The air-to-air rotary heat exchangers consist of a cylindrical rotor containing thousands of channels in a permeable storage mass. In rotary heat exchangers the heat exchange takes place through the accumulation of heat in the rotor.When the wheel cylinder rotates slowly, the exhaust air moves across half of the shell and transfers heat to the rotor matrix which accumulates it. The supply air, which moves across the other half, absorbs the accumulated heat.The wheel’s rotation speed is varied to control the heat recovered. The rotary heat exchanger can also be used for cooling recovery.