Fast-track air travel
When the first round of PFI contracts were awarded in 1997 the gun was fired in the race to be the first to complete. Amec romped home in 27 months at the Cumberland Infirmary, Carlisle, largely due to close team working and the use of simultaneous construction and design. The construction team, by necessity, has to make a major contribution to achieve solutions when concurrent construction and design takes place. The trust, working alongside the team, was also very proactive in achieving acceptable solutions.
The Amec design and build team engaged consulting engineers Donald Smith Seymour Rooley to assist with the design of the mechanical electrical and public health services. Environmental standards for different areas of the hospital are prescribed in Health Technical Memoranda resulting in the need for 40 individual systems within the 28 000 m2 complex.
Temperature, humidity, air velocity, air change rates and standards for filtration all vary according to function. Specifications varied enormously between ward, outpatient, administration and kitchen areas. Also, there were special requirements for operating theatres, audiology, laboratories and the hydrotherapy pool.
For general ventilation, Senior Hargreaves installed industry standard DW/144 galvanised steel ductwork. Value analysis was undertaken to ensure economic manufacture and installation. This enabled the design of bends, tees and special components to be standardised for efficient batch manufacture.
Wire supports were used for the standard ductwork in place of conventional bracketry; this reduced weight and cost and speeded installation. Manufacture and site work were closely coordinated to ensure installation within the specified time slots. The ductwork systems themselves are virtually maintenance free. Hygiene is a concern, so access doors for cleaning are provided at each change of direction or at 10 m intervals. Other unusual features include the use of specially attenuated ducts in the audiology department.
The potential for fire and the need to control smoke are serious concerns in any environment. This is especially so in hospitals, for which requirements are prescribed in NHS standard HTM81. Areas of special concern were the kitchens, linen stores and stairwells. The kitchen area is located on the lower ground floor, requiring the dual purpose dayto-day extract and emergency fire ducts to pass through three floors above, before discharging to atmosphere at roof level. Similarly, the linen stores, close to the wards, were distant from the point of discharge.
Senior Hargreaves developed a fully engineered and tested system for emergency smoke removal – the HFD series of fire resistant ducts. There are seven ductwork constructions in the series to meet differing site needs. These offer between 30 and 240 minutes resistance to fire, which has been performance tested and assessed by independent laboratories. This is achieved by the use of double skin armoured and insulated construction allied to the use of heavy duty supports positioned at more frequent intervals. All the fire ducts at Carlisle meet the most stringent four hour resistance criteria for stability and integrity with varying degrees of insulation.
Each three and four storey building is divided by a fire wall to create two fire compartments on each floor. Heavy duty fire doors, fire dampers in ordinary ventilation ducts and fire stopping, check fire spread, where other services pass through the barrier. Fire will therefore be confined to one fire compartment and evacuation can proceed via the protected stairwells and escape routes.
The kitchens and linen stores are high hazard areas. The ducts from these spaces pass through several fire zones. Specially constructed HFD exhausts contain heat smoke and products of combustion and prevent leakage and spread. The kitchen extract ducts have easy-clean inner surfaces and access doors at increased frequency for regular cleaning.
In the past it was common to use standard ductwork, plus insulating wraps, fire retardant board or other treatments, to improve fire resistance. This solution was, in my opinion, unsatisfactory due to problems of on-site quality control that could lead to inconsistency in application of insulation and dangers of damage by passing trades. Such installations also conferred only theoretical protection against an external duct fire.
The Cumberland Infirmary project has been a test bed, not for design philosophies or materials, but for a method of collaborative working. It has been a learning experience that has benefited all team members and that will be carried forward to future projects.
Copyright Wilmington Publishing Ltd. Apr 2001
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