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