1. Topics in Processing
Dr. C. L. Jones
Biosystems and Ag. Engineering

2. Psychrometrics – Lectures 5, 6, & 7
Assignment:
Read Chapter 9 in Henderson/Perry
HW # 3 Due 9/12/06
HW #4 Due 9/14/06
Dr. C. L. Jones
Biosystems and Ag. Engineering

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

4. 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 = kg/kg dry air
Rh = 14 %
Dew point = 3° C
Dr. C. L. Jones
Biosystems and Ag. Engineering

5. 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. = m^3/kg dry air
Humidity ratio = kg/kg dry air
Dew point = 21° C
Wet bulb = 25.6° C
Dr. C. L. Jones
Biosystems and Ag. Engineering

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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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