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Lecture Objectives

This lecture reviews psychrometrics, key variables like temperature and humidity, and their use in HVAC systems design. Topics include moist air thermodynamics, psychrometric quantities, and practice for quizzes. Learn about heating and cooling loads, dew point temperature, wet bulb temperature, and more. Understand the concept of relative humidity, latent vs. sensible heat, and the psychrometric chart. Examples on calculating sensible and latent heat, energy transfer equations, and the importance of calculating heating and cooling loads in building design are covered. Explore the principles of heating, cooling, humidification, and dehumidification processes in HVAC systems.

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Lecture Objectives

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  1. Lecture Objectives • Review: Psychrometrics (chart and quantities) • Provide example on how we use it for building systems design and analyses • Psychrometric and HVAC • Practice for the Quiz • Define Heating and Cooling Load

  2. Thermodynamics of Moist air orPsychrometrics Variables • Temperature • Relative Humidity • Absolute Humidity • Enthalpy (total energy) • Dew Point Temperature • Wet Bulb Temperature • ….

  3. Temperatures • Absolute Temperature (T) (K, R) • Dry-bulb temperature (t) [°F, °C] • Wet-bulb temperature (t*) • Dew-point temperature (td)

  4. Wet-bulb Dry-bulb Which temperature do you expect to be higher?

  5. Wet-bulb temperature (t*) • Temperature measured by a psychrometer • Lower than dry-bulb temperature • Evaporating moisture removes heat from thermometer bulb • The higher the humidity • Smaller difference between wet-bulb and dry-bulb temperature

  6. Dew-Point Temperature, td • Define temperature at which condensation happen • td is defined as temperature of that air at saturation • i.e. RH = 100% • Surfaces below the dew point temperature will have condensation • Measured with a chilled-mirror apparatus

  7. Absolute Humidityor Humidity Ratio • Humidity ratio (W) [lb/lb, g/kg, grains] [grains/lb = 1/7000 lb/lb]

  8. Humidity Ratio, W • Mass of water vapor/divided by mass of dry air • Orthogonal to temperature • Not a function of temperature • Most convenient form for calculations involving airflow • Very hard to measure directly

  9. Relative Humidity, RH or  • Relative humidity (RH, ) [%] • Ratio of partial pressure of water vapor to partial pressure of water vapor at same T and P at saturation • Strong function of temperature • For constant humidity ratio • Higher temperature, lower relative humidity • Saturation

  10. Enthalpy or Total Energy • Enthalpy h or i [J/kg] or [Btu/lb] • Defines amount of energy contend in moist air

  11. Enthalpy

  12. Psychrometric Chart

  13. Sensible vs. Latent Heat latent sensible

  14. ASHRAE Comfort Zone

  15. New ASHRAE Comfort Zone

  16. Psychrometric Chart • Make sure chart is appropriate for your environment • Figure out what two quantities you know • Understand their slopes on the chart • Find the intersection • Watch for saturation

  17. We will have our first Quizon Thursday • First 10 minutes of the class • An example is provided in the handout section of the course website • At the end of the class we will solve several examples

  18. If you know the dew point temperature (td) and the dry bulb temperature (t) for a sample of air • You can’t get the statepoint because the problem is overspecified (you know the RH = 100%, t and td). • You get the state point by the intersection of the t and td lines. • You get the state point by moving horizontally from td until you intersect the t line • You get the state point by moving vertically from td until you intersect the t line

  19. Examples: 1) You heat one pounds of air air A (T=50F, W=0.009 lbW/lbDA) to point T=80F and humidify it to RH 70%. What is the sensible, latent and total heat added to the one pound of air. 2) One pound of air D(T=90F, RH=30%) is humidified by adiabatic humidifier to 90% relative humidity. What is the temperature at the end of humidification process and how much water is added to the air.

  20. Process in HVAC systems • Heating • Cooling • Humidification • Dehumidification All these processes ca be quantified in Psychrometric Chart Also, all the these quantities can be with and without help of the Psychrometric Chart

  21. Equations for sensible energy transport by air • Energy per unit of mass Δhsensible = cp×ΔT [Btu/lb] cp - specific heat for air (for air 0.24 Btu/lb°F) • Heat transfer (rate) Qs = m × cp×ΔT [Btu/h] m - mass flow rate [lb/min, lb/h], m = V ×r V – volume flow rate [ft3/min or CFM] r – airdensity (0.076lb/ft3) Qs = 1.1 × CFM ×ΔT (only for IP unit system)

  22. Equations for latent energy transport by air • Energy per unit of mass Δhlatent = Δw×hfg[Btu/lbda] hfg - specific energy of water phase change (1000 Btu/lbw) • Heat transfer (rate) Ql = m ×Δw×hfg [Btu/h] Ql = 1000 × WaterFloowRate (only for IP units)

  23. Total energy transport calculation using enthalpies from chat • Energy per unit of mass Δh=h1-h2[Btu/lbda] • Heat transfer (rate) Qtotal = m ×Δh[Btu/h] Qtotal = Qsensible + Qlatent

  24. Why do we calculate heating and cooling loads? Heating and Cooling Loads To estimate amount of energy used for heating and cooling by a building Or To size heating and cooling equipment for a building

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