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Psychrometrics – Lecture 2

Psychrometrics – Lecture 2. Dr. C. L. Jones Biosystems and Ag. Engineering . Psychrometrics. Mollier vs. Traditional charts. Dr. C. L. Jones Biosystems and Ag. Engineering . Psychrometrics – State Points. Example: 35 °C dry bulb and 17° C wet bulb

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Psychrometrics – Lecture 2

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  1. Psychrometrics – Lecture 2 Dr. C. L. Jones Biosystems and Ag. Engineering

  2. Psychrometrics • Mollier vs. Traditional charts Dr. C. L. Jones Biosystems and Ag. Engineering

  3. Psychrometrics – 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° C Dr. C. L. Jones Biosystems and Ag. Engineering

  4. Psychrometrics – 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° C Dr. C. L. Jones Biosystems and Ag. Engineering

  5. Psychrometrics • Mollier vs. Traditional charts Dr. C. L. Jones Biosystems and Ag. Engineering

  6. Psychrometric Processes Application: condensation A 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. Dr. C. L. Jones Biosystems and Ag. Engineering

  7. Psychrometric 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% RH q = rate of heat transfer to air vapor mixture q = (Q/v) Δh Dr. C. L. Jones Biosystems and Ag. Engineering

  8. Psychrometric 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/s Dr. C. L. Jones Biosystems and Ag. Engineering

  9. Psychrometric 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 absorption Dr. C. L. Jones Biosystems and Ag. Engineering

  10. Psychrometric 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? Dr. C. L. Jones Biosystems and Ag. Engineering

  11. Psychrometric 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? Dr. C. L. Jones Biosystems and Ag. Engineering

  12. Psychrometric Processes: Addition of Water at any state • Changes enthalpy and humidity ratio • Mass of water: (eqtn 9.100) • Energy balance: (eqtn 9.101) Dr. C. L. Jones Biosystems and Ag. Engineering

  13. Psychrometric 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 plants? Dr. C. L. Jones Biosystems and Ag. Engineering

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