The logic of an Air Handling Unit (AHU) revolves around efficiently maintaining a desired supply air temperature and air quality within a conditioned space, primarily using sequencing logic to control its various components.
Core Logic: Maintaining Setpoint with Efficiency
At its heart, AHU logic aims to achieve and maintain a pre-defined supply air temperature setpoint while minimizing energy consumption and maximizing equipment lifespan. This is achieved by intelligently managing the AHU's components, such as:
- Cooling Coil: Used to cool the air.
- Heating Coil: Used to heat the air.
- Fans: Used to circulate air.
- Dampers: Used to control airflow (e.g., outside air, return air, exhaust air).
- Filters: Used to clean the air.
- Humidifiers/Dehumidifiers: Used to control humidity (less commonly included within the AHU itself).
The AHU controller constantly monitors the supply air temperature and compares it to the setpoint. Based on this comparison, it adjusts the operation of these components in a coordinated manner.
Sequencing Logic: The Brains of the Operation
The specific logic used to control the AHU is often referred to as "sequencing logic." This dictates the order and conditions under which different components are activated or modulated. Here's how sequencing logic typically works:
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Temperature Monitoring: The AHU continuously monitors the supply air temperature, return air temperature, and potentially outside air temperature.
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Setpoint Comparison: The measured supply air temperature is compared against the user-defined setpoint.
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Control Action: Based on the temperature difference (error), the controller initiates control actions according to a pre-defined sequence. This involves activating and modulating components in an efficient manner.
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Example 1: Cooling Demand: If the supply air temperature is too high, the controller might first increase the cooling coil valve opening, allowing more chilled water to flow through the coil and cool the air. If this is insufficient, it might increase the supply fan speed to deliver more cooled air.
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Example 2: Heating Demand: If the supply air temperature is too low, the controller might first increase the heating coil valve opening, allowing more hot water or steam to flow through the coil. If this is insufficient, it might decrease the amount of outside air being drawn in (as outside air is often colder than return air).
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Optimization: Modern AHU controllers often incorporate optimization algorithms to further enhance efficiency. This might involve modulating the supply fan speed to match the actual airflow demand, or adjusting the outside air damper position to leverage free cooling when the outside air temperature is suitable.
Example: Simple AHU Control Sequence
| Step | Condition | Action |
|---|---|---|
| 1 | Supply Air Temp > Setpoint + 2°F | Increase Cooling Coil Valve Opening |
| 2 | Supply Air Temp < Setpoint - 2°F | Increase Heating Coil Valve Opening |
| 3 | Outside Air Temp < Desired Temp & Supply Air Temp > Setpoint | Increase Outside Air Damper (Free Cooling) |
Note: This is a simplified example. Real-world AHU control sequences are often much more complex.
Key Considerations in AHU Logic Design
- Energy Efficiency: Prioritize strategies that minimize energy consumption.
- Occupant Comfort: Maintain consistent and comfortable temperature and humidity levels.
- Air Quality: Ensure adequate ventilation and filtration.
- Equipment Protection: Avoid operating components outside of their design limits.
- System Stability: Prevent oscillations and ensure smooth transitions between operating modes.
In summary, the logic of an AHU centers on maintaining the supply air temperature at a setpoint through a series of sequenced actions controlling various components to achieve optimal efficiency, comfort, and air quality.
