1. Chapter 13A: PSYCHROMETRIC PROPERTIES
Agami Reddy (rev- May 2017)
Definition and importance of psychrometrics
Gibbs-Dalton’s law of mixtures of perfect gases
Moist air properties of primary interest
Determining moist air properties from tables and equations
Correlations between properties
Psychrometric chart
- usefulness
- construction
- determining moist air properties
HCB 3-Chap 13A: Psych Properties

2. HCB 3-Chap 13A: Psych Properties
Psychrometrics
Definition: Study of the properties of mixtures of dry air and water vapor
Fundamental to most HVAC processes
Importance: Its proper understanding is critical for anyone involved in conditioning air supply to indoor spaces which need to be maintained at pre-specified temperature and humidity conditions
Ideal gas laws are adequate for determining properties since the partial pressure of water vapor is very small
Recall: Ideal gases are those that follow the law
pV=mRT
HCB 3-Chap 13A: Psych Properties

3. Change of Atmospheric Pressure with Altitude
Several different empirical equations exist. One can also use the ideal gas law.
Eq is one such correlation
Constant
H 1220 m (4000 ft)
H > 1220 m (4000 ft)
SI Units
IP Units a
29.92
99.436
29.42 b
-0.010
At Phoenix, elevation H=1120 ft, p= x 1120=28.57 “Hg=14.0 psi
HCB 3-Chap 13A: Psych Properties

4. HCB 3-Chap 13A: Psych Properties
Gibbs-Dalton’s Law of Mixtures
HCB 3-Chap 13A: Psych Properties

5. Important Moist Air Properties
(a) Dry bulb temperature: Tdb or DBT °C or °F
air temperature measured by a thermometer
(b) Partial pressure of water vapor (Pv or Pvap )
Do not confuse with saturated partial pressure Psat
which can be found from the steam tables
HCB 3-Chap 13A: Psych Properties

6. HCB 3- Chap 12B: Property Tables, Carnot Cycles and HX
Recall
HCB 3- Chap 12B: Property Tables, Carnot Cycles and HX

7. An expression for saturation pressure of water vapor
13.19
where temperature T is in absolute units
The critical pressure of water pc is 22.1 MPa (3226 psia),
and the critical temperature Tc is K ( °R)
And the parameter K is:
SI units:
IP units:
HCB 3-Chap 13A: Psych Properties

8. HCB 3- Chap 12B: Property Tables, Carnot Cycles and HX
Recall Property Diagram- note the critical point
Fig Pressure-volume diagram for water.
HCB 3- Chap 12B: Property Tables, Carnot Cycles and HX

9. HCB 3-Chap 13A: Psych Properties
Example
psia
Compare this value from that from the steam table: psia
HCB 3-Chap 13A: Psych Properties

10. HCB 3-Chap 13A: Psych Properties
(c) Relative humidity RH or
(13.8)
HCB 3-Chap 13A: Psych Properties

11. HCB 3-Chap 13A: Psych Properties
(d) Humidity Ratio or W is the ratio of mass of water vapor by mass of dry air
(kgw/kga , lbw/lba or grains/lba)
pounds/grains of water vapor per pound dry air
(7000 grains = 1 lbm)
(analogy to a sponge containing water)
HCB 3-Chap 13A: Psych Properties

12. HCB 3-Chap 13A: Psych Properties
(e) Wet Bulb Temperature (Twb, or WBT ) °C or °F
The humidity of the passing air affects how much water will evaporate, which affects temperature reading
Approximates adiabatic saturation process
Easy to measure
Water on sock evaporates, pushing the temperature of the sock
towards the adiabatic saturation temperature
Fig Apparatus for measuring wet and dry bulb temperatures
HCB 3-Chap 13A: Psych Properties

13. Moist Air Properties contd…
(f) Dew Point Temperature (Tdp, or TDP °C or °F)
the temperature at which the water vapor in the moist air would begin to condense if the moist air were cooled at constant pressure– the temperature T when pvap = psat
- When a surface reaches the dew point temperature of air, condensation begins
Condensation of water from air
on the outside of a cold glass of
water during summer
HCB 3-Chap 13A: Psych Properties

14. HCB 3-Chap 13A: Psych Properties
(g) Specific volume (v - m3/kg or ft3/lbm dry-air)
- the volume of moist air per unit mass of dry air
(h) Specific enthalpy (h or i J/kg or Btu/lbm dry-air)
- the enthalpy of moist air per unit mass of dry air
Specific enthalpy of moist air
h= hdry-air + hwater-vapor or
h= ha + W. hvap
(13.22)
HCB 3-Chap 13A: Psych Properties

