The Terracotta Army conquers London

Published:  03 February, 2008

Terracotta warrior
This close up o one of the exhibits in the British Museum shows clearly the intricate detail of the Terracotta Warriors, no two of which are the same.

The current Terracotta Army exhibition in the British Museum is the largest exhibition of this archaeological discovery outside of China itself — and necessitated the provision of a tightly controlled environment.

When the Chinese authorities allowed the British Museum to stage the highly successful Terracotta Army exhibition, featuring one of the most phenomenal archaeological discoveries of the twentieth century, they were insistent that a controlled environment should be provided during the seven months of the exhibition to prevent damage to the artefacts, many of which are well over 2000 years old.

The specification arrived — written in the Mandarin language! Not many building-services consultants can read Mandarin, so the Chinese experts at the British Museum stepped in. Perhaps fortunately for them, the technical requirements were in SI units — and specified a temperature of 22°C±2 K and a relative humidity of 45%±8%.

Challenge

The challenge for the British Museum’s retained building-services consulting engineers was to devise a means of maintaining these conditions in the Reading Room where the exhibition is staged, irrespective of the variation in load presented by display lighting and up to 400 visitors at any one time.

The starting point was the existing services for the Reading Room, which had been equipped with a mechanical ventilation system when the covered Great Court was developed in 2000. That also has to be the finishing point when the exhibition finishes in April.

The British Museum is a Grade 1 listed building, and permission was sought from the local authority and English Heritage for the temporary conversion to stage the exhibition. The main conditions of that permission were that nothing should be done to the historic fabric of the building and whatever work was carried out to stage the exhibition should be completely reversible.

TGA Consulting Engineers has worked with the British Museum on a variety of work over the last few years, and was given responsibility for this challenging project. The Stevenage office was responsible for the mechanical services and the Durham office for electrical.

David Reynolds, a partner with TGA, explains that the round Reading Room is served by two air-handling units installed in the ‘Spyder’ a labyrinth of passageways beneath the Reading Room. These units delivered a mixture of fresh air from the Great Court primary plant and recirculated air from the Reading Room via ductwork in floor penetrations below the desks and distributed through linear grilles in the tops of the desk units.

The exhibition is staged on a circular platform 42 m in diameter supported on a structural steel frame above the historic Reading Room desks and 2 m above the floor. The desk area of the Reading Room is enclosed on the underside of this platform.

David Reynolds’ practised eye immediately saw the possibility of using the enclosed space as an air plenum to supply the exhibition area above.

Another factor to be considered was the height of the top of the dome of the Reading Room above the floor of the exhibition — 15 m.

To avoid having to provide a uniform environment in such a huge volume of space, displacement ventilation units around the perimeter of the exhibition area were considered to deliver the required environmental conditions for the 1.8 m-high exhibition zone.

 Micro-climate

That idea turned out not to be practical for a number of reasons, so a solution based on grilles in the floor was devised. The Krantz circular grilles specified have a high induction ratio and create large isothermal plumes to provide a micro-climate in a zone 1.8 m deep. Air emerges from the grilles at 17°C and quickly mixes with air in the exhibition area to achieve a temperature of 21 to 22°C.

Air at high level is returned to plant within the plenum via an annular void around the exhibition area and the wall that encloses the reading room.

Excess air escapes through the occulus at the top of the Reading Room, which is part of original design and allows excess air to be released through non-return timber dampers.

To flesh out the basic design approach, TGA worked closely with Colman Moducel to modify the performance of the two existing air-handling units in the Spyder and to design and supply bespoke temporary air-handling units to be installed in the plenum to provide detailed control of the supply air.

The two AHUs in the Spyder deliver a total of 4 m3/s, made up of 50% fresh air and 50% recirculation. That 50% recirculation amounts to 2 m3/s, which at 10 l/s would have limited the number of visitors to 200 at any one time. By converting these units to full fresh air using surplus capacity designed into the primary plant, 4 m3/s can be delivered to the exhibition area to permit 400 visitors at a time — which turns out to be a good number for this exhibition.

That 4 m3/s of fresh air is supplied to four Colman Moducel AHUs on the floor of the Reading Room in the temporary enclosure. Each unit handles a quarter of the fresh air, and they operate only when the exhibition is open. Incoming fresh air is dehumidified as required by cooling it. A neat energy-saving trick devised by Ian Bull of Colman Moducel was to cut a small slot above the chilled-water coil to provide a bypass for about 10% of the fresh air so that sufficient moisture could be removed without unduly reducing the dry-bulb temperature, thus avoiding the need for reheating. The air then enters the plenum though a non-return damper. The plenum is maintained at a positive pressure of 25 Pa.

