Final answer:
Scale is the substance that commonly fouls condenser heat transfer surfaces.
Step-by-step explanation:
The substance that commonly fouls condenser heat transfer surfaces is scale.
Scale is formed when minerals in water precipitate and accumulate on the surface of the condenser. It can reduce the efficiency of heat transfer and eventually lead to clogging and decreased performance of the condenser.
For example, in a cooling tower, water is often recirculated and becomes concentrated with minerals. Over time, these minerals deposit on the condenser surfaces, forming scale.a