Most industrial refrigeration evaporators cool air in refrigerated spaces. Even in some evaporators that cool a liquid, the liquid (water or brine, for example) is often sent to a coil that cools air. In most cases, removal of moisture from air is a companion process to reducing the air temperature, so a knowledge of the properties of the air/water-vapor mixture is fundamental to understanding the behavior of air passing through an evaporator coil. This background equips the engineer to choose coil surface areas, air-flow rates, performance at part load, and to know how to accommodate special humidity control requirements.
A valuable tool in relating the properties of the air/water-vapor mixture is the psychrometric chart. The four psychrometric charts that will be presented first are:
Figure 6.18. Psychrometric chart, normal temperatures, SI units
Figure 6.19. Psychrometric chart, low temperatures, SI units
Figure 6.20. Psychrometric chart, normal temperatures, I-P units
Figure 6.21. Psychrometric chart, low temperatures, I-P units
To help identify the properties displayed on psychrometric charts, a skeleton chart is shown in Fig. 6.22. The horizontal scale is the dry-bulb temperature in °C (°F), which is different from the wet-bulb temperature to be described later. The vertical scale is the moisture content, W, in units of kg of water per kg of dry air (lb of water per lb of dry air).
Immediately, an inconsistency arises. How could any water be contained in dry air? A preferable designation is kg (lb) of water associated with a kg (lb) of dry air. The psychrometric chart is based on a unit mass of dry air which normally remains constant in processes, even when the amount of water increases or decreases. The curved line that forms the upper-left border of the chart is the saturation line, which relates the moisture content of saturated air to temperature. The region below and to the right of the saturation line is unsaturated air capable of holding additional water vapor.
Another indication of the moisture content is the relative humidity, expressed as a percentage. One curve of constant relative humidity is shown in Fig. 6.22. The various relative humidity curves simply proportion off the vertical distance from the base of the diagram, where the moisture content is zero, to the saturation curve. Thus, the 40% relative humidity line lies 40% of the vertical distance upward from the base to the saturation line.
The next property to be highlighted in Fig. 6.22 is the wet-bulb temperature, shown by lines that run downward to the right from the saturation line. The wet-bulb temperature is that measured by a thermometer whose bulb is wrapped with a wet wick. Moisture evaporates from the wet bulb if the air in contact with the bulb is unsaturated. The difference between dryand wet-bulb temperatures is a measure of the extent that the air is unsaturated. At temperatures below 0°C (32°F), the water on the bulb freezes, but.even then the sublimation of the ice to water vapor cools the wet bulb relative to the dry bulb.
The enthalpy scale is shown in Fig. 6.22 to the left of the saturation line. To determine enthalpy in kJ per kg of dry air (Btu per lb of dry air), locate the point on the psychrometric chart and follow the line of constant wet bulb to the enthalpy scale.
Lines of constant specific volume in m3/kg of dry air (ft3/lb of dry air) are nearly vertical on the chart.
We have implied that the lines of constant wet-bulb temperature and lines of enthalpy are coincident on the psychrometric chart. This statement is not correct, because there is a slight deviation of slope of the two lines. The charts shown in this chapter are accurate enough for most engineering work, but if greater precision is desired, other charts may be obtained that recognize the difference of the slope of the two lines. The ASHRAE Handbook5 presents a psychrometric chart where both the lines of constant wet-bulb temperature and constant enthalpy are shown on the chart. Another approach6 taken on charts prepared by this firm is to show lines of enthalpy deviation, which provide values to correct the enthalpy values when using the wet-bulb lines to locate the condition of the air.