Psychrometrics

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

• Mollier vs. Traditional charts

Dr. C. L. Jones Biosystems and Ag. Engineering
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Psychrometrics State Points

• Example 35C 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 m3/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
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Psychrometrics State Points

• Example 42C 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 m3/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
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Psychrometrics

• Mollier vs. Traditional charts

Dr. C. L. Jones Biosystems and Ag. Engineering
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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
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Psychrometric Processes Application power
requirement for heating hog confinement system

• 60 sows requiring 0.15 m3/sec and 16C
• Air coming in 10C, 40 RH
• q rate of heat transfer to air vapor mixture
• q (Q/v) ?h

Dr. C. L. Jones Biosystems and Ag. Engineering
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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, 90RH, 5 m3/s
• Air stream 2 40 C 20RH, 10 m3/s

Dr. C. L. Jones Biosystems and Ag. Engineering
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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
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Psychrometric Processes Adiabatic Process
Example

• Moisture content of a large container of coconut
  must be reduce by removing moisture from the
  container.
• Air at 40C, 25 rh will be passed through the
  container at 13 m3/s.
• What is the maximum rate of moisture removal?

Dr. C. L. Jones Biosystems and Ag. Engineering
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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
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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
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Psychrometric Processes Addition of Water at
any state

• Greenhouse plants like to be kept at 30C and 95
  rh.
• To achieve this condition, steam at 160C is
  injected into the ventilation air stream (20C,
  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