HUMIDIFICATION,
AIRCONDITIONING AND REFRIGERATION
Syllabus:
Definition,
psychrometric chart, wet-bulb theory, determination of humidity, humidifiers
and dehumidifiers, refrigeration.
Questions:
Q.1. Humidity chart and its uses - short note. (1999) [4]
Q.2. Write short note on psychrometric chart. (1998) [4]
Q.3. Write short note on humidifiers (1996) [6]
Q.4. Write short note on dehumidifiers. (1995) [4]
Q.5. What information does a psychrometric chart contain?
Write a note on dehumidifiers. (1994) [10]
Humidity
is concerned with the physical properties of mixtures of air and water vapor.
Humidity:
Humidity
is defined as the pounds (lb) of water vapor carried by 1 lb of dry air under
any given set of conditions. This quantity is also called the “humidity ratio”
Calculation
Say
mw moles of water vapor and ma moles of dry air are
enclosed inside a chamber then,
mole
fraction of water vapor in the chamber, 
and
and
mole
fraction of dry air in the chamber, 
Where,
pw and pa are the partial pressures of water vapour and
air respectively.
\ 
If
the total pressure, P = 1 atm. then, 
where
pw = p.
Since,
humidity has been defined as pounds of water vapor per pound of dry air it
follows that
humidity,
Saturated air
It is the
air in which the water vapour is in equilibrium with liquid water at the given
temperature and pressure.
In such a
mixture, the partial pressure of water-vapor in the water-air mixture is equal
to the vapor-pressure of pure water at that temperature.
If Hs
is the saturation humidity and 
the vapor pressure of
the liquid,
the vapor pressure of
the liquid,
Relative humidity, HR
It is defined as the ratio of the partial pressure of the
vapor (p) to the vapor-pressure of the liquid (p’) at the air-temperature.
It is usually expressed on a percentage basis, so
HR = 100 Þ
saturated air
HR = 0 Þ
vapor free air
By definition: 
Percentage humidity
It is the ratio of the actual humidity H to the saturation
humidity Hs, at the air temperature, also expressed on a percentage basis, or
Humid heat, Cs
It is the
heat energy necessary to increase the temperature of 1 g or 1 lb of dry air
plus whatever vapor it may contain by 10C or 10F. Thus
Cs = CpA + CpwH
Where CpA and Cpw are the specific heats of gas and vapor,
respectively (at constant pressure).
Unit of 
CpA = 0.24 Btu/oF/lb
Cpw = 0.45
Btu/oF/lb Cs
= 0.24 + 0.45 H
Enthalpy (E) of
an air-water vapor mixture is the enthalpy of 1 lb of dry air plus the enthalpy
of its accompanying water vapor.
Unit: It is
expressed as Btu / lb of dry air.
To calculate E, two reference states must be chosen, one for
gas and one for vapor.
For water: Liquid water at 320F (i.e. 00C)
and 1 atm pressure
For air: Dry air at 00F (i.e. -17.80C)
and 1 atm pressure.
Therefore, for an air-water vapor mixture containing H lb of
water vapor per pound of dry air and at a temperature of t0F, the
enthalpy may be calculated as follows:
Total enthalpy, E = sensible
heat of the vapor
+ the latent heat of the liquid at 320F
+ the sensible heat of the dry air.
E = CpA (t - 0) +
Hl0
+ HCpw (t - 32 )
0.24
t + H{1075.2 + 0.45 (t - 32)}
 |
E = 0.24
t + H (1060.8 + 0.45 t)
l0
= latent heat of vaporization of water at 320F
Unit of E = Btu / lb of dry air.
Humid volume, vH
is the total volume (cubic ft) of a unit mass (1 lb) of dry air plus whatever
vapor it may contain at 1 atm pressure and at air temperature.
From the gas laws in fps unit.
 |
T = absolute temperature [deg Rankin] = 460 + t (t in oF)
MA = average molecular weight of water
H = humidity
Derivation:
Say 1 lb of dry air is taken having humidity, H.
\
1 lb dry air will contain H lb of water vapor.
\
vH = volume of 1 lb dry air + volume of H lb water vapor
[all at 1
atm pressure and toF temp
Since 1 lb dry air =
(1/29) lb mole of dry air
º 
ft3 of dry
air [V = nRT/P]
ft3 of dry
air [V = nRT/P]
and H lb water vapor =
(H/18) lb-mole of water vapor
º 
ft3 of
water vapor
ft3 of
water vapor
\Total
volume of 1 lb dry air + H lb water vapor =
vH = 
.
.
If t = 320F (i.e. 00C) at
P = 1 atm
1 mole of any gas has a volume of 359 ft3
(i.e. 22.4 lit)
T = 460 + t = 460 + 32 = 4920R
P = 1 atm
n = 1 mole
V = 359 ft3
Since, PV = nRT or R/P = V/nT = 359/(1 x 492) =
359/492
* For dry air H = 0
vH = specific
volume of dry air
*For saturated air H = HS and vH = saturated volume.
So saturated volume
in ft3 of 1 lb of dry air plus that of the water vapor necessary to
saturate it.
