Module (1.2) psychometrics-air parameters-hvac_by ss-eng. juma

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Psychometrics:
AIR PARAMETERS
Module 02
Fundamentals of HVAC
By: Start Smart–Your Skills Solution

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THE IMPORTANCE OF HVAC:
1.4 AIR PARAMETERS
•
Figure (1.4) HVAC Process
Psychometrics:
is the study of the thermodynamic properties of
moist air. It is used extensively to illustrate and
analyze the characteristics of various air
conditioning processes and cycles.
Atmospheric Air:
It 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.

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1.4 AIR PARAMETERS
•
Moist Air:
The surface of the earth is surrounded by a layer
of air called the atmosphere, or atmospheric air.
From the point of view of psychrometrics, the
lower atmosphere, or homosphere, is a mixture
of dry air (including various contaminants) and
water vapor, often known as moist air.

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1.4 AIR PARAMETERS •
The composition of dry air is comparatively stable. It varies
slightly according to geographic location and from time to time.
The approximate composition of dry air by volume percent is the
following:
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. In 1949, a standard composition
of dry air was fixed by the International Joint Committee on
Psychrometric data. It is given in Table 1.1 and Figure (1.5).

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•
1.4 AIR PARAMETERS
Figure (1.5) HVAC
Process

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1.5 TEMPERATURE AND SCALES
•
The temperature of a substance is
a measure of how hot or cold it is.
Two systems are said to have
equal temperatures only if there is
no change in any of their
observable thermal
characteristics when they are
brought into contact with each
other. Various temperature scales
commonly used to measure the
temperature of various
substances are illustrated in
Figures

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1.5 TEMPERATURE AND
TEMPERATURE SCALES
•

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•

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Figure (1.6) Commonly Used Temperature Scales

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Dry bulb temperature (DBT)
•
Figure (1.7)
Thermometer/Relative Humidity
is the temperature of the moist air as
measured by a standard thermometer or
other temperature measuring instruments.
Figure (1.7)

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Wet-Bulb Temperature
•
Figure (1.8) Schematic of a Wet-
Bulb Thermometer
When unsaturated
moist air flows over the
wet bulb of the
psychrometer, liquid
water on the surface of
the cotton wick
evaporates, and as a
result, the temperature
of the cotton wick and
the wet bulb. Figure
(1.8)

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1.6 PSYCHROMETER
•
Figure (1.10) A psychrmeter

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1.6 PSYCHROMETER
•
Figure (1.11) A psychrmeter

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Humidity Ratio
•
Figure (1.8) Schematic of a Wet-Bulb Thermometer
The humidity ratio of
moist air w is the ratio of
the mass of water vapor
mw to the mass of dry air
ma contained in the
mixture of the moist air, in
lb / lb (kg/kg). The
humidity ratio can be
calculated as:

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Relative Humidity
•
Figure (1.9)
Meters

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Relative Humidity
•
Figure (1.9.1) Thermometer/Relative Humidity
Curves

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Relative Humidity
•
Figure (1.9.1)
Curves

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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.

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Dew-point temperature:
•

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1.6 PSYCHROMETER
•
A psychrometer is an instrument that permits one to determine
the relative humidity of a moist air sample by measuring its
dry-bulb and wet-bulb temperatures. Figure 2.4 shows a
psychrometer, which consists of two thermometers. The
sensing bulb of one of the thermometers is always kept dry.
The temperature reading of the dry bulb is called the dry-bulb
temperature. The sensing bulb of the other thermometer is
wrapped with a piece of cotton wick, one end of which dips
into a cup of distilled water.
The surface of this bulb is always wet; therefore, the
temperature that this bulb measures is called the wet-bulb
temperature. The dry bulb is separated from the wet bulb by a
radiation- shielding plate. Both dry and wet bulbs are
cylindrical. See Figure (1.10), Figure (1.11)

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1.6 PSYCHROMETER
•
Sling psychrometer
(becoming obsolete)
Sling psychrometer
Other types of psychrometric instruments:
1. Dunmore Electric Hygrometer
2. DPT meter
3. Hygrometer (Using horse’s or human hair)

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HUMIDITY MEASUREMENTS
Humidity sensors used in HVAC&R for direct
humidity indication or operating controls are
separated into the following categories:
mechanical hygrometers and electronic
hygrometers.
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.
Electronic Hygrometers
There are three types of
electronic hygrometers:
Dunmore resistance
hygrometers, ion-exchange
resistance hygrometers, and
capacitance hygrometers.

