chapter 2 revised

Anh văn Chuyên ngành Nhiệt English for thermal engineering TÀI LIỆU THAM KHẢO (1) 2017 ASHRAE Handbook—Fundamentals (SI) (2) Fundamentals of thermal-fluid science, Y A Çengel Chapter 2: Psychrometrics Psychrometry is the study of the thermodynamics of gasvapor mixtures CONTENTS (1) COMPOSITION OF DRY AND MOIST AIR (2) PERFECT GAS RELATIONSHIPS FOR DRY AND MOIST AIR (3) HUMIDITY PARAMETERS (4) THERMODYNAMIC WET-BULB AND DEW-POINT TEMP (5) OTHER PROPERTIES OF THE MOIST AIR (6) PSYCHROMETRIC CHARTS (7) TYPICAL AIR-CONDITIONING PROCESSES Chapter 2: Psychrometrics (1) COMPOSITION OF DRY AND MOIST AIR  Atmospheric air contains many gaseous (ˈɡeɪ.si.əs) components (kəmˈpəʊ.nənt) as well as water vapor and miscellaneous contaminants (e.g., smoke, pollen, and gaseous pollutants not normally present in free air far from pollution sources)  Dry air is atmospheric air with all water vapor and contaminants; removed  Moist air is a binary (two-component) mixture of dry air and water vapor Harrison (1965) lists the approximate percentage composition of dry air by volume as: nitrogen, 78.084; oxygen, 20.9476; argon, 0.934; neon, 0.001818; helium, 0.000524; methane, 0.00015; sulfur dioxide, to 0.0001; hydrogen, 0.00005; and minor components such as krypton, xenon, and ozone, 0.0002 Chapter 2: Psychrometrics (2) PERFECT GAS RELATIONSHIPS FOR DRY AND MOIST AIR When moist air is considered a mixture of independent perfect gases (i.e., dry air and water vapor), each is assumed to obey the perfect gas equation of state as follows: Dry air: Water vapor: where Chapter 2: Psychrometrics (3) HUMIDITY (hjuːˈmɪd.ə.ti) PARAMETERS Humidity ratio, w The humidity ratio of moist air w is the ratio of the mass of water vapor mw to the mass of dry air mda contained in the mixture of the moist air, in (kg/kgda) , kg/kgda The saturation humidity ratio, ws , kg/kgda 𝑝𝑤𝑠 −represents the saturation pressure of water vapor in the absence of air at the given temperature t Chapter 2: Psychrometrics (3) HUMIDITY PARAMETERS  Absolute humidity, 𝝆𝒘 𝑚𝑤 𝜌𝑤 = 𝑉 , kg/m3  Relative humidity, 𝛗 Relative humidity is the amount of moisture in the air compared to what the air can “hold” at that temperature 𝜌𝑤 𝑝𝑤 𝜑= × 100 = × 100, % 𝜌𝑤𝑠 𝑝𝑤𝑠 𝜑 = 0% - Dry air 𝜑 = 100%- Saturated air < 𝜑 < 100% - Moist air Chapter 2: Psychrometrics (4) THERMODYNAMIC WET-BULB AND DEW-POINT TEMPERATURE  Dry bulb temperature, 𝒕𝑫𝑩 Dry-bulb temperature can be measured using a normal thermometer freely exposed to the air but shielded from radiation and moisture  Wet bulb temperature, 𝒕𝑾𝑩 Thermometer’s bulb is covered by a wick that has been thoroughly wetted with water When the wet bulb is placed in an airstream, water evaporates from the wick, eventually reaching an equilibrium temperature