How does increasing the number of effects in a Multi-Effect Distillation (MED) system primarily improve its performance?

Increasing the number of effects in a Multi-Effect Distillation (MED) system primarily improves its performance by increasing its thermal efficiency. In MED, seawater is evaporated in a series of vessels called effects, each operating at progressively lower pressures and temperatures. The steam generated in one effect is then used as the heating medium for the next effect. By reusing the heat in this way, MED systems significantly reduce the amount of energy required to produce a given amount of desalinated water. Each effect essentially acts as a heat re-use stage. As the number of effects increases, more of the latent heat of vaporization is recovered and utilized to evaporate additional water. The latent heat of vaporization is the amount of heat required to convert a liquid into a gas at a constant temperature. This heat is substantial and is often the largest energy consumer in single-effect distillation. For example, in a single-effect system, all the heat used to evaporate the water is lost when the steam is condensed. However, in a multi-effect system with, say, ten effects, the latent heat is reused nine times before being rejected, resulting in a much lower energy consumption per unit of water produced. The performance ratio (PR), which is the ratio of the mass of distillate produced to the mass of steam supplied, increases proportionally with the number of effects, making the process more energy-efficient. Therefore, a larger number of effects translates directly into improved energy efficiency and reduced operating costs, although there are practical limits due to capital costs and diminishing returns in efficiency gains beyond a certain number of effects.