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HUMIDITY MEASUREMENTS
•

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•Relative Humidity and DEW Point

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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 vapour pressure or
humidity ratio (ordinate). The Left Hand Side of the
psychrometric chart is bounded by the saturation line. Figure
27.2 shows the schematic of a psychrometric chart.
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). See Figure (1.13)

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Temp. & Humidity Loggers

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Wireless Logging-Weather station

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•
Psychrometric chart
Figure (1.13) Schematic of a psychometric chart for a given barometric pressure

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DB-Dry Bulb Temperature

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DB-SENSIBLE COOLING

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SENSIBLE COOLING (Process O-A):
• During this process, the moisture
content of air remains constant but
its temperature decreases as it
flows over a cooling coil. For
moisture content to remain
constant, the surface of the cooling
coil should be dry and its surface
temperature should be greater than
the dew point temperature of air. If
the cooling coil is 100% effective,
then the exit temperature of air will
be equal to the coil temperature.
However, in practice, the exit air
temperature will be higher than the
cooling coil temperature. Figure
(2.1) shows the sensible cooling
process O-A on a psychrometric
chart. The heat transfer rate during
this process is given by:

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SENSIBLE HEATING (PROCESS O-B)

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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:

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WB-Wet Bulb Temp.

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RH%-Relative Humidity Lines

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COOLING AND DEHUMIDIFICATION
(PROCESS O-C):

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P-Point will indicate All
7-Properities of Air

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What is the Process each Direction
indicate?

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(PROCESS O-C):
When moist air is cooled below its dew-
point by bringing it in contact with a cold
surface as shown in Figure (2.3), some
of the water vapor in the air condenses
and leaves the air stream as liquid, as a
result both the temperature and humidity
ratio of air decreases as shown. This is
the process air undergoes in a typical air
conditioning system. Although the actual
process path will vary depending upon
the type of cold surface, the surface
temperature, and flow conditions, for
simplicity the process line is assumed to
be a straight line. The heat and mass
transfer rates can be expressed in terms
of the initial and final conditions by
applying the conservation of mass and
conservation of energy equations as
given below:
• By applying mass
• balance for the water:

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(PROCESS O-C):
• It can be observed that the cooling and
de-humidification process involves both
latent and sensible heat transfer
processes, hence, the total, latent and
sensible heat transfer rates (Qt, Ql and
Qs) can be written as:

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(PROCESS O-C):
By separating the total heat transfer rate from
the cooling coil into sensible and latent heat
transfer rates, a useful parameter called
Sensible Heat Factor (SHF) is defined. SHF is
defined as the ratio of sensible to total heat
transfer rate, i.e.,
From the above equation, one can deduce that a SHF of 1.0 corresponds
to no latent heat transfer and a SHF of 0 corresponds to no sensible heat
transfer. A SHF of 0.75 to 0.80 is quite common in air conditioning
systems in a normal dry-climate. Lower value of SHF, say 0.6, implies a
high latent heat load such as that occurs in a humid climate.

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(PROCESS O-C):
• The amount of moisture that is removed depends
on several factors including:
•The temperature of the
cooling fluid
•The depth of the coil
•Whether the fins are flat or
embossed
•The air velocity across the
coil.

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2.4HEATING AND HUMIDIFICATION
(PROCESS O-D):

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2.4HEATING AND HUMIDIFICATION
(PROCESS O-D):
During winter it is essential
to heat and humidify the
room air for comfort. As
shown in Figure (2.4), this
is normally done by first
sensibly heating the air
and then adding water
vapor to the air stream
through steam nozzles as
shown in the figure.

