Untitled Document

Mechanical Ventilation

This was incorporated for comfort during seasonal weather extremes when, for most normal office uses, he windows need o be closed. The building is well insulated; the glazing system has an average U value of 1.6W/m²K, and the external walls and roof 0.3W/m²K and 0.25W/m²K respectively. This does not include the external glazing o the west facade, so in effect the U value is better still. The main air-handling plant is in a two-storey plantroom at the 22nd floor ( just below the roof ). The central plant has variable air volume control to respond to the ventilation needs of the floor zones. Air is supplied from the floor via swirl diffusers recessed into a raised screed system which itself acts as a plenum. The floor plenum is divided into three zones, which in urn are fed with air from local risers, allowing all floors to be mechanically or naturally ventilated, with up to three tenant zones per floor. Mechanical ventilation is initiated by the BMS, although occupants can select individual zones within a floor in either mechanical or naturally ventilation mode by a simple wall-mounted zone controller.

Air is returned to the central plantroom via risers for heat recovery in winter. Perimeter radiators are provided with individual thermostatic radiator valves, sized for a -14^oC winter condition. Because the client has quite high internal equipment loads, and because tenants had to be offered reasonable equipment loads too, the building has a limited comfort cooling system.

The system is designed to 'peak lop', ie provide maximum internal temperatures of about 27^oC at external temperatures of 32^oC. In keeping with the environmentally friendly design, no refrigeration systems are used. Instead, cooling is based on spray coolers and desiccant thermal wheels, the latter regenerated using the district heating supply in winter provides the heat source for the air handlers and radiators.
The heat required to dry the desiccant thermal wheels in summer is essentially a of electricity generation for the local grid, and a such adds very little CO2 to the atmosphere that would not already be produced for electricity.

| Home | Case Study Index |

Users can choose natural or mechanical ventilation (with limited cooling in hot weather), and whether to have shading devices open or closed. It was felt desirable to give the occupants as much control over their own environment as sensible, plus guidance on the energy benefits of natural ventilation. The design team therefore decided to put the necessary controls and information on the window transom in each office module. These comprise green and red lights which, when illuminated, indicate whether natural or mechanical ventilation is recommended by the BMS, and simple rocker switches o close and open the windows and shades. The occupants can choose either, irrespective of the BMS recommendation. The new tower was designed to offer flexibility in layout, including open-plan, cellular offices either side of a central corridor and a mixture of hybrid cellular and open-plan layouts. Perhaps the most demanding arrangement was the full cellular office area, as it stops air flow through the east office zone into the west offices.
Natural Ventilation
Initially there was a double strategy for reducing draughts in the rooms. The principal solution was to protect all openable windows on the wet facade with a single-glazed weather screen suspended 1m from the internal double-glazed facade, which, acting together as a buffer zone, was the main protection against heat loss. This arrangement became known as the 'thermal flue', inducing cross-ventilation through the building when it is warm and if winds are weak- the
negative pressure created by the upwards motion of the air in the convection facade is additionally used on every storey to draw fresh air through the building from the east side, and therefore to naturally ventilate all workplaces in a controlled manner. The ventilation which occurs as a result of this convection facade renders the operation of a mechanical ventilation system superfluous in the summer and in the transitional periods between the seasons.
Ventilation, like solar protection, is regulated by the user. Because the option of opening and closing windows makes a vital contribution towards subjective well-being within the workplace. In order to ensure that solar protection and the windows are operated in the optimum manner, the building is also equipped with a central building management system, which operates these elements in the event of the users' absence.
	To ensure that the natural system works as well as possible, Arup carried out extensive analysis using in-house software to size the necessary passive ventilation elements. This included determining the optimum thermal flue depth ( 1.0m was chosen ) and its eight to the highest discharge point. The size and control of the flue's top and base dampers were also studied extensively.
The airflow paths and ventilation opening through the east and west facade windows were analyzed to ensure reasonable control of cross-ventilation of the adjacent offices, and his analysis was informed by a series of wind tunnel tests to determine the required pressure coefficients around he building for varying wind directions and strengths. A part of the facade package, a model test was undertaken on the windows to ensure that their pressure and flow characteristics as hey were opened met the performance parameters we specified.