CPD Programme
CPD Programme
The CIBSE Journal CPD Programme programme is free and can be used by any
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The properties of air
If there is a need to examine the performance of a ventilation or air
conditioning system, or simply a requirement to understand the condition
of the atmosphere in and around the built environment, it is important to
have an understanding of the properties of air, and specifically of humid air.
These psychrometric properties provide a common method of analysing
the changes in temperature and moisture content – and consequently
energy – that take place in every heating, ventilation or air conditioning
(HVAC) system in a building. This article will look at the basic parameters
that combine to provide a psychrometric toolkit. A later article will consider
the applications of these properties using the psychrometric chart.
A
ir is normally considered to be a definitions of the symbols). This equation example, using the information from Figure
mix of dry air and water vapour. allows calculation of the properties of gases 1, the gas constant for dry air is given by
‘Normal’ dry air itself is mainly under different pressures and temperatures. R0 /M = 8314.7 / 28.97 = 287 J.kg
-1
·K
-1
. The
nitrogen (N
2
) and oxygen (O
2
) The value of R, the specific gas constant, may molecular mass of water vapour is 18 and
with traces of argon (Ar), carbon dioxide be obtained for any gas by considering the so the specific gas constant may be obtained
(CO
2
) and other gases (see Table 1). Water molecular mass, M, of each of the constituent from 8314.7 / 18 = 461 J·kg
-1
·K
-1
vapour (H
2
O) constitutes about 0.40 per cent gases where R = R
0
/M and R
0
is the Universal These relationships can also be usefully
of the whole global atmosphere and typically Gas Constant of 8314.7 J·kg
-1
·mol
-1
K
-1
. For employed to examine the properties of
makes up one to four per cent of the mass other gas mixtures – as might be found,
Gas Molar Proportion Mass
of the combined air mixture at the Earth’s for example, in controlled specialist
mass fraction
surface. The molecular actions of the gases environments.
combining together have been determined
N
2
28 x 0.7803 = 21.86
The air dry bulb temperature, θ (°C), is
by the Gas Laws and it is assumed that for
O
2
32 x 0.2099 = 6.704
the temperature measured using a standard
the work in HVAC these ‘ideal gas law’
Ar 40 x 0.0094 = 0.376
thermometer that is shielded from any
relationships are valid. From these laws,
CO
2
44 x 0.0003 = 0.013
radiant heating or cooling effects. (And, of
the characteristic equation for any gas is
Average molecular mass 28.97
course, is most frequently sensed using an
given by pV = mRT (see box overleaf for
Table 1 – The Molecular Mass of Dry Air
electrical or electronic device.) >
www.cibsejournal.com April 2009 CIBSE Journal 55
CIBSEapr09 pp55-58 cpd.indd 55 4/2/09 3:08:40 PM
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