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Single Wheel vs. Dual Wheel Energy Recovery in DOAS

Single Wheel vs. Dual Wheel Energy Recovery in DOAS: A Technical Comparison

As Dedicated Outdoor Air Systems (DOAS) become the standard for high-performance buildings, the configuration of energy recovery components has moved to the forefront of mechanical design. Specifically, the choice between a Single Wheel and a Dual Wheel system determines the unit's ability to manage latent loads and maintain precise discharge air temperatures without excessive reheat.

The DOAS Challenge: Decoupled Loads

The primary purpose of a DOAS is to handle 100% of the outdoor air ventilation load, decoupling the latent load (moisture) from the sensible load (temperature) handled by terminal units (VRF, Chilled Beams, or WSHPs).

In humid climates, the latent load of outdoor air is the most energy-intensive component to treat. Energy recovery wheels (ERWs) are the first line of defense.

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 Sequence of Operations:

  1. Energy Exchange: Outdoor air passes through the enthalpy wheel, pre-cooling and pre-dehumidifying it using the building's exhaust air.
  2. Active Cooling: The air then hits the cooling coil (DX or Chilled Water) to reach the required dew point (typically 50°F - 55°F) for dehumidification.
  3. Reheat: Because 52°F air is too cold to deliver directly to occupants in many spaces, a reheat coil (Hot Gas Reheat or Electric) is used to bring the air up to a "neutral" 70°F.

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.

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.

The Dual Wheel Layout:

  • Wheel 1 (Enthalpy): Located at the intake to pre-treat outdoor air (Total Energy Recovery).
  • Cooling Coil: Dehumidifies the air to its dew point.
  • Wheel 2 (Sensible): Located after the cooling coil. This wheel transfers sensible heat from the incoming outdoor air (after Wheel 1) to the outgoing supply air (after the cooling coil).

The Technical Advantage:

  1. Free Reheat: The sensible wheel "borrows" heat from the outdoor air to warm the supply air to neutral temperatures.
  2. Free Pre-Cooling: By losing heat to the supply air, the outdoor air is further cooled before it even hits the cooling coil.
graph LR OA[Outdoor Air] --> W1(Enthalpy Wheel) W1 --> W2_Pre(Sensible Wheel - PreCool) W2_Pre --> CC(Cooling Coil) CC --> W2_Re(Sensible Wheel - Reheat) W2_Re --> SA[Supply Air] style CC fill:#00f,color:#fff style W2_Re fill:#f00,color:#fff

Comparative Performance Data

Metric Single Wheel Dual Wheel
Cooling Coil Load Baseline 15% - 25% Lower
Reheat Requirement High (Active HGRH) Zero (Passive Sensible)
Latent Capacity Good Excellent
Unit Footprint Compact Larger (Longer Cabinet)
First Cost Lower Higher (Approx. 20-30%)
Payback Period N/A 2 - 4 Years (Climate Dependent)

When to Choose Which?

Choose Single Wheel If:

  • Dry/Temperate Climates: Where the latent load is minimal and the air rarely needs deep dehumidification.
  • First-Cost Driven Projects: Where the budget cannot support the additional cabinet length and component cost.
  • Small Airflows: Under 2,000 CFM, where the complexity of two wheels may not scale economically.

Choose Dual Wheel If:

  • Humid Climates (ASHRAE Climate Zones 1A, 2A, 3A): Where dehumidification is the primary operational cost.
  • Chilled Beam / Radiant Projects: Where maintaining a supply air dew point below 53°F is non-negotiable to prevent condensation.
  • Net-Zero / LEED Gold Goals: Where maximizing EER and reducing active energy consumption is paramount.

Design Checklists

For Design Engineers:

  • [ ] Static Pressure: Account for the pressure drop of two wheels (typically 1.0" - 1.5" additional ESP total).
  • [ ] Purge Sectors: Ensure both wheels have purge sectors to prevent cross-contamination of exhaust air into the supply stream.
  • [ ] Control Logic: Use Variable Frequency Drives (VFDs) on the sensible wheel to modulate the supply air temperature exactly to the setpoint.

For Facility Managers:

  • [ ] Belt Maintenance: Dual wheels mean dual belts and motors. Check tension quarterly.
  • [ ] Cleaning: Enthalpy wheels are desiccant-coated; ensure filters are high-efficiency (MERV 13+) to prevent clogging the media.

Conclusion

While the Single Wheel system is a capable workhorse, the Dual Wheel configuration is the "Formula 1" of energy recovery. By utilizing the second wheel for passive reheat, designers can deliver neutral, dry air without the energy penalty of traditional systems.

Interested in optimizing your DOAS sequence of operations? Contact One Man Buzz for a design review.

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