What issue may occur if a coil-type high-pressure evaporator continuously requires an increase in steam pressure to maintain capacity?

The option indicating that heat transfer surfaces are layered with scale is the correct answer because scale buildup on the heat transfer surfaces of a coil-type high-pressure evaporator significantly impairs its efficiency. As scale accumulates, it acts as an insulator, reducing the overall heat transfer capability. Consequently, the evaporator needs to use higher steam pressure to achieve the same capacity of evaporation because the effective surface area for heat transfer is diminished.

When scale formation occurs, it not only increases the thermal resistance but also localizes overheating, which can lead to further operational problems such as tube failure. Therefore, if an evaporator is requiring a constant increase in steam pressure, it's often an indication that scale buildup has occurred and is negatively impacting the system's performance.

Other issues, such as low feedwater flow, excessive brine density, or trapped air, can certainly be problematic in a steam system but are less likely to directly manifest as a continuous need for higher steam pressures to maintain the same operational capacity. Instead, they may lead to different symptoms or operational inefficiencies, rather than a straightforward increase in steam requirements.