Psychrometrics. Outline. Psychrometrics. What is psychrometrics? Psychrometrics in daily life and food industry Psychrometric chart

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1 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 1

2 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 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% N 2 and ~20% O 2 (Ratio of N 2 :O 2 = 4:1) If 10 molecules of water are added to air, 8 molecules of N 2 and 2 molecules of O 2 are displaced Atomic weight gained by air = 10 x 18 = 180 Atomic weight lost by air = 8 x 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 2

3 Psychrometric Chart: 6 Quantities Dry bulb temperature, T db ( C) Temperature recorded by a regular thermometer Wet bulb temperature, T wb ( 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 (m 3 /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 T db & T wb, 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 (T dp ) 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 3

Hygrometers to Measure RH (and T wb, T dp ) Psychrometer Two thermometers; one has moist wick (sling or mounted with

similar to that in a bimetallic thermometer; inexpensive & not very accurate) Hair tension (human hair attached to

) Electronic Change in electrical resistance of LiCl or semiconductor Chilled mirror Optoelectronic mechanism; very

material) High RH: Sweating; promotes growth of mold during storage of foods Low RH: Static electricity 10 Hygrometers

4 Hygrometers to Measure RH (and T wb, T dp ) 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. wet wick dry bulb temp. handle is rotated Sling psychrometer 11 Psychrometric Chart (Low Temp.) 10 C to +55 C Page 819 of textbook Dry bulb Temperature ( C) Specific Volume (m 3 /kg dry air) 12 4

5 Psychrometric Chart (High Temp.) 20 C to 120 C Page 820 of textbook Dry bulb Temperature ( C) 13 Constant Dry Bulb Temperature Dry bulb Temperature ( C) 14 Const. Wet Bulb Temp. & Const. Enthalpy 15 5

6 Constant or Humidity Ratio 16 Constant Relative Humidity 17 Constant Specific Volume Specific Volume (m 3 /kg dry air) 18 6

7 Lines of Constant Psychrometric Parameters Dry bulb Temperature ( C) Specific Volume (m 3 /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) & reading the temp. at that point. Note: T db = T wb at this point < A 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 < A 21 7

8 Psychrometric Chart (Determining Properties) Given any two properties on the chart, the condition of airwatervapor mixture can be identified on the chart and hence the remaining properties can be determined. Dry bulb Temperature ( C) Exception: Constant enthalpy and constant wet bulb temperature lines are the same. Thus, given enthalpy & wet bulb temperature, we can not identify the point that depicts the properties of the air-watervapor mixture on the chart and hence the remaining properties can not be determined. 22 Mixing Two Streams of Air. A: m a kg/s. B: m b kg/s C: Conditions of mixture... L A to C : [m b /(m a + m b )]L... A to B L B to C : [m a /(m a + m b )]L A to B Dry bulb Temperature ( C) A C Specific Volume (m 3 /kg dry air) B Example: L A to B = 10 cm.. m a = 6 kg/s, m b = 2 kg/s Then, L A to C = [2/(2+6)]10 = 2.5 cm L B to C = [6/(2+6)]10 = 7.5 cm 23 Air + Tiny Particles of Product Cyclone Separator Dry Product Spray Dryer Wet Product Atomizer Moist Product (Atomized) C Heater Hot Dry Air B Warm Moist Air + Dry Product Blower Ambient air A Atomization involves breaking up a liquid product into tiny droplets by forcing the product & compressed air into an atomizer (disc with multiple slots at periphery that spins at a high rpm) at the TOP. This increases the surface area of the product, thereby increasing the rate of heat transfer, and thus the rate of evaporation. In this lab, we are using a 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 points B & C (product adds moisture to air at point B ) 24 8

9 Heating of Air (Constant or Humidity Ratio) Q: Why do we feel dry in a heated room?.. V Note : ma V' > A B Energy Balance: m. (H ) Q m. (H ) Dry bulb Temperature ( C) a A a B 25 Drying of Product (Constant Enthalpy & Wet Bulb Temp.) Adiabatic Process (Q = 0) If Q = 0 & work done = 0, then, H = Constant Part of sensible heat of air is converted to latent heat of water vapor; thus, temp. drops; m.c. inc. Water Balance: a B Dry bulb Temperature ( C) C m. (W ) m. (%moisture) m. (W ) p B a C 26 Heating Ambient Air & Drying a Product A: Ambient air B: Heated air C: Exit air (after heating product) Dry bulb Temperature ( C) C > A Energy Balance B 27 9

10 Further Applications of Psychrometrics: Heating, Cooling, Humidification, and Dehumidification Cooling & Humidification Humidification (Latent Heat Addition) Heating & Humidification Sensible Cooling Sensible Heating Cooling & dehumidification Heating & dehumidification Dehumidification (Latent Heat Removal) 28 Summary Psychrometric chart (6 quantities + dew point temp., T dp ) Dry bulb temperature, T db ( C) Wet bulb temperature, T wb ( C) Moisture content or specific humidity, W (kg water / kg dry air) Relative humidity, (%) Specific volume, V (m 3 /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. m.c. line (horizontal line); energy balance Drying of a product Const. enthalpy (inclined line const. T wb ); water balance 29 10

Week 4. Gas-vapor mixtures Air conditioning processes. ME 300 Thermodynamics II 1

Week 4. Gas-vapor mixtures Air conditioning processes. ME 300 Thermodynamics II 1 Week 4 Gas-vapor mixtures Air conditioning processes ME 300 Thermodynamics II 1 Today s Outline Gas-vapor mixtures Key definitions and measures ME 300 Thermodynamics II 2 Gas-vapor Mixtures At temperatures