15. HCB 3-Chap 13A: Psych Properties
Determining enthalpy: h
Perfect gas assumption: Cp = cte, Cv = cte and h = f(T)
(13.22)
HCB 3-Chap 13A: Psych Properties

16. HCB 3-Chap 13A: Psych Properties

17. HCB 3-Chap 13A: Psych Properties
For greater accuracy use table for Moist Air Properties
(note: this applies only to saturated conditions).
Table 13.3
HCB 3-Chap 13A: Psych Properties

18. HCB 3-Chap 13A: Psych Properties
Table of moist air properties used for more accurate calculation
Example: Find enthalpy at 60o F and 100% RH
From Table 13.3, h=26.47 Btu/lb
Compare this with that using equations
first determine W=0.011 from Table 13.3
From Eq. (13.23),
enthalpy h =0.24 x ( x 60)
= =26.36 Btu/lb
HCB 3-Chap 13A: Psych Properties

19. HCB 3-Chap 13A: Psych Properties
Definitions of Important Terms
Dew Point Temperature: The temperature of moist air saturated at the same pressure and humidity ratio. Or more simply the temperature at which water vapor will begin to condense from a sample of air.
Dry-Bulb Temperature: The temperature of air read on a standard thermometer indicating it's thermal state.
Enthalpy: The thermodynamic property defined as energy per unit mass of dry air commonly used to define the internal energy of moist air.
Humidity Ratio: The ratio of the mass of water vapor to the mass of dry air of a sample.
Relative Humidity: The ratio of mole fraction of water vapor in a given moist air sample to the mole fraction in a saturated air sample at the same temperature and pressure.
Specific Volume: The ratio of the total volume of air to the mass of dry air in a sample.
Wet-Bulb Temperature :The equilibrium temperature reached as water evaporates from a thoroughly wetted psychrometer wick into an airstream.
HCB 3-Chap 13A: Psych Properties

20. HCB 3-Chap 13A: Psych Properties
Correlations are used to determine certain properties knowing others:
Determining humidity ratio W when DBT and RH are known (along with total pressure)
Determining RH when DBT and WBT are known (along with total pressure)
Determining DPT knowing DBT (along with total pressure)
HCB 3-Chap 13A: Psych Properties

21. HCB 3-Chap 13A: Psych Properties
(a) Relationship between Humidity Ratio and DBT and RH
(13.15)
HCB 3-Chap 13A: Psych Properties

22. HCB 3-Chap 13A: Psych Properties
Psychrometric Chart
Extensively used by engineers
Chart displays DBT, WBT, W, RH, DPT, specific volume and enthalpy
Each point on the chart represents one moist air condition
Locating the point from known conditions will allow determining the other unknown conditions
One chart for a specified atmospheric pressure
Two additional independent states needed to fix state on chart
X-axis is dry-bulb temperature and
Y-axis is humidity ratio
HCB 3-Chap 13A: Psych Properties

23. HCB 3-Chap 13A: Psych Properties

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25. HCB 3-Chap 13A: Psych Properties
Fig. 13.6
HCB 3-Chap 13A: Psych Properties

26. Example: Use of Psych Chart
Locating other properties (IP units): Fill the table cells
DBT
WBT RH
Hum. Ratio
Enthalpy
Sp. Vol.
DPT
90 F
40%
60 F
55 F
70 F
0.01
75 F
0.011
HCB 3-Chap 13A: Psych Properties

27. HCB 3-Chap 13A: Psych Properties
Example
Locating other properties (IP units)
DBT
WBT RH
Hum. Ratio
Enthalpy
Specific volume
DPT
90 F
71 F
40%
0.0122 35
14.1
63 F
60 F
55 F 72
0.0081
23.8
13.3 51
70 F 62 65
0.01 28
13.5 57
75 F 60
0.011 30
13.7
Undetermined
HCB 3-Chap 13A: Psych Properties

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30. HCB 3-Chap 13A: Psych Properties
Outcomes
Knowledge of the definition and scope of psychrometrics
Understanding of the Gibbs-Dalton’s law of partial pressures
Understanding of the important moist air properties
Knowledge of how to use tables and equations to determine moist air properties under different conditions
Understanding of the relationships and correlations between certain properties
Understanding the importance of using psychrometric charts
Knowledge of how psychrometric Charts are generated
Be able to use the psychrometric chart to determine individual moist air properties
HCB 3-Chap 13A: Psych Properties