called the wet-bulb temperature Chapter 2: Psychrometrics (4) THERMODYNAMIC WET-BULB AND DEW-POINT TEMPERATURE - The wick should be saturated - The air velocity around the wick should be from 5-10 m/s - Unsaturated air: 𝑡𝐷𝐵 >𝑡𝑊𝐵 - As 𝜑 = 100%  𝑡𝐷𝐵 = 𝑡𝑊𝐵 = 𝑡𝐷𝑃 - At the difference between 𝑡𝐷𝐵 and 𝑡𝑊𝐵 depends on the relative humidity  A sling psychrometer Chapter 2: Psychrometrics (4) THERMODYNAMIC WET-BULB AND DEW-POINT TEMPERATURE  Dew point temperature, 𝒕𝑫𝑷 The dew-point temperature 𝒕𝑫𝑷 is defined as the temperature at which condensation begins when the air is cooled at constant pressure 10 Chapter 2: Psychrometrics (7) TYPICAL AIR-CONDITIONING PROCESSES 7.4 Adiabatic Mixing of Water Injected into Moist Air Steam or liquid water can be injected into a moist airstream to raise its humidity, as shown in below figure If mixing is adiabatic, the following equations apply: 𝑚ሶ 𝑑𝑎 ℎ1 + 𝑚ሶ 𝑤 ℎ𝑤 = 𝑚ሶ 𝑑𝑎 ℎ2 𝑚ሶ 𝑑𝑎 𝑤1 + 𝑚ሶ 𝑤 = 𝑚ሶ 𝑑𝑎 𝑤2 Therefore ℎ2 − ℎ1 ∆ℎ = = ℎ𝑤 , kJ/g w 𝑤2 − 𝑤1 ∆𝑤 29 Chapter 2: Psychrometrics (7) TYPICAL AIR-CONDITIONING PROCESSES 7.4 Adiabatic Mixing of Water Injected into Moist Air Example 5: Moist air at 20°C dry-bulb and 8°C thermodynamic wet-bulb temperature is to be processed to a final dew-point temperature of 13°C by adiabatic injection of saturated steam at 110°C The rate of dry airflow is kgda/s Find the final dry- bulb temperature of the moist air and the rate of steam flow ℎ2 − ℎ1 ∆ℎ = = ℎ𝑔 = 2.691, kJ/g w 𝑤2 − 𝑤1 ∆𝑤 30 Chapter 2: Psychrometrics (7) TYPICAL AIR-CONDITIONING PROCESSES 7.5 Space Heat Absorption and Moist Air Moisture Gains Air conditioning required for a space is usually determined by (1) the quantity of moist air to be supplied, and (2) the supply air condition necessary to remove given amounts of energy and water from the space at the exhaust condition specified 31 Chapter 2: Psychrometrics (7) TYPICAL AIR-CONDITIONING PROCESSES 7.5 Space Heat Absorption and Moist Air Moisture Gains The quantity qs denotes the heat gain in the space, or sensible heat gain The quantity of σ 𝑚ሶ 𝑤 is called as Moisture gain Assuming steady-state conditions, governing equations are 𝑚ሶ 𝑑𝑎 ℎ1 + 𝑞𝑠 + ෍ 𝑚ሶ 𝑤 ℎ𝑤 = 𝑚ሶ 𝑑𝑎 ℎ2 𝑚ሶ 𝑑𝑎 𝑤1 + ෍ 𝑚ሶ 𝑤 = 𝑚ሶ 𝑑𝑎 𝑤2 or 𝑞𝑠 + ෍ 𝑚ሶ 𝑤 ℎ𝑤 = ℎ2 − ℎ1 𝑚ሶ 𝑑𝑎 ෍ 𝑚ሶ 𝑤 = 𝑤2 − 𝑤1 𝑚ሶ 𝑑𝑎 32 Chapter 2: Psychrometrics (7) TYPICAL AIR-CONDITIONING PROCESSES 7.5 Space Heat Absorption and Moist Air Moisture Gains Example 6: Moist air is withdrawn from a room at 25°C dry-bulb temperature and 19°C thermodynamic wet-bulb temperature The sensible rate of heat gain for the space is kW A rate of moisture gain of 0.0015 kgw/s occurs from the space occupants This moisture is assumed as saturated water vapor at 30°C Moist air is introduced into the room at a dry-bulb temperature of 15°C Find the required