2. Introduction
• Atmospheric air makes up the environment in almost every type of air
conditioning system. Hence a thorough understanding of the properties of
atmospheric air and the ability to analyze various processes involving air is
fundamental to air conditioning design.
• Psychrometry is the study of the properties of mixtures of air and water
vapor.
• Atmospheric air is a mixture of many gases plus water vapor and a number
of pollutants (Fig. 2.1). The amount of water vapor and pollutants vary
from place to place.
• The concentration of water vapor and pollutants decrease with altitude, and
above an altitude of about 10 km, atmospheric air consists of only dry air.
The pollutants have to be filtered out before processing the air. Hence,
what we process is essentially a mixture of various gases that constitute air
and water vapor. This mixture is known as moist air.
2
4. 4
• The moist air can be thought of as a mixture of dry air and
moisture. For all practical purposes, the composition of dry air
can be considered as constant. The composition of dry air is
given table below.
• Based on the above composition the molecular weight of dry
air is found to be 28.966 and the gas constant R is 287.035
J/kg.K.
Introduction
5. • As mentioned before the air to be processed in air conditioning
systems is a mixture of dry air and water vapor.
• While the composition of dry air is constant, the amount of
water vapor present in the air may vary from zero to a
maximum depending upon the temperature and pressure of the
mixture (dry air + water vapor).
• At a given temperature and pressure the dry air can only hold a
certain maximum amount of moisture.
• When the moisture content is maximum, then the air is known
as saturated air, which is established by a neutral equilibrium
between the moist air and the liquid or solid phases of water.
• For calculation purposes, the molecular weight of water vapor
is taken as 18.015 and its gas constant is 461.52 J/kg.K.
5
Introduction
6. Methods for estimating properties of moist air
• In order to perform air conditioning calculations, it is essential
first to estimate various properties of air.
• It is difficult to estimate the exact property values of moist air
as it is a mixture of several permanent gases and water vapor.
• However, moist air up to 3 atm. pressure is found to obey
perfect gas law with accuracy sufficient for engineering
calculations.
• Since in most cases the pressures involved are low, one can
apply the perfect gas model to estimate psychometric
properties.
6
7. Basic gas laws for moist air:
• According to the Gibbs-Dalton law for a mixture of perfect
gases, the total pressure exerted by the mixture is equal to the
sum of partial pressures of the constituent gases.
• According to this law, for a homogeneous perfect gas mixture
occupying a volume V and at temperature T, each constituent
gas behaves as though the other gases are not present (i.e.,
there is no interaction between the gases). Each gas obeys
perfect gas equation. Hence, the partial pressures exerted by
each gas, p1,p2,p3 … and the total pressure pt are given by:
• where n1,n2,n3,… are the number of moles of gases 1,2,3,…
7
8. • Applying this equation to moist air.
where
p = pt = total barometric pressure
pa = partial pressure of dry air
pv = partial pressure of water
8
Basic gas laws for moist air:
9. Important psychrometric properties:
• Dry bulb temperature (DBT) is the temperature of the moist air as
measured by a standard thermometer or other temperature measuring
instruments.
• Saturated vapor pressure (psat) is the saturated partial pressure of
water vapor at the dry bulb temperature. This is readily available in
thermodynamic tables and charts.
• ASHRAE suggests the following regression equation for saturated
vapor pressure of water, which is valid for 0 to 100oC
• where psat = saturated vapor pressure of water in kiloPascals
• T = temperature in K
• The regression coefficients c1 to c6 are given by:
• c1 = -5.80022006E+03, c2 = -5.516256E+00, c3 = -4.8640239E-02
• c4 = 4.1764768E-05, c5 = -1.4452093E-08, c6 = 6.5459673E+00
9
10. • Relative humidity (Φ) is defined as the ratio of the mole
fraction of water vapor in moist air to mole fraction of water
vapor in saturated air at the same temperature and pressure.
Using perfect gas equation we can show that:
• Relative humidity is normally expressed as a percentage.
When Φ is 100 percent, the air is saturated.
10
Important psychrometric properties:
11. • Humidity ratio (W): The humidity ratio (or specific humidity)
W is the mass of water associated with each kilogram of dry
air. Assuming both water vapor and dry air to be perfect gases,
the humidity ratio is given by:
• Substituting the values of gas constants of water vapor and air
Rv and Ra in the above equation; the humidity ratio is given by:
11
Important psychrometric properties:
12. • For a given barometric pressure pt, given the DBT, we can find
the saturated vapor pressure psat from the thermodynamic
property tables on steam.
• Then using the above equation, we can find the humidity ratio
at saturated conditions, Wsat.
