Design of Evaporators and Condensers

3-3 F inned tube condenser operated with cooling air 

Condensation Organic liquids without inert gases 1,500 

The design of natural convection evaporators is difficult because a complex interrelationship between the liquid circulation rate due to density differences and heat transfer coefficients exists. The circulation flow rate depends on the amount of evaporated liquid and is not controlled by an external device as in forced circula- 

After the beginning of boiling vapor bubbles lead to a bubble flow which results in plug and slug flow with increasing vapor fraction and finally annular and spray flow can be observed. 

It is understandable that the heat transfer coefficients are different in these zones because of different flow patterns. 

---

Evaporating / -propanol from an n-heptane liquid is feasible, but the boiling points are very close. When we explore the design of evaporators and condensers in Chapter 4 we will learn the implications of the boiling points are very close. In theory any difference in boiling points can be exploited for a gas/liquid separation. But in practice the process may be too expensive. It is prudent to consider other methods. 

Thermal and mechanical design of heat exchangers (condensers and evaporators) is presented earlier in this section. 

A knowledge of the thicknesses of flowing liquid films is of importance in a wide range of practical problems involving film flow. Such problems include the calculation of heat transfer in evaporators and condensers, mass transfer in film-type equipment, the design of overflows and downcomers, etc. 

There are many reactors in industry that use evaporative cooling. The liquid in the reactor boils to remove reaction heat. The vapor leaving the reactor is condensed in an external heat exchanger, and the liquid is returned to the reactor. Clearly the vapor phase is important in these autorefrigerated reactors. In addition to chemical kinetics, the vapor-liquid equilibrium properties influence the design of the reactor-condenser system. 

To design the evaporator, compressor, condenser,and auxiliary equipment one must know tire rate of circulation of refrigerant/w. This is determined from the rate of heat absorption in tire evaporator by the equation ... 

The prior and substantial contributions of Frank L. Rubin (Section Editor, Sixth Edition) and Dr. Kenneth J. Bell (Thermal Design of Heat Exchangers, Condensers, Reboilers), Dr. Thomas M. Elynn (Cryogenic Processes), and E. C. Standiford (Thermal Design of Evaporators, Evaporators), who were authors for the Seventh Edition, are gratefully acknowledged. 

As the vapour pressure curve shows, the boiling point of a compound can be lowered by reducing the total pressure this also has consequences on equipment design. According to the law of Boyle and Mariotte, p V = constant, the volume of gases increases under reduced pressure. These higher values have to be taken into account when calculating evaporation and condensation areas. 

Another approach intensifies distillation by combining two columns into one, a so-called dividing wall column (Figure 3.8). This arrangement obviates a second separate column and its evaporator and condenser. The unit features a vertical wall in the middle part of the column, creating a feed and draw-off section in this part of the column. The dividing wall, which is designed to be gas- and liquid-sealed, permits... 

Typically, the temperature drop between the evaporator and condenser of a heat pipe is of particular interest to the designer of heat pipe thermal control systems and is most readily found by utilizing an analogous electrothermal network. Figure 12.4 illustrates the electrothermal analogue for the heat pipe illustrated in Fig. 12.1. As shown, the overall thermal resistance is composed of nine different resistances arranged in a series-parallel combination. These nine resistances can be summarized as follows ... 

F. Chopard, C Marvillet, and J. Pantaloni, Assessment of Heat Transfer Performance of Rectangular Channel Geometries Implications on Refrigerant Evaporator and Condenser Design, Proc. 3rd UK Nat. Heat Transfer Conference, I. Chem. E. Symp. Ser. 129, 2, pp. 725-733,1992. 

Evaporators and condensers can also be reliably designed with the available methods of calculation and generally do not require investigations in representative test apparatus. 

The second property is the water vapor permeability. When the garment is impermeable to water vapor, our sweat cannot evaporate, and condensation of water occurs in the air layer between the skin and garment. This may lead to conductive heat loss when exposed to cold. It is important for sports clothing designers to know the locations where people sweat most. Smith and Havenith (2012) provide sweat maps of the body. 

Design a vapor compression cycle using ammonia as the refrigerant that will be used to maintain the temperature of a stream at - 20 °C. The evaporator is air cooled and the ambient temperature may be assumed to be 25 °C. Assume a compressor efficiency of 80% and allow for 2 °C of temperature difference between streams in the evaporator and condenser for practical heat transfer. 

The second method involves a predictive method that determines VLE and liquid-liquid equihbrium (LLE), which are important for designing evaporators and condensers. Another method is a powerful tool for parameter determination and optimization by Levenberg and Marquardt [28]. This method can be used to develop complex models for process engineering purposes which require an adaptation of parameters to experimental results. The final analysis method deals with the pinch-point method developed by Linnhoff etal. [29]. This technique aims to identify the maximum possible heat recovery and the minimum energy requirement of a thermal or chemical process [30-32]. The apphcation of this method to fuel cell systems is explained in the final part of this section. 

The determination of VLE or LLE is important for designing evaporators and condensers. Bubble formation must be avoided to guarantee proper nozzle functionality. Equations (22.37) and (22.38) involve strong simplifications, such as the assumption of the ideal gas and ideal liquid phase. The complete description of the... 

---