UM 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:
Como funciona
- Structure: 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.
- Types:
- 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.
- Efficiency: Sensible heat recovery can achieve up to 85% efficiency, while enthalpy wheels may add 10-15% more by recovering latent heat.
Benefits
- Energy Savings: 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.
- Applications: Common in commercial buildings, hospitals, schools, and gyms where high ventilation rates are needed.
Key Considerations
- Maintenance: 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.
Example
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.