Air Conditioning Clinic Psychrometry One of the Fundamental Series TRG-TRC001-EN
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Psychrometry One of the Fundamental Series A publication of The Trane Company— Worldwide Applied Systems Group
Preface 3V\FKURPHWU\ $ 7UDQH $LU &RQGLWLRQLQJ &OLQLF Figure 1 The Trane Company believes that it is incumbent on manufacturers to serve the industry by regularly disseminating information gathered through laboratory research, testing programs, and field experience. The Trane Air Conditioning Clinic series is one means of knowledge sharing. It is intended to acquaint a nontechnical audience with various fundamental aspects of heating, ventilating, and air conditioning.
Contents period one The Psychrometric Chart ................................... 1 Properties of Air ......................................................... 2 Constructing a Simple Psychrometric Chart .............. 8 Effect of Sensible Heat and Moisture Changes ....... 15 period two Air Mixtures ......................................................... 18 period three Sensible Heat Ratio ........................................... 22 period four Air Quantity ..........................................
iv TRG-TRC001-EN
period one The Psychrometric Chart notes 3V\FKURPHWU\ SHULRG RQH 7KH 3V\FKURPHWULF &KDUW Figure 2 Psychrometry is the science dealing with the physical laws of air – water mixtures. When designing an air conditioning system, the temperature and moisture content of the air to be conditioned, and the same properties of the air needed to produce the desired air conditioning effect, must be known. Once these properties are known, the air conditioning task can be determined.
period one The Psychrometric Chart notes 3V\FKURPHWULF &KDUW Figure 3 Properties of Air At first glance, the psychrometric chart appears to be an imposing network of lines. When properly used, however, it provides valuable information about the properties of air. During this session, the psychrometric chart and its use in solving many air conditioning problems will be explained.
period one The Psychrometric Chart notes 'U\ %XOE 7KHUPRPHWHU Figure 5 Dry-bulb temperatures are read from an ordinary thermometer that has a dry bulb. :HW %XOE 7KHUPRPHWHU Figure 6 Wet-bulb temperatures are read from a thermometer whose bulb is covered by a wet wick. The difference between the wet-bulb temperature and the drybulb temperature is caused by the cooling effect produced by the evaporation of moisture from the wick.
period one The Psychrometric Chart notes &RQGHQVDWLRQ 2FFXUV DW 'HZ 3RLQW Figure 7 The third property, dew-point temperature, is the temperature at which moisture leaves the air and condenses on objects, just as dew forms on grass and plant leaves. )RJ 2FFXUV :KHQ $LU ,V 6DWXUDWHG Figure 8 When the dry-bulb, wet-bulb, and dew-point temperatures are the same, the air is saturated. It can hold no more moisture.
period one The Psychrometric Chart notes 5HODWLYH +XPLGLW\ 5HODWLYH +XPLGLW\ $PRXQW RI PRLVWXUH WKDW D JLYHQ DPRXQW RI DLU LV KROGLQJ $PRXQW RI PRLVWXUH WKDW D JLYHQ DPRXQW RI DLU FDQ KROG Figure 9 The fourth property, relative humidity, is a comparison of the amount of moisture that a given amount of air is holding, to the amount of moisture that the same amount of air can hold, at the same dry-bulb temperature.
period one The Psychrometric Chart notes +XPLGLW\ 5DWLR Figure 11 Finally, humidity ratio describes the actual weight of water in an air – water vapor mixture. In other words, if one pound of air were wrung completely dry, comparing the weight of the water to the weight of the dry air would yield its humidity ratio. Humidity ratio can be expressed as pounds of moisture per pound of dry air, or as grains of moisture per pound of dry air. There are 7000 grains of water in a pound.
period one The Psychrometric Chart notes 6XPPHU 'HVLJQ &RQGLWLRQV s ) '% GU\ EXOE s ) :% ZHW EXOE Figure 13 For example, let's assume that the summer design conditions are 95°F dry bulb and 78°F wet bulb. What is the relative humidity, humidity ratio, and dew point? ) 3RLQW RI ,QWHUVHFWLRQ ) ) Figure 14 Only one point on the psychrometric chart represents air with both of these conditions.
