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Dehumidification and the Psychrometric Chart

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Dehumidification and the Psychrometric Chart Est.1978 Technical Bulletin 3 Dehumidification and the Psychrometric Chart I NTRODUCTION R ELATIVE HUMIDITY The psychrometric chart has been well documented in a wide Relative humidity is a misapplied term. It is often used in variety of technical textbooks and journals. This technical bulletin place of absolute humidity. The key is the word “relative.” To will not attempt to cover the chart in detail, but, will highlight understand this concept, a law of nature must be Reviewed. those features of the chart which apply to refrigerant type Air is a compressible fluid and its volume is represented by dehumidification applications. It will define the terms which the following equation: form the nucleus of properly applying a dehumidifier. v=K(T/P) T HE CHART V = Volume Figure 1 shows a typical psychrometric chart. Dry Bulb T = Temperature temperatures are shown on the chart as vertical lines. The P = Pressure horizontal lines represent Dew Point temperatures. Lines K = Constant 9 4 14.5 CU. FT. 8 4 representing Wet Bulb temperatures are the straight diagonal 85 90 95 100 105 7 180 4 lines sloping downward from left to right. 6 4 5 170 4 4 The curve forming the top edge of the chart is called the 4 3 4 160 ) 0 8 B 2 L 4 “saturation curve”. Air in a condition that falls on any point / U 1 T 4 150 B ( along this curve is totally saturated with moisture. Any additional 0 N 4 IO T 9 A 3 140 moisture added could not be absorbed and would remain in a R s U 8 T 3 e A 5 r S 7 T 7 u 3 t liquid state as condensation. A 130 Y a 6 P r L 3 A e H 5 T 3 p N 120 E 4 m The sweeping curved lines that follow the saturation curve are 3 e H 3 0 R T 3 7 % 0 9 relative humidity lines expressed as percentages. These b 110 2 l R 3 I H u A R 1 3 % Y 0 B R lines represent the degree of volume displaced 8 0 100 D y 3 H F R r % O 9 5 0 D by moisture with Respect to the 2 6 7 D W N 8 90 6 2 e U 2 H t O R B total air volume. % P 7 0 u 5 2 6 2 lb R T E P 4 e 80 2 0 6 m E H p R R 3 y e % U 2 t 0 r i 5 at T e d S i u I 2 r 70 2 v r m es O u M 5 u H 1 5 R 2 C H F 0% n e 4 O 8 o v 1 0 i i 60 S 2 t t N a a I 7 r l Figure 1 1 9 0 e A 1 u5 at R R H G 6 S R 1 % 50 30 5 1 45 2 4 1 1 40 0 H 1 3 4 R 1 % 1 20 0 1 2 1 35 30 9 30 8 25 H 10% R 20 7 Dew Point Temperatures 10 0 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 12.5 CU. FT. 13.0 CU. FT. DRY BULB °F 13.5 CU. FT. 14.0 CU. FT. As the air temperature increases, its total volume increases This is important to understand because water damage and decreases on reduction of temperature. Pressure has the occurs at an absolute humidity concentration regardless of its opposite effect. As pressure increases volume decreas es. relative humidity. This is known as the constant Dew Point Temperature. Water, however, is not compressible. Therefore given a specific amount, it will always occupy the same amount of volume. S ENSIBLE AND LATENT HEATING AND COOLING Figure 2 illustrates how this applies to the psychrometric chart. As moisture laden air is heated or cooled the air volume There are four types of energy changes when heat of moisture changes but the moisture does not. Thus there is a change in is added or removed. Sensible heat occurs when heat is relative humidity, without a change in actual water content. added without the addition or reduction of moisture. Sensible cooling is the reverse. Latent heat, also known as humidification, is the addition of moisture without changing the dry bulb tem - perature. Latent cooling or dehumidification is the removal of moisture. Figure 3 shows how these are displayed on the chart. 