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Service Application ManualSAM Chapter 630-16Section 3ATHE PSYCHROMETRIC CHART AND ITS USEPsychrometry is an impressive word which is defined as the measurement of the moisture content of air. In broader terms itis the science and practices associated with atmospheric air mixtures, their control, and the effect on materials and humancomfort. This can be accomplished through use of psychrometric tables or a psychrometric chart. The tables are somewhatmore accurate, but the chart is accurate enough for all practical purposes and is much easier to use.Before we start to explain the psychrometric chart, let us review a few of the principles on which it is based.First, atmospheric air is a mixture consisting of dry air and water vapor in varying relative amounts. It is best to think of dryair and water vapor separately, for they behave independently of one another. In this chapter, when dry air is referred to, itwill mean the air part; when water vapor or moisture is referred to, it will mean the water part; and when referring to just air,it will mean the mixture of dry air and water vapor.Dry air is itself a mixture of several gases, mainly nitrogen and oxygen, but these do not change in their proportions, andthey behave so much the same, that we can consider dry air as if it were a single gas.We know dry air only as a gas. It can be liquefied, but at very low temperatures, about 300 F below zero. In our atmosphereit is superheated three or four hundred degrees, or far above its condensing or boiling temperature.Thus, when we warm or cool dry air, we add or remove sensible heat only, and are not concerned with latent heat. Dryair obeys the gas laws of Charles, and Boyle, which state that if a gas, such as air, is heated, it expands and its densityor weight per cubic foot becomes less, provided the pressure remains constant. Conversely, if we cool dry air, it becomesheavier per cubic foot, again provided that its pressure remains constant. Moreover, the temperatures, densities, volumes,and pressures all vary proportionately.Therefore, problems connected with cooling or heating dry air are rather simple, for when we cool it we remove its sensibleheat of about 1/4 of a Btu per pound of dry air per degree, and when we heat dry air, we add that same amount of sensibleheat.Picture a room at normal temperatures. It is filled with a mixture of dry air and water vapor. Each fill the room, so theirvolumes are equal. However, their densities are far different because the water vapor is a very light gas. At ordinary roomtemperatures, it constitutes only about 1/2 to 1–1/2 per cent of the total weight of the mixture which we call air.Air, in addition to having weight, also exerts a pressure, called barometric pressure, which is usually measured in inches ofmercury above a perfect vacuum. Standard barometric pressure at sea level is 29.921 inches of mercury, which is equivalentto 14.696 pounds per square inch absolute.The pressure exerted by the air, or barometric pressure, is caused by both the dry air and the water vapor. Most of the totalpressure is from the air, but some is from the water vapor. As is true of the weight, or density, the pressure from the watervapor is usually only about 1/2 to 1–1/2 per cent of the total pressure.Copyright 1959, 2009, By Refrigeration Service Engineers Society.-1-

It is essential to always keep in mind that air consists of two separate gases, dry air and water vapor, which act independently,each according to its individual properties, just as if the other were not there. The total density of air is the sum of thedensities of the dry air and water vapor; and the total pressure is the sum of the partial pressure of the dry air and the partialpressure of the water vapor.Although the water vapor is a very small part of air, as far as density and pressure are concerned, it is the part whichcomplicates the psychrometric processes and calculations. For example, the amount of water vapor in the air varies widelyfrom time to time according to temperature, and the proportion of water vapor in the air must be regulated in order to providecomfortable conditions.It would not be necessary to regulate the amount of water vapor in the air if it remained as a superheated gas, and providingcomfortable conditions would not be difficult, for we would only have to control the dry bulb temperature of the air. It’s notthat simple, however, because at the temperatures at which we normally work, the water vapor sometimes condenses intowater. When this occurs, a large amount of latent heat must be removed, and as you know, the latent heat per pound ofwater is extremely large compared to the sensible heat. Consequently, when water vapor is condensed out of the air, a largeamount of cooling capacity is required.In cooling air, such as in summer air conditioning, we are chiefly concerned with:1. Cooling the dry air, which is comparatively simple and involves sensible cooling only. (The water vapor is such asmall part of the air that we usually ignore its sensible heat.)2. Controlling the amount of water vapor in the air. This involves condensing out some of the water vapor, thus bringingits large latent heat into the problem.3. Circulating and distributing the air, which involves changes in density and volume.4. Mixing together air from two different sources, at different temperatures, and different percents of water vapor.These four processes, along with others, involved in the controlling of atmospheric conditions for human comfort, are commonlyreferred to as psychrometric processes. In order to arrive at the proper amounts of humidification or dehumidification andheating or cooling necessary to effect these changes, we make use of the psychrometric tables or charts.THE USE OF THE PSYCHROMETRIC TABLEThe psychrometric properties in Table 54T03 have been compiled through countless laboratory experiments and mathematicalcalculations and are the basis for what we know as the psychrometric chart.Copyright 1959, 2009, By Refrigeration Service Engineers Society.-2-