period one The Psychrometric Chart notes Notice that the point of intersection falls between two relative humidity curves: 40% and 50%. By interpolation, the relative humidity at this condition is approximately 47%. Constructing a Simple Psychrometric Chart To better understand the psychrometric chart and show why the lines intersect as they do, we will construct a simple chart.
period one The Psychrometric Chart notes 3ORWWLQJ 6DWXUDWLRQ 3RLQWV +XPLGLW\ 5DWLR JUDLQV OE OE RI GU\ DLU RI GU\ DLU +XPLGLW\ 5DWLR JUDLQV Figure 16 'U\ %XOE 7HPSHUDWXUH ) These saturation points can be plotted on a chart with dry-bulb temperature along the horizontal a
period one The Psychrometric Chart notes 'U\ %XOE :HW %XOE DQG 'HZ 3RLQW $ +XPLGLW\ 5DWLR JUDLQV OE OE RI GU\ DLU RI GU\ DLU +XPLGLW\ 5DWLR JUDLQV GU\ EXOE ZHW EXOE GHZ SRLQW UHO KXPLGLW\ % & ) ) ) ) ) ) ) ) ) ' ) ) ) ) $ ZHW EXOE ) GHZ SRLQW % & ' 'U\ %XOE 7HPSHUDWXUH ) Figure 18 Another fact ab
period one The Psychrometric Chart notes $GGLQJ 6HQVLEOH +HDW +XPLGLW\ 5DWLR JUDLQV OE OE RI GU\ DLU RI GU\ DLU +XPLGLW\ 5DWLR JUDLQV VDWXUDWLRQ ) GHZ SRLQW ) ZHW EXOE 'U\ %XOE 7HPSHUDWXUH ) Figure 19 Additionally, the dew-point temperature does not change as the dry-bulb temperature changes, provided that the moisture content of the air remains the same.
period one The Psychrometric Chart notes 5HODWLYH +XPLGLW\ &XUYHV UHODWLYH KXPLGLW\ +XPLGLW\ 5DWLR JUDLQV OE OE RI GU\ DLU RI GU\ DLU +XPLGLW\ 5DWLR JUDLQV Figure 20 'U\ %XOE 7HPSHUDWXUH ) Additional curves can be added to the chart to represent relative humidity conditions that are less than 100%.
period one The Psychrometric Chart notes 'HWHUPLQLQJ :HW %XOE /LQHV +XPLGLW\ 5DWLR JUDLQV OE OE RI GU\ DLU RI GU\ DLU +XPLGLW\ 5DWLR JUDLQV %, &, ', $, $ % & ' 'U\ %XOE 7HPSHUDWXUH ) Figure 22 To complete this basic chart, the wet-bulb temperature lines must be added. Once again, at a saturated condition the wet-bulb, dry-bulb and dew-point temperatures are equal.
period one The Psychrometric Chart notes 3URSHUWLHV RI $LU +XPLGLW\ 5DWLR JUDLQV OE OE RI GU\ DLU RI GU\ DLU +XPLGLW\ 5DWLR JUDLQV GHZ SRLQW KXPLGLW\ UDWLR ZHW EXOE GU\ EXOE UHODWLYH KXPLGLW\ Figure 23 'U\ %XOE 7HPSHUDWXUH ) The psychrometric chart now defines these five properties of air: dry-bulb temperature (vertical lines), humidity ratio and dew-point temperature (ho
period one The Psychrometric Chart notes (IIHFW RI $GGLQJ 6HQVLEOH +HDW Figure 25 Effect of Sensible Heat and Moisture Changes When either the sensible heat content or the moisture content of air changes, the point on the psychrometric chart that represents the original air condition moves to a position that represents the new condition of temperature and/or humidity. For example, if sensible heat is added to air, the air condition moves horizontally to the right.
