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Psychrometrics Outline

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Psychrometrics Outline Psychrometrics Outline • What is psychrometrics? • Psychrometrics in daily life and food industry • Psychrometric chart – Dry bulb temperature, wet bulb temperature, absolute humidity, relative humidity, specific volume, enthalpy – Dew point temperature • Mixing two streams of air • Heating of air and using it to dry a product 2 Psychrometrics • Psychrometrics is the study of properties of mixtures of air and water vapor • Water vapor – Superheated steam (unsaturated steam) at low pressure – Superheated steam tables are on page 817 of textbook – Properties of dry air are on page 818 of textbook – Psychrometric charts are on page 819 & 820 of textbook • What are these properties of interest and why do we need to know these properties? 3 Psychrometrics in Daily Life • Sea breeze and land breeze – When and why do we get them? • How do thunderstorms, hurricanes, and tornadoes form? • What are dew, fog, mist, and frost and when do they form? • When and why does the windshield of a car fog up? – How do you de-fog it? Is it better to blow hot air or cold air? Why? • Why do you feel dry in a heated room? – Is the moisture content of hot air lower than that of cold air? • How does a fan provide relief from sweating? • How does an air conditioner provide relief from sweating? • When does a soda can “sweat”? • When and why do we “see” our breath? • Do sailboats perform better at high or low relative humidity? Key factors: Temperature, Pressure, and Moisture Content of Air 4 Do Sailboats Perform Better at low or High RH? • Does dry air or moist air provide more thrust against the sail? • Which is denser – humid air or dry air? – Avogadro’s law: At the same temperature and pressure, the no. of molecules in a given volume is the same, no matter what the gas is – Air has ~80% N2 and ~20% O2 (Ratio of N2:O2 = 4:1) – If 10 molecules of water are added to air, 8 molecules of N2 and 2 molecules of O2 are displaced – Atomic weight gained by air = 10 x 18 = 180 – Atomic weight lost by air = 8 x 28 + 2 x 32 = 288 – Thus, there is a net decrease in weight and hence density – So, humid air is less dense than dry air Thus, there is more thrust for the sail with dry air and hence sailboats perform better at low RH 5 Psychrometrics in the Food Industry • Heating or cooling of air – To heat or cool a product • Mixing different streams of air • Drying a food product using hot and dry air – Drier the air, better the drying efficiency 6 Psychrometric Chart: 6 Quantities • Dry bulb temperature, Tdb (°C) – Temperature recorded by a regular thermometer • Wet bulb temperature, Twb (°C) – Temp. of a thermometer with air blowing over a moist wick on its bulb • Moisture content or specific humidity, W (kg water / kg dry air) – Amount of moisture in air (also called, absolute humidity) • Relative humidity (RH), (%) – Ratio of amount of moisture in air to max. amt. of moisture air can hold • Specific volume, V’ (m3/kg dry air) – Volume of moist air per unit mass of dry air (specific volume = 1/density) • Enthalpy, H (kJ/kg dry air) – Energy content of air Note 1: Each psychrometric chart is created at some constant pressure (most are for atmospheric pressure). So, psychrometric charts can not be used to analyze processes in which the pressure changes. Note 2: Human comfort zone is at ~70-80 °F & ~40-60% RH 7 Measurement of Wet Bulb Temperature • Place a moist wick over the bulb of a mercury thermometer • Blow air at high speed over the wick • High energy water molecules from the wick evaporates since vapor pressure of water vapor near the wick is higher than that of the bulk surrounding air • Latent heat for evaporation (of high energy water molecules) is removed from the wick, causing a decrease in temperature • As the temperature of the wick decreases, sensible heat from air flows to it • Equilibrium is attained when latent heat lost from the wick equals sensible heat flowing into the wick Note 1: If the relative humidity of the surrounding air is 100%, moisture will NOT evaporate from the wick and hence the reading of the wet bulb & dry bulb thermometers will the same Note 2: Greater the difference between Tdb & Twb, lower the RH of the surrounding air Note 3: This evaporative cooling principle provides cooling of water in an earthen pot 8 Dew Point Temperature (Tdp) • It is the temperature at which moisture in a mixture of water vapor and air begins to condense (or form “dew”) when cooled Q: Why does a soda can “sweat”? Q: When and why do we “see” our breath? 