1. Psychrometric analysis Revision of principles - Central plant
for
UBCLQT-40-3: Building & Engineering Systems Performance
and
UBLLSM-20-3: Mechanical Services

2. Psychrometric quantities
temperature
dry bulb, t [oC d.b.]
wet bulb, t’ [oC w.b.]
relative humidity,  [%rh] = avp / svp
percentage saturation, μ [% sat.] = ms/mss
moisture content, x [kg.kg-1dry air]
specific volume, v [m3.kg-1 dry air] = 1/
specific enthalpy, h [kJ.kg-1 dry air]
dew point temperature, td [oC]

3. Psychrometric chart Lines of constant d.b. temp.
Dry bulb temperature, t

4. Psychrometric chart Lines of constant moisture content
Moisture content, x

5. Psychrometric chart percentage saturation , 
Lines of constant relative humidity, 
(percentage saturation)

6. Psychrometric chart Wet bulb temperature, t’
Lines of constant w.b. temp.
Wet bulb temperature, t’
5oC wb
5oC db

7. Psychrometric chart Lines of constant specific volume
specific volume, v

8. Psychrometric chart Specific enthalpy, h ‘Lines’ of constant

9. Psychrometric chart
Dew point temperature, td oC

10. State point
Any two psychrometric quantities can be used to define a state point. All other properties can be read from the chart
50 %rh
26.5 kJ.kg-1
0.82 m3.kg-1
8.9oC db
kg.kg-1
14oC db

11. Exercise - complete Table 1

12. Exercise – Solutions

13. Ideal processes Isothermal humidification Adiabatic humidification
Sensible heating
Sensible cooling
Adiabatic dehumidification
Isothermal dehumidification

14. Real processes - Sensible Heating
Heating load, QH = ma (h2 - h1)
h2 - h1 1 2 
Heater Battery

15. Heating load, QH = ma (h2 - h1)
Real processes - Sensible Heating
Plant Loads
Example 1:
Air flows at a rate of 3.42 kg.s-1 into a heater battery.
If the initial condition of the air is state 5 (see Table 1) and the air leaves the heater battery at a temperature of 31 oC d.b., determine the load on the heater battery in kW.
Heating load, QH = ma (h2 - h1)

16. Real processes - Sensible cooling
Cooling load, QC = ma (h1 - h2)
(Nominal)
h1 - h2 2 1 
Cooler Battery

17. Real processes - Cooling & Dehum.1 2 
Cooler Battery

18. Real processes - Cooling & Dehum.
Cooling load, QC = ma (h1 - h2)
(Nominal)
h1 - h2 1 2 
Cooler Battery

19. Real processes - Cooling & Dehum.
Cooling load, QC = ma (h1 - h2)
(Nominal)
h1 - h2 1 2 
Cooler Battery

20. Contact Factor
Cooling load, QC = ma (h1 - h2)
h1 - h2 1 2 s
ADP t

21. Real processes – Cooling with dehum
Plant Loads
Example 2:
A cooler battery takes a flow of 1.5 kg/s of air at 25oC d.b. with a moisture content of 11.3 g/kg and dehumidifies to obtain a moisture content of air leaving the battery of 8 g/kg.
If the cooler contact factor is 85%, determine the apparatus dew point and the nominal cooling load in kW.
Cooler Contact Factor,

22. Real processes - adiabatic humidifier
Adiabatic process - where no energy transfer occurs
i.e. h1 = h2  Load = 0 2 1
Air Washer

23. Real processes - steam injection
 h 2 1
Steam humidifier

24. Humidifier saturation efficiency
ADP 2 1

25. Humidifier saturation efficiency,
Real processes – Humidification
Plant Loads
Example 3:
Air enters an air washer at 31oC d.b., 14.4oC w.b. and leaves with a moisture content of kg/kg d.a. Determine the saturatiion efficiency of the air washer.
What factors influence the saturation efficiency of humidifiers generally?
Humidifier saturation efficiency,

26. Room heat loads Summer room loads:-
Sensible heat gain + Latent heat gain
Sensible Heat Ratio 1
Room ratio line R S
tR - tS

27. Room heat loads Winter room loads:-
Sensible heat loss + Latent heat gain
Sensible Heat Ratio 1
Room ratio line R S
tS - tR

28. Room heat loads
Example 4: The room is to be kept at state 4 using a flow of 1.50 kg.s-1 of air at state 6. Determine the room sensible and latent loads in kW.
The specific heat capacity of moist air is 1.02 kJ.kg-1K-1

29. Central plant AHU - 100% fresh air
Summer cycle 2 4 6

30. Central plant AHU - 100% fresh air
Summer cycle 2
Cooler battery 8 4 6

31. Central plant AHU - 100% fresh air
Summer cycle 2
Cooler battery
+ Re-heater battery 8 4 6

32. Central plant AHU - 100% fresh air
Summer cycle O
Cooler battery
+ Re-heater battery C R S
Room ratio line (RRL)

33. Central plant AHU - 100% fresh air
Winter cycle 3 7 5

34. Central plant AHU - 100% fresh air
Winter cycle
Pre-heater battery 3 7 5 10

35. Central plant AHU - 100% fresh air
Winter cycle
Pre-heater battery
+ steam humidifier 3 9 7 5 10

36. Central plant AHU - 100% fresh air
Winter cycle
Pre-heater battery
+ steam humidifier 3
+ re-heater battery 9 7 5 10

37. Central plant AHU - 100% fresh air
Winter cycle
Pre-heater battery
Room ratio line (RRL)
+ steam humidifier R
+ re-heater battery B S O A

38. Central plant AHU - 100% fresh air
Winter cycle
Pre-heater battery
Room ratio line (RRL)
+ adiabatic humidifier R
+ re-heater battery B S O A

39. Mixing AM A xA BM M xM B xB tB tM tA

40. Real processes - Mixing
Example 5: Air in state 3 is mixed with air in state 4 to obtain mixed air, state M, with a dry bulb temperature of 25oC.
If the mass flow rate of mixed air is 1.45 kg/s, determine the flow rates of consituent air flows.

41. Central plant AHU - recirculation air2 6 4 C C’ M
Summer cycle

42. Central plant AHU - recirculation airS M
Room ratio line (RRL)
Winter cycle