EVERYTHING ABOUT PSYCHROMETRY USED IN HVAC
Psychrometry is term which is
generally used to study the environmental air.
Basically to design one of the
best HVAC system one must understand all about psychrometry.
As we know that managing a
comfortable temperature which brings comfort to human is achieved only if we
study the behaviour of air.
Well psychrometry is a branch
of engineering science which deals with the study of the air.
Basically air atmospheric condition is not
fixed all the time .It keeps on changing with the zones , sessions , country ,
state , even a small region has different air atmospheric conditions.
So depending on this condition
a HVAC system, design and execution is decided .
which need a strong foundation of
psychrometry knowledge.
Before playing with
temperature we need to understand the properties of the air which will surely
helps to achieve our goal of maintaining a comfortable temperature.
WHAT IS A PSYCHROMETRY
PSYCHROMETRY is basically the
study of atmospheric air which help us to understand its different properties.
Basically the atmospheric air
that surrounds us in not only air but it is a mixture of dry air and water
vapour which is called as moist air.
The study of moist air is
called as psychrometry.
It is a Greek word which means
study of air.
Well we are clear what is psychrometry
and what are we going to deal with. As it is a study of atmospheric air which is a
mixture of dry air and water vapour , which has certain properties which has been
found after study of the atmospheric air.
So lets understand it properties.
PROPERTIES OF AIR
- DRY BULB TEMPERATURE
- WET BULB TEMPERATURE
- DEW POINT TEMPERATURE
- HUMIDITY RATIO/SPECIFIC HUMIDITY
- RELATIVE HUMIDITY
- SATURATED AIR
- ENTHALPY OF AIR
This are the properties which
are very important to find , in order to maintain comfortable temperature .
Well to maintain a best design
in HVAC it is necessary to calculate the amount of sensible heat and latent
heat , which help us to understand what type of system one must select to
overcome this heat loads .
Well it is not necessary to
find all properties of the following but only two parameter are enough to
understand the other properties using a either a psychrometry graph or other
platforms like psychrometric software.
To find this two properties of air we use a device which
is called as hygrometer or sling psychrometer .
But before that it is very important
to study this properties….
So lets start!!!!!!!!!!!
DRY BULB
TEMPERATURE (DBT)
DRY BULB TEMPERATURE which is
also called as DBT .
It is a normal temperature of
air which is indicated by thermometer. This temperature will measure the
sensible heat of air.
DBT lines are shown vertical in the graph.
To understand a what is DBT
lets take an example of a normal thermometer which indicates your body temperature
in the same way DBT is used to measure the atmospheric normal temperature .
WET BULB
TEMPERATURE (WBT)
WET BULB TEMEPARTURE is also
called as WBT.
Temperature sensed by a
thermometer whole bulb which indicated the temperature is wrapped with the water
soaked wick , which is rapidly moving in air .
WBT lines are shown inclined on the psychometric graph.
Most important thing is that WBT IS always have less temperature than that
of the DBT.
Have confusion ????
Lets clear it..
Lets understand the WBT
process by an example
So when you take a shower and
come out of the bathroom you have a some glimpse of water stuck to your body
and when you come in contact with air you will find cold whereas without a glimpse
of water you will not feel so cold .
I hope you got it which is dry
bulb and which is wet bulb temperature!
If not , lets clear it ,well
from above example the period when we feel cold after taking shower is called a
WBT.
And when we move without a glimpse
of water we feel less cold as compared to after shower that is DBT.
Well hope we are clear what is
DBT and WBT .
Basically this parameter or
properties are very important to find so that we can achieve other properties
by graph or other software.
DO YOU HAVE
QUESTION WHY WE FEEL COOL WHEN WE COME OUT OF SHOWER?????
Well we have a water particles
on our body , that means we are wet , so when the atmospheric air comes in contact
with our body it absorbs the water from your body which is called as evaporation
.
This evaporation of water from
our body generate a coolness which is one of the principle of air conditioning.
Similarly earthen pots which
has a pores in it . when we put water on the surface of the earthen pots the
process of evaporation takes place which cools the water fill in the pots.
This one of the example of
natural cooling process using evaporation..
Hope we are clear with its so
lets move on to the next properties..
DEW POINT
TEMPERATURE (DPT)
Dew point temperature is a temperature
at which water vapour in the atmosphere begin to condense, if the air was
cooled at constant pressure.
It is the temperature at which air moisture or water
vapour leaves the air and condenses on an object.
DPT are shown horizontal in the psychometric graph.
To understand it we have an
example like dew form on grass and plant leaves.
During winter you will find
the objects in early morning is surrounded with droplets of water which is
presence of atmospheric air.
Similarly if you remove a
bottle of water from refrigerator you will find the droplet of water around it
after sometimes.
Hope we have understood with
this example ,what it is….
HUMIDITY RATIO
It is also called as specific
humidity or moisture content .
It can be expressed as the
mass of water vapour present in kg of
dry air.
HUMIDITY RATIO are shown horizontal in the psychometric graph.
Generally expressed as grains/kilogram
of dry air.
This is very important
parameter to find out the latent heat present in the atmosphere which helps to
decide how to overcome the heat load .
1lb =7000grains.
RELATIVE HUMIDITY (RH)
Relative humidity is basically
a comparison of actual to ideal parameter.
I know you may be confused by this
wording don’t worry we will resolve it.
It is a ratio of actual amount
of water vapour present in atmosphere air to the ideal amount of water vapour
or maximum possible water vapour present in atmospheric air.
RELATIVE HUMIDITY are shown with a curve in the psychometric graph.
It is expressed in %
Generally 50% of RH is suitable
for human comfort ……
SATURATED AIR
Saturated air is the
temperature at which dry bulb ,wet bulb and dew point temperature are same or
equal ,then the air is said to be saturated.
SATURATED AIR are shown with a curve on the top curve of the .psychometric graph.
This curve is also called as saturation curve.
It can hold no moisture when
the air is in saturated condition.
Saturation is the extreme condition
of the atmospheric air when it remove the moisture content in it.
ENTHALPY
Enthalpy is a type of energy
which content in object according to thermodynamics in the same way atmospheric
air too have some energy content in it .
ENTHALPY are shown inclined in the psychometric graph.
And the atmospheric air is
mixture of water vapour and dry air.
Hence total enthalpy is the
addition of dry and water vapour enthalpy.
H total =H dry air + H water
vapour.
Now let see the real physcromtric chart.
PSYCHOMETRIC CHART
PSYCHOMETRIC SOFTWARE
Just put the value of DBT and any one other parameter like WBT or RH or humidity ratio etc will get the result as shown.
This is basically carrier soft ware which I generally used for faster process.
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