How does fouling factor affect the heat transfer rate of a cooling coil in an AHU?

Fouling factor decreases the heat transfer rate of a cooling coil in an Air Handling Unit (AHU). Fouling factor is a measure of the thermal resistance caused by the accumulation of deposits, such as scale, rust, algae, or other contaminants, on the heat transfer surfaces of the coil. These deposits act as an insulating layer, increasing the resistance to heat flow between the air and the refrigerant inside the coil. The higher the fouling factor, the greater the thermal resistance, and consequently, the lower the heat transfer rate. This reduced heat transfer rate means that the cooling coil cannot effectively cool the air passing through it, leading to reduced cooling capacity and increased energy consumption. For example, a clean cooling coil might have a high heat transfer coefficient, allowing it to efficiently cool the air to the desired temperature. However, if fouling occurs, the deposits on the coil surface create an additional thermal resistance, reducing the overall heat transfer coefficient. As a result, the air leaving the coil will be warmer than desired, and the system will have to work harder (consume more energy) to maintain the desired space temperature. In design calculations, the fouling factor is considered to account for the expected degradation in performance over time and to ensure the cooling coil is sized appropriately to meet the cooling load requirements even with some level of fouling.