4. System improvement and energy analysis of evaporators
It is observed that the present set-up installed in the factory is not optimum with regard to the surface area of the evaporators and the amount of exhaust surface area of the quadruple effect evaporators is higher than the optimum area required by 36 % . It for the same amount of water to be evaporated, a quintuple effect evaporator system is designed such that the total surface area of the evaporators is minimized; the amount of steam required is reduced by 9 ton/h . A schematic of the proposed quintuple effect evaporator is shown in Fig. 4 . It is observed that total surface area of the new design is less then that of the original by 7%. In case of expansion or new plants it would be worthwhile to opt for an optimal design with quintuple effect evaporators. The new design results in annual saving of fuel costs of Rs. 4.8 million. The incremental capital cost likely to have a payback within a year.
In order to assess the efficiency of the evaporative process and to find out ways to improve it, energy analysis is applied to the evaporators. According to Ahern [7], energy is defined as the work that is available in a gas, fluid or mass as a result of its non- equilibrium condition relative to some reference frame. It calculates the true thermodynamic efficiency of the process and is based on the second law of thermodynamics. As energy is an explicit property at steady state condition, its value can be calculated at any point in an energy system from the other properties that are determined from an energy balance on each process in the system. The general expticit equation that is used under conditions where the gravitational and moment terms in the general equation can be neglected is shown in eq.1
The terms h and s denote the enthalpy and the entropy of the substance and the terms with a supscript ‘r’ represent the reference condition. The amount of energy lost in any process is represented by the percentage of energy lost which is defined as shown as shown in eq.2
The reference condition that has been chosen to perform the energy analysis is
The relation given by Taner [8] are used to estimate the entropy and enthalpy of the sugar solution. A graph between the temperature of the evaporator vessel and the cumulative amount of energy lost is drawn for both the cases and shown in fig.5 . It can 33.5 to 17.6MW if the quadruple effect is modified to a quintuple effect.