3.2.2. Avoidable/Unavoidable Exergy Destruction

Some values of exergy loss rates in each part, which are called unavoidable exergy destruction ( . *E UN D*.*k* ), cannot be reduced owing to technical limitations such as accessibility, cost of materials and making.

In order to calculate the unavoidable exergy destruction, some assumptions for the unavoidable conditions of the components are needed, which are usually obtained by researchers' experiences with component operations. In this study, these assumptions are selected from references [12,16,27–29], and these conditions for each component are written in Table 2. To calculate the unavoidable exergy destruction of the kth component, all components are set on their best possible conditions (unavoidable conditions), and the unavoidable loss rates for the kth component are obtained from the equation below [29]:

$$
\dot{E}\_{D,k}^{LIN} = \dot{E}\_{P,k} \left(\frac{\dot{E}\_{D,k}}{\dot{E}\_{P,k}}\right)^{LIN} \tag{23}
$$

Avoidable exergy loss in the kth part is obtained by the subtraction of the unavoidable portion from the exergy loss rate of that part (Equation (23)). The avoidable exergy loss rate is recoverable and can be decreased, so designers should localize this sector to improve system performance.

$$
\dot{E}\_{Dk} = \dot{E}\_{Dk}^{IN} + \dot{E}\_{Dk}^{AV} \tag{24}
$$


**Table 2.** The assumed conditions for performing an advanced exergy analysis under real, unavoidable and ideal conditions [12,18,28,30].
