Psychrometrics defined

Rearranging equation 32 and defining the ratio h -a/iky-ci-Cf ) as r, the psychrometric ratio, give... 

Define the dry-bulb temperature, wet-bulb temperature, and humid volume of humid air. Given values of any two of the variables plotted on the psychrometric chart (dry-buib and wet-bulb temperatures, absolute and relative humidity, dew point, humid volume), determine the remaining variable values and the specific enthalpy of the humid air. Use the psychrometric chart to carry out material and energy balance calculations on a heating, cooling, humidification, or dehumidification process involving air and water at 1 atm. 

The following paragraphs define and describe the different properties of humid air at 1 atm that appear on the psychrometric chart. Once you know the values of any two of these properties, you can use the chart to determine the values of the others. We will use the abbreviation DA for dry air. 

The outlet temperature and humidity determined in this manner are represented by a point on the psychrometric chart. If a lower value of Ts is assumed, a higher outlet humidity would be calculated, yielding another point on the chart. The set of ail such points for a specified and m i th defines a curve on the chart, known as the adiabatic saturation curve. If the three stated assumptions are valid, the final state of air undergoing an adiabatic humidification must lie on the adiabatic saturation curve that passes through the inlet state on the psychrometric chart. 

The relationships among the psychrometric states, loads, and sensible heat ratio are shown graphically in Fig. 3. For a given pair of entering and leaving air states, sensible, latent, and total loads are proportional to the differences in temperature, humidity ratio, and enthalpy, respectively, as shown in the figure. The SHR is defined by the slope of the line connecting the two points. 

The interrelationships between air and water vapor are called psychrometric properties [3]. Changes in these properties are shown by a psychrometric chart. A sample of the chart is given in Figure 24.2. The psychrometric terms given in the chart are defined as follows. 

In heat transfer applications, either the liquid stream or the gas stream can be cooled. The most common application, in which the liquid stream is cooled, is a water cooling tower. Although there is sensible heat transfer between the warmer water and the cooler air, in this device the warm water primarily is cooled by evaporation of some of the water into the air stream. Both the heat and mass are transferred in the same direction, from the water to the air stream. Exit air from the water cooling tower may be assumed to be saturated with water vapor. Kern reports tests showing the exit air is 95% to 99% saturated [2]. Exit air enthalpy can be obtained from a psychrometric table. The inlet air condition usually is defined by the dry-bulb and wet-bulb temperatures. For practical design purposes the inlet air is assumed to be saturated with water at the wet-bulb temperature, because the enthalpy of this air is sufficiently close to the theoretically accurate adiabatic saturation temperature. 

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