What effect does fouling of the heat transfer surfaces have on the temperature rise of water in a heat exchanger designed to heat liquid water?

Fouling of heat transfer surfaces refers to the accumulation of deposits, such as dirt, scale, or biological growth, on the surfaces through which heat is exchanged. This buildup acts as an insulating layer that hinders the efficient transfer of heat between the two fluids in the heat exchanger.

When fouling occurs, the thermal resistance at the heat transfer interface increases, meaning that for a given heat input, less heat is effectively transferred to the liquid water. As a result, the temperature rise of the water will be reduced because the fouling material decreases the effective heat transfer area and increases the temperature difference required to transfer the same amount of heat. Therefore, the fouling directly leads to a decrease in the temperature rise of the water being heated, confirming that this is the correct answer.

Overall, the fouling of the heat transfer surfaces negatively impacts the performance of a heat exchanger by reducing its ability to efficiently transfer heat, thereby leading to a decrease in the temperature rise of the heated liquid.