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Introduction to Psychrometric charts
by
Salim Deramchi
Email: salim.deramchi@bsria.co.uk
1
What is a Psychrometric chart?
Psychrometric is the study of moist air
2
Atmospheric Air
A mix of "dry" air and water vapour
Dry Air is a mixture
Mainly nitrogen and oxygen
Traces of argon, krypton, neon, helium, xenon, carbon dioxide, methane and others
3
NATURAL PSYCHROMETRY
Rain Snow
Hail
Fog/Mist
Dew Ground Frost
4
Why psychrometric charts?
5
Thermal comfort
6
TO AVOID
CONDENSATION
SWEATING
7
Where can psychrometric chart be used
8
CENTRALISED
CAV & VAV
LOCAL
PARTIALLY
CENTRALISED
UNIT
+
Design of AIR-CONDITIONING
SYSTEMS
+
& SPLIT
9
10
Types of psychrometric charts
11
12
13
CIBSE GUIDE C Tables
14
The properties of the psychometric chart
15
The properties to be discussed
Dry bulb temperature Wet bulb temperature Moisture content (kg Water / kg Dry air)
Percentage of relative humidity (this is shown
on psychrometric charts as percentage saturation)
Specific enthalpy
16
Dry bulb
The temperature which can be measured by a thermometer
17
WET-BULB TEMPERATURE
Dry-bulb
Temperature
Wet-bulb
depression
Wet-bulb
Temperature
18
Used to infer the amount of water moisture in the air (Water content)
Specific enthalpy
Specific enthalpy is the amount of energy per kg of air at a specific
condition
19
So lets determine some parameters
20
Dry Bulb Vertical Lines
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Dry bulb temperature 10 20 30
Wet Bulb Diagonal Lines
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6
11
16
21
Dew point Horizontal Lines
23
Dew
po
int
tem
per
atu
re
V
erti
cal L
ines
Relative humidity
It is the Ratio of water vapour compared to the maximum amount
the air can hold
24
Exercise
Determine all the parameters at the temperatures of 20Cdb and 20%RH
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26
Exercise
Dry
bu
lb
Dew Point Moisture content
Dew Point temperature
Exercise
Determine all the parameters at the temperatures of 22Cdband 15 Cwb
27
47%
Dew-point Temperature = 10 C 22 C
15 C
Enthalpy
41.2 kJ/kg
Moisture Content 7.6 g/kg
28 28
COMFORT
ZONE
Arabian Desert
Hong
Kong
UK
Comfort
Zone
DESIGN CONDITIONS
Singapore
29
Basic Formulae
Sensible Heat = mass x specific heat x temperature change
Flow flow
qs = m x cp x Dt
30
Exercise
Heat 2m3/s of air entering at 10OCdb and 7OCwb
to 30O C & determine the heater duty
31
32
33
10
7
30
a b
Ha=22.5
Hb=42.5
Exercise
Determine coil duty to cool 3m3/s of air at 30OCdb and 20
OCwb to 20OCdb
34
35
30
20
a
b
Hb=47
Ha=57
20
END
Any questions?
37
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