SAMPLE RH = P 1. where. P 1 = the partial pressure of the water vapor at the dew point temperature of the mixture of dry air and water vapor

Transcription

1 moisture starts to condense out of the air. The temperature at which this happens is called the dew point temperature, or the saturation temperature. What is commonly called saturation pressure or condensing pressure when referring to refrigerants is called vapor pressure when referring to water vapor. Thus, vapor pressure can be defined as the pressure exerted by water vapor in a space at its corresponding dew point temperature. Dalton s Law of partial pressures states that the total pressure exerted by a mixture of gases is the sum of the partial pressures of the gases contained in the mixture. The total pressure of air, or standard atmospheric pressure, is in. Hg, so the partial pressure of dry air only is in. Hg minus the vapor pressure at the corresponding dew point temperature. At a saturation temperature of F, for example, the vapor pressure is in. Hg (see Table 26-2). The pressure of the dry air, therefore, is the remaining in. Hg ( ). Relative humidity Vapor pressure tables are very useful in calculating the relative humidity of air at partially saturated conditions. The term relative humidity (RH) refers to the ratio of the actual partial pressure of water vapor at a given condition to its saturation pressure at the same temperature. Expressed mathematically: where RH = P 1 P s P 1 = the partial pressure of the water vapor at the dew point temperature of the mixture of dry air and water vapor P s = the saturation pressure of the water vapor corresponding to the dry-bulb temperature. Don t confuse relative humidity with specific humidity or absolute humidity, both of which are based on the actual weight of water vapor mixed in the air (per pound of dry air and per cubic foot of air, respectively). In other words, the term specific humidity refers to the weight of water vapor (expressed in pounds or grains) that could be contained in a pound of dry air at a given condition. The term absolute humidity refers to the weight of water vapor (expressed in pounds or grains) contained in 1 ft 3 of a mixture of dry air and water vapor at a given condition. 8

2 LESSON 26 Specific humidity Look at the two columns in Table 26-1 labeled Moisture content (per pound of dry air). They refer to the amount of moisture, by weight, that is required to saturate 1 lb of dry air at the given dew point temperature. This weight of water can be expressed either in pounds of water per pound of dry air or in grains of water per pound of dry air. As stated in the previous paragraph, it is called the specific humidity, or sometimes the humidity ratio. A grain of water is approximately one drop. There are 7,000 grains of water to 1 lb of water, so you simply divide the number of grains by 7,000 to convert to pounds. Specific volume To find the specific volume and density of a mixture is a little more complicated. To do this, you first must go back to one of the fundamental Gas Laws, which states that absolute pressure (in pounds per square foot) times volume (in cubic feet) equals weight (in pounds) times temperature (in absolute degrees Fahrenheit) times the perfect gas constant for that particular gas. This is expressed mathematically as PV = WRT. The R in this equation is the gas constant. The abbreviations P, V, W, and T stand for pressure, volume, weight, and temperature, respectively. Recall that specific volume is expressed in cubic feet per one pound of air. Since air is considered to be a perfect gas, the equation above can be transformed to fit all conditions at in. Hg. The resulting equation is: where V a = 53.3 (4 + t).7( P 1 ) V a = the specific volume of the air t = the dry-bulb temperature of the mixture P 1 = the partial pressure of the water vapor at the dew point temperature of the mixture 53.3 = R, the gas constant for air (4 + t) = the absolute temperature of the air.7 = the factor for changing inches of mercury to pounds per square foot

3 Heat content (enthalpy) The energy contained in air is commonly called its heat content. However, the term enthalpy has largely replaced heat content in engineering terminology. The enthalpy of a quantity of air is the sum of the enthalpies of the dry air and the moisture contained in the mixture. Sensible heat ratio The sensible heat ratio is the ratio of the sensible cooling load to the total load. It is expressed as a percentage of total capacity. For example, if you have an air conditioning cooling load of 120,000 Btu and a sensible cooling load of,000 Btu, then the sensible heat ratio is:,000 Btu 100 = 75% 120,000 Btu USING PSYCHROMETRIC CHARTS There are many types of psychrometric charts, each with its own advantages. All serve basically the same function. The chart to be used should be chosen for temperature range and type of application. The examples used in the following discussion, which explains how a typical psychrometric chart is constructed, covers a dry-bulb temperature range of 20 to 120 F and a wet-bulb temperature range of 20 to 95 F. Look at Figure The vertical lines extending from the bottom to the top of the chart are constant dry-bulb lines. That is, every point on any given line has the same dry-bulb temperature the temperature indicated on the dry-bulb scale along the bottom of the chart. If you were told to plot only a dry-bulb temperature of 95 F on the chart, it could fall at any point on the vertical line extending upward from 95 F on the dry-bulb scale. Now look at Figure Recall that a wet-bulb temperature is the temperature shown by a thermometer that has a bulb covered with a wetted wick, over which 20 Dry-bulb temperature, F FIGURE Constant dry-bulb temperature lines Wet-bulb F temperature, FIGURE Constant wet-bulb temperature lines