period one The Psychrometric Chart notes (IIHFW RI $GGLQJ 0RLVWXUH Figure 27 On the other hand, if moisture is added to air without changing the dry-bulb temperature, the air condition moves upward along a dry-bulb temperature line. (IIHFW RI 5HPRYLQJ 0RLVWXUH Figure 28 Finally, if moisture is removed from the air without changing its dry-bulb temperature, the air condition moves downward along a dry-bulb temperature line.
period one The Psychrometric Chart notes 5HPRYLQJ 6HQVLEOH +HDW DQG 0RLVWXUH Figure 29 Put all of these changes together on one chart and they show the direction the air condition will move when the dry-bulb temperature or moisture content is altered. 5HPRYLQJ 6HQVLEOH +HDW DQG 0RLVWXUH $ $PHULFDQ 6WDQGDUG ,QF Figure 30 In actual practice, however, both the dry-bulb temperature and moisture content of the air generally change simultaneously.
period two Air Mixtures notes 3V\FKURPHWU\ SHULRG WZR $LU 0L[WXUHV Figure 31 Before an air conditioning problem can be analyzed on the psychrometric chart, the conditions of the air to be cooled or heated must be known. 'HWHUPLQLQJ (QWHULQJ $LU &RQGLWLRQV PL[WXUH & VXSSO\ IDQ RXWGRRU DLU 2$ % FRROLQJ FRLO $ UHFLUFXODWHG DLU 5$ UHFLUFXODWHG DLU 5$ Figure 32 The air entering the cooling coil may be 100% recirculated (A), 100% outdoor (B), or a mixture of the two (C).
period two Air Mixtures notes 'HWHUPLQLQJ (QWHULQJ $LU &RQGLWLRQV RXWGRRU DLU % % $ $ UHFLUFXODWHG DLU ) ) ) ) Figure 33 If outdoor air B is mixed with recirculated air A, the conditions of the resulting mixture are found somewhere on a straight line connecting the two points. If the mixture is half and half, this condition falls on the midpoint of the line between A and B.
period two Air Mixtures notes The outdoor air quantity in this example constitutes 25% of the mixture, while the recirculated air makes up the remaining 75%. The next step is to determine the dry-bulb temperature of the air mixture. 'HWHUPLQLQJ (QWHULQJ $LU &RQGLWLRQV ) Õ ) ) Õ ) PL[WXUH ) Figure 35 This is done by multiplying the dry-bulb temperature of each air condition by its percentage and summing the results.
period two Air Mixtures notes 'HWHUPLQLQJ (QWHULQJ $LU &RQGLWLRQV ) ) % % & & $ $ ) ) ) ) ) ) Figure 36 Returning to the psychrometric chart, point C, at which the 83.75°F dry-bulb temperature falls on the line from A to B, represents the conditions of the air mixture: 83.75°F DB and approximately 70°F WB.
period three Sensible Heat Ratio notes 3V\FKURPHWU\ SHULRG WKUHH 6HQVLEOH +HDW 5DWLR Figure 37 This period is devoted to understanding the term sensible heat ratio and how it is represented on the psychrometric chart. The ratio of sensible heat gain to total heat gain is one of the most important factors affecting air conditioning system requirements.
period three Sensible Heat Ratio notes (IIHFW RI 5HPRYLQJ /DWHQW +HDW Figure 39 Conversely, if only latent heat is removed, the line moves vertically downward along a constant dry-bulb temperature line. This results in a lower moisture content or humidity ratio. 5HPRYLQJ 6HQVLEOH DQG /DWHQW +HDW Figure 40 If both sensible and latent heat are removed from the air, the resulting air condition will be to the left and below the initial condition.
