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

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

3 Dr. C. L. Jones Biosystems 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 = kg/kg dry air –Rh = 14 % –Dew point = 3° C

4 Dr. C. L. Jones Biosystems 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. = m^3/kg dry air –Humidity ratio = kg/kg dry air –Dew point = 21° C –Wet bulb = 25.6° C

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

6 Dr. C. L. Jones Biosystems 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.

7 Dr. C. L. Jones Biosystems 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

8 Dr. C. L. Jones Biosystems 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

9 Dr. C. L. Jones Biosystems 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

10 Dr. C. L. Jones Biosystems 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?

11 Dr. C. L. Jones Biosystems 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?

12 Dr. C. L. Jones Biosystems 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)

13 Dr. C. L. Jones Biosystems 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?