Psychrometry and Air Conditioning Processes g

BBSE2005: Air Conditioning
g and Refrigeration
g
I
http://www.hku.hk/bse/bbse2005/
Psychrometry and
Air Conditioning
g Processes
Dr. Sam C M Hui
Department of Mechanical Engineering
The University of Hong Kong
E-mail:
E
mail: cmhui@hku.hk
cmhui@hku hk
Dec 2010
Contents
• Introduction to Psychrometry
y
y ((Handout Chapter
p 1)*
)
• Psychrometric Processes (Handout Chapter 2)*
• Practical Use of Psychrometric Chart
• Computer Software for Psychrometric Calculation
and Analysis
[* Acknowledgment: Denco Air Conditioning (UK)]
Introduction to Psychrometry
• Basics
• The atmosphere
• Water vapour
• Saturated vapour pressure
• Also, Appendix - Thermodynamic Basics
• Perfect gas laws
• 1st law of thermodynamics
• Conservation of energy
Introduction to Psychrometry
• Psychrometry
• The study of atmospheric air and its associated
water vapour
• Dryy air and moist air
• Dalton’s law of partial pressures
• Standard atmospheric pressure = 101.325 kPa
• Saturated
S t t d vapour pressure
• Max. ppressure of water vapour
p
that can occur at
any given temperature
Introduction to Psychrometry
• Psychrometric Chart (Theory)
•
•
•
•
•
Moisture content (g), or absolute humidity (w)
Relative humidity (rh or RH)
Percentage saturation (μ)
Wet-bulb temperature (twb)
Specific volume (v)
• (See the illustration on psychrometric chart)
Can y
you read them
from the chart?
Enthalpy
Psychrometric Chart
Wet-bulb
temperature
Relative
humidity
Dew-point
Dew
point
temperature
Humidity
ratio
Dry-bulb
temperature
Specific
volume
Sling psychrometer
Introduction to Psychrometry
• The Psychrometric Equation
•
•
•
•
Dew-point temperature (tdp)
Specific enthalpy (h)
Specific volume (v)
Density (ρ)
• Do you know how to find out the moist air
properties using the psychrometric chart?
Introduction to Psychrometry
• Commonly used psychrometric charts
• ASHRAE psychrometric chart
• CIBSE psychrometric chart
• Why are they slightly different?
• Can yyou find out the differences?
Psychrometric Processes
• Common processes:
• Sensible cooling / sensible heating
• Cooling and dehumidification / heating and
humidification
• Humidification / dehumidification
• Evaporative
i cooling
li / chemical
h i l dehydration
d h d i
• Typical devices:
• Cooling/heating coils
• Humidifiers / dehumifiers
Basic psychrometric processes
3
7
5
1
2
6
8
4
Process 0-1: Sensible heating
P
Process
00-2:
2 Sensible
S ibl cooling
li
Process 0-3: Humidifying
Process 0-4: Dehumidifying
y g
Process 0-5: Heating and humidifying
Process 0-6: Cooling and dehumidifying
P
Process
00-7:
7 Cooling
C li andd humidifying
h idif i
Process 0-8: Heating and dehumidifying
Psychrometric processes
Sensible cooling/heating
Adiabatic dehumidification
Cooling and dehumidification
Evaporative cooling
Cooling coil
Cooling and dehumidification
Cooling and dehumidification
Simple air conditioning cycle
C you draw
Can
d
suchh a cycle
l for
f Hong
H
Kong
K
summer conditions?
di i ?
- Outdoor: DBT = 33 ºC; WBT = 28 ºC; flow = 20% of supply air
- Indoor:
I d
DBT = 25 ºC;
ºC %RH = 50%
- Air leaving cooling coil: DBT = 13 ºC; %RH = 95%
Psychrometric Processes
•
•
•
•
•
•
•
Sensible heating coils
Cooling coils
Humidifiers
Water spray types
Steam humidifier
Room p
psychrometric
y
process
p
Mixing air streams
Psychrometric Processes
• Calculations:
•
•
•
•
•
Sensible heat ratio (SHR)
Space cooling load
Cooling coil
coil’ss load/capacity
Humidification capacity
Mixing processes
• Principles of heat balance & conservation of mass
Sensible and latent cooling loads
Practical Use of Psych. Chart
• Examples of psychrometric charts
•
•
•
•
•
ASHRAE
CIBSE
Carrier
Mr. S K Wang (similar to Trane)
The chart used in Mainland China (upside down)
• [From Mollier diagram: many users in Scandinavia,
Eastern Europe, and Russia]
• Do you
o know
kno ho
how to construct
constr ct these charts?
Psychrometric Software
• ASHRAE Psychrometric Analysis CD-ROM
CD ROM
(2002) [AV 697 P97]
• ArchiSci Software - PSYCHWIN (an
evaluation version can be downloaded)
• Psychrometric Chart (PSY) software (Free for
download)
• Daikin's
D iki ' Free
F Psychrometrics
P h
t i tool
t l
Analysis of Hong Kong climate on a psychrometric chart
Analysis of HVAC operation strategy