TABLE 54T03 Thermodynamic Properties of Air based on one pound of air at a total pressure of 29.921 in.hg. (Atmospheric pressure)Copyright 1959, 2009, By Refrigeration Service Engineers Society.-3-

TABLE 54T03 (cont) Thermodynamic Properties of Air based on one pound of air at a total pressure of 29.921 in.hg. (Atmosphericpressure)Copyright 1959, 2009, By Refrigeration Service Engineers Society.-4-

TABLE 54T03 (cont) Thermodynamic Properties of Air based on one pound of air at a total pressure of 29.921 in.hg. (Atmosphericpressure)This table, and in fact the whole psychrometric chart, is based on standard atmospheric pressure, also called barometricpressure, of 29.921 inches of mercury. This is the total pressure of the mixture of dry air and water vapor, each of which hasits own partial pressure, and these pressures must be added together to get the total pressure. If we were to add water vaporto a cubic foot of dry air at standard atmospheric pressure, the total pressure would then be greater than 29.921 inches ofmercury. To balance this and keep the same total pressure of 29.921 inches of mercury, we would have to let a little air outbefore we add the water vapor.Since water vapor is very light compared to air, if we let the heavier air out and replace it with the lighter water vapor, thedensity of the mixture will be less than that of the dry air alone. If we look at the density of air at 60 F on Table 54T03, wefind that its density at saturation is .0750 lbs/cu ft, while the density of dry air at 60 F is .0763 lbs/cu ft, or .0013 lbs morethan the saturated air.VAPOR PRESSUREWater, like any other liquid such as ammonia, sulfur dioxide, or other refrigerants, has different boiling points at differentpressures. These pressures are very low, however, compared to most refrigerants. Table 54T06 shows the pressuretemperature relationship for water. The pressures are in inches of mercury absolute (not vacuum) and the temperatures arein degrees Fahrenheit.Copyright 1959, 2009, By Refrigeration Service Engineers Society.-5-

TABLE 54T06 Saturated Water VaporCopyright 1959, 2009, By Refrigeration Service Engineers Society.-6-

TABLE 54T06 (Cont.) Saturated Water VaporThe water vapor in ordinary air is very light and has a very low pressure. If we start to cool the mixture, the water vapor getsdenser (specific volume decreases) until it finally gets down to the temperature at which moisture starts to condense out ofthe air. The temperature at which this happens is called the dew point. We could also call this the condensing temperatureor the boiling temperature, for it is the temperature at which water vapor condenses or water evaporates at its correspondingpressure.What is commonly called the saturation pressure or condensing pressure with refrigerants is called vapor pressure whenreferring to water vapor. Thus, we can define the vapor pressure of water as the pressure exerted by the water vapor in aspace at its corresponding dew point temperature.Copyright 1959, 2009, By Refrigeration Service Engineers Society.-7-