4 LESSON 26 air passes at about 1,000 ft/min. In Figure 26-2, constant wet-bulb lines run downward at an angle of about from horizontal. They extend from the wetbulb scale, along the curved left edge of the chart, to the right margin. All points on any given wet-bulb line are at the same wet-bulb temperature. If you were told to plot only a 75 F wet-bulb temperature on the chart, it could fall anywhere on the wet-bulb line extending diagonally downward from 75 F on the wet-bulb scale. Figure 26-3 shows constant dew point lines. Note that the wet-bulb scale along the curved left edge of the chart in Figure 26-2 and the dew point scale in Figure 26-3 are the same. Constant dew point lines, however,run horizontally from left to right. All points on any given constant dew point line correspond to the temperature shown on the dew point scale along the curved left margin of the chart. The vertical scale along the right margin of the chart is the specific humidity scale. It shows the weight of water vapor (in grains) in 1 lb of dry air. Constant specific humidity lines also are horizontal, and coincide with constant dew point lines. Thus, you can see that the amount of water vapor in the air depends on the dew point of the air. It was noted previously that wet-bulb temperatures and dew point temperatures share the same scale along the curved left edge of the chart. Since the only condition under which wet-bulb and dew point temperatures are the same is at saturation, this outer curved line represents a relative humidity of 100%. Constant relative humidity lines, shown in Figure 26-4, decrease in value as you move downward, away from the saturation line. The values for these relative humidity lines are given in percentages, and are shown near the right margin of the chart. FIGURE Constant relative humidity lines Now look at Figure 26-5 at the top of the next page, which shows constant specific volume lines. These lines are at an angle of about from horizontal, and increase in value as you move from left to right. Remember that the density of air is the reciprocal of Dew point temperature, F 100% % % % % % % % 20% 10% Specific humidity, grains of water per pound of dry air FIGURE Constant dew point temperature and specific humidity lines

5 specific volume. For example, at a saturation temperature of 65 F, the specific volume of 1 lb of air is 13. ft 3 /lb. Its density, therefore, can be determined as follows (see also Table 26-1 to confirm your calculations): 1 = lb / 13. ft 3 ft3 /lb Figure 26-6 shows constant enthalpy lines. Note that these lines are merely extensions of the wet-bulb lines shown in Figure 26-2, since the total heat content of the air depends on the wet-bulb temperature. The enthalpy scale at the far left of the chart shows the total heat content of the air, expressed in Btu/lb of dry air. Values increase from approximately 7.2 Btu/lb at a wet-bulb temperature of 20 F to approximately 58.6 Btu/lb at a wet-bulb temperature of 92 F. Figure 26-7 (on the foldout page) shows how a FIGURE Constant specific volume lines complete psychrometric chart is put together. Its construction consists of all of the skeleton charts shown in Figures 26-1 through 26-6, superimposed on one another. That is, all of the constant lines in the six skeleton charts appear in this one psychrometric chart, and all have the same relative positions. You now have a number of lines that cross each other on the composite psychrometric chart. If you plot a point on a constant dry-bulb line, that point will correspond to different values on the constant lines for wet-bulb temperature, dew point, relative humidity, specific volume, and enthalpy. Since any two of these constant lines cross at only one point on the chart, you can plot this point exactly if you know any two properties of the air. From this point, you can then move along respective constant lines to find other properties of the air. Once you learn how to use a psychrometric chart, obtaining the values you need by reading them FIGURE Constant enthalpy lines from their respective scales is not as difficult as it looks. This method is not quite as accurate as using psychrometric tables and performing mathematical calculations, but it is much faster. And, in general, the degree of accuracy will be close enough for your purposes Enthalpy, Btu/lb of dry air