Entropy generation minimization of combined heat and mass transfer devices

Abstract

This paper details a simple procedure by which the entropy generation in simultaneous heat and mass exchange devices can be minimized. Effectiveness for these devices is defined and a new parameter, 'modified heat capacity rate ratio' is introduced. It is found that the entropy generation of a combined heat and mass exchange device is minimized (at constant value of effectiveness) when the modified heat capacity rate ratio is equal to one irrespective of the value of other independent parameters. Several typical examples of the cooling towers have been studied to illustrate this concept. A practical application of the concept is also illustrated using a humidification-dehumidification desalination system.

Energy Effectiveness of Simultaneous Heat and Mass Exchange Devices

“…Figure 9 illustrates the comparison for a situation in which HCR = 1. This condition is called the thermally balanced condition, at which remanent irreversibility is minimum (Narayan et al, 2010a). Since the temperature profiles and the humidity levels are balanced, all the various definitions of effectiveness are very similar.…”

“…This further demonstrates the concept of thermal balancing. In simple heat exchangers, this concept is well known and corresponds to the balancing of the temperature profiles (Bejan, 1996;Narayan et al, 2010a).…”

“…In other words, a heat exchanger is said to be thermally 'balanced' at a heat capacity rate ratio of one. This concept of thermodynamic balancing, even though very well known for heat exchangers, was only recently extended to HME devices (Narayan et al, 2010a). It is important to establish a reliable definition for the heat capacity rate ratio for an HME in order to understand its influence on selecting the appropriate definition of effectiveness.…”

“…What is the significance of the heat capacity rate ratio to HME devices and how can it be defined without any approximations about fluid properties? What is the analogy of a balanced heat exchanger (which minimizes entropy generation (Narayan et al, 2010a)) in the case of a heat and mass exchanger? How does thermal balancing affect the effectiveness definitions?…”

Split-feed counterflow reverse osmosis for brine concentration

“…Another important aspect 5 of this technology is that the feed streams flow in opposite directions. The configuration minimizes the local difference in osmotic pressure across the membrane, which leads to increased efficiency by reducing entropy generation, much in the same way that balancing temperature differences in a counterflow heat exchanger increases the efficiency of heat transfer [27,28]. CFRO modules can be configured in several different cascading configurations to produce product streams at a wide variety of conditions.…”