Question

Use of multiple effect evaporator results in:

• A. Increase in steam economy only
• B. Decrease in steam economy only
• C. Increasing evaporating capacity only
• D. Increase in both steam economy and evaporating capacity

Hint:

Think of multiple effect evaporators as recycling energy. The heat that would be wasted in a single effect is reused in subsequent effects to evaporate more liquid, thus saving steam (improving economy) and increasing the total amount evaporated (increasing capacity for a given steam input).

Answer 1

The Correct Option is D

Step 1: Understand the concept of a multiple effect evaporator.
A multiple effect evaporator is a system consisting of several evaporators connected in series, where the vapor from one effect is used as the heating medium for the next effect. This cascading use of energy improves the efficiency of the evaporation process. Step 2: Define steam economy and evaporating capacity.
Steam economy: The mass of water evaporated per unit mass of steam supplied to the first effect. A higher steam economy indicates better energy utilization.
Evaporating capacity: The total mass of water evaporated per unit time by the entire evaporator system. A higher evaporating capacity means more product can be processed.
Step 3: Analyze how using multiple effects impacts steam economy.
In a single effect evaporator, one kg of steam typically evaporates approximately one kg of water (ideally). However, in a multiple effect system, the vapor produced in the first effect (heated by the initial steam) is used to heat the second effect, causing further evaporation. This continues through subsequent effects. As a result, the total amount of water evaporated by the entire system is significantly more than the amount of steam supplied to the first effect, leading to a substantial increase in steam economy. For an \( N \)-effect evaporator, the ideal steam economy can approach \( N \). Step 4: Analyze how using multiple effects impacts evaporating capacity.
For a given heat transfer area in each effect and a specific overall temperature drop across the system, adding more effects generally allows for a greater total evaporation rate. This is because the total temperature difference is distributed across multiple stages, enabling efficient heat transfer in each stage. While adding more effects can sometimes slightly reduce the capacity of each individual effect due to lower temperature differences across each effect compared to a single effect system with the same total temperature difference, the overall evaporating capacity of the multiple effect system is typically higher than that of a single effect evaporator using the same amount of heating steam and having the same heat transfer area as one effect of the multiple effect system. Furthermore, to achieve the same evaporating capacity as a multiple-effect system, a single-effect system would require significantly more steam. However, it's important to note that adding too many effects can eventually lead to diminishing returns in terms of capacity increase and can increase capital costs and operational complexity. The evaporating capacity of a multiple-effect system is primarily enhanced when compared to a single-effect system operating with the same steam consumption. To achieve a higher evaporating capacity with a single-effect evaporator, the steam consumption would need to be proportionally higher. Considering the options provided, the primary advantage of multiple effect evaporators is the significant improvement in steam economy while also allowing for a higher overall evaporating capacity for a given steam input. Step 5: Evaluate the given options.
Option 1 (Increase in steam economy only): While steam economy increases, evaporating capacity also generally increases for a given steam input.
Option 2 (Decrease in steam economy only): This is incorrect; steam economy increases.
Option 3 (Increasing evaporating capacity only): While evaporating capacity increases for a given steam input compared to a single effect, steam economy also significantly improves.
Option 4 (Increase in both steam economy and evaporating capacity): This is the most accurate description of the benefits of using a multiple effect evaporator.
Step 6: Select the correct answer.
Use of multiple effect evaporator results in an increase in both steam economy and evaporating capacity (when compared to a single effect evaporator operating with the same steam consumption).