1. Chapter 13B: PSYCHROMETRIC MOIST AIR PROCESSES
Agami Reddy (rev- May 2017)
Overall conservation equations
Adiabatic mixing of air streams
Sensible heating and cooling
Adiabatic cooling
Cooling and dehumidification
- Cooling coil process line
- Bypass factor
-Apparatus dew point
Heating and humidification
Various conditioning processes
HCB 3-Chap 13B: Psych Processes

2. HCB 3-Chap 13B: Psych Processes
Conservation Equations
Fig. 13.8
(13.26)
(13.27)
(13.28)
HCB 3-Chap 13B: Psych Processes

3. HCB 3-Chap 13B: Psych Processes
Adiabatic Mixing of Air Streams
Resulting in:
(13.33)
(13.34)
Fig. 13.9
HCB 3-Chap 13B: Psych Processes

4. HCB 3-Chap 13B: Psych Processes
12.65
HCB 3-Chap 13B: Psych Processes

5. HCB 3-Chap 13B: Psych Processes

6. HCB 3-Chap 13B: Psych Processes
Contd.
HCB 3-Chap 13B: Psych Processes

7. HCB 3-Chap 13B: Psych Processes
Sensible Heating or Sensible Cooling
(13.36)
(13.37)
HCB 3-Chap 13B: Psych Processes

8. HCB 3-Chap 13B: Psych Processes
Example 13.9: Sensible Heating
Fig
HCB 3-Chap 13B: Psych Processes

9. HCB 3-Chap 13B: Psych Processes
Contd.
HCB 3-Chap 13B: Psych Processes

10. HCB 3-Chap 13B: Psych Processes
HVAC calculations usually involve volume flow rates: Volumetric flow rates change during A/C
Sensible heating example : An air handling unit conditions (i.e., in this case heats) 1000 CFM outdoor air from 40 F and 40 % RH to 70 F. Determine the outlet CFM considering the change in specific volume with temperature
Solution: From the Psychrometric chart:
v at 40 F = 12.6 ft3/lbm, and
v at 70 F = 13.4 ft3/lbm
Hence: outlet cfm = 1000 cfm x 13.4/12.6= 1063 cfm
(a 6.3% increase in volumetric flow rate)
HCB 3-Chap 13B: Psych Processes

11. HCB 3-Chap 13B: Psych Processes
Example 13.12: Residential Evaporative Cooler
Fig
HCB 3-Chap 13B: Psych Processes

12. HCB 3-Chap 13B: Psych Processes
Contd.
HCB 3-Chap 13B: Psych Processes

13. HCB 3-Chap 13B: Psych Processes
Cooling and Dehumidification
(13.41)
(13.42)
Fig
(13.43)
(13.44)
HCB 3-Chap 13B: Psych Processes

14. HCB 3-Chap 13B: Psych Processes
Example 13.14: Cooling of Moist Air
Fig
HCB 3-Chap 13B: Psych Processes

15. HCB 3-Chap 13B: Psych Processes
Contd.
HCB 3-Chap 13B: Psych Processes

16. HCB 3-Chap 13B: Psych Processes
Determining Sensible and Latent Loads Separately (contd...)
HCB 3-Chap 13B: Psych Processes

17. HCB 3-Chap 13B: Psych Processes
Simplified Approach:
Let us solve the previous Example using these equations:
HCB 3-Chap 13B: Psych Processes

18. HCB 3-Chap 13B: Psych Processes
Simplified Equations used by Industry Professionals:
Amount of water condensed: lb/h
4.5 =(0.075 lb/ft3) x (1 ft3/min) x 60 min/h
Sensible heat equation: Btu/h
=1.08
Where T is in 0F
1.08=(0.075 lb/ft3) x (1 ft3/min) x 60 min/h x 0.24 Btu/lb-oF
Latent heat equation: Btu/h
Where W is in grains/lb
0.68= (1075 Btu/lb)/7000 gr/lb x 4.5
HCB 3-Chap 13B: Psych Processes

19. Cooling Coil Process Line
(13.47)
HCB 3-Chap 13B: Psych Processes

20. Also called “coil dew point temp.”
(13.48)
Fig
HCB 3-Chap 13B: Psych Processes

21. HCB 3-Chap 13B: Psych Processes
For previous example 13.14
Contd.
Manufacturers do not design coils with zero by-pass factors because higher contact factor increase fan pressure drop across the coil and hence require higher fan electric power.  Typical ranges for the BP factors:
for coils in residences
for those in commercial buildings.
HCB 3-Chap 13B: Psych Processes

22. HCB 3-Chap 13B: Psych Processes
Heating and Humidification
(13.51)
(13.52)
Fig
HCB 3-Chap 13B: Psych Processes

23. HCB 3-Chap 13B: Psych Processes
Example 13.15: Warming and Humidification
and specific volume =12.65 ft3/lba
HCB 3-Chap 13B: Psych Processes

24. HCB 3-Chap 13B: Psych Processes
Contd. 2 1
HCB 3-Chap 13B: Psych Processes

25. HCB 3-Chap 13B: Psych Processes
Lookup values: 
Saturated steam enthalpy at 212°F, hvap = Btu/lbw (which is equal to hliq,2).
From psychrometric chart,
h1 = 13 Btu/lba, v1 = ft3/lba,
W1 = lbw/lba, h3=30 Btu/lba
Fig
HCB 3-Chap 13B: Psych Processes

26. HCB 3-Chap 13B: Psych Processes
Contd.
HCB 3-Chap 13B: Psych Processes

27. HCB 3-Chap 13B: Psych Processes
Fig Different psychrometric processes
HCB 3-Chap 13B: Psych Processes

28. HCB 3-Chap 13B: Psych Processes
Outcomes
Understanding of the laws of conservation applicable to conditioning moist air streams
Knowledge of the important air conditioning processes common in HVAC
Be able to solve problems involving various air conditioning processes
Be able to plot the various air conditioning processes on a psychrometric chart and solve related problems
Understanding the concepts of coil sensible heat factor, coil dew point temperature, and by-pass factor
Understanding the directional process changes of various air conditioning processes as plotted on a psychrometric chart
HCB 3-Chap 13B: Psych Processes