9 4 14.5 CU. FT. 8 4 85 90 95 100 105 7 180 92 F 4 ° 6 78 F 4 ° 5 170 70°F 4 4 4 3 4 160 ) 0 8 B 2 L 4 / U 1 T 4 150 B ( 0 N 4 IO T 9 A 3 140 R U 8 T 3 A 5 80% RH S 7 7 T 3 60% RH A 130 Y 40% RH P 6 L 3 A H 5 T 3 N % 120 E 4 3 0 H 8 3 0 R 3 7 % 0 9 110 2 % R 3 0 I H A R 6 1 3 % Y 0 8 R 0 100 D 3 H R F % O 9 5 0 2 6 7 88 GR. D N 8 90 6 2 U 2 H R O % P 7 0 78°F92°F 2 70°F 5 6 2 R E P 4 80 2 0 6 E H R R 3 % U 2 0 5 T S 2 70 I 2 % O 5 M 5 0 1 H Figure 2 4 F 2 R 0% O 8 4 1 0 60 S 2 N 7 I 1 9 0 A 1 5 R H G 6 R 1 % 50 30 5 1 45 2 4 1 1 40 0 H 1 3 4 R 1 % 1 20 0 1 2 1 35 30 9 30 8 25 H 10% R 20 7 10 0 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 12.5 CU. FT. 13.0 CU. FT. DRY BULB °F 13.5 CU. FT. 14.0 CU. FT. TECHNICAL BULLETIN 3 Dehumidification and the Psychrometric Chart Rarely will these occur as shown but will rather be a mixture By obtaining the starting and finishing grains per pound, the of them. A refrigerant dehumidification system is a combination amount of moisture to be removed can be calculated. The of sensible and latent cooling and sensible heating. First the amount of moisture to be removed is the difference between system cools the air to reduce the dry bulb temperature to the these two values known as GR. dew point. Then latent cooling reduces the absolute humidity and finally the air is reheated increasing its dry bulb temperature. Figure 5 shows how a dehumidification system was sized. The Figure 4 graphs this process. ambient design was 91°F dry bulb and 78°F wet bulb. The desired indoor value was 80°F dry bulb and 50% relative D EHUMIDIFIER SIZING humidity. The outside ambient has a moisture content of 124 grains and the indoor design has 78 grains. Thus the required To properly apply a dehumidification system, the amount of moisture removal rate is 124-78 = 46 grains per pound of dry air. moisture to be removed must be calculated. For most applications the only information available is the dry bulb and relative humidity or dry bulb and wet bulb temperatures. The psy - chrometric chart is used to plot these two values by finding their intersection and then following the horizontal line to the right to determine the moisture content in grains per pound. 9 4 14.5 CU. FT. 8 4 85 90 95 100 105 7 180 4 6 4 5 170 4 4 4 3 4 160 ) 0 8 B 2 L 4 / U 1 T 4 150 B ( 0 N 4 Figure 3 IO T 9 A 3 140 R U 8 T 3 A 5 S 7 7 C T 3 A. Sensible Heating A 130 Y P 6 L 3 A H 5 B. Sensible Cooling T 3 N 120 E 4 3 C. Humidification H 3 0 B R A 3 7 % 0 9 110 2 R D. Dehumidification 3 I H A R 1 3 % Y 0 8 R 0 100 D 3 H R F % O 9 5 0 2 6 7 D N 8 90 6 2 U 2 H R O % P 7 0 D 5 2 6 2 R E P 4 80 2 0 6 E H R R 3 % U 2 0 5 T S 2 70 I 2 O 5 M 5 1 H 2 F R 0% O 8 4 1 0 60 S 2 N 7 I 1 9 0 A 1 5 R H G 6 R 1 % 50 30 5 1 45 2 4 1 1 40 0 H 1 3 4 R 1 % 1 20 0 1 2 1 35 30 9 30 8 25 H 10% R 20 7 10 0 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 12.5 CU. FT. 13.0 CU. FT. DRY BULB °F 13.5 CU. FT. 14.0 CU. FT. 9 14.5 CU. FT. Figure 4 4 8 4 85 90 95 100 105 7 180 4 6 4 5 170 4 QT = Total Cooling A-B 4 4 3 4 160 ) 0 8 B 2 QS = Sensible Cooling C-B L 4 / U 1 T 4 150 B ( 0 N 4 QL = Latent Cooling A-C O I T 9 A 3 140 R U 8 T 3 A 5 WA = Specific Humidity A (room air) S 7 7 T 3 A 130 Y P 6 L 3 WB = Specific Humidity C (supply air) A H 5 L T 3 Q N 120 E 4 3 H 3 0 R T = Total Temperature Rise B-D 3 7 % 0 9 110 2 T R 3 I Q H A R 1 3 % Y 0 A Entering Air WA GR = WA-WB 8 R 0 100 D 3 H R F % O 9 5 0 2 6 7 D N S 8 90 6 2 U 2 H Q R O % P 7 0 5 2 6 2 R E P ∆GR 4 80 2 0 6 E H R R 3 % U 2 0 5 T S 2 70 I 2 O 5 M 5 1 H 2 R D F 0% O 8 4 1 0 60 S 2 N 7 I WB 1 9 0 B Reheated A 1 5 C R H G 6 R Air (Supply) 1 Leaving % 50 30 5 Evaporator 1 45 Coil 2 4 1 1 40 0 H 1 3 4 R 1 % 1 20 0 1 2 1 35 30 9 30 8 25 H 10% R 20 7 10 0 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 12.5 CU.
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