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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 T rane Company— W orldwide Applied Systems Group.
Preface © 1999 American Standard Inc. All rights reserved TRG-TRC001-EN ii The T rane 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.
TRG-TRC001-EN iii Contents period one The Psychrometric Chart ................................... 1 Properties of Air ......................................................... 2 Constructing a Simple Psychrometric Chart .............. 8 Effect of Sensible Heat and Moisture Changes .
iv TRG-TRC001-EN.
TRG-TRC001-EN 1 notes period one The Psychrometric Chart Psychrometry is the science dealing with the physical laws of air – water mixtures. When designing an air conditioning system, the temperatur.
2 TRG-TRC001-EN notes period one The Psychrometric Chart 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 .
TRG-TRC001-EN 3 period one The Psychrometric Chart notes Dry-bulb temperatures are read from an ordinary thermometer that has a dry bulb. Wet-bulb temperatures are read from a thermometer whose bulb is covered by a wet wick.
4 TRG-TRC001-EN notes period one The Psychrometric Chart 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. When the dry-bulb, wet-bulb, and dew-point temperatures are the same, the air is saturated .
TRG-TRC001-EN 5 period one The Psychrometric Chart notes 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.
6 TRG-TRC001-EN notes period one The Psychrometric Chart 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.
TRG-TRC001-EN 7 period one The Psychrometric Chart notes 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? Only one point on the psychrometric chart represents air with both of these conditions.
8 TRG-TRC001-EN notes period one The Psychrometric Chart Notice that the point of intersection falls between two relative humidity curves: 40% and 50%.
TRG-TRC001-EN 9 period one The Psychrometric Chart notes These saturation points can be plotted on a chart with dry-bulb temperature along the horizontal axis and humidity ratio along the vertical axis. When several saturation points are plotted, the curve created resembles the relative humidity curves of the psychrometric chart.
10 TRG-TRC001-EN notes period one The Psychrometric Chart Another fact about saturated air should be discussed before we proceed. Assume a volume of moist air has the initial conditions indicated in column D of the table. The air has a 90 °F dry-bulb temperature and a 60 °F dew point.
TRG-TRC001-EN 11 period one The Psychrometric Chart notes 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. Merely heating the air does not change its moisture content.
12 TRG-TRC001-EN notes period one The Psychrometric Chart Additional curves can be added to the chart to represent relative humidity conditions that are less than 100%. The curves shown are at 10% intervals and represent humidity conditions ranging from completely saturated air to completely dry air .
TRG-TRC001-EN 13 period one The Psychrometric Chart notes T o 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. Therefore, the wet-bulb temperature lines start at the saturation curve.
14 TRG-TRC001-EN notes period one The Psychrometric Chart The psychrometric chart now defines these five properties of air: dry-bulb temperature (vertical lines), humidity ratio and dew-point temperature (horizontal lines), relative humidity (curved lines) and wet-bulb temperature (diagonal lines).
TRG-TRC001-EN 15 period one The Psychrometric Chart notes 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 .
16 TRG-TRC001-EN notes period one The Psychrometric Chart 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.
TRG-TRC001-EN 17 period one The Psychrometric Chart notes 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. In actual practice, however , both the dry-bulb temperature and moisture content of the air generally change simultaneously .
18 TRG-TRC001-EN notes 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. The air entering the cooling coil may be 100% recirculated ( A ), 100% outdoor ( B ), or a mixture of the two ( C ).
TRG-TRC001-EN 19 period two Air Mixtures notes 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 .
20 TRG-TRC001-EN notes period two Air Mixtures The outdoor air quantity in this example constitutes 25% of the mixture, while the recirculated air makes up the remaining 75%.
TRG-TRC001-EN 21 period two Air Mixtures notes 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.
22 TRG-TRC001-EN notes period three Sensible Heat Ratio 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.
TRG-TRC001-EN 23 period three Sensible Heat Ratio notes Conversely , if only latent heat is removed, the line moves vertically downward along a constant dry-bulb temperature line.
24 TRG-TRC001-EN notes period three Sensible Heat Ratio 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 .
TRG-TRC001-EN 25 period three Sensible Heat Ratio notes 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.
26 TRG-TRC001-EN notes period three Sensible Heat Ratio 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.
TRG-TRC001-EN 27 period three Sensible Heat Ratio notes 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.
28 TRG-TRC001-EN notes period three Sensible Heat Ratio 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.
TRG-TRC001-EN 29 period three Sensible Heat Ratio notes 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.
30 TRG-TRC001-EN notes period four Air Quantity Next, we will determine the flow rate of supply air necessary to maintain a given set of design room conditions.
