Savings are Bank of England’s main interest
UK: The Bank of England is looking after its own savings following the replacement of its ageing air conditioning chillers as part of a £6.5m refurbishment project.
The UK’s central bank, and the model on which most modern central banks have been based, was established in 1694, making it the second oldest central bank in the world.
The two-year project involved replacing three long-serving centrifugal chillers and two reciprocating chillers with a combined capacity of 4.2MW and installing new high efficiency variable speed Carrier chillers. Despite the enhanced capacity of 5.8MW, energy savings following the project are estimated at around £3,000 a week.
Alongside the installation of new chillers, the project involved replacing six roof-top cooling towers – four for chilled water and two for the Bank’s stand-by generators – and replacement of an 11kV/3.3kV electrical sub-station supplying the chillers.
Motors and drives in the building’s main and secondary air handling units were also upgraded. On the controls side, the existing BMS was enhanced to provide better monitoring and analysis capability.
Consultant AECOM was commissioned to produce a detailed performance specification for the chiller replacement and related works, which the main contractor, Interserve Engineering Services, then carried forward as the basis for the chiller and other equipment supply tenders.
Crucially, the design was based on a N+1 redundancy approach in relation to the critical cooling load, which includes the Bank’s data centre facility, considered to be of national importance.
Installation without interruption
A key challenge for the team was to deliver the project while the iconic Grade l listed building in the heart of the City of London continued to function as normal, with full occupancy, without any interruption of cooling to the building or vital services.
“It is akin to servicing a jet airliner whilst in flight,” said Robert Foster, head of the Bank of England’s operations and maintenance department, who was responsible for the project. “Work on the progressive project had to be carefully managed over the 24-month changeover period to ensure continuity of cooling throughout. Due to its scale and importance for the Bank’s operations, it was a major undertaking requiring a high degree of planning collaboration and close working between all the parties.”
Providing cooling for the building’s base load is a Carrier AquaEdge 23XRV water-cooled screw chiller. Equipped with a variable speed drive and hermetic motors, it was manufactured and shipped from the US.
Two water-cooled Carrier AquaForce 30XW units, with twin rotor screw compressors, provide additional cooling on top of the base load. The fourth chiller, a smaller Carrier AquaForce 30XW1052 unit, is the lead chiller during the winter months. Heat is recovered from the chiller to supply the low temperature panel heating system throughout the Bank.
The controls philosophy was an important part of the design, the aim being to even-out chiller run-time to help reliability and minimise power use. At its heart is a Sauter BMS which sequences the chillers according to building load and ensures system resilience, via redundancy. The complete system is fully integrated and visualised across the Bank’s BMS network.
Ensuring effective communications between all parts of the system proved to be one of the most technically complex aspects of the project. An initial problem stemmed from different routing protocols between the US and European manufactured plant. However, the team traced the source of the problem and established full communications.
The new system wherever possible makes use of existing headers and chilled water circuits. However, the project was made more complex due to the logistical challenge presented by the underground plant room and sub-station. The physical structure of the 1930s building meant that access was severely restricted.
“The underground plant room is a bit like a ship’s hold,” explained Robert Foster. “There is a hydraulic lift for access, but the space is restricted and difficult to operate in, particularly given the size of the equipment requiring removal and replacement.”
To manage this, the project required a 16-phase sequencing strategy, covering all steps needed to prepare for decommissioning and removal of existing cooling plant, and installation of the new chillers and cooling towers, controls, pumps and power systems. Hot tapping and line stopping of the existing system plus installation of temporary headers was essential to ensure an uninterrupted chilled water supply throughout the project.
The work involved roof access using cranes, which required road closures. A unique consideration impacting deliveries of plant and materials was the unannounced arrival (for security reasons) at the Bank of deliveries of bullion and cash. On occasion, this necessitated items being placed in temporary store offsite until onsite access was restored.
Existing chillers were decommissioned and removed in a staged process, and new replacement chillers – dismantled into four sections – positioned in place. In disassembling the chillers, Carrier ensured that refrigerant circuits were retained intact to ensure the factory-finished integrity of the system. Once elements were in place within the plant room, they were reassembled and replacement chillers connected up and commissioned.
Mr Foster, who also manages the Bank’s utility budget, says that reducing energy costs and related carbon emissions was a key driver. “We wanted to ensure that not only was resilience enhanced, but power consumption and ongoing running costs were significantly reduced. We have a responsibility both to the environment and society at large to minimise the environmental impact of our buildings and reduce expenditure on energy.”
The result of all the elements of the refurbishment is an estimated 48% saving on cooling energy, which translates into a saving of some £150,000 a year, and £3m over the 20-year design lifetime of the chiller plant. This has reduced the Bank’s carbon emissions by 717tonnes of CO2 equivalent per year, representing an 8% reduction in overall emissions.
Despite the complexity and unique challenges, the two year project was delivered on time and within-budget. The plant room is now state-of-the-art in terms of technology, resilience and energy efficiency.
The Bank has confirmed that the Carrier chillers at the heart of the project have performed well against the high design specification. They are now realising the savings from the efficiency improvements, which will obviously continue to accrue over the lifetime of the plant, despite the increase in intensity of building use.