The thermal wheel comes round again
11 May 2015
Once regarded as a high cost specialist heating component for very large space applications, the thermal wheel is now seen as a highly economical form of energy saving and is rapidly becoming an integral part of larger air handling units, particularly in new buildings. Andrew Kirk of Reznor UK Ltd explains.Thermal wheel technology has been around for over 30 years, but never has its importance been greater than in the current climate of stringent building regulations and ever-tighter standards in building services provision. Why? Because it has the capacity to turn a standard efficiency air handling unit into a very high efficiency unit, with the consequent energy savings and reduced carbon emissions that implies.
Once the preserve of specialist applications because of their relatively higher cost compared with more traditional cross-plate heat exchangers or run around coils, thermal wheels are the leading component of a trend towards heat recovery as route to energy savings. They are particularly effective for handling large air volumes where higher efficiencies are required.
Thermal wheels can recover around 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 emissions reduction for all heating equipment, it would be surprising if an air handling unit 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.
Thermal wheels explained
For standard heating, ventilation and air conditioning installations, the thermal wheel (or rotor) is constructed from aluminium. Very thin aluminium sheet 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 air handling unit so that 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 very 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 given 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.
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 air handling unit 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.
Versatile and recyclable
The applications of thermal wheels in air handling units are extremely versatile. They have been used successfully in what are usually regarded as aggressive environments, such as marine applications or paint spray plant. Although a continual overspray of paint particles would be expected to clog the wheel, the robust nature of the aluminium matrix, together with regular planned maintenance and cleaning using air, 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.
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 chiller plant, with significant carbon and capital savings.
It is hard to imagine air handling units 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.