Which disadvantage is associated with once-through boilers?

Once-through boilers are designed to heat water and convert it into steam in a continuous flow without the need for large water storage capacities typical of traditional boilers. One of the significant disadvantages of once-through boilers is their vulnerability to scale deposits, particularly due to the complete evaporation of water that occurs inside the system.

In a once-through boiler, water is heated rapidly and converted to steam almost instantaneously. As water evaporates, any minerals and contaminants present in the water can concentrate in the remaining liquid. Once the water is fully evaporated, these concentrated minerals can precipitate out, leading to the formation of scale deposits on heat transfer surfaces. This scaling can be detrimental, as it reduces the efficiency of heat transfer and can cause overheating, which may lead to equipment failure or increased maintenance costs.

In contrast, traditional boiler systems can manage scale deposits more effectively because they have a water treatment system and allow for blowdown processes that help manage mineral concentrations by removing some water with concentrated impurities. Therefore, the once-through design's operational method inherently makes it more susceptible to the formation of scale due to the continuous conversion of water to steam and the subsequent concentration of solubles in the system.