Dew point is the temperature at which a mixture of air and
water must be cooled (at constant humidity) in order to become saturated (i.e.
to be in equilibrium with liquid water at the dew point).
Unit It has unit
of temperature (oF) or (oC).
HUMIDITY CHART OR
PSYCHROMETRIC CHART
Figure 1.
The humidity
characteristics of air are best shown graphically in a psychrometric or
humidity chart. Such charts can be found in various handbooks. See figure 1.
The basic curves of the humidity chart are shown in a
simplified version in the above figure.
·
These curves are graphic representations of the
relationship between the temperature and humidity of the air-water vapour
system at constant pressure.
·
The temperature
is shown in the horizontal axis (i.e. abscissa) and the vertical axis (i.e.
coordinate) represents humidity.
1. Saturation humidity:
The curve
CDE represents the saturation humidity where the percentage humidity is 100%
(i.e. HA = 100% along this line). Under this conditions the air is
completely saturated with moisture.
2. Dew point:
Any point,
A on the chart represents a humidity H, at a certain temperature (810F).
By reading along the humidity coordinates to the right point H1 is
found, which is the humidity of the sample. By following the H1A
line to the left of A, the line intersects the saturation curve (CDE) at point
C; the corresponding temperature at point C is the dew point. At this point (C) the air is saturated with water
vapour.
[N.B. Let a sample of
unsaturated air, under the conditions represented by point A is brought into
contact with liquid water under adiabatic conditions (i.e. no heat is received
from or given up to the surroundings during the operation; i.e. heat exchange
will occur only between the wet surface and air).
Since the air is not
saturated, water will evaporate from the wet surface into the air. Thus the
humidity of air will increase and latent heat of evaporation will be taken by
the wet surface, hence the temperature of the wet surface will decrease.
Eventually, an equilibrium state will reach where the heat transferred becomes
equal to the heat of vaporization, and the temperature stabilizes. This
temperature is called wet bulb
temperature. The actual temperature of air is called dry bulb temperature.]
3. Determination of humidity from wet-bulb and dry-bulb
temperatures:
The
wet-bulb temperature is a function of the temperature and humidity of the air
and thus can be employed as a means of measuring humidity. For this purpose, a
second type of curve is super-imposed on the temperature-humidity curves of the
psychrometric chart. This is the constant
wet-bulb temperature line (or adiabatic
cooling line).
For example let us assume a wet-bulb temperature of 540F
and a dry-bulb temperature of 600F. The 540F adiabatic
cooling line is followed until it intersects the saturation humidity curve at
an humidity of H2. From 600F dry-bulb temperature, if we
vertically come up then 600F line will intersect the 540F
line at F and the corresponding humidity is H3. so the % humidity of
the air is H3 / H2 x 100. And the humidity is H3.
MEASUREMENT OF HUMIDITY
The humidity
of a stream of air may be found by measuring either the dew point or wet-bulb
temperature or by direct-absorption
methods.
Dew point method:
If a
cooled, polished disk is inserted into an air of unknown humidity and the
temperature of the disk gradually lowered, the disk reaches a temperature at
which mist condenses on the polished surface. The temperature at which this
mist just forms is the temperature of equilibrium between the vapor in the air
and the liquid phase. It is therefore the dew
point. A check on the reading is obtained by slowly increasing the disk
temperature and noting the temperature at which the mist just disappears. From
the average of the temperatures of mist formation and disappearance, the dew
point of that air sample is determined. From the dew-point the humidity can be read from the humidity chart.
[N.B. Let the dew
point is 600F. If a vertical line is drawn towards the
saturation (100% humidity) line then it will cut at point C. At point C the
humidity can be read from the humidity
axis (e.g. H1).]
Psychrometric
methods:
A very
common method of measuring the humidity is to determine simultaneously the wet-bulb temperature and dry-bulb temperatures. From these
readings the humidity is found by locating the psychrometric line (adiabatic
cooling line) intersecting the saturation line at the observed wet-bulb
temperature and following the psychrometric line to its intersection with the
ordinate of the observed dry-bulb temperature. [See the Determination of
humidity by wet and dry-bulb temperature]
Direct methods.
The vapour
content of a gas can be determined by direct analysis, in which a known volume
of gas is drawn through an appropriate analytical device.
HUMIDIFICATION
It is often necessary to prepare air that will have a known temperature and known humidity.
·
This can be accomplished by bringing the air
into contact with water under such conditions that the desired humidity is
reached.
·
Then the temperature of the air is raised to the
desired dry-bulb temperature.
Theory
Figure 2
This is
shown in figure 2. The point A
represents the entering air, whose initial dry-bulb temperature is t1
and whose humidity is H1. It is desired to convert this air to an
air of dry-bulb temperature t2
and humidity of H2 (point
B). This can be achieved by two methods.
Method I:
In this method the air is first given the desired humidity
by treatment with water to give the conditions represented by point C (wet-bulb
temperature t3) and then heating to t2. The path of the
air from A to B is ACB.
