CPD Programme
1
1
5
120 125 130 135 140
1
1
0
PERCENTAGE SATURATION
Figure 2 – Standard CIBSE Psychrometric Chart
1
0
5
90 80 70 60 50 40 30 20
0·030
1
0
0
0.90
0·029
95 30
0·028
90
1
40
0·027
85 0·026 1
35
0·025
0.2
0.1
SENSIBLE/TOTAL HEAT
80
0.3 RATIO FOR WATER
0·024 1
30
0.4 ADDED AT 30°C 7
5
0·023
0.5 -
1
0.6
g
70
0·022 1
25
0.7 k
J
.
k
65
0·021
0.8 25
0.9
1. 0 A
L
P
Y
/
60
0·020 1
20
0.9
0·019
0.8
0.7
55
0·018
1
1
5
0.6
T
50 0·017
0.5 T
E
N
1
10
0.4 S
P
E
C
I
F
I
C
E
N
T
H
0.85 0·016 N
0.3
45
20
O
0.2
40 0·015
0.1
1
05
R
E
C
35
/
˚
C
(
S
L
I
N
G
)
0·014
0·013
I
S
T
U
O
1
0
0
30
WET- BULB TEMPERATURE / ˚C (SLING) P
E
R
A
T
U
R
E
0·012
M
15
25
T
E
M
0·011
95
L
B
0·010
20
1
5
W
E
T
-
B
U
0·009
90
10
0·008
1
0 85
0.80
0·007
5
5
0·006 8
0
0 0·005
0
0·004 75
-5
0.75
-5
0·003
-
1
0
-10
0·002
70
0·001
0·000
-10 -5 0 5 10 15 20 25 30 35 40 45 50 55 60
DRY - BULB TEMPERATURE / ˚C
45 50 55 60 65
20 25 30 35 40
-10-5 0510 15
SPECIFIC ENTHALPY /
kJ.kg
-1
The psychrometric chart
The parameters described so far are combined The application of the chart to processes With thanks to Dr Roger Legg for assistance
to produce the psychrometric chart, which and systems will be explored in a future CPD with the material for this article
dates back to empirical work undertaken article. © Tim Dwyer 2009
by James Glaisher in Greenwich in the
first half of the 19th century. Glaisher had
Temperature – Celsius °C – normal symbol θ (formerly ‘t’)
problems reliably determining the wet bulb
Absolute Temperature – Kelvin K (normal symbol T) – Kelvin is also frequently used to
temperature – these problems arose from the
denote a temperature difference
differences between screen and sling wet bulb
temperatures. The chart was later refined by
Energy – joule J
1J = 1kg·m
2
·s
-2
= 1 N·m
others in the US, including the famous Willis
energy to heat one gram of dry cool air by 1K
Carrier at the beginning of the 20th century,
by which time the uncertainties of wet
Power – watt W (normal symbol P)
bulb temperature measurement were well 1W = 1J·s
-1
= 1kg·m
2
·s
-3
understood. The form of the psychrometric
Pressure – pascals Pa normal symbol p
chart has changed little from the early
1 Pa = 1 N·m
-2
(force per area)
versions – the most visually significant
Bar is frequently used as the unit 1 Bar = 1000mbar = 100000 Pa = 100kPa
alteration being the changes in the units to
Density – kg m
-3
of TOTAL air (normal symbol ρ)
SI for the CIBSE Chart, see Figure 2.
The chart provides a convenient way of
Specific volume – m
3
·kg
-1
of DRY air plus any associated water vapour (normal symbol v)
examining the changes in the dependent
Other symbols (not units)
parameters in air as the air undergoes a
V = volume, m
3
process in a space (for example, heating and
m = mass, kg
humidification from occupants) or as it is
R = Gas constant, J·kg
-1
·K
-1
conditioned in an HVAC system. If any two Subscripts
properties are known, then the others may
v = water vapour
be read from the chart – so, for example, in
s = saturated
the field this would typically be wet bulb and
da = dry air
dry bulb temperature measured with a sling See Section 6 of CIBSE Guide C 2007 for standard units, symbols and notation.
psychrometer.
www.cibsejournal.com April 2009 CIBSE Journal 57
CIBSEapr09 pp55-58 cpd.indd 57 4/2/09 3:09:01 PM
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