Effect of Dry bulb Temperature in Air cooled heat exchanger
he Dry Bulb Temperature is the single most critical environmental factor affecting the performance of an Air Cooled Heat Exchanger (ACHE).
In simple terms: The Dry Bulb Temperature is the ambient air temperature measured by a standard thermometer. It’s the “normal” air temperature we experience and report in weather forecasts.
Operational Consequences and “Pinch Point”
The reduced heat transfer capability has several direct consequences:
1. Higher Process Outlet Temperature
This is the most immediate and critical effect. If the ACHE is designed to cool a fluid from 120°C to 60°C on a 20°C day, on a 40°C day, the outlet temperature might only drop to 80°C or higher. This can be a major problem for downstream processes that require a specific fluid temperature.
2. Reduced Condensation (for Condensing Services)
If the ACHE is used as a condenser (e.g., condensing steam or hydrocarbons), a higher air temperature raises the pressure and temperature at which condensation begins. This can lead to:
- Reduced Condensate Yield: Less vapor is condensed.
- Higher Back-Pressure: The operating pressure of the upstream system (e.g., a turbine or reactor) increases, which can reduce efficiency or capacity.
3. The Approach Temperature Widens
The Approach Temperature is the difference between the cooled process outlet temperature and the ambient dry bulb temperature.
Approach = Process Outlet Temp - Dry Bulb Temp
As the dry bulb temperature rises, the process outlet temperature also rises, causing the approach temperature to widen. A wider approach indicates less efficient operation. The ACHE is “pinched” and cannot bring the process fluid as close to the air temperature.
4. Increased Energy Consumption
To compensate for the reduced performance, plant operators often have to increase the fan speed (using more electricity) to force more air across the tubes. This increases the operating cost. In some cases, even maximum fan speed cannot achieve the desired cooling on an extremely hot day.
5. Potential for Process Trip or Shutdown
In extreme cases, the inability to cool the process fluid to a safe temperature can trigger alarms and automatic shutdowns to protect the equipment and ensure process safety. This leads to lost production and revenue.
Design Implications and Mitigation Strategies
Because of this critical dependency, ACHEs are carefully designed for a specific Design Dry Bulb Temperature.
- Design Point: This is typically a high ambient temperature (e.g., the temperature that is exceeded only 1-2% of the year in that location). Designing for the absolute highest temperature would be prohibitively expensive.
- Trade-off: There is always a trade-off between capital cost (a larger, more robust ACHE) and operational risk (reduced performance on hot days).
Strategies to Mitigate High Dry Bulb Temperature Effects:
- Oversizing the Exchanger: Designing with extra surface area provides a “safety margin” for hot days.
- Fan Control:
- Variable Frequency Drives (VFDs): Allow fan speed to be precisely controlled to increase airflow when needed.
- Adjustable Pitch Fans: The blade angle can be changed to move more or less air.
- Misting Systems: Spraying a fine mist of water into the incoming air stream. The evaporation of the water cools the air (utilizing the wet-bulb temperature, which is lower than the dry-bulb), significantly boosting performance. This is a very effective but water-consuming solution.
- Recirculation (To Be Avoided): On hot days, it’s critical to prevent hot air discharged from the ACHE from being sucked back into the fan inlets, as this drastically reduces performance.
| Aspect | Effect of High Dry Bulb Temperature |
| Heat Transfer Rate | Decreases due to a smaller temperature difference (ΔT). |
| Process Outlet Temp | Increases, potentially exceeding design/safety limits. |
| Condensation Efficiency | Decreases, leading to lower yield and higher pressure. |
| Approach Temperature | Widens, indicating lower thermal efficiency. |
| Energy Consumption | Increases as fans must work harder to compensate. |
| Risk of Overheating | Increases, potentially causing process trips or shutdowns. |