## 2 Methods  
### Thermodynamic Parameter Setting and Calculation  
ORC is usually used in low-medium temperature waste heat recovery. Table 1 gives the temperatures of waste gases from process equipment in the low-medium temperature range. Figure 1 depicts the typical process of an ORC, in which dry and isentropic working fluids are always used as working fluids. After isentropic expansion process to produce work, the dry working fluid with a positive slope of the saturation vapor curve is at a superheated state and the isentropic working fluid with a nearly infinitely large slope is at a saturated state. This avoids blade damage of the turbine caused by wet vapor. To simplify the calculation, the following assumptions were made. (1) All processes are quasi-stationary processes. (2) The pressure loss of pipes and heat exchangers are ignored. The efficiencies of the expander and pump are 0.8. (3) There is no superheating in ORC [47]. Table 2 gives some parameter settings for thermodynamic calculation. In our analysis, tube-in-tube heat exchanger was adopted due to the following advantages. 1. The structure is simple and the heat transfer area can be increased or decreased freely. Because it is composed of standard components, no additional processing is required for installation. \begin{table}
\begin{tabular}{c c} \hline
**Type of Heat Source** & **Temperature (\({}^{\circ}\)C)** \\ \hline Steel Industry & 190–400 [48] \\ Glass Industry & 140–200 [48] \\ Chemical Industry & 230–340 [48] \\ Kraft Pulping Mill & 164–238 [49] \\ Steam Boiler Exhaust & 230–480 [50] \\ Gas Turbine Exhaust & 370–540 [50] \\ Reciprocating Engine Exhaust & 320–590 [50] \\ Heat treating furnace & 430–650 [50] \\ Drying and baking ovens & 230–590 [50] \\ Exhaust Gases Exiting Recovery Devices in Gasfired Boilers, Ethylene Furnaces, etc. & 70–230 [50] \\ Drying, Baking, and Curing Ovens & 90–230 [50] \\ \hline \end{tabular}
\end{table} 
Table 1: Typical waste heat temperature at low-medium temperature range from various sources. Figure 1: Typical process of an organic Rankine cycle (a dry fluid as working fluid). It has a high heat transfer efficiency. It is a counter-flow heat exchanger. Appropriate cross-section size can be set for improving the fluid speed and increasing the heat transfer coefficient of both sides of the fluid. 3. It has a wide working range. The shape can be changed arbitrarily according to the installation position to facilitate installation. In evaporator, organic working fluid flows in inner tube and flue gas flows in outer tube. Water is used in condenser and its temperature is 15 \({}^{\circ}\)C, which is the same as the ambient temperature. When the mass flow rate and inlet temperature of heat source are given, the mass flow rate of organic working fluid and outlet temperature of heat source can be determined by pinch point temperature difference method. Similarly, the mass flow rate and outlet temperature of cooling water can be determined by giving inlet temperature of cooling water. The parameters can be calculated by the following equations. The mass flow of organic working fluid,  
\[m_{\text{wf}}=\frac{m_{\text{g}}c_{\text{p,g}}\big{(}T_{\text{g,i}}-T_{\text{E} }-\Delta T_{\text{E}}\big{)}}{h_{3}-h_{2}}, \tag{1}\]  
The outlet temperature of heat source,  
\[T_{\text{g,o}}=T_{\text{g,i}}-\frac{m_{\text{wf}}(h_{3}-h_{1})}{m_{\text{g}}c _{\text{p,g}}}, \tag{2}\]  
The mass flow of cooling water,  
\[m_{\text{w}}=\frac{m_{\text{wf}}(h_{5}-h_{6})}{c_{\text{p,w}}\big{(}T_{\text{ C}}-\Delta T_{\text{C}}-T_{\text{w,i}}\big{)}}, \tag{3}\]  
The outlet temperature of cooling water,  
\[T_{\text{w,o}}=T_{\text{w,i}}+\frac{m_{\text{wf}}(h_{4}-h_{5})}{m_{\text{w}}c _{\text{p,w}}}, \tag{4}\]  
In evaporator and condenser, the heat transfer includes single-phase and vapor-liquid phase process. During single-phase heat transfer process, heat convection coefficients of organic working fluid in tube and fluid outside the tube can be calculated according to Dittus-Boelter equation.