• It is to be noted that, W is a function of both total barometric
pressure and vapor pressure of water.
12
Important psychrometric properties:
13. • Dew-point temperature: If unsaturated moist air is cooled at
constant pressure, then the temperature at which the moisture
in the air begins to condense is known as dew-point
temperature (DPT) of air. An approximate equation for dew-
point temperature is given by:
• where Φ is the relative humidity (in fraction). DBT & DPT are
in oC.
• Of course, since from its definition, the dew point temperature
is the saturation temperature corresponding to the vapor
pressure of water vapor, it can be obtained from steam tables
or using Eqn. above.
13
Important psychrometric properties:
14. • Properties such as humidity ratio, enthalpy and specific
volume are based on 1 kg of dry air.
• This is useful as the total mass of moist air in a process varies
by the addition/removal of water vapor, but the mass of dry air
remains constant.
• Degree of saturation μ: The degree of saturation is the ratio
of the humidity ratio W to the humidity ratio of a saturated
mixture Ws at the same temperature and pressure, i.e.,
14
Important psychrometric properties:
15. • Enthalpy: The enthalpy of moist air is the sum of the enthalpy
of the dry air and the enthalpy of the water vapor.
• Enthalpy values are always based on some reference value.
For moist air, the enthalpy of dry air is given a zero value at
0oC, and for water vapor the enthalpy of saturated water is
taken as zero at 0oC.
• The enthalpy of moist air is given by:
where
– cp = specific heat of dry air at constant pressure, kJ/kg.K
– cpw = specific heat of water vapor, kJ/kg.K
– t = Dry-bulb temperature of air-vapor mixture, oC
15
Important psychrometric properties:
16. • where
W = Humidity ratio, kg of water vapor/kg of dry air
ha = enthalpy of dry air at temperature t, kJ/kg
hg = enthalpy of water vapor at temperature t, kJ/kg
hfg = latent heat of vaporization at 0oC, kJ/kg
• The unit of h is kJ/kg of dry air. Substituting the approximate
values of cp and hg, we obtain:
16
Important psychrometric properties:
17. • Humid specific heat: From the equation for enthalpy of moist
air, the humid specific heat of moist air can be written as:
• where
cpm = humid specific heat, kJ/kg.K
cp = specific heat of dry air, kJ/kg.K
cpw = specific heat of water vapor, kJ/kg
W = humidity ratio, kg of water vapor/kg of dry air
• Since the second term in the above equation (w.cpw) is very
small compared to the first term, for all practical purposes, the
humid specific heat of moist air, cpm can be taken as 1.0216
kJ/kg dry air.K
17
Important psychrometric properties:
18. • Specific volume: The specific volume is defined as the number
of cubic meters of moist air per kilogram of dry air.
• From perfect gas equation since the volumes occupied by the
individual substances are the same, the specific volume is also
equal to the number of cubic meters of dry air per kilogram of
dry air, i.e.,
18
Important psychrometric properties:
19. Psychrometric chart
• A Psychrometric chart graphically represents the
thermodynamic properties of moist air.
• Standard psychrometric charts are bounded by the dry-bulb
temperature line (abscissa) and the vapor pressure or humidity
ratio (ordinate). The Left Hand Side of the psychrometric chart
is bounded by the saturation line.
• Psychrometric charts are readily available for standard
barometric pressure of 101.325 kPa at sea level and for normal
temperatures (0-50oC).
• ASHRAE has also developed psychrometric charts for other
temperatures and barometric pressures (for low temperatures:
-40 to 10oC, high temperatures 10 to 120oC and very high
temperatures 100 to 120oC).
19
21. • The basic features of the psychrometric chart are illustrated in
Fig. shown below.
• The dry-bulb temperatures are shown on the horizontal axis,
and the specific humidity is shown on the vertical axis.
• Some charts also show the vapor pressure on the vertical axis
since at a fixed total pressure P there is a one-to-one
correspondence between the specific humidity v and the vapor
pressure Pv.
• On the left end of the chart, there is a curve (called the
saturation line) instead of a straight line.
• All the saturated air states are located on this curve. Therefore,
it is also the curve of 100 percent relative humidity. Other
constant relative-humidity curves have the same general shape.
21
Psychrometric chart
22. Psychrometric chart
• Lines of constant wet-bulb
temperature have a downhill
appearance to the right.
• Lines of constant specific
volume (in m3/kg dry air) look
similar, except they are steeper.
• Lines of constant enthalpy (in
kJ/kg dry air) lie very nearly
parallel to the lines of constant
wet-bulb temperature.
Therefore, the constant wet-
bulb-temperature lines are used
as constant-enthalpy lines in
some charts.