Dalton’s Law of partial pressures states that the total pressure exerted by a mixture of gases is the sum of the partialpressures of the gases in the mixture.The total pressure of air, or standard atmospheric pressure, is 29.921 inches of mercury, so the partial pressure of the dryair only is 29.921 inches of mercury minus the vapor pressure at the corresponding dew point temperature.RELATIVE HUMIDITYThese vapor pressure tables are very useful in calculating the relative humidity of air at partially saturated conditions. Theterm “relative humidity” refers to the ratio of the actual partial pressure of the water vapor at a given condition to its saturationpressure at the same temperature.This can be expressed in the equation:RH P1/PsWhere:P1 Partial pressure of the water vapor at the dew point temperature of the mixture of dry air and water vapor,Ps Saturation pressure of the water vapor corresponding to the dry bulb temperature.Relative Humidity should not be confused with Specific Humidity and Absolute Humidity, which are based on the weight ofwater vapor per pound of dry air and per cubic foot of dry air, respectively. In other words, the term “Specific Humidity” isused in referring to the weight of water vapor in pounds or grains contained in the air at a given condition, for each pound ofdry air at that same condition. The term “Absolute Humidity” refers to the pounds or grains of water vapor contained in onecubic foot of a mixture of dry air and water vapor at a given condition.SPECIFIC HUMIDITYThe two columns in Table 54T03 headed “Moisture Content (per pound of dry air)” refer to the amount of moisture by weightthat is required to saturate one pound of dry air at the given dew point temperature. This weight of water is expressedeither in pounds of water per pound of dry air or in grains of water per pound of dry air, and is called the Specific Humidity,or Humidity Ratio. A grain of water is approximately one drop, and there are 7,000 grains of water to one pound of water.Therefore, to convert grains of water to pounds of water, we simply divide the number of grains by 7,000. The term “Grainsof Moisture” is used in psychrometric calculations because the larger numbers are handled more easily than the six placedecimals necessary to evaluate pounds of moisture per pound of dry air. A table giving pounds of moisture per 100 poundsof air could be used to simplify these calculations. To do this we would merely move the decimal point two places to the right,and make all calculations on the basis of 100 pounds of dry air rather than one pound of dry air.PER CENT OF SATURATIONAnother term which is sometimes confused with Relative Humidity is the term “Percent of Saturation” (or percentagehumidity) which is 100 times the ratio of the weight of water vapor actually held, to the weight of the water vapor necessaryto saturate one pound of dry air at that dry bulb temperature. This can be expressed in an equation:Percent of Saturation w1/wsWhere:w1 Specific Humidity at dew point temperature of mixture of dry air and water vapor,Copyright 1959, 2009, By Refrigeration Service Engineers Society.-8-

ws Specific Humidity at Saturation (the dew point equals the dry bulb).If we want to calculate both the relative humidity and percent of saturation at a dry bulb temperature of 95 F and a dew pointtemperature of 60 F, we take the vapor pressures from Table 54T06 for the dew point temperature and dry bulb temperatureto obtain the relative humidity and we take the specific humidities from Table 54T06, or the psychrometric chart, for thesesame temperatures to obtain the Percent of Saturation, as follows:R.H. P1/Ps or .5216/1.6607 31.4%% Saturation w1/ws or .01105/.03662 30.2%From this we can see there is a difference in values between these two humidities. Since the percent of saturation is basedon the weight which is not affected by temperature changes, and the relative humidity is based on volume (pressure andvolume are dependent upon each other) which is affected by temperature change, the percent of saturation is the moreaccurate of the two. However, the difference is slight, so the use of relative humidity is permissible where extreme accuracyis not of prime importance.SPECIFIC VOLUMETo find the specific volume and density of this mixture is a little more complicated. In order to do this, we must first go backto the fundamental gas law which states that the Absolute Pressure in pounds per square foot times the Volume in cubicfeet equals the Weight in pounds times the Temperature in absolute degrees Fahrenheit times the Perfect Gas Constant forthat particular gas. This can be expressed as:PV WRTSince air is considered a perfect gas, and the specific volume is expressed in cu ft per one pound of air, we can transformthis