64. Which of the following acids is preferable and most commonly used for scale removal from the heating surfaces of the jacket-water heated submerged-tube evaporator?

Citric or dilute sulfuric acid is preferred for scale removal due to its effectiveness and reduced risk of corrosion compared to concentrated mineral acids. Scale primarily consists of calcium and magnesium salts, which are effectively dissolved by milder acids that minimize damage to evaporator metal surfaces. Concentrated acids are too corrosive and unsuitable for routine maintenance.