TRG-TRC001-EN 31 period four Air Quantity notes T o 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.
32 TRG-TRC001-EN notes period four Air Quantity 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. Locate the mixed air conditions C on the psychrometric chart (82.
TRG-TRC001-EN 33 period four Air Quantity notes 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.
34 TRG-TRC001-EN notes period four Air Quantity 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.
TRG-TRC001-EN 35 period four Air Quantity notes For this example, the supply airflow is calculated as follows: The cooling coil must cool and dehumidify 3,430 cfm of air from the entering condition C to the supply air condition D to maintain the desired room conditions.
36 TRG-TRC001-EN notes period four Air Quantity 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.
TRG-TRC001-EN 37 notes period five T ons of Refrigeration 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 . Another property of air , enthalpy , must now be defined.
38 TRG-TRC001-EN notes period five T ons of Refrigeration 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.
TRG-TRC001-EN 39 period five T ons 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.
40 TRG-TRC001-EN notes period five T ons of Refrigeration 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.
TRG-TRC001-EN 41 period five T ons of Refrigeration notes 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.
42 TRG-TRC001-EN notes period six Psychrometric Analyses 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. In the previous example, the sensible heat ratio was based on full load or design load conditions .
TRG-TRC001-EN 43 period six Psychrometric Analyses notes 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.
44 TRG-TRC001-EN notes period six Psychrometric Analyses 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.
TRG-TRC001-EN 45 period six Psychrometric Analyses notes 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.
46 TRG-TRC001-EN notes period six Psychrometric Analyses 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.
TRG-TRC001-EN 47 period six Psychrometric Analyses notes 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 .
48 TRG-TRC001-EN notes period six Psychrometric Analyses 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.
TRG-TRC001-EN 49 period six Psychrometric Analyses notes 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.
50 TRG-TRC001-EN notes period six Psychrometric Analyses At part load, when the SHR of the room is reduced from 0.80 to 0.70, the V A V system responds by reducing the quantity of 56.5 °F air supplied to the room to match the reduced sensible heat gain.
TRG-TRC001-EN 51 period six Psychrometric Analyses notes The psychrometric chart is a visual tool that helps designers find solutions to many common HV AC problems by plotting conditions on the chart. T oday , many of these same problems can be quickly solved by computers, which can often eliminate the need for a graphical solution altogether .
52 TRG-TRC001-EN notes Let’ s review some of the main concepts from this clinic on psychrometry . The lines of the psychrometric chart represent five physical properties of air: dry bulb, wet bulb, dew point, humidity ratio, and relative humidity .
TRG-TRC001-EN 53 period seven Review notes In Period T wo, a method was discussed to determine the resulting properties of an air mixture. By plotting the conditions of the outdoor air and recirculate.
54 TRG-TRC001-EN notes period seven Review 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.
TRG-TRC001-EN 55 period seven Review notes 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 .
56 TRG-TRC001-EN notes period seven Review For more information, refer to the following references: ■ T rane Air Conditioning Manual ■ ASHRAE Handbook—Fundamentals ■ Fundamentals of Thermodynamics and Psychrometrics, ASHRAE self- directed learning course ■ Psychrometrics: Theory and Practice, ASHRAE Visit the ASHRAE Bookstore at www .
TRG-TRC001-EN 57 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.
58 TRG-TRC001-EN 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.
TRG-TRC001-EN 59 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 6a 95 °F × 0.20 = 19 °F 80 °F × 0.80 = 64 °F Mixed-Air T emperature = 19 °F + 64 °F = 83 °F b See Figure 90.
60 TRG-TRC001-EN Answers 8a The SHR line crosses the saturation curve at 56 °F WB (See Figure 92.) b 60 °F DB, 58 °F WB (See Figure 92.) 9a b See Figure 93.
TRG-TRC001-EN 61 Answers 10 58.3 °F WB (See Figure 94.) 11 ) ) 6 + 5 6 + 5 Figure 93 SHR 80,000 Btu/hr 80,000 Btu/hr .
62 TRG-TRC001-EN Answers 12 See Figure 95. 13 a See Figure 96. b c d Refrigeration Load 4.5 7,000 cfm × 33.2 24.2 – () × 283,500 Btu/hr == 283,500 Btu/hr 12,000 Btu/hr/ton 23.
TRG-TRC001-EN 63 Answers e f g ) ) 6 + 5 6 + 5 ) ) ) ) Figure 96 Airflow (cfm) 156,000 Btu/hr 1.
64 TRG-TRC001-EN 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.
TRG-TRC001-EN 65 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.
The T rane 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/A V -.
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