A £6.5m project to replace long-serving chillers has achieved outstanding resilience and dramatic reductions in energy consumption and running costs
A two-year project replacing three ageing centrifugal chillers and two reciprocating chillers for the Bank of England has wrought significant energy and carbon savings, according to manufacturer Carrier.
The £6.5 million project has seen installation of new variable speed Carrier chillers with a capacity of 5.8 MW – 1.6 MW greater than their predecessor units. Despite increased intensity of use, energy savings following the project are estimated at around £3,000 a week.
Alongside the installation of the new chillers, the project involved replacing six rooftop cooling towers: four for chilled water and two for the Bank’s stand-by generators, along with replacement of the electrical sub-station supplying the cooling system.
The motors and drives in the building’s AHUs were also upgraded, while the existing Building Management System was enhanced, to enable better monitoring and analysis.
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 used as the basis for the chiller and other equipment supply tenders.
The design was based on an N+1 redundancy approach to the critical cooling load, which includes the Bank’s data centre, considered to be of national importance.
Five manufacturers competed for the chiller contract, with Carrier awarded the tender to supply four chillers on the basis of value, energy efficiency, engineering quality and ability to deliver to project deadlines.
The team was required to deliver the project while the Grade One Listed building in the City of London continued to function as normal, at full occupancy, without any interruption of cooling to the building.
“It is akin to servicing a jet airliner whilst in flight,” says 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.”
One of the new units is Carrier’s AquaEdge 23XRV water-cooled screw chiller, with variable speed drive and hermetic motors. At the time the order was placed, the 23XRV was only being manufactured in the US, requiring the unit to be shipped to the UK for installation.
The variable speed drive offers the ability to adjust compressor speed to meet the ambient conditions and load, thereby conserving energy.
The 23XRV provides cooling for the building’s base load. Given its exceptional efficiency, this means that a substantial proportion of the Bank’s cooling load is delivered by the most efficient available means, Carrier notes.
Two water-cooled AquaForce 30XW units, based on twin rotor screw compressors, provide additional cooling on top of the base load, providing both headroom for expansion and the specified N+1 resilience. The fourth chiller is a smaller AquaForce unit, which acts as the lead chiller during the winter months. Heat is recovered from the chiller to supply the low-temperature panel heating system throughout the Bank.
A Sauter Building Management System sequences the chillers according to building load and ensures system resilience, via redundancy. Ensuring effective communications between all parts of the system proved to be one of the most technically complex aspects of the project, not least overcoming the different routing protocols between the US and European manufactured plant.
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,” says Mr 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, ranging from decommissioning and removal of existing cooling plant to 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, Carrier notes.
The project also had to accommodate the unannounced arrival at the Bank of deliveries of bullion and cash (for security reasons), necessitating items being placed in temporary store offsite, until onsite access was restored.
While the existing chillers were decommissioned and removed in a staged process, 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 the project is a win-win for both.”
A three-pronged approach to energy reduction was used, comprising: reduction of demand through improved design and building use; improved efficiency due to new and upgraded plant and equipment; and improved management due to better monitoring and control.
The result is an estimated 48 per cent saving on cooling energy, saving £150,000 a year. The energy reduction has reduced the Bank’s carbon emissions by 717 tonnes CO2e per year, representing an 8 per cent reduction in overall emissions.
As a result of the chiller upgrade and other related building services improvements, the DEC energy efficiency rating has been improved from E to D.