Psychrometrics – Lecture 1

1. Psychrometrics – Lecture 1 • Reading: • Chapter 9 in Henderson/Perry 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 = 31.9602 – 6270.3605/T – 0.46057lnT (for 255.38 to 273.16 K) ln(psat/R’) = (A + BT + CT2 + DT3 + ET4)/(FT – GT2) (for 273.16 to 2533.16 K) K = C + 274.15 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 = 0.8724 m^3/kg, • h1 = 55.8 kJ/kg, • h2 = 76.3 kJ/kg Dr. C. L. Jones Biosystems and Ag. Engineering