22
23. • For saturated air, the dry-bulb,
wet-bulb, and dew-point
temperatures are identical (Fig.
14–15).
• Therefore, the dew-point
temperature of atmospheric air
at any point on the chart can be
determined by drawing a
horizontal line (a line of ω =
constant) from the point to the
saturated curve.
• The temperature value at the
intersection point is the dew-
point temperature.
23
Psychrometric chart
24. EXAMPLE: The Use of the Psychrometric Chart
• Consider a room that contains air at 1 atm, 35°C, and 40
percent relative humidity. Using the psychrometric chart,
determine
a) the specific humidity,
b) the enthalpy,
c) the wet-bulb temperature,
d) the dew-point temperature, and
e) the specific volume of the air.
24
25. a) The specific humidity is
determined by drawing a
horizontal line from the
specified state to the right
until it intersects with the
ω axis. At the intersection
point we read
ω = 0.0142 kg H2O/kg dry
air
25
EXAMPLE: The Use of the Psychrometric Chart
26. b) The enthalpy of air per unit mass of dry air is determined by
drawing a line parallel to the h constant lines from the
specific state until it intersects the enthalpy scale, giving
h =71.5 kJ/kg dry air
c) The wet-bulb temperature is determined by drawing a line
parallel to the Twb = constant lines from the specified state
until it intersects the saturation line, giving Twb = 24°C
d) The dew-point temperature is determined by drawing a
horizontal line from the specified state to the left until it
intersects the saturation line, giving Tdp = 19.4°C
e) The specific volume per unit mass of dry air is determined by
noting the distances between the specified state and the v =
constant lines on both sides of the point. The specific volume
is determined by visual interpolation to be v = 0.893 m3/kg
dry air
26
29. Measurement of psychrometric properties:
• Based on Gibbs’ phase rule, the thermodynamic state of moist
air is uniquely fixed if the barometric pressure and two other
independent properties are known.
• This means that at a given barometric pressure, the state of
moist air can be determined by measuring any two
independent properties.
• One of them could be the dry-bulb temperature (DBT), as the
measurement of this temperature is fairly simple and accurate.
• The accurate measurement of other independent parameters
such as humidity ratio is very difficult in practice.
• Since measurement of temperatures is easier, it would be
convenient if the other independent parameter is also a
temperature.
29
30. • Of course, this could be the dew-point temperature (DPT), but
it is observed that accurate measurement of dew-point
temperature is difficult.
• In this context, a new independent temperature parameter
called the wet-bulb temperature (WBT) is defined.
• Compared to DPT, it is easier to measure the wet-bulb
temperature of moist air.
• Thus knowing the dry-bulb and wet-bulb temperatures from
measurements, it is possible to find the other properties of
moist air.
30
Measurement of psychrometric properties:
31. AIR-CONDITIONING PROCESSES
• Maintaining a living space or an
industrial facility at the desired
temperature and humidity requires
some processes called air-
conditioning processes.
• These processes include simple
heating (raising the temperature),
simple cooling (lowering the
temperature), humidifying (adding
moisture), and dehumidifying
(removing moisture). Sometimes
two or more of these processes are
needed to bring the air to a desired
temperature and humidity level.
31
32. Simple Heating and Cooling (ω = constant)
• The amount of moisture in
the air remains constant
during this process since
no moisture is added to or
removed from the air.
• That is, the specific
humidity of the air remains
constant (ω = constant)
during a heating (or
cooling) process with no
humidification or
dehumidification.
32
33. • Notice that the relative humidity of
air decreases during a heating
process even if the specific humidity
ω remains constant.
• This is because the relative humidity
is the ratio of the moisture content to
the moisture capacity of air at the
same temperature, and moisture
capacity increases with temperature.
• Therefore, the relative humidity of
heated air may be well below
comfortable levels, causing dry skin,
respiratory difficulties, and an
increase in static electricity.
33
Simple Heating and Cooling (ω = constant)
34. • A cooling process at constant
specific humidity is similar to
the heating process, except the
dry-bulb temperature
decreases and the relative
humidity increases during such
a process.
• Cooling can be accomplished
by passing the air over some
coils through which a
refrigerant or chilled water
flows.
34
Simple Heating and Cooling (ω = constant)
35. Sensible cooling:
35
• Figure below shows
the sensible cooling
process O-A on a
psychrometric chart.
• The heat transfer rate
during this process is
given by:
36. • Sensible heating (Process
O-B): During this process,
the moisture content of air
remains constant and its
temperature increases as it
flows over a heating coil.
