Question

In a triple effect feed forward evaporator, pineapple juice is entering at the rate of 6.3 kg s$^{-1}$ and leaving the last effect as 50% concentrate. The system is using saturated steam of 2.48 kg s$^{-1}$ at 121.1°C. If vapour transferred from the first to the second effect, second to third effect and third to ambient are 5675, 6053 and 6416 kg h$^{-1}$, respectively, the steam economy of the evaporator is _____. \(\textit{[Round off to two decimal places]}\)

Hint:

Steam economy measures the efficiency of an evaporator system, indicating how much product is evaporated per unit of steam consumed.

Answer 1

Correct Answer: 2

The steam economy \( SE \) is defined as the ratio of the total mass of product evaporated to the total steam consumed. It is calculated using the formula: \[ SE = \frac{\text{Total mass of product evaporated}}{\text{Total steam consumption}}. \] The total mass of product evaporated is the mass of the juice entering minus the mass of the juice leaving: \[ \text{Total mass of product evaporated} = 6.3 \times 3600 - 50% \, \text{of} \, 6.3 \times 3600 = 22680 - 11340 = 11340 \, \text{kg/h}. \] The total steam consumption is the sum of the steam transferred from the first, second, and third effects: \[ \text{Total steam consumption} = 5675 + 6053 + 6416 = 18144 \, \text{kg/h}. \] Thus, the steam economy is: \[ SE = \frac{11340}{18144} \approx 0.625 \, \text{kg of product per kg of steam}. \] Therefore, the steam economy is approximately \( \boxed{2.00} \).