. efrigeration is the process of cooling below ambient environmental temperature. Air conditioning (AC) is the process of treating air to simultaneously control its temperature, humidity, cleanliness, and distribution. Psychrometrics is the science of air/water vapor mixtures. It is used to study air at its various stages in the AC process and determine how the air moves from one state to another. The AC process changes the psychrometric condition of air and may involve cooling, heating, humidification, or dehumidification. This chapter provides a basic overview of psychrometrics as the underlying thermodynamic process involved in the application of certain technologies addressed in ensuing chapters in this Section. Examples include condensing methods, cooling towers, and desiccant dehumidification.

Figure 34-1 is a psychrometric representation of the basic AC processes. The arrows point to the direction on the chart that is followed to track the changing condition of the air as each of the processes occur. Directions A, C, E, and G are the four basic elemental AC processes: humidifying, heating, dehumidifying, and cooling, respectively. These four lines form four quadrants much like the four directions on a compass. Any point lying within these quadrants represents a combination of processes. For example, just as a point lying between north and east is in a northeast quadrant, any point lying between A and C represents heating and humidifying combined.

Fig. 34-1 Psychrometric Representation of Basic AC Processes.


A = Humidifying only B = Heating and humidifying C = Sensible heating only D = Heating and dehumidifying (or chemical dehumidifying) E = Dehumidifying only F = Cooling and dehumidifying G = Sensible cooling only H = Evaporative cooling only

For the purposes of HVAC engineering, all aspects of these processes must be considered. Of primary interest for the ensuing chapters in this section are the cooling and dehumidification processes.

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