The heat transfer rate
during this process is given
by:
• where cpm is the humid
specific heat (≈1.0216 kJ/kg
dry air) and ma is the mass
flow rate of dry air (kg/s).
36
Sensible heating:
37. Heating with Humidification
• Problems associated with
the low relative humidity
resulting from simple
heating can be eliminated
by humidifying the
heated air.
• This is accomplished by
passing the air first
through a heating section
(process 1-2) and then
through a humidifying
section (process 2-3), as
shown in the Fig.
37
38. • The location of state 3 depends on how the humidification is
accomplished.
• If steam is introduced in the humidification section, this will
result in humidification with additional heating (T3 > T2).
• If humidification is accomplished by spraying water into the
airstream instead, part of the latent heat of vaporization
comes from the air, which results in the cooling of the heated
airstream (T3 < T2).
• Air should be heated to a higher temperature in the heating
section in this case to make up for the cooling effect during
the humidification process.
38
Heating with Humidification
44. Cooling with Dehumidification
• The specific humidity of
air remains constant
during a simple cooling
process, but its relative
humidity increases.
• If the relative humidity
reaches undesirably high
levels, it may be necessary
to remove some moisture
from the air, that is, to
dehumidify it. This
requires cooling the air
below its dew point
temperature.
44
48. Adiabatic Mixing of Airstreams
• Many air-conditioning
applications require the
mixing of two airstreams.
• This is particularly true for
large buildings, most
production and process plants,
and hospitals, which require
that the conditioned air be
mixed with a certain fraction
of fresh outside air before it is
routed into the living space.
48
49. • The heat transfer with the surroundings is usually small, and
thus the mixing processes can be assumed to be adiabatic.
Mixing processes normally involve no work interactions, and
the changes in kinetic and potential energies, if any, are
negligible.
• Then the mass and energy balances for the adiabatic mixing of
two airstreams reduce to
49
Adiabatic Mixing of Airstreams
51. Example 3: Adiabatic mixing
• Saturated air leaving the cooling section of an air-conditioning
system at 14°C at a rate of 50 m3/min is mixed adiabatically
with the outside air at 32°C and 60 percent relative humidity at
a rate of 20 m3/min. Assuming that the mixing process occurs
at a pressure of 1 atm, determine the specific humidity, the
relative humidity, the dry-bulb temperature, and the volume
flow rate of the mixture.
51
55. Cooling Towers
• Power plants, large air-conditioning systems, and some
industries generate large quantities of waste heat that is often
rejected to cooling water from nearby lakes or rivers.
• In some cases, however, the cooling water supply is limited or
thermal pollution is a serious concern. In such cases, the waste
heat must be rejected to the atmosphere, with cooling water
recirculating and serving as a transport medium for heat
transfer between the source and the sink (the atmosphere).
• One way of achieving this is through the use of wet cooling
towers.
• Cooling towers also are frequently employed to provide
chilled water for applications other than those involving power
plants.
55
56. Cooling Tower
• A schematic diagram of a
forced-convection,
counterflow cooling tower is
shown in Fig. below. The
warm water to be cooled
enters at 1 and is sprayed
from the top of the tower.
• The falling water usually
passes through a series of
baffles intended to keep it
broken up into fine drops to
promote evaporation.
• Atmospheric air drawn in at
3 by the fan flows upward,
counter to the direction of
the falling water droplets.
56
57. • As the two streams interact, a small fraction of the water stream evaporates
into the moist air, which exits at 4 with a greater humidity ratio than the
incoming moist air at 3.
• The energy required for evaporation is provided mainly by the portion of
the incoming water stream that does not evaporate, with the result that the
water exiting at 2 is at a lower temperature than the water entering at 1.
• Since some of the incoming water is evaporated into the moist air stream,
an equivalent amount of makeup water is added at 5 so that the return mass
flow rate of the cool water equals the mass flow rate of the warm water
entering at 1.
• For operation at steady state, mass balances for the dry air and water and an
energy balance on the overall cooling tower provide information about
cooling tower performance.
• In applying the energy balance, heat transfer with the surroundings is
usually neglected. The power input to the fan of forced-convection towers
also may be negligible relative to other energy rates involved.
57
Cooling Towers
58. Example: Cooling of a Power Plant by a
Cooling Tower
• Cooling water leaves the
condenser of a power plant and
enters a wet cooling tower at
35°C at a rate of 100 kg/s.
Water is cooled to 22°C in the
cooling tower by air that enters
the tower at 1 atm, 20°C, and
60 percent relative humidity
and leaves saturated at 30°C.
