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 or Psychrometrics
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)

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

6. 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

8. 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

10. Absolute Humidity or Humidity Ratio
Humidity ratio (W) [lb/lb, g/kg, grains]
[grains/lb = 1/7000 lb/lb]

11. 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

13. Relative Humidity, RH or 
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

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

16. Enthalpy

17. Psychrometric Chart

18. Sensible vs. Latent Heat

19. ASHRAE Comfort Zone

20. New ASHRAE Comfort Zone

21. 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

22. We will have our first Quiz on 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

23. 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

24. 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.

25. 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

26. 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 – air density (0.076lb/ft3)
Qs = 1.1 × CFM × ΔT (only for IP unit system)

27. 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)

28. 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

29. Why do we calculate heating and cooling loads?
10/21/2003
Heating and Cooling Loads
Why do we calculate 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
ARE 346N