Primarily, there are two main types of Air Handling Units (AHUs) used in HVAC systems to regulate airflow: draw-through and blow-through AHUs. Each type differs in how the fan is positioned relative to the cooling coil and other components.
Draw-Through AHUs
In a draw-through AHU, the fan is located downstream of the cooling coil, meaning it pulls air through the filters, cooling coil, and other components before discharging it into the ductwork.
- Working Principle: The fan creates a negative pressure, drawing air across the cooling coil.
- Advantages:
- More even air distribution across the coil, leading to better heat transfer efficiency.
- Less prone to stratification issues.
- Disadvantages:
- Potential for condensate carry-over if the fan speed is too high.
- Fan motor may be exposed to colder air, which could affect its lifespan (though this is usually mitigated by design).
Blow-Through AHUs
In a blow-through AHU, the fan is located upstream of the cooling coil, meaning it pushes air through the filters, cooling coil, and other components.
- Working Principle: The fan creates a positive pressure, forcing air through the cooling coil.
- Advantages:
- Reduced risk of condensate carry-over because the air velocity entering the coil is more uniform.
- Fan motor operates in a warmer environment, potentially improving its efficiency and lifespan in some applications.
- Disadvantages:
- Potential for uneven air distribution across the coil, possibly reducing heat transfer efficiency.
- Increased risk of air stratification if not properly designed.
Comparison Table
| Feature | Draw-Through AHU | Blow-Through AHU |
|---|---|---|
| Fan Location | Downstream of cooling coil | Upstream of cooling coil |
| Airflow | Fan pulls air through the coil | Fan pushes air through the coil |
| Air Distribution | More even | Potentially uneven |
| Condensate Carry-over | Higher risk | Lower risk |
| Air Stratification | Less prone | More prone |
Both draw-through and blow-through AHUs are effective, and the choice between them depends on specific application requirements, including space constraints, desired air distribution, and energy efficiency considerations. In addition to these two main types, AHUs can be further classified based on their size, configuration (e.g., horizontal, vertical), and specific features, such as energy recovery systems or specialized filtration.
