1. Psychrometrics – Lecture 1
Reading:
Chapter 9 in Henderson/Perry
Additional handouts given in class
HW#12 Due 4/21
Dr. C. L. Jones
Biosystems and Ag. Engineering

2. Psychrometry
Study of physical and thermal properties of air and water vapor mixtures.
Simply put: the science of drying
Dr. C. L. Jones
Biosystems and Ag. Engineering

3. Psychrometrics – Ideal Gas Law
Where:
p = absolute pressure
V = volume
N = number of kg-moles
R = universal gas constant
T = absolute temperature (K)
Dr. C. L. Jones
Biosystems and Ag. Engineering

4. Psychrometrics – Ideal Gas Law
For a fixed number of moles of an ideal gas:
p1V1/T1=p2V2/T2
For Mixtures…Dalton’s Law pg. 238
Dr. C. L. Jones
Biosystems and Ag. Engineering

5. Psychrometrics – Definition of Terms
Atmospheric air is always a mixture of dry air and water vapor. Physical and thermal quantities describe the state of the mixture…
Dry-bulb temperature: true temp. of the mixture
Saturation pressure: pressure where liquid and vapor forms of water are in equilibrium… evaporation rate = condensation rate
Use standard steam table or calculate (9.18)
ln psat = – /T – lnT (for to K)
ln(psat/R’) = (A + BT + CT2 + DT3 + ET4)/(FT – GT2) (for to K)
K = C
Dr. C. L. Jones
Biosystems and Ag. Engineering

6. Psychrometrics – more terms
Humidity ratio: mass of water vapor per unit mass of dry air
Relative humidity: measure of amount of water air can hold at a certain temperature
Specific Volume: vol. per unit mass of dry air at the state of the mixture…iow…space occupied by air…greater volume at warmer temperatures
Dr. C. L. Jones
Biosystems and Ag. Engineering

7. Psychrometrics – yet more terms
Dew-Point Temperature: temperature at which water will begin to condense out of moist air. (constant humidity ratio and pressure)
Wet-bulb Temperature: determined when air is circulated past a wet sensor tip. Temp. at which water evaporates and brings the air to saturation. (constant enthalpy) (adiabatic process)
Enthalpy: heat energy content of moist air
Dr. C. L. Jones
Biosystems and Ag. Engineering

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

9. Psychrometrics Mollier vs. Traditional charts pg 252-253
Dr. C. L. Jones
Biosystems and Ag. Engineering

10. Psychrometrics – State Points
Find a point using two properties…
Can find all of the other 5 properties
This point is called the “state point”
Example: 25°C dry bulb and 18° C wet bulb
Find enthalpy, sp. vol., humidity ratio, rh, dewpoint
Dr. C. L. Jones
Biosystems and Ag. Engineering

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

12. Psychrometric Processes Application: condensation
A carton of eggs was removed from the refrigerator and left on the kitchen counter. The refrigerator was set at a temperature of 5oC and the kitchen at 24oC, 30% relative humidity.
Will condensation occur (i.e., "sweat" appear on eggs)?
(def. of dew point: temperature at which water will begin to condense out of moist air)
SOLUTION
The corresponding dew-point temperature at 24oC and 30% relative humidity is 4.5oC. Since 4.5oC < 5oC, condensation will not occur.
Dr. C. L. Jones
Biosystems and Ag. Engineering

13. Psychrometric Processes Application: power requirement for heaters
Example 9.10 pg. 258
Mass flow rate m = Q/v,
Q=vol. flow rate,
v=spec. vol. of air
q = m(Δh) where q = rate of heat transfer
So: q = (Q/v) Δh
Dr. C. L. Jones
Biosystems and Ag. Engineering

14. Psychrometric Processes
Application example: power requirement for heaters
Example 9.10 pg. 258
v = m^3/kg,
h1 = 55.8 kJ/kg,
h2 = 76.3 kJ/kg
Dr. C. L. Jones
Biosystems and Ag. Engineering