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

Psychrometrics – Lecture 5 Reading: Chapter 9 in Henderson/Perry Dr. C. L. Jones Biosystems and Ag. Engineering

Psychrometrics – Lectures 5, 6, & 7 Assignment: Read Chapter 9 in Henderson/Perry Dr. C. L. Jones Biosystems and Ag. Engineering

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

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

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

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

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

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

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

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

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

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

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

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

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