A new approach to temperature control

Located opposite the north western perimeter of the Tower of London, Ten Trinity Square is a Grade II listed building built between 1915 and 1922. Since 1977, the building, famous for its decorative roof feature once described as a 'wedding cake', has housed Willis Corroon Group, one of the world's leading professional service providers of risk management solutions and risk transfer expertise through insurance and reinsurance broking and related specialised consultancy services.

Until recently, the building's cooling system centred around four evaporative condensing cooling towers located on the roof. The condensing units had reached the end of their useful life and building services engineering company, Concept Design and Project Management (CDPM) was employed to recommend a new installation. CDPM decided to use adiabatic dry coolers to replace the existing condensing units because they would remove the problems inherent in the 'wet' system, including potential for legionnaires disease; they would remove the need for exhaustive water treatment programmes; they would dramatically reduce energy consumption and overall maintenance loads.

In conjunction with specialist mechanical contractors, MJN Colston and Thermal Transfer Technology Ltd (3T), a design was drawn up for the installation of two full V adiabatic dry coolers to serve the building's heat pump air conditioning system. A further half V adiabatic dry cooler would serve an existing water chiller on the roof, supplying water to the supply air handling units throughout the building.

Inside the building, the temperature maintenance system comprises one condensing water loop with 720 heat pumps controlling the air conditioning requirements of 30,000 m2 of office space over 7 floors. Boilers in the basement add heat if required. Each heat pump has its own temperature controller and these are set from a number of central positions throughout the building.
On the roof, the full V coolers have eight fans arrayed over two coils (the half V cooler has four fans over one coil), cooling the system water as it passes through the coil. Each set of four fans has a controller that switches the fans on and then reacts to temperature changes within the water system, providing a voltage signal to a Yaskawa 616P5 variable speed drive which controls the speed of the fans with great accuracy, maintaining the temperature of the water within very close tolerances.

The adiabatic dry cooler was specified because, with 415 Volt three-phase inverter drives, the cooling capacity of the unit would be controlled using minimal supply current. The increased duty requirement is covered by using an adiabatic system, which, in combination with the installed control system, varies the inverter speeds automatically during the adiabatic stages.
The introduction of inverter control enables the cooling fans to run only at the speed at which they are required to run to bring the water temperature down to the required level, rather than being full on or full off. This brings a significant energy saving, not only because of the running speed of the fan, but because the inverter ramps the fan speed up gradually, avoiding the need for a large power surge to get over the motor's inertia and bring it up to full speed immediately.

The Yaskawa 616P5 is a high performance general-purpose inverter that will bring maximum efficiency to any motor but that offers optimum energy saving for fans or pumps by calculating power requirements at a high speed, according to load size and motor constants. At the same time, real-time auto tuning compensates for temperature fluctuations.
Variable speed control through the Yaskawa 616P5, in conjunction with adiabatic cooling, has proven to be the optimum solution for this difficult application in an historical building, with minimal operating space available and many floors requiring constant temperature control.

Released April 1999