During construction of a new five-star London hotel, a heat recovery system which exceeded the energy efficiency requirements of the building regulations was chosen. Phil Bullock reports
ECE UK says there is a growing demand for heat recovery ventilation systems since changes to energy policy were implemented by government in 2006.

The new Approved Document Part L in April 2006 is the cornerstone of government energy policy, and lays down some demanding yet achievable challenges for the construction sector.

Heat recovery ventilation systems, as the name suggests, recover the heat energy in the exhaust air, and transfer it to the fresh air as it enters the building.

At the heart of the heat recovery ventilation system is the air-handling unit, which incorporates the heat recovery device. This device can transmit the heat in two ways: by heat conduction (recuperative) or by heat storage (regenerative).

Modern buildings today are by design made more airtight, and as a result less well ventilated. Opening a window does provide ventilation but the building's heat and humidity will then be lost in the winter and gained in the summer. This is undesirable for the indoor climate and for energy efficiency, therefore the need for heat recovery becomes obvious.

With this in mind, when designing and building a new five-star hotel in the shadows of St Paul's Cathedral in London, Grange Hotels decided to go beyond the Approved Document Part L. It wanted further to reduce the impact on its energy usage, running costs and CO2 emissions by insisting on a heat recovery system that could provide an efficiency in excess of 90%.

Varying needs

ECE UK proposed its RG reversing regenerator air-handling unit because it uses the free or recycled sources of heating, cooling, dehumidification and humidification to produce acceptable internal conditions. Also it retains control with varying internal needs and external influences, and is simple in design and operation.

Another feature of this regenerative unit that appealed to the hotel was the lack of need for inlet or discharge dampers, atmosphere-side filtration to protect the cell packs or frost heaters to protect the atmosphere-side filters plus the associated pipe work, insulation, wiring, controls and labour etc. This means the size of the primary plant such as boilers, refrigeration equipment and their associated services is reduced.

Because of this, Grange Hotels' total expenditure on the system and its revenue expenditure on fuel will be less than on a conventional system.

Fresh air

The ventilation system design for the hotel was such that one RG unit was required for each of the six zones. Each RG unit provides fresh air to and extracts the stale air from the rooms in each zone. The control of each RG unit is by the building management system.

The core of each RG unit is the two cell packs, which are separated by a damper section. The cell packs are constructed from corrugated high-purity aluminium sheets layered to form airways. And the damper section consists of four dampers linked in pairs to a drive motor.

The unit onboard controls are capable of controlling the dampers via the internal thermostats, one mounted in the supply air path and one mounted in the extract air path. These controls are generally only used if the system design does not call for external controls or if the external controls should fail.

The principle of operation of the RG unit is for the extract air to be passed through a cell pack for 60 seconds. This cell pack absorbs the heat from the extract air. For that same 60 seconds, the fresh air is drawn in via the other cell pack, which emits the heat from the cell pack to the air.

After each 60 seconds, the dampers will change position to reverse the airflows through the cell packs. This will continue to happen every minute until each of the cell packs is fully charged, which will take about ten minutes.

This operation, called cycling for recovery, will continue until the space temperature set point has been reached. When this happens, the dampers will stop cycling and remain in their current position until the space temperature drops below its set point lower limit when the cycling for recovery phase will start again.

The heating operation of the RG units described previously would raise a winter ambient temperature of -4C to a supply air temperature of 22.5C when provided with extract air at a temperature of 24˚C.

The overall machine efficiency of each RG unit when heating is calculated to be 94.64%. Also, during the summer months, the RG unit can provide cooling. During this mode it would have an approximate overall machine efficiency of 60%.

Hence, the design criterion as defined by Grange Hotels was exceeded.

Phil Bullock is UK sales support
manager for ECE UK