thermodynamic wet-bulb temperature and volume flow rate of the supply air 33 Chapter 2: Psychrometrics (7) TYPICAL AIR-CONDITIONING PROCESSES 7.5 Space Heat Absorption and Moist Air Moisture Gains Example 6: The specific enthalpy of the added water vapor is hg = 2555.58 kJ/kgw ∆ℎ + 0.0015 × 2555.58 = = 8555 kJ/kg w ∆𝑤 0.0015 ∆ℎ ∆𝑤 = 8.555 kJ gw −protractor Parallel to this reference line, draw a straight line on the chart through state The intersection of this line with the 15°C drybulb temperature line is state Thus, 𝑡𝑊𝐵 = 14.0°C 34 Chapter 2: Psychrometrics (7) TYPICAL AIR-CONDITIONING PROCESSES 7.5 Space Heat Absorption and Moist Air Moisture Gains Example 6: The flow of dry air is 𝑚ሶ 𝑑𝑎 𝑞𝑠 + σ 𝑚ሶ 𝑤 ℎ𝑤 + 0.0015 × 2555.58 = = = 0.873, 𝑘𝑔𝑑𝑎 /s ℎ2 − ℎ1 53.9 − 39.2 At state 1, 𝑣1 = 0.829 m3/kgda Therefore, supply volume 𝑉1 = 𝑣1 × 𝑚ሶ 𝑑𝑎 = 0.829 × 0.873 = 0.724 m3/s 35 Psychrometric charts online http://daytonashrae.org/psychrometrics /psychrometrics_si.html http://www.flycarpet.net/en/psyonline 36 Psychrometric charts online http://daytonashrae.org/psychrometrics /psychrometrics_si.html 37 Psychrometric charts online http://www.flycarpet.net/en/psyonline 38 The end 39 Homework The air in a room is at atm, 32C, and 60 percent relative humidity Use the psychrometric chart or available software Determine (a) the specific humidity; (b) the enthalpy (in kJ/kgda); (c) the wet-bulb temperature; (d ) the dew-point temperature; (e) the specific volume of the air (in m3/kgda) 40 Homework Air enters a 30-cm-diameter cooling section at atm, 35C, and 60 percent relative humidity at 120 m/min The air is cooled by passing it over a cooling coil through which cold water flows The water experiences a temperature rise of 8C The air leaves the cooling section saturated at 20C Determine (a) the rate of heat transfer, (b) the mass flow rate of the water, and (c) the exit velocity of the airstream 41 Homework Guidance: State 1: w1 = 0.02144 kgv/kgda ; h1 = 90.19 kJ/kgda ; v1 = 0.903 m3/kgda ; State 2: w2 = 0.0147 kgv/kgda ; h2 = 57.4 kJ/kgda ; v2 = 0.85 m3/kgda a The rate of heat transfer 𝑄ሶ = 𝑄ሶ 𝑎,𝑙𝑜𝑠𝑡 = 𝑄ሶ 𝑤,𝑔𝑎𝑖𝑛𝑒𝑑 𝑄ሶ = 𝑚ሶ 𝑑𝑎 ℎ𝑎,𝑜 − ℎ𝑎,𝑖 𝑉ሶ = ℎ − ℎ𝑎,𝑜 𝑣1 𝑎,𝑖 b The mass flow rate of water ҧ 𝑄ሶ = 𝑄ሶ 𝑤,𝑔𝑎𝑖𝑛𝑒𝑑 = 𝑚ሶ 𝑤 𝐶𝑝,𝑤 𝑡𝑤,𝑜 − 𝑡𝑤,𝑖 c The outlet velocity of airstream 𝑚ሶ 𝑑𝑎 = 𝑚ሶ 𝑑𝑎,𝑖 = 𝑚ሶ 𝑑𝑎,𝑜 𝑉1ሶ 𝑉ሶ2 𝜔2 𝑓 = = = → 𝜔2 𝑣1 𝑣2 𝑣2 42 Homework Two airstreams are mixed steadily and adiabatically The first stream enters at 32C and 40 percent relative humidity at a rate of 20 m3/min, while the second stream enters at 12C and 90 percent relative humidity at a rate of 25 m 3/min Assuming that the mixing process occurs at a pressure of atm, determine the specific humidity, the relative humidity, the drybulb temperature, and the volume flow rate of the mixture 43 ...