Psychrometrics – Lecture 2PsychrometricsPsychrometrics – State PointsSlide 4Slide 5Psychrometric Processes Application: condensationPsychrometric Processes Application: power requirement for heating hog confinement systemPsychrometric Processes: MixturesPsychrometric Processes: Adiabatic ProcessPsychrometric Processes: Adiabatic Process ExampleSlide 11Psychrometric Processes: Addition of Water at any stateSlide 131Dr. C. L. JonesBiosystems and Ag. EngineeringPsychrometrics – Lecture 22Dr. C. L. JonesBiosystems and Ag. EngineeringPsychrometrics•Mollier vs. Traditional charts3Dr. C. L. JonesBiosystems and Ag. EngineeringPsychrometrics – State Points•Example: 35°C dry bulb and 17° C wet bulb–Find enthalpy, sp. vol., humidity ratio, rh, dewpoint–Enthalpy, h = 48 kJ/kg dry air–Sp. Vol. = 0.88 m^3/kg dry air–Humidity ratio = 0.0046 kg/kg dry air–Rh = 14 %–Dew point = 3° C4Dr. C. L. JonesBiosystems and Ag. EngineeringPsychrometrics – State Points•Example: 42°C dry bulb and 30% rh–Find enthalpy, sp. vol., humidity ratio, rh, dewpoint–Enthalpy, h = 83.5 kJ/kg dry air–Sp. Vol. = 0.915 m^3/kg dry air–Humidity ratio = 0.0156 kg/kg dry air–Dew point = 21° C–Wet bulb = 25.6° C5Dr. C. L. JonesBiosystems and Ag. EngineeringPsychrometrics•Mollier vs. Traditional charts6Dr. C. L. JonesBiosystems and Ag. EngineeringPsychrometric Processes Application: condensationA pallet containing cartons of strawberries are removed from a cooler. The cooler is kept at 4° C. The room temperature is 27° C with a relative humidity of 20%. Will condensation form on the cartons of berries?Dewpoint for 27° C and 20% rh is 2.5° C which is less than 4° C. Therefore, condensation will not condense.Rule: if the object coming into the warm area is at a temperature less than the dew point of the warm area, water will condense on the colder object.7Dr. C. L. JonesBiosystems and Ag. EngineeringPsychrometric Processes Application: power requirement for heating hog confinement system–60 sows requiring 0.15 m^3/sec and 16°C–Air coming in: 10°C, 40% RHq = rate of heat transfer to air vapor mixtureq = (Q/v) Δh8Dr. C. L. JonesBiosystems and Ag. EngineeringPsychrometric Processes: Mixtures•State Point of an air-vapor mixture from mixing 2 air streams at different state points lies on a line connecting the 2 initial state points. (proof on page 259)•Air stream #1: 20 °C, 90%RH, 5 m^3/s •Air stream #2: 40 °C 20%RH, 10 m^3/s9Dr. C. L. JonesBiosystems and Ag. EngineeringPsychrometric Processes: Adiabatic Process•Adiabatic: a procedure whereby there is a change from one state to another without heat exchange between system and surroundings•Follows the wet bulb line, H changes, h stays the same •Examples: evap. cooling, dehumidification by absorption10Dr. C. L. JonesBiosystems and Ag. EngineeringPsychrometric Processes: Adiabatic Process Example•Moisture content of a large container of coconut must be reduce by removing moisture from the container. •Air at 40°C, 25% rh will be passed through the container at 13 m^3/s. •What is the maximum rate of moisture removal?11Dr. C. L. JonesBiosystems and Ag. EngineeringPsychrometric Processes: Adiabatic Process Example•If the original weight and moisture content of the coconut was 500 kg at 75% (wb), what will be the moisture content after 15 minutes of aeration?12Dr. C. L. JonesBiosystems and Ag. EngineeringPsychrometric Processes: Addition of Water at any state•Changes enthalpy and humidity ratio •Mass of water: (eqtn 9.100) •Energy balance: (eqtn 9.101)13Dr. C. L. JonesBiosystems and Ag. EngineeringPsychrometric Processes: Addition of Water at any state•Greenhouse plants like to be kept at 30°C and 95% rh. •To achieve this condition, steam at 160°C is injected into the ventilation air stream (20°C, 30% rh). •Mass flow rate of the air stream is 1800 kg/min and the mass flow rate of the steam is 5 kg/min. •Will this system achieve the ideal conditions for the
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