Thermal wheels for air-handling units used to be regarded as a specialist luxury. But, in the face of tougher new building regulations, they are coming into their own and becoming an increasingly popular solution. That, explains Andrew Patch of Reznor UK, is because they simply save energy and reduce carbon emissions
Thermal wheel technology has been with us for more than 30 years now, but never has its importance been greater because of stringent building regulations and ever-tighter standards in building-services provision.

Why? Because thermal wheels have the capacity to turn a
standard-efficiency air-handling unit (AHU) into a high-efficiency unit, with the consequent energy savings and the reduced carbon emissions that implies.

Once the preserve of specialist applications - because of their
relatively higher cost than more traditional cross-plate heat exchangers or runaround coils - thermal wheels are now the
leading component of a trend towards heat recovery. They are particularly effective for handling large air volumes where higher efficiencies are required.

Thermal wheels can recover about 85% of heat from ventilation air, transferring it to incoming fresh air, which then needs minimal additional heating to reach the required temperature for the building.

Given the strict legislative requirements for energy efficiency, and carbon-emission reduction for all heating equipment, it would be surprising if an AHU could now be fully compliant without some form of heat-recovery capacity. Thermal wheels do the job more efficiently than most, and can have a significant positive impact on carbon footprint and fuel costs.

Special seals

For standard heating, ventilation and air-conditioning installations, the thermal wheel (or rotor) is constructed from aluminium. Thin aluminium sheeting is wound in alternating flat and corrugated layers to form an ultra-strong and rigid matrix containing thousands of small air passages.

The wheel is sited within the AHU, so the fresh-air and exhaust-air streams pass in a counterflow direction. The wheel rotates slowly, passing from the exhaust-air section to the fresh-air section, the two being separated by special seals. As the matrix rotates, warm exhaust air passes through the narrow channels and a large percentage of heat from the air is absorbed by the highly conductive aluminium. The matrix moves into the fresh-air flow and the absorption process is reversed - that is, cooler incoming air flows over both sides of the aluminium wheel and through the narrow channels where it absorbs heat from the matrix.

This process is known as regenerative heat transfer. The rotation speed for heat absorption and transfer is 10rpm.

Unlike most other heat exchanger technology, the thermal wheel also allows the transfer of moisture from exhaust air to the incoming air.

This can be important in air-conditioned premises where the humidity of the internal air is controlled. The wheel is also equally effective for applications where recovery of cooling energy is required, such as in summer,or where permanent cooling is necessary.

Ecological solution

While the energy efficiency of thermal wheels with AHUs is a given, there are other environmentally advantageous reasons to fit them.

First, the thermal wheel's small footprint allows the size of the AHU to be reduced, cutting capital expenditure and increasing the pay-back period. Typically, the wheel is only 200mm deep in the direction of air flow and is easily incorporated into the AHU without significantly increasing its length. By comparison, a large plate-heat exchanger could require an additional two-metre section to be added to the AHU.

Fitting a thermal wheel also reduces the size of the heating or cooling coil and therefore allows a reduced capacity boiler and/or cooling condenser to be fitted. Where Building Regulations require part, or all of, the energy requirement to be met from renewable-energy sources, the reduced load where thermal wheels are fitted makes these options far more viable.

The applications of thermal wheels in AHUs are extremely versatile. They have been used successfully in what are usually regarded as aggressive environments, such as marine applications or paint-spray plants.

Although a continual overspray of paint particles would be expected to clog the wheel, the robust nature of the aluminium matrix, together with automated cleaning system using air or water - or a combination of both - maintains the wheel at peak efficiency during a long operational life.

Since the wheel rotates very slowly, there is little wear during its lifetime, and the aluminium can be recycled at the end of its working life.

New developments

Although the technology is not new, innovations have been applied to thermal wheels to enhance performance even further.

In particular, wheels are now available with a sorption coating for superior temperature and humidity transfer.

Specialist wheels can significantly reduce the size of a chiller plant, with significant carbon and capital savings.

It is hard to imagine AHUs in the future being installed without some form of integral heat recovery.

Thermal wheels offer high efficiency and versatility - and a deceptively simple solution to the continuing quest for economic and proven methods of reducing our carbon footprint.