Psychrometrics Lecture 1 Reading Chapter 9 in HendersonPerry

Psychrometrics – Lecture 1 • Reading: – Chapter 9 in Henderson/Perry Dr. C. L. Jones Biosystems and Ag. Engineering 1

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 2

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 3

Psychrometrics – Ideal Gas Law – For a fixed number of moles of an ideal gas: • p 1 V 1/T 1=p 2 V 2/T 2 • For Mixtures…Dalton’s Law pg. 238 Dr. C. L. Jones Biosystems and Ag. Engineering 4

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. 46057 ln. T (for 255. 38 to 273. 16 K) ln(psat/R’) = (A + BT + CT 2 + DT 3 + ET 4)/(FT – GT 2) (for 273. 16 to 2533. 16 K) K = C + 274. 15 Dr. C. L. Jones Biosystems and Ag. Engineering 5

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 6

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 7

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

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

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 10

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

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 5 o. C and the kitchen at 24 o. C, 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 24 o. C and 30% relative humidity is 4. 5 o. C. Since 4. 5 o. C < 5 o. C, condensation will not occur. Dr. C. L. Jones Biosystems and Ag. Engineering 12

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 13

Psychrometric Processes • Application example: power requirement for heaters • Example 9. 10 pg. 258 – v = 0. 8724 m^3/kg, – h 1 = 55. 8 k. J/kg, – h 2 = 76. 3 k. J/kg Dr. C. L. Jones Biosystems and Ag. Engineering 14