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Atmospheric
air is a mixture of various gases and water vapour.
The invisible water vapour in the air is called humidity.
The amount of water that can be contained in a pound of air depends
upon ......
– temperature of the air
– pressure of the air
At a constant pressure, the warmer the air the amore water vapour
it can retain. If air at a certain temperature has absorbed all
the moisture it can hold at that temperature, it is said to be saturated.
The amount of water present, or the degree of saturation is evaluated
in terms of Relative Humidity or Saturation ratio.
Hence, Relative Humidity is the actual water content in % of that
of a saturated volume.
When air is 50% saturated, it contains only one half the amount
of water than it can contain at the same temperature and pressure.
As the relative humidity approaches 100%, the air can take on less
and less moisture and at 100% relative humidity, that air cannot
hold more water.
If air is saturated with water vapour at a given temperature, a
drop in the temperature will lead to condensation of water in the
form of droplets. The temperature at which moisture condenses out
is the dewpoint temperature.
Measurement of RH
Relative Humidity is determined by means of wet bulb and dry bulb
thermometers.
The dry bulb temperature is the temperature of air as determined
by a standard thermometer.
The wet bulb temperature is determined by tying a wet wick over
the bulb dipped in a reservoir containing distilled water. Airflow
around the wick causes the evaporation of moisture thus lowering
the temperature and producing a reading lower than that on the dry
bulb thermometer.
The comparison between the two readings given us the measurement
of water vapour in the air.
The lesser the difference - the wetter the air.
The greater the difference - The dryer the air.
The readings can be plotted on a chart known as the psychometric
chart from where the properties of air vapour mixture like relative
humidity, absolute humidity, dewpoint can be directly determined.
Using a Psychrometric Chart
A psychrometric chart is nothing but a graphic presentation of several
interrelated air parameters brought together.
A simplified psychrometric chart is shown in the figure -
Vertical lines represent dry bulb temperature.
The diagonal lines sloping from left to right show the wet bulb
temperature. Horizontal lines represent the humidity ratio and are
represented as grains of moisture per pound of dry air (7000 grains
= 1 pound)
The lines curving upward from left to right are levels of relative
humidity.
The left curved margin of the chart is the saturation of 100% relative
humidity.
On a particular day if the dry bulb and wet bulb temperatures are
known then by plotting these readings on the chart all parameters
can be found out.
It can be used to plot changes needed to transform an initial state
to a final state. Given a required change, the initial and desired
conditions are plotted, then the chart can determine the energy,
temperature and moisture content changes, that will take place.
The process of heating, cooling and dehumidifying and chemical dehydration
can be explained with the help of the psychrometric chart.
Heating
When sensible heat is applied to air the temperature increases.
However, there is no change in the moisture content of the air.
This effect is shown on the psychrometric chart as a straight horizontal
line starting at the left and extending to the right. The Dry bulb
reading increases, wet bulb reading increases the dewpoint remains
unchanged, the RH is lowered however the moisture content of the
air remains unchanged.
Cooling and Dehumidifying
When air is cooled the capacity to hold water decreases, the extra
water vapour condenses. In the process both sensible heat and latent
heat are removed and the process takes place along a line sloping
downward and to the left. Change occur in dry bulb, wet bulb and
in dewpoint temperature. Relative Humidity may or may not change.
Chemical Dehydration
In chemical dehydration the air is brought in contact with a chemical
which either absorbs or adsorbs moisture from the air. The heat
thus liberated is added to the air and is approximately equal to
the latent heat of vaporization of the moisture removed. The process
is indicated by a line sloping downward approximately along the
wet-bulb line. The slope may be either greater or less than the
wet bulb line depending upon whether heat is stored, liberated,
or absorbed in the process.
Methods of Dehumidification
Principally there are three methods of dehumidification:-
... Over compression
... Refrigeration Dehumidification
... Sorption Dehumidification
Using Compression to Dry Air
Air compression suggests a method of reducing moisture content in
air. With air as compressed, partial pressure of the water vapour
in the water gas mixture is raised to the point where moisture can
be condensed from the air at a higher temperature. This approach
is some times quite practical for very small volumes of air but
the cost of compressed equipment, BHP requirement and the amount
of cooling water required for after-cooling make it very impractical
for large volumes of air.
Reducing Temperature to Dry Air
Another method most commonly employed is the reduction of moisture
in the air by means of reducing the temperature. By examination
of the dew point alone or saturation curve on the psychrometric
chart. It can readily be seen that as the temperature of the air
is lowered, the amount of moisture it can hold is reduced considerably.
Thus by cooling the air below the dew point, the moisture contained
in that air can be condensed out and some of the moisture vapour
removed in liquid form, but cooling to very low temperature makes
the refrigeration process impractical, as it requires a great deal
of subsequent re-heating. The reduction in air temperature is also
limited by the freezing point of water condensing on the cooling
coil, which in some designs is tried to be offset by complicated
brine spray and liquid lithium chloride type systems available using
a combination of refrigeration and adsorbent liquid but these are
very bulky and involve complicated control systems for the proper
maintenance of solution density.
Using Sorbents to Dry Air
Sorbents are solid or liquid materials which have the property of
extracting and holding other substances (usually water vapour) brought
into contact with them.
Sorbents can be classified into two general categories.
* Absorbents
* Adsorbents
Absorbent : A sorbent which changes either physically,
chemically or both during the sorption process.
Lithium Chloride is an example of solid absorbent. When water is
absorbed on this material it changes to a hydrated state.
In liquid sorption dehumidification system, the air is passed through
sprays of a liquid sorbent such as lithium chloride or glycol solution.
The sorbent in an active state has a vapour pressure below that
of the air to be dehumidified and absorbs moisture from the air
stream. The sorbent solution during the process of absorption becomes
diluted with moisture which during regeneration is given up to an
air stream in which the solution is heated.
Typically absorbent used is lithium chloride in either liquid form
or as solid crystals in a honey comb shell.
Adsorbent : A sorbent which does not change physically
or chemically during the sorption process. Adsorbents are normally
granular beads or solids with porous structures making it capable
of holding large amounts of water on their surface.
The principle behind desiccant dehumidification is that the desiccant
is exposed to moisture laden air, from where it is extracted by
the desiccant and held. The saturated desiccant is heated, which
drives off the collected moisture into the exhaust air stream. The
regenerated desiccant is ready for use again. Thus a continuous
cycle of sorption and regeneration can be set up giving very low
dew points.
Typical absorbents used are Silica gel, Molecular Sieve and Activated Alumina.
Thus it can be seen that chemical dehumidifiers based on the principle
of physical adsorption offer the most simple direct and economical
method of humidity control.
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