or out of the internal space.

A particular benefit is that its casing is internally lined with thermal/acoustic material. This ensures that air quality is achieved with reduced noise levels. And, as the unit is non-powered, harnessing natural ventilation, it can, in appropriate applications, meet the air quality requirement demands of the building more energy efficiently.

In the case of the Trox FSL-B-60 there is significant installation flexibility. Units can be installed under a sill, or as the sill, at high level, or as part of a window head detail, vertically at the side of a window or integrated appropriately into curtain walling.

Decentralisation

At a more ambitious level, we are now seeing both new build projects and major refurbishments (principally in Germany) where the architect has moved away from the traditional centralised air handling approach. In these instances, air conditioning equipment units are installed at the perimeter, in the external walls, to provide partial or complete decentralisation.

This decentralised ventilation approach is often confused with natural ventilation. So, to clarify, I refer here to mechanical, perimeter systems, designed to carry out functions such as ventilation, heating, cooling, dehumidification and/or heat recovery. All of the building’s HVAC capability is carried within the fabric of the external walls, doing away with the need for a plant room or other centralised HVAC capability.

Key advantages of these decentralised ventilation systems for the architect can include reduced constructional cost and complexity. There is no requirement for air distribution ducts, or fire protection dampers that would otherwise be fitted in ductwork. Risers are not required either, as the building services equipment is typically installed in fascias/perimeters. Footprint/ cost for the central plant room is also saved.

By eliminating the need for a false ceiling void, and facilitating a smaller slab to slab height, the overall height of a new building can be reduced whilst maintaining the same floor area. As the construction costs increase linearly with building height, the saving, in the case of an office block of 30 occupied floors, could be approximately 20% compared with centralised systems.

The installation of units at the perimeter also provides maximum flexibility for interior space utilisation.

A new build example is Capricorn House in Düsseldorf. This landmark building, designed with a primary energy requirement 20% below the German energy saving regulations, incorporates Trox decentralised ventilation sill units. Designed into the façade of the building (creating optimum flexibility for use of interior space) the units incorporate latest generation heat recovery technology. They deliver individually controllable airflow and harness the benefits of free cooling to provide comfort conditions in a highly efficient way without the need for centralised plant.

There are also examples of retrofit projects.

When the Franziska-Hager school in Prien, Germany, underwent modernisation, a decentralised approach was adopted to meet challenging energy efficiency requirements. The solution provided by Trox combined SCHOOLAIR-B air handling units installed horizontally in front of window sills, with SCHOOLAIR-D units installed under the ceiling, directly near the lintel area, and on the outer façade.

Working together they exchange up to a maximum of 266l/s of air per hour in the classrooms, with heating provided on a needs-driven basis. The units are equipped with automatic room control, which regulates the ventilation phases depending on the CO2 concentration. The decentralised technology enables rooms to be regulated on an individual basis to minimise operating costs. The incorporation of an integrated heat exchanger means that the air handling units are supplied efficiently, with water as the energy transfer medium.

Another advantage of decentralised ventilation systems in refurbishment projects is that this is often the only feasible option for introducing ventilation and comfort cooling in buildings designed without air conditioning in mind.

The specific challenges of the technology must be factored in however. For example, when considering comfort conditions, it needs to be recognised that, as the supply air di
scharge is typically sill mounted, the supply air temperatures could lead to levels of discomfort for individuals sat nearest to the units where there are very high cooling capacities. In addition, due to the numerous decentralised ventilation openings in the façade of the building, the effects of the wind pressure on the building cannot be ignored. For this reason, control systems for perimeter ventilation units need to compensate by shifting the fan operating point in order to avoid the generation of draughts.

With these technical issues taken into consideration however, these are another façade-based option for system designers.

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