Colman Moducel also supplied a further four AHUs to handle the return air. These units cool the return air as required and can introduce humidification. The high return temperature of 28 to 30°C makes cooling very efficient. Each unit handles 1.55 m3/s.

The desks in the Reading Room are protected by being encased in foam and plywood.

Chilled water for the recirculation AHUs is provided by a new Carrier chiller on the roof of the energy centre. It provides 225 kW of cooling and was installed on a base alongside to chillers that were installed five years ago.

The layout of the Krantz grilles in the floor of the exhibition area had to satisfy the aesthetic requirements of the exhibition designer, integrate with the structural steelwork — and deliver the required airflow pattern in the exhibition space.

David Reynolds explains that around 160 diffusers are generally set out in square groups of four in a tartan pattern. The air distribution is asymmetrical off the corners of each square to create extensive isothermal plumes. There is also a ring of diffusers around the circumference of the exhibition area. This arrangement was finally arrived at after about six design iterations.

 Environmental trial

All the design work was based on type-testing data carried out by Krantz in its development laboratories in Germany. That the design met the specification for the exhibition was verified by an environmental trial before the exhibits were delivered, with lighting loads and heat from visitors simulated by changing set points.

The whole system is controlled by the British Museum’s existing Trend building-management system.

The exhibition opened on 13 September, and its opening time has recently been extended to 10 hours a day because of its popularity.

TGA Consulting Engineers began the design work in the middle of 2006, with installation contractor Romec Building Services starting work on site towards the end of March. The exhibition area became available in July.

Ashley Pursey, project manager with Romec, tells us that although the project did not present any particular technical difficulties, it was logistically very challenging.

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<tr><td><p class="caption"><b></b> <i></i> The Terracotta Exhibition is staged in a temporary gallery in the Reading Room of the British Museum, which is situated in the centre of the Great Court. </p></td></tr>



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In particular, deliveries were only permitted between 8 and 10 a.m., with access through a single set of double doors at the front of the building. Delivery of some larger pieces of equipment was permitted to start at 6.30 a.m.

The air-handling units were delivered in modules. The recirculation units were broken down into seven pieces and the fresh-air units into four. They were assembled on supports on the floor of the Reading Room, followed by the steelwork to support the exhibition floor. While this work was going on, the modifications to the AHUs in the Spyder were made and flexible connections prepared to connect them to the new fresh-air units in the plenum.

To deliver such a mission-critical project in such a tight timescale and within these demanding physical constraints, close collaboration between all parties was essential.

 Association

TGA Consulting Engineers has a long-standing association with the British Museum dating back to 1992, from both its Durham and Stevenage offices. That association continues with TGA’s recent re-appointment to a new 5-year framework agreement to continue to provide mechanical and electrical engineering services for projects throughout the British Museum — building on involvement with over 200 projects over the last 15 years. Romec has a 5-year contract for the provision of mechanical and electrical services.

Contractor Romec, TGA and Moducel have also worked together before at the British Museum, notably on a project to provide a fully air-conditioned close-control environment for a suite of galleries in the King Edward Building, which reopened in October 2006 after a refurbishment project.

The team of David Reynolds of TGA, Ashley Pursey of Romec and Ian Bull of Colman Moducel worked together on the Japanese galleries and continued that effective relationship with the services for the Terracotta exhibition.

 About the Terracotta Army exhibition

‘The First Emperor: China’s Terracotta Army’ is described by the British Museum as a once-in-a-lifetime exhibition that explores one of the greatest archaeological discoveries of the twentieth century and provides an insight into China’s first Emperor, Qin Shihuangdi, and his legacy.

He introduced the idea of a unified state and effectively created China in 221 BC as the oldest surviving political entity in the world.

It was in 1974 that the head of the first of the terracotta warriors was discovered by chance by local farmers drilling a well, one of over 8000 larger-than-life clay soldiers (they are generally over 1.8 m tall) created to protect the Emperor’s tomb in his afterlife.

 The vast scale of the Terracotta Army protecting the tomb of the first emperor of China has steadily emerged since the site was first discovered in 1974.

The Terracotta Army has travelled outside of China on only a few occasions, and the current display at the British Museum is the largest ever outside of China.

 



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