The narrow plan offices house one of Nationwide Trust's computer based call centres handling customer enquiries for their personal loan business. The building was not originally designed with air conditioning in mind, but given its now intensive IT load, which includes a server room feeding a number of locations throughout the region, air conditioning had become essential. "Previously it was a very hot building, even with all the windows open the building was unbearable in the summer," says Bob Smith from Nationwide Trust.
The offices are arranged over three floors, with the reception, a small area of office space, server room and internal plant room located on the ground floor. The upper two floors are predominantly open-plan work space, although there is also an enclosed meeting room and a restroom on the second floor. Design conditions were typically 21°C for the server room and 23°C for the office spaces, though the occupants prefer it nearer the 25°C mark.
The close proximity of the housing meant there were a number of restrictions. "The original proposal to locate air-cooled plant in the prime car-parking spaces behind the building was rejected by planning. Nor would they allow anything to be mounted externally on the walls because of the visual impact," says Smith.
As a result all the plant had to be located internally. However there were two major constraints put on the design of the cooling plant. First, permission had been granted to replace four existing windows with louvred air intakes and discharge grilles, but this only amounted to a combined face area of around 8·75 m2. Second, the noise break-out limits were set at 40 dB(A) for any one hour from Monday to Saturday, and 34 dB(A) on Sunday.
Proposals previously submitted had been to follow a conventional route and install four air-cooled condensers for the variable refrigerant flow (vrf) system and two large air-cooled condensers for the server room units. The noise generated by the required plant combined with the available louvre area meant that there was a maximum face velocity at which air could be passed through the louvres without exceeding the levels stipulated in the planning consent – depending on the airflow path immediately prior to or after the louvre. Advice from acoustic consultants suggested that the velocity would be below 3 m/s, with the most conservative recommendation being 2 m/s. Under this proposal the maximum airflow rate was based upon a face velocity of 2·5 m/s, equating to around 11 m3/s intake and discharge. This limiting volume would clearly compromise the installation of the proposed standard air-cooled solution.
John Noad were appointed as services consultants to develop an alternative strategy. "It was realised that the airflow rate for the four air-cooled condensers for the vrf system alone would exceed 12 m3/s," explains Stuart East of John Noad. In addition to this the condenser fans provided would not have been capable of developing sufficient pressure to overcome any additional attenuation required.
The solution
As an alternative the designers considered using Mitsubishi's WR2, water-cooled vrf system, which runs along the same principles as the Mitsubishi R2 set-up originally proposed, differing only by the use of a water loop as the heat rejection medium. The design incorporates three WR2 condensers, each with a capacity of 29·1 kW, for the vrf office system in combination with two 14·5 kW Uniflair upflow units installed in the server room. The plantroom acts as a plenum with fresh air drawn in through attenuators at either end to two dry coolers tailor-made by ACP to match the odd-sized window-cum-louvres through which they discharge.
"If we'd gone for the air-cooled plant we would have needed four of the 29 kW units, but because the water is a more stable heat rejection medium, we're able to squeeze a bit more out of it, and reduce the amount of plant needed," comments Stephen Newman of AMP Air Conditioning who were involved in the design and commissioning of the system.
With all systems connected to the common water circuit, the excess heat generated all-year-round from the server room can be used to warm the office spaces during the heating season, making the condensing boiler virtually redundant. "If we'd gone for a conventional design the 40 kW of heat energy being rejected from the comms room would just go to waste," explains Newman. "But we are now using as much of that energy as possible to heat the rest of the building when required."
A Trend controller maintains the water loop temperature at 15-35°C (although this has been allowed to creep up to nearer 50°C), should the temperature of the water loop begin to rise the dry coolers kick-in and the fan speed is increased or decreased to match the required heat rejection rate. Conversely should the water temperature start to drop too low a standby 30 kW condensing boiler, linked to the air cooler circuit, is activated. "In reality this boiler has scarcely been triggered," says Newman.
The water loop also significantly reduces the amount of refrigerant required for the systems compared to a similar air-cooled solution and provides advantages in terms of the noise restrictions – the air blast coolers are provided with fans and attenuators specifically selected for quietness of operation.
The dry coolers are also fitted with variable speed drives to allow the maximum airflow rate to be achieved to meet the noise criteria. The 11 m3/s was the theoretical maximum, but the with the inverters the airflow rate can be set higher or lower to match noise levels measured with the system up and running. For the majority of the year the condenser fans will meet the cooling demands operating at substantially reduced speeds and hence reduced noise levels.
Compared with the original air-cooled proposal the system requires more plant space and a greater initial capital outlay, "however it was generally accepted that the original proposal would not have satisfied the noise requirements of the planning approval," says East.
Office spaces
The low floor to ceiling heights on the top floor, around 2·2 m, combined with the restricted ceiling void depths added to the complications of retrofitting the comfort cooling and heating system. Fresh air is ducted to each floor via individual ahus installed at each level. Each of the three condensers on the vrf circuit serves ceiling mounted cassettes on all floors, thereby balancing peak load requirements across the building. Occupant control has been provided in the office spaces via wall-mounted controllers, although this is restricted to a deadband limit.
The future
"The watchword is practical sustainability," says Newman, "you can't stop people air conditioning buildings but this approach does allow you to reduce the overall energy consumption of such a building." The penetration of heat pumps in the UK market is now substantial with around 75% of the 8000 vrv/vrf systems sold a year being heat pumps. "There are many market drivers, what we're trying to do is create a package that makes sense financially to the landlord and the tenant, which is simple, risk free and practical" Newman adds.
AMP has now packaged the idea into what it calls the 'fluid building'. "The landlord pays for the water loop, and the tenants then pay for the floor by floor system," says Newman. "It's very difficult to decide over the lifetime of the building who is going to be in it, this way you can make your building as flexible as possible". This is a philosophy he is now successfully selling.
The next WR2 project being tackled by AMP and John Noad is for Land Securities at their central London office refurbishment in Soho Square. The building has over 6000 m2 of modern office space and is due for completion later this year. This will comprise 23 systems, over eight floors and follows the same design principles as the Nationwide building with the exception that the air entering the dry coolers passes through an adiabatic pre-cooler to boost the heat rejection capacity within the constraints of a limited plant space at roof level.
Newman's next challenge is to increase the coefficient of performance for the system by incorporating ground water as a renewable energy source. His target is to create a system that, running all year, would have a C02 contribution lower than that of a boiler doing the heating. "I haven't got one off the ground or rather into the ground yet, but watch this space."
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Building Sustainable Design
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