8/3/2019 Introduction to the Psychrometric Chart
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INTRODUCTION TOTHEPSYCHROMETRICCHART
OverviewBeforecommencingwith thedesignphase, it isimportant
tolook at theclimate youare going tobe designingfor andassess
thethermalcomfort
conditionsthroughoutthe year.The
psychrometric chart is a good analysis tool to use for this
purpose. As wehave seen in the first part of this tutorial,
atmospheric factors, namely the airtemperature and the moisture in
the air pretty much define thermal comfort inthe presence of air
movement and radiation. The psychrometric chartrepresents the state
of a given atmosphere by a point which gives the dry-bulb,
wet-bulb, relative humidity, specific volume and saturation
temperatureof the atmosphere.
The figure below represents a typical psychrometric chart. The
psychrometric
chart differs only by altitude or barometric pressure.
Psychrometric TermsThe chart might seem a littleoverwhelming in
the number offactors it represents and theirrelationship to one
another butwe will look at its componentsindividually in the
following order:
1. Saturation humidity line:
Absolute humidity (AH)is the vapor content of
air, given in grams of water vapour per kg of air, i.e. g/kg.
Air at agiven temperature and pressure can support only a certain
amountof moisture and no more. This is referred to as
thesaturationhumidity (SH). If this is plotted on a graph against
the dry bulb (air)temperature (DBT), we have the basis of the
psychromeytric chart
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