CPD Programme
CPD Programme
The CIBSE Journal CPD Programme programme is free and can be used by any
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their careers. Continuing professional institutions, who should record CPD activities
development (CPD) means the systematic in accordance with their institution’s guidance.
maintenance, improvement and broadening Simply study the module and complete the
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Applying the psychrometric
relationships
The CPD article in the April edition of CIBSE Journal considered the
majority of the properties of moist air that may be determined from a
pair of coincident properties using the psychrometric chart. This article
will continue to develop the formulae that may be used to calculate
the properties and consider the comparative accuracy of using a chart
compared with simple calculations
In the April article some subscripts were inconsistent in the explanation of the equation for moisture content, g (kg·kg
-1
da
)
The final relationship should be g = 0.622 p
v
/(p
at
– p
v
)
A
rguably the one process that A = 6.66x10
-4
K
-1
where θ’ ≥ 0 deg C The final volume of the
determines whether a system A = 5.94x10
-4
K
-1
where θ’ < 0 deg C psychrometric story?
is air conditioning, as opposed Where p
s
’ is the saturation vapour An element of the standard psychrometric
to simply ventilation, is the pressure at the wet-bulb temperature that chart that causes widespread uncertainty is
control of air moisture content. So a key can be determined from tables or from the the specific volume and more particularly its
requirement of psychrometric analysis is rather lengthy but suitably accurate empirical relationship with density. Many texts simply
the ability to establish the vapour pressure equation that is used to determine saturation relate one as the inverse of the other -however
that in turn can be used to calculate the air vapour pressure, p
s
(kPa)
[2]
there is a convention in the HVAC&R world
moisture content. log p
s
= 30.5905 – 8.2 log (θ + 273.16) + that, for humid air, this is not the case.
To establish the vapour pressure in air, 2.4804 x 10
–3
(θ + 273.16) – [3142.31 / The specific volume of moist air,
p
v
(kPa) the relationship based on Sprung’s (θ + 273.16)] v (m
3
·kg
-1
da
), is the volume of 1 kg dry air
original experiments with in the late 19th and this is ideally suited to use in a and its associated water vapour, so using the
century
[1]
known as the psychrometric spreadsheet or web tool (and is valid for gas law pV = mRT,
equation may be used temperatures above 0 deg C). Note that the v = V /m
a
= R
a
T /p = R
a a
T /(p
at
– p
v
)
p
V
= p
s
’ - p
at
A (θ – θ’) saturation vapour pressure is not dependent = 287·T /(p
at
– p
v
)
where, for a sling wet-bulb temperature, upon the atmospheric pressure. where V is the volume occupied by the >
www.cibsejournal.com August 2009 CIBSE Journal 49
CIBSEaug09 pp49-52 cpd.indd 49 7/23/09 4:06:19 PM
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