A thermal wheel, also known as a rotary heat exchanger, or rotary air-to-air enthalpy wheel, or heat recovery wheel, is a type of energy recovery heat exchanger positioned within the supply and exhaust air streams of an air-handling system or in the exhaust gases of an industrial process, in order to recover the heat energy. Other variants include enthalpy wheels and desiccant wheels. A cooling-specific thermal wheel is sometimes referred to as a Kyoto wheel. A thermal wheel consists of a circular honeycomb matrix of heat-absorbing material, which is slowly rotated within the supply and exhaust air streams of an air-handling system. As the thermal wheel rotates, heat is picked up from the exhaust air stream in one half of the rotation and given up to the fresh air stream in the other half of the rotation. Thus waste heat energy from the exhaust air stream is transferred to the matrix material and then from the matrix material to the fresh air stream, raising the temperature of the supply air stream by an amount proportional to the temperature differential between air streams, or “thermal gradient”, and depending upon the efficiency of the device. Heat exchange is most efficient when the streams flow in opposite directions, since this causes a favorable temperature gradient across the thickness of the wheel. The principle of course works in reverse, and “cooling” energy can be recovered to the supply air stream if so desired and the temperature differential allows.
The heat exchange matrix may be aluminum, plastic, or synthetic fiber. The heat exchanger is rotated by a small electric motor and belt drive system. The motors are often inverter speed-controlled for improved control of the leaving air temperature. If no heat exchange is required, then the motor can be stopped altogether.
Because of the nature of thermal wheels in the way that heat is transferred from the exhaust air stream to the supply air stream without having to pass directly through an exchange medium, the gross efficiencies are usually much higher than that of any other air-side heat recovery system. The shallower depth of the heat exchange matrix, as compared to that, say, for a plate heat exchanger, means that the pressure drop through the device is normally lower in comparison. Generally, a thermal wheel will be selected for face velocities between 1.5 and 3.0 meters per second (4.9 and 9.8 ft/s), and with equal air volume flow rates, gross “sensible” efficiencies of 85% can be expected. Although there is a small extra energy requirement to rotate the wheel, the motor energy consumption is usually very low and has little effect upon the seasonal efficiency of the device. In addition, the ability to recover “latent” heat, depending upon the materials and coatings used, can improve gross efficiencies by 10–15%.