period three Sensible Heat Ratio notes +HDW DQG 0RLVWXUH 7UDQVIHU VXSSO\ DLU UHWXUQ DLU VHQVLEOH KHDW ODWHQW KHDW Figure 41 Imagine conditioned supply air as a sponge. As it enters a room, it absorbs heat and moisture. The amount of heat and moisture absorbed depends on the temperature and humidity of the supply air. This “sponge,” the supply air, must be cool enough to pick up the room's excess sensible heat gain and dry enough to pick up the room's excess latent heat (i.e., moisture.
period three Sensible Heat Ratio notes 5HPRYLQJ 6HQVLEOH DQG /DWHQW +HDW FRROHU GDPSHU FRROHU GDPSHU ZDUPHU GULHU ZDUPHU GULHU Figure 42 When the required amount of sensible and latent heat are not properly removed from the room, the desired room conditions cannot be maintained. For example, if too much sensible heat and not enough latent heat are removed, the room feels cold and damp. On the psychrometric chart, room conditions move up and to the left.
period three Sensible Heat Ratio notes +HDW *DLQ Figure 43 This relationship between the conditions and quantity of the supply air can be described using the analogy of maintaining a constant temperature within a container of water. In this illustration, the container of water is capable of absorbing heat. The amount of heat it absorbs is called heat gain. To maintain the water temperature at a constant 75°F, any heat gain must be offset by mixing cool water with the water already in the container.
period three Sensible Heat Ratio notes 6HQVLEOH +HDW 5DWLR 6+5 6+5 6HQVLEOH +HDW *DLQ 6HQVLEOH +HDW *DLQ /DWHQW +HDW *DLQ Figure 44 The sensible heat ratio, abbreviated as SHR, refers to the comparison of sensible heat gain to total heat gain (sensible heat plus latent heat). Once this ratio is known, an SHR line can be drawn on the psychrometric chart.
period three Sensible Heat Ratio notes 'UDZLQJ DQ 6+5 /LQH QH OLQH +5 OL 6+5 LQGH[ SRLQW $ $ Figure 46 Assume that room design conditions (A) are 78°F DB and 65°F WB, and that the sensible heat ratio is calculated as 0.80. That is, sensible heat gain represents 80% of the total (sensible plus latent) heat gain. The SHR line is found by aligning the index point with the 0.80 marking on the sensible-heat-ratio scale and drawing a line from the index point to the saturation curve.
period three Sensible Heat Ratio notes 'UDZLQJ DQ 6+5 /LQH ' & LQGH[ SRLQW % O OLQHH 6++5 6 ) ) Figure 48 Sensible-heat-ratio lines for other conditions are drawn in the following manner. Assume that room design conditions are 80°F DB and 60% RH, and that the SHR is calculated as 0.60. First, line up the index point with the 0.60 marking on the sensible-heat-ratio scale and draw a line.
period four Air Quantity notes 3V\FKURPHWU\ SHULRG IRXU $LU 4XDQWLW\ Figure 49 Next, we will determine the flow rate of supply air necessary to maintain a given set of design room conditions. &RLO &XUYHV FRLO FXUYHV FRLO FXUYHV Figure 50 Before proceeding, one more set of curves on the psychrometric chart must be identified. These curved lines represent the changes in dry-bulb and wet-bulb temperatures as air passes through a “typical” cooling coil.
period four Air Quantity notes 'HWHUPLQLQJ 6XSSO\ $LUIORZ 67(3 &DOFXODWH WKH VHQVLEOH KHDW UDWLR 6+5 %WX KU VHQVLEOH KHDW JDLQ %WX KU ODWHQW KHDW JDLQ 6+5 %WX KU %WX KU Figure 51 To demonstrate how the required supply airflow is determined, assume that a room’s sensible heat gain is 80,000 Btu/hr and its latent heat gain is 20,000 Btu/ hr. First, divide the sensible heat gain by the total heat gain. The resulting sensible heat ratio (SHR) is 0.80.
