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A heat recovery wheel in an air handling unit (AHU) is a device that improves energy efficiency by transferring heat and sometimes moisture between incoming fresh air and outgoing exhaust air. Here's a concise explanation:

仕組み

• 構造: The heat recovery wheel, also called a rotary heat exchanger, thermal wheel, or enthalpy wheel, is a rotating cylindrical matrix typically made of aluminum or a polymer, often coated with a desiccant (e.g., silica gel) for moisture transfer. It has a honeycomb structure to maximize surface area.
• Operation: Positioned between the supply and exhaust air streams in an AHU, the wheel rotates slowly (10-20 RPM). As it turns, it captures heat from the warmer air stream (e.g., exhaust air in winter) and transfers it to the cooler air stream (e.g., incoming fresh air). In summer, it can pre-cool incoming air.
• 種類:

  • Sensible Heat Wheel: Transfers only heat, affecting air temperature without changing moisture content.
  • Enthalpy Wheel: Transfers both heat (sensible) and moisture (latent), using a desiccant to adsorb and release water vapor based on humidity differences. This is more effective for total energy recovery.

• 効率: Sensible heat recovery can achieve up to 85% efficiency, while enthalpy wheels may add 10-15% more by recovering latent heat.

利点

• エネルギー節約: Pre-conditions incoming air, reducing heating or cooling loads, especially in climates with large indoor-outdoor temperature differences.
• Improved Air Quality: Supplies fresh air while recovering energy from exhaust air, maintaining indoor comfort.
• アプリケーション: Common in commercial buildings, hospitals, schools, and gyms where high ventilation rates are needed.

Key Considerations

• メンテナンス: Regular cleaning is critical to prevent dirt or clogs from reducing efficiency. Filters should be replaced, and the wheel inspected for buildup.
• Leakage: Slight cross-contamination between air streams is possible (Exhaust Air Transit Ratio <1% in well-maintained systems). Overpressure on the supply side minimizes this risk.
• Frost Prevention: In cold climates, wheel frosting can occur. Systems use variable speed control (via VFD), preheating, or stop/jogging to prevent this.
• Bypass Dampers: Allow the wheel to be bypassed when heat recovery isn’t needed (e.g., during mild weather), saving fan energy and extending wheel life.

例

In a hospital AHU, a heat recovery wheel might pre-heat incoming winter air (e.g., from 0°C to 15°C) using exhaust air (e.g., 24°C), reducing the heating system’s workload. In summer, it could pre-cool incoming air (e.g., from 35°C to 25°C) using cooler exhaust air.

Limitations

• Space: Wheels are large, often the biggest AHU component, requiring careful installation planning.
• Cross-Contamination: Not ideal for applications requiring complete air stream separation (e.g., labs), though modern designs minimize this.
• Cost: Initial cost is high, but energy savings often justify it in high-ventilation settings.