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Psychrometrics – Lecture 1. Reading: Chapter 9 in Henderson/Perry Handouts. 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

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psychrometrics lecture 1
Psychrometrics – Lecture 1
  • Reading:
    • Chapter 9 in Henderson/Perry
    • Handouts

Dr. C. L. Jones

Biosystems and Ag. Engineering

psychrometry
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
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 law4
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
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
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
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
Psychrometrics
  • Mollier vs. Traditional charts

Dr. C. L. Jones

Biosystems and Ag. Engineering

psychrometrics9
Psychrometrics
  • Mollier vs. Traditional charts pg 252-253

Dr. C. L. Jones

Biosystems and Ag. Engineering

psychrometrics state points
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

psychrometrics11
Psychrometrics
  • Mollier vs. Traditional charts

Dr. C. L. Jones

Biosystems and Ag. Engineering

psychrometric processes application condensation
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
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
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