period four Air Quantity notes 'HWHUPLQLQJ 6XSSO\ $LUIORZ 67(3 3ORW URRP RXWGRRU DQG HQWHULQJ FRQGLWLRQV ) Õ ) ) Õ ) PL[WXUH ) % & $ 6+5 Figure 53 Plot the outdoor air B (95°F DB, 78°F WB) and indoor air A (78°F DB, 65°F WB) conditions on the psychrometric chart. Then calculate the mixed-air conditions using the method learned in Period 2. 95°F × 0.25 = 23.75°F 78°F × 0.75 = 58.50°F Mixed-Air Temperature = 23.75°F + 58.50°F = 82.
period four Air Quantity notes 'HWHUPLQLQJ 6XSSO\ $LUIORZ 67(3 ,GHQWLI\ VXSSO\ DLU FRQGLWLRQV % & 6+5 $ ' ) ) Figure 54 The third step is to determine the required supply air conditions. This is where the coil curves are used. Using the curvature of the nearest coil line as a guide, draw a curve from the mixed-air condition C until it intersects the SHR line.
period four Air Quantity notes 'HWHUPLQLQJ 6XSSO\ $LUIORZ 67(3 6ROYH WKH VXSSO\ DLUIORZ HTXDWLRQ 6XSSO\ $LUIORZ 6HQVLEOH +HDW *DLQ Õ 5RRP '% ¤ 6XSSO\ '% Figure 55 With the supply air conditions known, the next step is to calculate the specific quantity of air (cfm or cubic feet per minute) needed to satisfy the room heat gains. The required supply airflow is determined using the following formula, where the sensible heat gain is expressed in Btu/hr and the two temperatures are in °F.
period four Air Quantity notes 'HWHUPLQLQJ 6XSSO\ $LUIORZ 67(3 6ROYH WKH VXSSO\ DLUIORZ HTXDWLRQ %WX KU FIP Õ ) ¤ ) Figure 56 For this example, the supply airflow is calculated as follows: 80,000 Btu/hr Supply Airflow (cfm) = = 3,430 cfm 1.085 × ( 75°F – 56.
period four Air Quantity notes $UELWUDULO\ 8VLQJ ) 6XSSO\ $LU %WX KU VHQVLEOH KHDW JDLQ %WX KU ODWHQW KHDW JDLQ 6+5 %WX KU %WX KU Figure 58 Some designers prefer to set the supply air temperature at 55°F or use a 20°F temperature differential (Room DB – Supply DB) without regard for the actual sensible heat ratio of the room. Using our same example, let’s examine how this has the potential for creating a problem.
period five Tons of Refrigeration notes 3V\FKURPHWU\ SHULRG ILYH 7RQV RI 5HIULJHUDWLRQ Figure 60 The psychrometric chart can also be used to determine the total load on the refrigeration equipment, expressed in Btu per hour or tons of refrigeration. One ton equals 12,000 Btu/hr. :KDW LV (QWKDOS\" 7KH WRWDO KHDW HQHUJ\ LQ RQH SRXQG RI DLU %WX OE DW LWV SUHVHQW FRQGLWLRQ (QWKDOS\ K 6HQVLEOH +HDW /DWHQW +HDW Figure 61 Another property of air, enthalpy, must now be defined.
period five Tons of Refrigeration notes 'HWHUPLQLQJ 7RQV RI 5HIULJHUDWLRQ 67(3 )LQG HQWKDOSLHV HQWHULQJ DQG OHDYLQJ FRLO % % & & ' ' $ $ Figure 62 Using the previous example for calculating supply airflow, the first step is to determine the enthalpies of the air entering and leaving the cooling coil.
period five Tons of Refrigeration notes Realize that 4.5 is not a constant! It is the product of density of air and the conversion factor of 60 minutes per hour. The density of air at “standard” conditions (69.5°F DB dry air at sea level) results in the value 4.5. Air at other conditions and elevations will cause this factor to change. Density = 0.075 lb/ft 3 0.075 × 60 min/hr = 4.
