Mechanically assisted ventilation
Rotating cowls was the system adopted by Michael Hopkins and Partners with Ove Arup and Partners in the Nottingham University Jubilee Campus (Figure 12.9). This ventilation system is the successor to Hopkins' and Arup's innovations at the Inland Revenue HQ also in Nottingham, and Portcullis House, Westminster. These led to a low pressure mechanical system linked to heat recovery via a thermal wheel which recovers 84 per cent of the exhaust heat.
The mechanical system requires 51 000 kWh per year and this is supplied by 450 m2 monocrystalline photovoltaic cells. The ventilation
system uses 100 per cent fresh air throughout the year. Air is introduced directly into the roof mounted air handling units where it passed through electrostatic filters. From here it is blown down vertical shafts into traditional floor voids and thence to teaching rooms via low pressure floor diffusers. Exhaust air uses the corridor as the extract path from where it rises under low pressure via a staircase to the roof air handling unit (AHU) for heat recovery then expelled through the cowl. The vane on the cowl ensures that the extract vent faces the leeward side according to the direction of the wind, as in the traditional oast houses of Kent (Figure 12.10).
In most commercial and institutional buildings it is unlikely that natural ventilation on its own will be adequate. A degree of mechanical assistance is necessary to achieve an adequate rate of movement around the building. Mechanical assistance should not be confused with air conditioning which is a much more complex operation.
Mechanical ventilation involves air flow and movement provision using fans and air and possibly supply/extract ducts. Such a system may be able to act as the heating system in winter. However, in its basic form, no cooling system is incorporated and therefore the lowest air temperature which can be supplied is usually restricted to ambient conditions. Air conditioning involves the cooling of the air using a refrigeration system. More precise control over air temperature and humidity can be achieved this way but usually only within a sealed
Typical system for a naturally ventilated office
Air handling units (AHUs) Jubilee Campus
Jubilee Campus, University of Nottingham
Air handling units (AHUs) Jubilee Campus building. In many temperate climates, the thermal inertia of a building structure, combined with controlled air flow, should be sufficient to avoid excessive overheating except for a few hours each year. Immediately air conditioning is specified, energy use is likely to increase substantially.
As mentioned the inclusion of mechanical reinforcement of natural ventilation is the first step in the mixed mode direction. There are at least four types of mixed-mode ventilation:
• Contingency - mechanical ventilation is added or subtracted from the system as necessary.
• Zoned - different ventilation systems are provided for different portions of the building depending upon needs.
• Concurrent - natural and mechanical systems operate together.
• Changeover - natural and mechanical systems operate as alternatives (but often turn out to be concurrent because of difficulties in zoning or changeover point control).
If mechanical ventilation is to be used to aid summer comfort levels, the following tactics are recommended:
• draw external air from the cool side of the building;
• consider drawing air through cooler pipes or ducts (for instance located underground) to reduce and stabilise its temperature; ground water cooling is becoming increasingly popular;
• ensure supply air is delivered to the required point of use efficiently to provide the most beneficial cooling effect but without uncomfortable draughts;
• ensure extracted air optimises heat removal by taking the most warm and humid air;
• integrate use and positioning of mechanical systems with natural air flow;
• in highly polluted city centre locations, air filtration down to PM5 (particulate matter down to 5 microns) is essential;
• employ night-time purging of the building to precool using lowest temperature ambient air.
The last of these options offers many potential benefits since the air delivered to the space can achieve a lower temperature than ambient external conditions. This is particularly the case where cooler night-time air is passed over the building's thermal mass (often the floor slab) which retains the ability to cool incoming daytime air. Further 'natural cooling' alternatives to air conditioning are summarised on pages 151-154.
An increasingly popular option is 'displacement ventilation'. In this case air at about one degree below room temperature is mechanically supplied at floor level at very low velocity, usually about 0.2 metres per second. This air is warmed by the occupants, computers or light
fittings, etc. causing it to rise and be extracted at ceiling level. Air quality and comfort levels can be more easily controlled using this system. However, not all rooms may be suitable for this strategy and therefore it should be specified only where appropriate.
Portcullis House is one of the most prestigious buildings to use displacement ventilation (Figures 12.11 and 12.12). A mechanically assisted ventilation system serves a network of linked floor plenums drawing air from ducts in the facade to provide 100 per cent external air to each room. The system incorporates high efficiency heat recovery from solar gain, the occupants, electrical equipment and room radiators. Exhaust air is carried by ducts expressed externally in the steeply pitched roof and expelled through a series of chimneys designed to enhance the stack effect. Heat recovery is by means of a roof mounted rotary hygroscopic heat exchanger or 'thermal wheel' with 85 per cent efficiency which is fed by air return ducts which follow the profile of the roof. This thermal wheel is also able to recover winter moisture from exhaust air, reducing the load on humidifiers (Figure 12.13).
Adjacent to Westminster Bridge, Portcullis House (Figure 12.1) is situated in one of the most heavily polluted locations in London. Ventilation air is drawn in at the highest possible level, well above the high concentration zone of particulate matter from vehicle exhausts. This outside air is fed into the underfloor plenum and the displacement ventilation is assisted by buoyancy action. The brief specified a temperature of 22°C plus or minus 2°so, when necessary, the ventilation air can be cooled by ground water in two bore holes at a steady 14°C. Buoyancy ventilation is assisted by low power fans. The full fresh air system is able to serve all rooms equally, despite the diversity of function. This is essential for a long-life building which may undergo numerous internal changes.
An outstanding example of displacement ventilation being inserted into a refurbished building is afforded by the Reichstag. By a slender majority the German Parliament decided to move to Berlin and to rehabilitate the Reichstag. Norman Foster was invited to submit a design in a limited competition which he won.
The debating chamber uses displacement ventilation drawing air again from high level above low level pollution such as PM10s (it is now considered that PM5 should be the health threshold). The chamber floor comprises a mesh of perforated panels covered by a porous carpet. The whole floor, therefore, is a ventilation grille. Large ducts under the floor enable air to be moved at low velocity, which reduces noise and minimises the power for fans (Figure 12.14).
Finally, the critical design issues concerning mechanical ventilation involve:
• the sizing and routing of ducts to minimise resistance and thus keep fan size to a minimum;
• the positioning of diffusers in relation to plan and section of rooms;
• the size of diffusers to minimise noise;
• the inclusion of devices to stop the spread of fire.
Continue reading here: Cooling strategies
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