How does the 'approach temperature' of a cooling tower affect the overall system efficiency?

The approach temperature of a cooling tower significantly affects the overall system efficiency. The approach temperature is defined as the difference between the cold water temperature leaving the cooling tower and the entering wet-bulb temperature of the air. A lower approach temperature indicates that the cooling tower is performing more effectively, delivering colder water to the condenser. Colder condenser water results in a lower condensing temperature and pressure in the refrigeration cycle. Lower condensing pressure reduces the workload on the compressor, which in turn reduces energy consumption and increases the overall Coefficient of Performance (COP) of the refrigeration system. Conversely, a higher approach temperature means the cooling tower is less effective at cooling the water, leading to higher condensing temperatures and pressures, increased compressor workload, and decreased system efficiency. For example, if a cooling tower has an approach temperature of 5°F, it is cooling the water to within 5°F of the entering wet-bulb temperature. If the wet-bulb temperature is 70°F, the cooling tower is delivering 75°F water to the condenser. If the approach temperature increases to 10°F, the cooling tower would deliver 80°F water, leading to higher condensing pressures and reduced efficiency. Factors affecting approach temperature include airflow rate, water flow rate, wet-bulb temperature, and the cleanliness of the cooling tower fill. Therefore, maintaining a low approach temperature is crucial for maximizing the efficiency of the entire HVAC system.