period five Tons of Refrigeration notes 6HQVLEOH DQG /DWHQW &RLO /RDGV ODWHQW ORDG % % VHQVLEOH ORDG $ $ ' ' & & Figure 65 The psychrometric chart can also be used to determine the sensible and latent components of the coil’s refrigeration load. First, draw a right triangle though the coil entering and leaving air conditions. The vertical leg represents the amount of moisture removed by the coil, i.e.
period five Tons of Refrigeration notes 6HQVLEOH DQG /DWHQW &RLO /RDGV Õ FIP Õ ¤ %WX KU WRQV RI UHIULJHUDWLRQ VHQVLEOH Õ FIP Õ ¤ %WX KU WRQV RI UHIULJHUDWLRQ ODWHQW Figure 66 By determining the enthalpy values for these three points, the same equation can be used to calculate both the sensible and the latent portions of the coil’s refrigeration load. Sensible Refrigeration Load = 4.5 × 3,430 cfm × ( 29.6 – 23.
period six Psychrometric Analyses notes 3V\FKURPHWU\ SHULRG VL[ 3V\FKURPHWULF $QDO\VHV Figure 67 Now we will look at a few ways that the psychrometric chart can help us analyze air conditioning systems. For simplicity, we will limit our examples to systems serving a single zone. 6+5 DW )XOO /RDG &RQGLWLRQV % % & & ' ' ) $ $ ) Figure 68 In the previous example, the sensible heat ratio was based on full load or design load conditions.
period six Psychrometric Analyses notes 6+5 &KDQJHV ZLWK 5RRP /RDG )XOO /RDG 6+5 %WX KU %WX KU %WX KU 3DUW /RDG 6+5 %WX KU %WX KU %WX KU Figure 69 For example, assume that at full load the room is subject to an 80,000 Btu/hr sensible heat gain and a 20,000 Btu/hr latent heat gain. The full-load sensible heat ratio is 0.80.
period six Psychrometric Analyses notes 6+5 DW 3DUW /RDG &RQGLWLRQV %WX KU ï ) ² 6XSSO\ '% FIP % % 6XSSO\ '% ) ' ' ' ',, $ $,, & & $ $ 5 6+ 6+5 ) ) Figure 71 In response to the reduction in room sensible heat gain, the coil capacity is throttled, raising the supply air temperature from D to D’ to balance the new room sensible heat gain.
period six Psychrometric Analyses notes &RQVWDQW 9ROXPH 6\VWHP ' 2$ & % 6$ PRGXODWLQJ FRROLQJ FRLO 5$ $ FRQVWDQW TXDQWLW\ RI YDULDEOH WHPSHUDWXUH DLU Figure 72 This is the manner in which a constant-volume, variable-temperature system with a modulating coil performs. It provides a constant quantity of air to the room and responds to part-load conditions by varying the supply air temperature.
period six Psychrometric Analyses notes (IIHFW RI $GGLQJ 5HKHDW %WX KU ï ) ² 6XSSO\ '% FIP 6XSSO\ '% ) % % & ' ' ) ) $ $ ( ( 6+5 ) ) Figure 74 Using the part-load conditions from the previous example, the room’s sensible heat gain is reduced from 80,000 Btu/hr to 47,000 Btu/hr while the latent heat gain remains the same.
period six Psychrometric Analyses notes (IIHFW RI $GGLQJ 5HKHDW % % & & ' ' ) ) 6+5 6+5 $ $ ( ( ) ) Figure 75 This supply air mixes with room air along the part-load 0.70 SHR line from E to A, arriving at the desired room conditions A. Now, if the room’s latent heat gain were also changed, the resulting room conditions would not fall exactly on A, but on the appropriate SHR line that runs through E.
period six Psychrometric Analyses notes varying the supply air temperature as the two airstreams mix downstream of the air handler. The face-and-bypass dampers are controlled by the room dry-bulb thermostat. The cooling coil is allowed to “run wild,” causing the air that does pass through it to be cooled more at partial airflows.
