The outside ambient has a moisture content of124 grains and the indoor design has 78 grains. Theamount of moisture to be removed is the difference betweenthese two values known as GR.įigure 5 shows how a dehumidification system was sized.The ambient design was 91F dry bulb and 78F wet bulb.The desired indoor value was 80F dry bulb and 50% rela-tive humidity. Dehumidificationīy obtaining the starting and finishing grains per pound, theamount of moisture to be removed can be calculated. Thepsychrometric chart is used to plot these two values by find-ing their intersection and then following the horizontal lineto the right to determine the moisture content in grains perpound.Ī. For most appli-cations the only information available is the dry bulb and rel-ative humidity or dry bulb and wet bulb temperatures. To properly apply a dehumidification system, the amount ofmoisture to be removed must be calculated. Then latent cooling reduces theabsolute humidity and finally the air is reheated increasingits dry bulb temperature. A refrigerant dehumidification system is a combi-nation of sensible and latent cooling and sensible heating.First the system cools the air to reduce the dry bulb temper-ature to the dew point. Rarely will these occur as shown but will rather be a mixtureof them. Figure 3 shows how these aredisplayed on the chart.
Latent cooling or dehumidificationis the removal of moisture. Latent heat, also known ashumidification, is the addition of moisture without changingthe dry bulb temperature. Sensible heat occurs when heat isadded without the addition or reduction of moisture.Sensible cooling is the reverse. There are four types of energy changes when heat of mois-ture is added or removed. This is known as the constant DewPoint Temperature. This is important to understand because water damageoccurs at an absolute humidity concentration regardless ofits relative humidity. Thus there is achange in relative humidity, without a change in actual watercontent. As moisture laden air is heated or cooled the air vol-ume changes but the moisture does not. Therefore given a spe-cific amount, it will always occupy the same amount of vol-ume.įigure 2 illustrates how this applies to the psychrometricchart. FT.Īs the air temperature increases, its total volume increasesand decreases on reduction of temperature. V = Volume T = Temperature P = Pressure K = Constantġ4.0 CU. The key is the word "relative." Tounderstand this concept, a law of nature must be Reviewed.Air is a compressible fluid and its volume is represented bythe following equation: It is often used inplace of absolute humidity. R ELATIVE HUMIDITYRelative humidity is a misapplied term. Theselines represent the degree of volume displaced by moisturewith Respect to the total air volume.
The sweeping curved lines that follow the saturation curveare relative humidity lines expressed as percentages. Any addi-tional moisture added could not be absorbed and wouldremain in a liquid state as condensation. Air in a condition that falls on any pointalong this curve is totally saturated with moisture. The curve forming the top edge of the chart is called the"saturation curve". Lines repre-senting Wet Bulb temperatures are the straight diagonallines sloping downward from left to right. The hori-zontal lines represent Dew Point temperatures. Dry Bulb tem-peratures are shown on the chart as vertical lines. T HE CHARTFigure 1 shows a typical psychrometric chart. It will definethe terms which form the nucleus of properly applying adehumidifier. This techni-cal bulletin will not attempt to cover the chart in detail, but,will highlight those features of the chart which apply torefrigerant type dehumidification applications.
I NTRODUCTION The psychrometric chart has been well documented in awide variety of technical textbooks and journals. Dehumidification and the Psychrometric Chart