Psychrometrics lecture 1
Sponsored Links
This presentation is the property of its rightful owner.
1 / 14

Psychrometrics – Lecture 1 PowerPoint PPT Presentation


  • 94 Views
  • Uploaded on
  • Presentation posted in: General

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

Download Presentation

Psychrometrics – Lecture 1

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript


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

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

    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


  • Login