Single Wheel vs. Dual Wheel vs. Heat Pipes in DOAS
As Dedicated Outdoor Air Systems (DOAS) become the standard for high-performance buildings, selecting the right energy recovery configuration is critical. The choice between Single Wheel, Dual Wheel, and Horse Shoe Heat Pipes (HSHP) determines your ability to manage latent loads without high operational costs.
1. Single Wheel Configuration (Total Energy Recovery)
In a standard single-wheel DOAS, a single Enthalpy Wheel (desiccant-coated) is used to exchange both sensible and latent energy between the exhaust air and the incoming outdoor air.
The Downside: You are paying to cool the air down to its dew point, and then paying again (or using waste heat) to heat it back up via active reheat coils.
2. Dual Wheel Configuration (The "Wrap-Around" Effect)
The dual-wheel system introduces a second wheel—a Sensible-Only Wheel—to eliminate the need for active reheat while significantly reducing the cooling coil load.
- Wheel 1 (Enthalpy): Located at the intake to pre-treat outdoor air.
- Cooling Coil: Dehumidifies the air to its dew point.
- Wheel 2 (Sensible): Transfers sensible heat from the incoming warm/dry air (after Wheel 1 but before the cooling coil) to the cold/saturated supply air (after the cooling coil).
3. Horse Shoe Heat Pipe (HSHP): The Passive Hybrid
For projects where maintenance of rotating parts is a concern, the Horse Shoe Heat Pipe offers a robust, passive alternative to the dual-wheel system.
How it Works:
The HSHP consists of a sealed loop containing a refrigerant. It is "wrapped around" the cooling coil: 1. Pre-Cool Section: Sits in the air stream before the cooling coil. It evaporates the internal refrigerant by absorbing heat from the outdoor air, pre-cooling it. 2. Reheat Section: Sits after the cooling coil. The refrigerant condenses here, releasing that absorbed heat into the cold supply air, providing "free" reheat.
Technical Comparison Data
| Metric | Single Wheel | Dual Wheel | Single Wheel + HSHP |
|---|---|---|---|
| Active Reheat | High (HGRH/Elec) | Zero (Passive) | Zero (Passive) |
| Moving Parts | 1 Wheel | 2 Wheels | 1 Wheel (HSHP is passive) |
| Control Precision | Moderate | Excellent (via VFD) | Moderate (Fixed) |
| Maintenance | Standard | Higher | Lower |
Interactive DOAS Configurator
DOAS System Configurator
Validated Psychrometric Performance Analysis
DOAS Internal Layout
Energy Recovery
13.5 Tons
Mechanical Load
15.0 Tons
Total Sys Impact
28.5 Tons
Design Checklists
For Design Engineers:
- [ ] Pressure Gradient: Ensure the supply fan is upstream of the energy wheels to maintain a positive pressure gradient for the purge sectors.
- [ ] HSHP Tilt: Verify if the heat pipe requires a specific tilt for gravity-assisted refrigerant flow (on older models).
- [ ] Static Pressure: Account for the significant pressure drop of HSHP fins (often 0.5" - 0.8" w.g. additional).
Conclusion
Whether you choose the high-efficiency Dual Wheel or the robust HSHP, the goal is the same: eliminate the energy penalty of active reheat. By utilizing the second wheel or a wrap-around pipe for passive heat transfer, you can deliver dry, neutral air at a fraction of the operational cost.
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Thank you for such in brief technical comparison
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