1 dr. c. l. jones biosystems and ag. engineering psychrometrics – lecture 2
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Dr. C. L. JonesBiosystems and Ag. Engineering
Psychrometrics – Lecture 2
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Dr. C. L. JonesBiosystems and Ag. Engineering
Psychrometrics
• Mollier vs. Traditional charts
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Dr. C. L. JonesBiosystems and Ag. Engineering
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
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Dr. C. L. JonesBiosystems and Ag. Engineering
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
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Dr. C. L. JonesBiosystems and Ag. Engineering
Psychrometrics
• Mollier vs. Traditional charts
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Dr. C. L. JonesBiosystems and Ag. Engineering
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.
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Dr. C. L. JonesBiosystems and Ag. Engineering
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
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Dr. C. L. JonesBiosystems and Ag. Engineering
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
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Dr. C. L. JonesBiosystems and Ag. Engineering
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
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Dr. C. L. JonesBiosystems and Ag. Engineering
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?
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Dr. C. L. JonesBiosystems and Ag. Engineering
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?
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Dr. C. L. JonesBiosystems and Ag. Engineering
Psychrometric Processes: Addition of Water at any state
• Changes enthalpy and humidity ratio
• Mass of water: (eqtn 9.100)
• Energy balance: (eqtn 9.101)
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Dr. C. L. JonesBiosystems and Ag. Engineering
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?