period six Psychrometric Analyses notes (IIHFW RI 0L[HG $LU %\SDVV % % 6+5 $ & $,, & ' ' ) ) ( ( $ $ ) ) Figure 78 This supply air E mixes with room air along the part-load 0.70 SHR line, arriving at the resulting new room conditions A’. While the quantity and temperature of supply air are suitable to absorb the room’s sensible heat gain, they are unable to completely absorb the latent heat gain.
period six Psychrometric Analyses notes &DOFXODWLQJ 3DUW /RDG $LUIORZ %WX KU ï ) ² ) FIP % % & $ $,, & $ $ ' ' 5 6+ 5 +5 6+ 6 ) ) Figure 80 At part load, when the SHR of the room is reduced from 0.80 to 0.70, the VAV system responds by reducing the quantity of 56.5°F air supplied to the room to match the reduced sensible heat gain. The part-load sensible heat gain of 47,000 Btu/hr and the constant supply air temperature, 56.
period six Psychrometric Analyses notes 6RIWZDUH 7RROV Figure 81 The psychrometric chart is a visual tool that helps designers find solutions to many common HVAC problems by plotting conditions on the chart. Today, many of these same problems can be quickly solved by computers, which can often eliminate the need for a graphical solution altogether. Still, a basic understanding of psychrometric principles is required to use these tools, and is fundamental to the science of air conditioning.
period seven Review notes 3V\FKURPHWU\ SHULRG VHYHQ 5HYLHZ Figure 82 Let’s review some of the main concepts from this clinic on psychrometry.
period seven Review notes 'HWHUPLQLQJ 0L[HG $LU &RQGLWLRQV ) ï ) ) ï ) PL[WXUH ) % % ) ) $ $ & & ) ) ) ) ) ) Figure 84 In Period Two, a method was discussed to determine the resulting properties of an air mixture.
period seven Review notes 'HWHUPLQLQJ &RLO /HDYLQJ &RQGLWLRQV 6XSSO\ $LUIORZ 6HQVLEOH +HDW *DLQ ï 5RRP '% ² 6XSSO\ '% % % & & FRLO FXUYH ' ' LQHH /LQ 5 / 6 +5 6+ $ $ VXSSO\ '% VXSSO\ '% Figure 86 After determining the entering air conditions for the coil and the slope of the SHR line, the coil curves were used to find the required supply air conditions. This point (D) was established by the intersection of the coil curve and the SHR line.
period seven Review notes 3V\FKURPHWULF $QDO\VLV % % & & $ $ ' ' 6$ 6+5 OLQH & ' % 2$ 5$ $ Figure 88 The resulting psychrometric chart plot represents the changes that a volume of air undergoes as it travels through a typical air conditioning system. In this illustration, recirculated air A is mixed with outdoor air B, producing a mixed air condition C.
period seven Review notes Figure 89 For more information, refer to the following references: ■ Trane Air Conditioning Manual ■ ASHRAE Handbook—Fundamentals ■ Fundamentals of Thermodynamics and Psychrometrics, ASHRAE selfdirected learning course ■ Psychrometrics: Theory and Practice, ASHRAE Visit the ASHRAE Bookstore at www.ashrae.org.
Quiz Questions for Period 1 1 Given air conditions of 80°F DB and 60% RH, find the humidity ratio and dew-point temperature. 2 Given air conditions of 85°F DB and a humidity ratio of 90 grains/lb, find the relative humidity and wet-bulb temperature. 3 Given air conditions of 74°F WB and 60% RH, find the dry-bulb temperature and humidity ratio. 4 Given air conditions of 80°F DB and a humidity ratio of 64 grains/lb, find the relative humidity and wet-bulb temperature.
Quiz Questions for Period 4 9 Given a room with a 42,000 Btu/hr sensible heat gain and a 56,000 Btu/hr total heat gain (excluding ventilation heat gain); and indoor design conditions of 80°F DB and 50% RH: a Determine the sensible heat ratio. b Draw the SHR line.
