Abstract
Simultaneous heat and mass exchange devices such as cooling towers, humidifiers and dehumidifiers are widely used in the power generation, desalination, air conditioning, and refrigeration industries. For design and rating of these components it is useful to define their performance by an effectiveness. In this paper, several different effectiveness definitions that have been used in literature are critically reviewed and an energy based effectiveness which can be applied to all types of heat and mass exchangers is defined. The validity and the limitations of the various effectiveness definitions are demonstrated by way of several examples including direct and indirect contact, parallel and counterflow heat and mass exchangers. The limiting case of a simple heat exchanger is also discussed. The importance of thermal balancing in minimizing entropy production and its implications for optimization and design of these devices is dealt with in detail. The application of the energy effectiveness to heat-exchanger-like -NTU correlations is also examined using a detailed numerical model.
Highlights
A simultaneous heat and mass exchanger (HME) is a device that is used to transfer energy by both heat and mass transfer between two fluid streams at different temperatures and concentrations
Thermal contact between the fluid streams will occur through direct contact of the streams if mass is transferred between them or through indirect contact via a heat transfer surface if the mass transfer is associated with phase change in just one stream
It should be noted that they neglect the effect of evaporation on mw and assume that the saturation enthalpy curve for moist air is linear with temperature
Summary
A simultaneous heat and mass exchanger (HME) is a device that is used to transfer energy by both heat and mass transfer between two fluid streams at different temperatures and concentrations. In cooling tower literature (Cheremisinoff and Cheremisinoff, 1981; Mandi et al, 2005), an effectiveness is commonly defined based on the temperature change of one of either the air or the water streams, typically, the change in water temperature. Jaber and Webb (1989) proposed a modified definition of effectiveness based on an analogy between counterflow heat exchangers and counterflow cooling towers They defined the maximum possible heat transfer rate as the product of the minimum modified mass flow rate (mmin) and the maximum air side enthalpy potential difference: mmwod. It should be noted that they neglect the effect of evaporation on mw and assume that the saturation enthalpy curve for moist air is linear with temperature This definition is examined in more detail in a succeeding section (Sec. 2.2). This paper seeks to answer the following questions: Can a single definition of effectiveness be given which will apply to all types of HME? Can this definition be used for developing reliable ε-NTU models? For what situations can one apply the existing definitions 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?
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