Creating an atmosphere

Air conditioning might be a prerequisite for offices these days, but what do you do when the local electricity supply can’t cope with the demands? This was the dilemma facing consultant Singler Building Services when it was asked to draw up plans to upgrade the air conditioning system at a large office block in Leicester city centre.

Electrically powered systems require a substantial power supply near to, or in, the building. The cost of installing new power supplies and site limitations in Leicester city centre meant a new sub-station was out of the question. Instead, Singler opted for a gas-powered system. This is the largest such installation in Europe, with 90% of the power requirement provided by natural gas.

The Mitsubishi Heavy Industries air conditioning system was supplied by 3D Air Sales and includes a total of 224 indoor units and more than 5000 m of pipe. The old chiller was removed from the main roof and replaced with 15 GHP 45 kW systems, allowing electrical power to be made available for other uses in the building. Three Mitsubishi GHP 560 (56 kW) condensing units were installed on the roof of the building’s single-storey wing.

The Mitsubishi GHP VRF system behaves in a similar way to a conventional electric-powered VRF two-pipe system – all on cooling or all on heating.

Instead of using electricity, the GHP compressor is driven by an engine powered by natural gas. The engine speed is varied to provide capacity control, dependent on the combined demand of the connected indoor units. This is similar to the variable speed inverter control on conventional electric VRF systems.

The engine coolant is circulated through a heat exchanger to transfer waste heat energy into the refrigerant, increasing the coefficient of performance of the heat pump operation. This means there is a constant source of heat energy from the engine that enables the heating performance to be maintained in ambient temperatures down to -15°C.

Shut-down periods for defrost are eliminated, and the building user benefits from 100% heating performance in all conditions.

A further benefit of the heat energy from the engine is the reduction of initial warm-up time from a cold start. At 60 dBA, the noise levels are similar to the electrically powered VRF equivalent.

The application of GHP to new or refurbished buildings fits well in the selection of cooling and heating equipment within the parameters of Part L – the GHP carbon emissions for heating are claimed to be 30% lower than the emissions for a condensing boiler. The system runs on R407C.

At James House in Leicester, the 224 indoor units maintain a constant temperature. The system has allowed the installers to retain the existing induction units to supply fresh air. Fan coil units and ducting have been concealed within a purpose-built central bulkhead on each floor, covering the majority of the occupied area.

Wall-mounted units are installed in other office areas. There is a pre-set central control system for each floor, concealed in a riser cupboard, and diagnostic information can be obtained from the condensing units using a laptop.