Method II
The air is preheated to such an initial temperature t4 that when cooled along an adiabatic cooling
line it will reach the desired humidity. It is then reheated to the desired
temperature. This corresponds to the path ADCB in figure 2.
HUMIDIFYING EQUIPMENT
Humidifying equipment essentially contains
(i)
a device for heating the air, either before or after
humidification. These are usually coils or banks of finned tubes.
(ii)
a device to bring air into contact with water.
An apparatus for
Method-II (i.e. path ADCB)
(i)
The air is first drawn over finned coils M and heated
as indicated by the line AD in figure-2
(ii)
It then passes through water sprays N and is
adiabatically cooled and humidified (lineDC, figure-2). The pumps that takes water from the reservoir below the
sprays and discharges back to the sprays may also deliver water to a pipe T
that is slotted so as to give a curtain of water to eliminate most of the
entrained water-droplets before the air enters a set of eliminator baffles P,
where the last entrained water is removed. In the pump discharge there may be a
heater or a provision for steam injection to adjust the water temperature.
(iii)
A second set of coils Q performs the final reheating
(line CB, fig.-2). The final
temperature of the air may be regulated by controlling the steam in the second
set of coils or by controlling a bypass damper S.
(iv)
A fan R draws the air through the apparatus and
discharges it to the point of use.
Apparatus for Method-I:(ACB path)
The apparatus for carrying out the process corresponding to
the path ACB of fig-2 is very similar
to that of shown in the fig-3.
Instead of the coils M of the fig-3
steam is injected directly into the water as it is pumped to the sprays N,
maintaining it at the temperature t3 of fig-2. The rest of the apparatus is exactly similar to that of
shown in fig-3.
DEHUMIDIFIERS
Sometimes
warm saturated air is required to be dehumidified for instance, dehumidifying
the air discharged from a dryer so that it may be reused.
DEHUMIDIFIED COOLER-CONDENSER
Figure 4
|
This can be done by bringing the moist air in contact with
cold liquid. The temperature of the air is lowered below the dew point, water vapour condenses, and
the humidity of the gas is reduced. After dehumidification the gas can be reheated
to its original dry-bulb temperature.
Equipment for dehumidification may utilize
(i) A direct
spray of coarse liquid droplets into the gas. This may be accomplished by
passing the air through sprays in an apparatus very similar to that of figure -3, except that the heater M is
unnecessary.
(ii) A spray of
water on refrigerated coils or other cold surface, or
(iii) Condensation
on a cold surface with no water spray. This is accomplished by passing a cold
fluid through the inside of finned tubes arranged in banks through which air is
blown. The outside surface of the metal tubes must be below the dew point so
that water will condense out of the air. (Figure-4).
Air conditioning requirements for building and residences
are apparatus in which air must be dehumidified to a point that calls for
cooling it to temperatures so low that water for this purpose is rarely
available. So, such system is supplied with artificially cooled water or a
refrigerant. The refrigerating system may be contained inside the unit itself
or may be entirely separate.
COOLING TOWERS
When warm liquid is brought into contact with unsaturated
gas, part of the liquid is vaporized and the liquid temperature drops. This
cooling of the liquid is the purpose behind many water-air contact operations.
This cooling is accomplished by bringing the warm water into
contact with unsaturated air under such conditions that the air is humidified
and the water brought approximately to the wet-bulb temperature. This method is applicable only in those cases
where the wet-bulb temperature of the air is below the desired temperature of
the exit-water.
Water is cooled in large quantities in
(i)
Spray ponds,
(ii)
Natural draft cooling towers or
(iii) Forced
draft cooling towers.
SPRAY PONDS
Water is sprayed from a spray nozzle into small droplets to
produce large surface area with air. The sprays are fitted over a basin to
catch the water, to reduce the water loss, such an arrangement is usually known
as a spray pond.
Advantage: Such
ponds are convenient for small capacities or where water loss is high due to
wind.
Disadvantage: The
power consumption for pumping the water through nozzle is appreciable.
NATURAL-DRAFT COOLING TOWERS
Two types of equipment are used:
(a) atmospheric
circulation type
(b) chimney type
(a) In the atmospheric-circulation
type (fig-5a) the circulation of air through the tower is in horizontal
direction rather than in vertical direction.
The water is distributed by allowing it to fall over baffles
of various types. Water is distributed over the tower by means of troughs, and
louvers are provided along the sides to prevent excessive amounts of water
being carried away as spray by the wind.
These towers may be 20-50 ft high, and 8 to 16 ft wide in
the direction of the prevailing wind), the length is dependent on the amount of
water cooled.
(b) The chimney type natural-draft
cooling towers air is warmed by the water and therefore may produce upward
draft (fig. 5b). The sides of such tower are completely enclosed all the way to
the top except for air inlets near the bottom.
The grid material, which distributes the water, is confined
to a relatively short section in the lower part of the tower.
(c) Forced draft
towers (fig. 6) use fans for air
circulation. These are usually called “forced-draft” towers if the fans are at
the bottom, and “induced-draft” towers if the fans are at the top.
The latter is the preferred design because it avoids return
of saturated air back into the tower.
