Performance of Cooling tower in Summer Vs Winter
The performance of a cooling tower varies dramatically between summer and winter due to the fundamental principles of how it operates. Understanding this difference is key to its operation and control.
The core principle to remember is:
- A cooling tower cools water by evaporating a small portion of it.
- The driving force for evaporation and cooling is the wet-bulb temperature (WBT) of the incoming air.
- The theoretical limit of how cold the water can get is the ambient wet-bulb temperature.
Performance in Summer
Summer conditions are characterized by high dry-bulb and high wet-bulb temperatures.
- Cooling Capacity (Efficiency):
- Reduced. The tower’s ability to reject heat is diminished because the temperature difference (ΔT) between the hot water and the cool ambient air is smaller.
- The approach (outlet water temperature – wet-bulb temperature) will widen, meaning the outlet water will be warmer for a given load.
- Result: The tower must work much harder to achieve the same cooling effect. It may struggle to reach design water temperatures, potentially reducing the efficiency of the process or chiller it serves.
- Operation:
- Fans run at or near 100% speed to maximize airflow and evaporation.
- Water flow rates are typically at their maximum.
- Water consumption is at its highest due to maximum evaporation rates.
- Key Challenge: Maintaining Load. The primary goal is to reject the required heat load despite the unfavorable conditions. This often means higher energy costs from fans and pumps.
Performance in Winter
Winter conditions are characterized by low dry-bulb and very low wet-bulb temperatures.
- Cooling Capacity (Efficiency):
- Greatly Increased. The temperature difference (ΔT) between the hot water and the cold air is very large.
- The approach becomes very narrow. The tower can easily cool water to temperatures much lower than design, often exceeding its design capacity.
- Result: The tower is extremely effective and can handle the load with minimal effort.
- Operation:
- Fan cycling or variable speed drives (VSDs) are used to reduce airflow and prevent over-cooling.
- Water flow may be reduced or bypassed.
- Water consumption is lower because less evaporation is needed to achieve the cooling effect.
- Key Challenge: Freeze Prevention. This is the most critical operational issue in winter. Ice formation can damage the tower’s structure, fill, and fans. Strategies to prevent freezing include:
- Fan Cycling: Turning fans off to let the warm water raise the air temperature inside the tower.
- Airflow Modulation: Using VSDs or louvers to precisely control airflow.
- Water Flow Recirculation: Using internal weirs or valves to ensure a continuous, strong flow of warm water to all parts of the tower, especially the exposed areas like intake louvers.
- Heaters: Using immersion heaters in the cold water basin.