M O D U L E 1 PULLOUT
Basics of PSYCHROMETRICS
Psychrometrics
is a branch of
thermodynamics
which looks at the
properties of moist
air – essentially, how
air and water vapour
interact with each
other. Psychrometrics
is fundamental to
the operation of all
refrigeration and air
conditioning systems.
The basic science behind
psychrometrics was established in
the 1800s, and the psychrometric
chart was first revealed to the public
in a technical paper presented to
the American Society of Mechanical
Engineers by Willis H Carrier, who
went on to found the Carrier
company (which most in the industry
today will still be familiar with).
Willis Haviland
Carrier,
1876-1950
When going
home by train
in the evening
from the office, Carrier would so
immerse himself in his problems
that he frequently went through
his home station. This happened
so often that he was driven to
moving his home to the terminal
station on the line.
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H VAC& R S K I LL S WO R K S H O P Figure 1 – a sling psychrometer, used for measuring dry and wet bulb temperatures
But how well do
we all understand it?
Air is made up of a mixture of gases – mostly
nitrogen and oxygen, but also carbon dioxide and
some other inert gases.
Air also contains a small amount of water vapour.
In temperate climates, the amount of water
vapour in air is usually more than 1% by weight.
In extreme conditions (such as in the tropics),
however, it can approach 3%.
There are two important temperature concepts
that need to be understood in psychrometrics
– those of dry bulb and wet bulb temperatures.
Dry bulb temperature is the temperature as
measured by a regular thermometer, expressed
in degrees Celsius (°C). This is the most
commonly used measure of temperature – it’s
usually assumed that a temperature is a dry bulb
temperature unless stated otherwise.
Wet bulb temperature is the temperature as
measured by a thermometer with its bulb covered
by a wet piece of cloth (known as a wick) and
exposed to moving air. At relative humidities
below 100%, water evaporates from the wick,
cooling the bulb of the thermometer below the
dry bulb (or ambient) temperature. Wet bulb
temperature is also expressed in degrees
Celsius (°C).
Dry and wet bulb temperatures can be measured
using thermometers, or specially designed digital
instruments.
Figure 1 shows a device traditionally used
to measure dry and wet bulb temperatures:
a ‘sling psychrometer’, which features two
thermometers that has been specifically
designed for measuring dry and wet bulb
temperatures simultaneously.
Notice the cloth covering the bulb of the
bottom thermometer, and the flexible joint at
the other end of the thermometers. The device
works by wetting the cloth then spinning the
two thermometers around to generate air
movement.
From these two readings we can calculate the
amount of water vapour in the air.
Air temperature and air pressure both have an
effect on how much water vapour air can hold
– generally, warmer and lower pressure air will be
Proudly sponosored by ebmpapst
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AIRAH PSYCHROMETRIC CHART
Barometric pressure 101.325kPa
Figure 2: the psychrometric chart
able to hold more water vapour, and
colder, higher pressure air will be able
to hold less water vapour.
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When it’s referred to
in refrigeration and
air conditioning
calculations, water
vapour content
in air is often
expressed
in terms
of
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‘relative humidity’. Relative humidity is the ratio of
the amount of moisture in the air to the maximum
amount of moisture the air could hold at the same
temperature.
Relative humidity:
(amount of moisture in the air) : (maximum
amount of moisture the same air could hold)
The level of relative humidity can have a significant
effect on human comfort and storage conditions
inside refrigerated spaces.
Luckily, moist air follows known thermodynamic
laws and behaves in a predictable way, so we can
make accurate calculations about the effects of
adding or removing heat from the air, or adding or
removing moisture from it.
Calculations to do with air temperature and
water vapour content are usually plotted on the
psychrometric chart: (Figure 2).
Psychrometrics, and the psychrometric chart, can
be used to identify air quantities and equipment
capacities, and what needs to be done to the air
(adding or removing heat or moisture) to meet
particular conditions.
The lines on the chart are taken from data tables,
which in turn have been calculated using the laws
of thermodynamics.
The horizontal axis represents the dry bulb
temperature of the air, while the vertical axis
represents the moisture content of the air (expressed
here as grams of moisture per kilogram of air, or g/kg).
The curved lines (or ‘saturation curves’) represent
relative humidity (percentages up to 100% are
marked for each line), and wet bulb temperatures
can be plotted along each curve.
Dry bulb and wet bulb temperatures can be
measured using thermometers as described above,
or by using specially designed digital instruments.
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Other
important terms
Dew point – temperature at which
water vapour starts to condense
from the air
Sensible heat – heat due to
temperature difference only
Latent heat – heat due to
moisture difference only
Total heat – the sum of sensible
heat and latent heat
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Enthalpy – the ‘heat energy’
contained in moist air
Specific volume – the amount of
space occupied by air at the given
temperature and humidity
Figure 3 – effect of various psychrometric processes
Once the dry and wet bulb temperatures are known,
values for moisture content, relative humidity, specific
volume and enthalpy can be read off the chart (see
sidebar for more information on these terms).
Figure 3 gives an overview of the effects of adding
or removing heat or moisture from a system.
The consequences of various changes to
the system can also be predicted using the
psychrometric chart.
Where psychrometrics is most helpful is in the
process of designing a system and selecting the
right capacity equipment to use in it.
If heat or moisture is added to or removed from
a system, a new point will be able to be plotted
on the chart. The chart can be used to assess
whether the new point will, for example, cause
condensation to form inside a space, or raise
humidity to uncomfortable levels.
Barometric pressure, or air pressure,
changes depending on altitude. It is
lower at the peak of Mount Everest
than it is at sea level, for example.
Barometric pressure can also have an
What do we use it for?
We usually
know the
conditions
we want to
be able to
achieve inside
the refrigerated or air
conditioned space. We
can plot these points on the
psychrometric chart.
We can also plot other
points on the psychrometric
chart – a common example
is the temperature of the air
outside, which needs to be
considered when designing an
air conditioning system.
effect on psychrometric calculations.
The standard psychrometric chart
is drawn with a standard sea level
barometric pressure of 101.325 kPa in
Further reading
The distance between points on the
psychrometric chart can tell us a lot about the
requirements for a system, and designers can use
this information to solve problems such as:
• Selecting the right equipment
for a job
• Working out system capacity
• Checking comfort levels within an air
conditioned space
• Calculating when an air conditioning
system can benefit from using an
economy cycle / full outside air
• Checking that a system that has
been installed is working properly
• Verifying the performance
of a piece of equipment
• Calculating where problems
with condensation may occur
• Manually checking the output
of computer programs or cooling
load calculations s
mind, though it is usable at pressures
AIRAH Building blocks of air conditioning – psychrometrics courses
ranging from 98.3 kPa to 105.1 kPa
AIRAH technical manual DA09 Air Conditioning Load Estimation
without introducing significant errors.
AIRAH building blocks courses are run around the country, or can be delivered in-house
to your company. Visit www.airah.org.au for more information.
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Next Month’s Workshop — Refrigerant handling