Answers 1 93 grains/lb, 64.8°F DP 2 49.5% RH, 70.5°F WB 3 85°F DB, 110 grains/lb 4 41% RH, 64°F WB 5 56°F DP, 32% RH 6 a 4,000 cfm = 0.20 20,000 cfm 95°F × 0.20 = 19°F 80°F × 0.80 = 64°F Mixed-Air Temperature = 19°F + 64°F = 83°F b See Figure 90. ) ) ) ) ) ) ) ) Figure 90 c 7 a 69.4°F WB 6,000 cfm = 0.15 40,000 cfm 95°F × 0.15 = 13.5°F 78°F × 0.85 = 66.3°F Mixed-Air Temperature = 13.5°F + 66.3°F = 79.
Answers ) ) ) ) ) ) ) ) Figure 91 8 a The SHR line crosses the saturation curve at 56°F WB (See Figure 92.) b 60°F DB, 58°F WB (See Figure 92.) ) ) ) ) ) ) 5 +5 6+ 6 ) ) Figure 92 9 a 42,000 Btu/hr SHR = = 0.75 56,000 Btu/hr b See Figure 93.
Answers +5 6+ 6 ) ) Figure 93 10 80,000 Btu/hr SHR = = 0.80 80,000 Btu/hr + 20,000 Btu/hr 58.3°F WB (See Figure 94.) +5 6+ 6 ) ) ) ) ) ) Figure 94 11 TRG-TRC001-EN 1,000 Btu/hr Supply Airflow (cfm) = = 40 cfm 1.
Answers 12 See Figure 95. Refrigeration Load = 4.5 × 7,000 cfm × ( 33.2 – 24.2 ) = 283,500 Btu/hr 283,500 Btu/hr = 23.6 tons of refrigeration 12,000 Btu/hr/ton % %WWXX OOEE % %WWXX OOEE Figure 95 13 a b See Figure 96. 95°F × 0.25 = 23.75°F 78°F × 0.75 = 58.50°F Mixed-Air Temperature = 23.75°F + 58.50°F = 82.25°F Mixed-Air Conditions = 82.25°F DB, 67.7°F WB (See Figure 96.
Answers ) ) +5 6+ 6 ) ) ) ) Figure 96 e 156,000 Btu/hr Airflow (cfm) = = 6,847 cfm 1.085 × ( 78°F – 57°F ) f h 1 – h 2 = 32.1 – 23.7 = 8.4 Btu/lb g Refrigeration Load = 4.5 × 6,847 cfm × 8.4 Btu/lb = 258,817 Btu/hr 258,817 Btu/hr = 21.
Glossary ASHRAE American Society of Heating, Refrigerating and Air-Conditioning Engineers coil curves These represent the changes in dry-bulb and wet-bulb temperatures as air passes through a “typical” cooling coil. constant-volume system A type of air-conditioning system that varies the temperature of a constant volume of air supplied to meet the changing load conditions of the space. dew-point temperature The temperature at which moisture leaves the air and condenses on surfaces.
Glossary saturation curve This represents the moisture content that constitutes complete saturation of air at the various dry-bulb temperatures. saturation point The maximum amount of water vapor that one pound of dry air can hold at a given dry-bulb temperature. sensible heat Heat that causes a change in the air’s dry-bulb temperature with no change in moisture content. sensible heat ratio (SHR) latent) heat gain.
The Trane Company Worldwide Applied Systems Group 3600 Pammel Creek Road La Crosse, WI 54601-7599 www.trane.com An American Standard Company Literature Order Number TRG-TRC001-EN File Number E/AV-FND-TRG-TRC001-1099-EN Supersedes ED-FND-TRG-TRC001-199-EN Stocking Location Inland-La Crosse Since The Trane Company has a policy of continuous product improvement, it reserves the right to change design and specifications without notice.