Neglecting the power input to
the fan, determine (a) the
volume flow rate of air into the
cooling tower and (b) the mass
flow rate of the required
makeup water. 58
61. Temperature Measurements
During the measurement of air temperatures, it is
important to recognize the meaning of the terms
accuracy, precision, and sensitivity.
1. Accuracy: is the ability of an instrument to indicate
or to record the true value of the measured quantity.
The error indicates the degree of accuracy.
2. Precision: is the ability of an instrument to give the
same reading repeatedly under the same conditions.
3. Sensitivity: is the ability of an instrument to indicate
change of the measured quantity.
61
62. • Liquid-in-glass instruments, such as mercury or
alcohol thermometers, were commonly used in
the early days for air temperature measurements.
A typical air temperature indication system includes
sensors, amplifiers, and an indicator.
Sensors: Air temperature sensors needing higher
accuracy are usually made from resistance temperature
detectors (RTDs) made of platinum, palladium, nickel,
or copper wires.
62
63. Sensors:
• The electrical resistance of these resistance
thermometers characteristically increases when the
sensed ambient air temperature is raised; i.e., they
have a positive temperature coefficient
• Many air temperature sensors are made from
thermistors of sintered metallic oxides, i.e.,
semiconductors.
63
64. Amplifier(s): The measured electric signal from the
temperature sensor is amplified at the solid state
amplifier to produce an output for indication.
Indicator: An analog-type indicator, one based on
directly measurable quantities, is usually a moving
coil instrument.
64
65. HUMIDITY MEASUREMENTS
Humidity sensors used in HVACR for direct humidity
indication or operating controls are separated into the
following categories: mechanical hygrometers and
electronic hygrometers.
65
66. Mechanical Hygrometers
• Mechanical hygrometers operate on the principle that
hygroscopic materials expand when they absorb water
vapor or moisture from the ambient air. They contract
when they release moisture to the surrounding air.
• Such hygroscopic materials include human and animal
hairs, plastic polymers like nylon ribbon, natural fibers,
wood, etc. When these materials are linked to mechanical
linkages or electric transducers that sense the change in
size and convert it into electric signals, the results in these
devices can be calibrated to yield direct relative humidity
measurements of the
ambient air. 66
67. Electronic Hygrometers
There are three types of electronic hygrometers:
i.Dunmore resistance hygrometer
ii. ion-exchange hygrometer
resistance hygrometer
iii. capacitance hygrometer
67
68. i. Dunmore Resistance Hygrometer
This instrument depends on the change in resistance
between two electrodes mounted on a hygroscopic
material. Figure. Below shows a Dunmore resistance
sensor.
68
69. • The electrodes could be a double-threaded winding
of noble-metal wire mounted on a plastic cylinder
coated with hygroscopic material. The wires can
also be in a grid-type arrangement with a thin film
of hygroscopic material bridging the gap between
the electrodes.
69
70. • At a specific temperature, electric resistance
decreases with increasing humidity.
• Relative humidity is generally used as the humidity
parameter, for it must be controlled in the indoor
environment.
70
71. • The time response to accomplish a 50 percent
change in relative humidity varies directly according
to the air velocity flowing over the sensor and also is
inversely proportional to the saturated vapor
pressure.
71
72. • If a sensor has a response time of 10 s at 70°F
(21°C), it might need a response time of 100 s at
10°F (12°C). Because of the steep variation of
resistance corresponding to a change in relative
humidity, each of the Dunmore sensors only covers
a certain range of relative-humidity measurements.
72
74. ii. Ion-Exchange Resistance Hygrometer
• The sensor of a ion-exchange resistance electric
hygrometer is composed of electrodes mounted on a
baseplate and a high-polymer resin film, used as a
humidity-sensing material, cross-linking the
electrodes.
• Humidity is measured by the change in resistance
between the electrodes.
74
75. • When the salt contained in the humidity sensitive
material bridging the electrodes becomes ion
conductive because of the presence of water vapor
in the ambient air, mobile ions in the polymer film
are formed.
• The higher the relative humidity of the ambient air,
the greater the ionization becomes, and therefore,
the greater the concentration of mobile ions. On the
other hand, lower relative humidity reduces the
ionization and results in a lower concentration of
mobile ions. 75
76. iii. Capacitance Hygrometer
• The commonly used capacitance sensor consists of a
thin-film plastic foil. A very thin gold coating covers
both sides of the film as electrodes, and the film is
mounted inside a capsule. The golden electrodes and
the dividing plastic foil form a capacitor.
• Water vapor penetrates the gold layer, which is
affected by the vapor pressure of the ambient air
and, therefore, the ambient relative humidity. The
number of water molecules absorbed on the plastic
foil determines the capacitance and the resistance
between the electrodes.
76