9 Hygrometers to Measure RH (and Twb, Tdp) • Psychrometer – Two thermometers; one has moist wick (sling or mounted with fan) • Mechanical – Metal-paper coil (paper strip attached to metal coil; coil changes shape with moisture; dial rotates similar to that in a bimetallic thermometer; inexpensive & not very accurate) – Hair tension (human hair attached to spring & dial; hair swells as RH inc.) • Electronic – Change in electrical resistance of LiCl or semiconductor • Chilled mirror – Optoelectronic mechanism; very accurate • Capacitive, resistive, thermal conductivity, gravimetric • Other (Change in weight, volume or transparency of a material) High RH: Sweating; promotes growth of mold during storage of foods Low RH: Static electricity 10 Hygrometers (contd.) wet bulb temp. dry bulb temp. wet wick handle is rotated Sling psychrometer 11 Psychrometric Chart (Low Temp.) ‐10 °C to +55 °C Page 819 of textbook factor heat Sensible air) Content dry (g/kg Dry bulb Temperature (°C) 3 Moisture Specific Volume (m /kg dry air) 12 Psychrometric Chart (High Temp.) 20 °C to 120 °C Page 820 of textbook air) Content dry (g/kg Moisture Dry bulb Temperature (°C) 13 Constant Dry Bulb Temperature factor heat Sensible Dry bulb Temperature (°C) 14 Const. Wet Bulb Temp. &Const.Enthalpy Temp. Bulb Wet Const. 15 Sensible heat factor Constant Moisture Content or HumidityRatio Contentor Constant Moisture Moisture Content 16 (g/kg dry air) Sensible heat factor Constant RelativeHumidity 17 Sensible heat factor Constant Specific Volume factor heat Sensible Specific Volume 3 (m /kg dry air) 18 Lines of Constant Psychrometric Parameters factor heat Sensible air) Content dry Dry bulb Specific Volume (g/kg 3 Moisture Temperature (°C) (m /kg dry air) 19 Dew Point Temperature Dew point temp. of air at “A” is determined by moving horizontally to the left and intersecting the 100% RH line (saturation temp. line) & factor reading the temp. at that point. heat A Note: Tdb = Twb at this point < Sensible * air) Content dry (g/kg Moisture 20 Cooling Air Below its Dew Point Temperature When warm air contacts a cold surface that is below its dew point temp., moisture from the air condenses onto the surface of the cold surface factor heat A < * Sensible air) Content dry (g/kg Moisture 21 Psychrometric Chart (Determining Properties) Given any two properties on the chart, the condition of air- watervapor mixture can be identified on the chart and hence the remaining properties can be determined. air) Exception: Constant enthalpy dry and constant wet bulb (g/kg temperature lines are the same. Thus, given enthalpy & wet bulb Content temperature, we can not identify Moisture the point that depicts the properties of the air-watervapor mixture on the chart and hence the remaining properties can not be determined. Dry bulb Temperature (°C) 22 Mixing Two Streams of Air . A: m kg/s . a B: mb kg/s B C: Conditions of mixture .. factor L : [m /(m + m )]*L A to C ..b a .b A to B C * L : [m /(m + m )]*L heat B to C a a b A to B A * * Sensible Example: L = 10 cm ..A to B ma = 6 kg/s, mb = 2 kg/s air) Then, L = [2/(2+6)]*10 = 2.5 cm Content A to C dry LB to C = [6/(2+6)]*10 = 7.5 cm Dry bulb Specific Volume (g/kg 3 Moisture Temperature (°C) (m /kg dry air) 23 Spray Dryer “Atomization” involves Air + Tiny breaking up a liquid Wet Product product into tiny droplets Particles Heater of Product Blower by forcing the product & compressed air into an atomizer (disc with Atomizer Hot Dry Air multiple slots at periphery B that spins at a high rpm) at Cyclone Ambient air the TOP. This increases the Separator A surface area of the product, Moist Product thereby increasing the rate (Atomized) of heat transfer, and thus the rate of evaporation. In Dry this lab, we are using a Product nozzle at the CENTER instead of a true atomizer at the TOP. Spray dryer calculations involve: 1. Energy balance equation for air between points ‘A’ & ‘B’ (heater adds energy to air at point ‘A’) 2. Water balance equation for air between C points ‘B’ & ‘C’ (product adds moisture to air at point ‘B’) Warm Moist Air + Dry Product 24 Heating of Air (Constant Moisture Content or Humidity Ratio) Q: Why do we feel dry in a heated room? . factor . V Note : m heat a V' Sensible > air) * * Content A B dry .. Energy Balance: ma (HA ) Q ma (HB ) Dry bulb (g/kg Moisture Temperature (°C) 25 Drying of Product (Constant Enthalpy & Wet Bulb Temp.) Adiabatic Process (Q = 0) If Q = 0 & work done = 0, factor then, H = Constant Part of sensible heat of air heat is converted to latent heat of water vapor; thus, temp. Sensible drops; m.c. inc. C* air) * Content B dry . .. Water Balance: ma (WB ) mp (%moisture) ma (WC ) Dry bulb (g/kg Moisture Temperature (°C) 26 Heating Ambient Air & Drying a Product A: Ambient air B: Heated air C: Exit air (after heating product) factor heat Sensible C* > air) * Energy Balance * Content A B dry Dry bulb (g/kg Moisture Temperature (°C) 27 Further Applications of Psychrometrics: Heating, Cooling, Humidification, and Dehumidification Humidification (Latent Heat Addition) Cooling & Humidification Heating & Humidification Sensible Cooling * Sensible Heating Cooling & dehumidification Heating & dehumidification Dehumidification (Latent Heat Removal) 28 Summary • Psychrometric chart (6 quantities + dew point temp., Tdp) – Dry bulb temperature, Tdb (°C) – Wet bulb temperature, Twb (°C) – Moisture content or specific humidity, W (kg water / kg dry air) – Relative humidity, (%) – Specific volume, V’ (m3/kg dry air) – Enthalpy, H (kJ/kg dry air) • Mixing of two streams of air – Straight line split in the inverse ratios of mass flow rates • Heating of air – Const.
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