An air-to-air heat exchanger transfers heat between two separate air streams without mixing them. It typically consists of a heat-conductive core (like a series of thin metal or plastic plates or tubes) where one airstream (e.g., warm indoor air) passes over one side, transferring its heat to the core, while the other airstream (e.g., cold outdoor air) passes over the opposite side, absorbing that heat.
Here’s how it works:
- Warm Air Input: Warm, stale indoor air (from a building) enters the exchanger.
- Heat Transfer: As this air flows through the core, it transfers heat to the core’s walls, which are made of a conductive material like aluminum.
- Cold Air Input: Simultaneously, cold, fresh outdoor air flows through adjacent channels in the core, picking up heat from the core’s walls.
- Exhaust and Supply: The now-cooled indoor air is exhausted outside, while the warmed outdoor air is supplied into the building.
The process can reverse in cooling mode (e.g., in summer), where cool indoor air transfers its "coolness" to warm outdoor air. The airstreams are kept separate to avoid contamination, often using counterflow or crossflow designs to maximize efficiency. Efficiency can reach 50-80%, depending on the design and conditions.
Common types include:
- Plate heat exchangers: Use stacked plates for heat transfer.
- Heat pipe exchangers: Use sealed tubes with a working fluid that evaporates and condenses to transfer heat.
- Rotary wheel exchangers: Use a rotating wheel to transfer heat and sometimes moisture.
It’s used in HVAC systems to save energy by pre-conditioning incoming air, reducing the load on heating or cooling systems.