A part of our great engineering heritage will soon be restored to her
former glory. The SS Great Britain's restoration will be completed next
month, but protecting her iron hull against further deterioration has
required the installation of sophisticated climatic controls. Drives play
an important role
When she was launched in 1843, Brunel's SS Great Britain was the world's
largest screw propelled, wrought iron, steamship. Built in the Great
Western Dockyard in Bristol, she had a distinguished history as the
world's first ocean liner, carrying emigrants to Australia and as a troop
ship during the Crimean War but came to an ignominious end as a storage
hulk in the Falkland Islands. Now she is back home in Bristol and nearing
the completion of an £11.3m restoration supported by the Heritage Lottery
Fund. From July, visitors will have full access to all parts of the ship
- as no passenger of her heyday could - and will be able to see her in
all her Victorian splendour.
A major aspect of the restoration is to stabilise her wrought iron hull
and protect it from the ravages of further corrosion. The most innovative
part of that work consists of constructing a sealed glass plate around
the waterline of the ship to form a giant airtight chamber surrounding
the ship's lower hull. The glass plate will be covered with a 50mm layer
of water, so that the ship appears to be floating at anchor.
Beneath the glass plate, moisture will be removed from the air using a
specially designed air-handling unit operating 24 hours per day, 7 days
per week. In the process section of this unit, air will be passed through
desiccant in a hydroscopic wheel to remove excess moisture and take the
humidity within the dock down to RH 20% - about the same as the Arizona
desert. Within the unit's regeneration section, direct fired gas heaters
dry the desiccant, and the air is passed via the heat recovery system to
the exhaust port. A similar but larger unit will be installed within the
ship's boiler room to control the ship's internal temperature and
humidity and thus protect the hull both externally and internally from
corrosion.
The temperature and humidity control system is being provided by Tour
Andover Controls, South West division. The company specified a 15kW
Danfoss VLT 6000 drive to power the process supply fan for the enclosed
dock, controlling the volume rates of flow between 5.7m3/s and 1.7m3/s,
and ensuring an energy efficient operation when ambient humidity is low.
The larger system within the ship itself will utilise a 55kW drive to
provide up to 13m3/s of controlled air circulation.
Control is exercised via wireless temperature and humidity sensors, three
along each side of the ship and 16 inside her. Every 30 seconds, readings
are transmitted to the receiver in the ship's plant room and relayed to
the ship's Management System via the Ethernet communications system. PID
control averages out the humidity reading and controls the dryer burner
rate. Fan speed is linked to the burner rate and the fans are run at full
speed when the regeneration burner rate exceeds 20% output.
When the ship and museum are closed and output to the regeneration burner
drops below 20%, the fan operates at a pre-set low speed. The circulating
air temperature is controlled by an indirect gas fired heater battery and
a water cooled chiller battery in the air path to maintain temperature
between 16OC and 23OC. Should the cooling requirement exceed 15% at times
when the visitor centre is open, then fan speed is increased to maximum
to achieve a more rapid return to the preferred control zone.
The first dryer unit, the size of a shipping container, was lowered into
the dock on December 8 2004. The larger, second unit has been installed
within the ship's boiler room and the completion date for the programme
of works is scheduled for mid July, 2005. This will be in tim