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2.5 COOLING & HUMIDIFICATION
(PROCESS O-E):

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(PROCESS O-E):
As the name implies, during this process, the air temperature drops and
its humidity increases. This process is shown in Figure (2.5). As shown
in the figure, this can be achieved by spraying cool water in the air
stream. The temperature of water should be lower than the dry-bulb
temperature of air but higher than its dew-point temperature to avoid
condensation (TDPT < Tw < TO).

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(PROCESS O-E):
It can be seen that during this process there is sensible heat
transfer from air to water and latent heat transfer from water
to air. Hence, the total heat transfer depends upon the water
temperature. If the temperature of the water sprayed is equal
to the wet-bulb temperature of air, then the net transfer rate
will be zero as the sensible heat transfer from air to water will
be equal to latent heat transfer from water to air. If the water
temperature is greater than WBT, then there will be a net heat
transfer from water to air.
If the water temperature is less than WBT, then the net heat
transfer will be from air to water. Under a special case when the
spray water is entirely re-circulated and is neither heated nor
cooled, the system is perfectly insulated and the make-up water is
supplied at WBT, then at steady-state, the air undergoes an
adiabatic saturation process, during which its WBT remains
constant

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2.6 HEATING AND DE-HUMIDIFICATION
(PROCESS O-F):

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2.6 HEATING AND DE-
HUMIDIFICATION (PROCESS O-F):
This process can be achieved by using a hygroscopic material, which
absorbs or adsorbs the water vapor from the moisture. If this process
is thermally isolated, then the enthalpy of air remains constant, as a
result the temperature of air increases as its moisture content
decreases as shown in Figure (2.6). This hygroscopic material can be
a solid or a liquid. In general, the absorption of water by the
hygroscopic material is an exothermic reaction, as a result heat is
released during this process, which is transferred to air and the
enthalpy of air ……………….increases.

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2.7 MIXING OF AIR STREAMS:

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2.7.1 MIXING OF AIR STREAMS:
• Mixing of air streams at different states is commonly
encountered in many processes, including in air
conditioning. Depending upon the state of the
individual streams, the mixing process can take place
with or without condensation of moisture.
• Without condensation: Figure (2.7.1), (2.7.2) shows
an adiabatic mixing of two moist air streams during
which no condensation of moisture takes place. As
shown in the figure, when two air streams at state
points 1 and 2 mix, the resulting mixture condition 3
can be obtained from mass and energy balance.
• From the mass balance of dry air and water vapor:

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2.7.2 MIXING OF AIR STREAMS:

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2.8 AIR WASHERS:
• An air washer is a device for conditioning air.
As shown in Figure (2.8), in an air washer air
comes in direct contact with a spray of water
and there will be an exchange of heat and
mass (water vapor) between air and water.
The outlet condition of air depends upon the
temperature of water sprayed in the air
washer. Hence, by controlling the water
temperature externally, it is possible to control
the outlet conditions of air, which then can be
used for air conditioning purposes.:

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2.8 AIR WASHERS:

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2.8 AIR WASHERS:
• In the air washer, the mean temperature of water droplets
in contact with air decides the direction of heat and mass
transfer. As a consequence of the 2nd law, the heat transfer
between air and water droplets will be in the direction of
decreasing temperature gradient. Similarly, the mass
transfer will be in the direction of decreasing vapor
pressure gradient. For example,
A. Cooling and dehumidification: tw < tDPT.
• Since the exit enthalpy of air is less than its inlet value,
from energy balance it can be shown that there is a transfer
of total energy from air to water. Hence to continue the
process, water has to be externally cooled. Here both latent
and sensible heat transfers are from air to water. This is
shown by Process O-A in Figure (2.11).

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2.8 AIR WASHERS:
• In the air washer, the mean temperature of water
droplets in contact with air decides the direction of heat
and mass transfer. As a consequence of the 2nd law,
the heat transfer between air and water droplets will be
in the direction of decreasing temperature gradient.
Similarly, the mass transfer will be in the direction of
decreasing vapor pressure gradient. For example,
B. Adiabatic saturation: tw = tWBT.
• Here the sensible heat transfer from air to water is
exactly equal to latent heat transfer from water to air.
Hence, no external cooling or heating of water is
required. That is this is a case of pure water
recirculation. This is

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2.8 AIR WASHERS:
• In the air washer, the mean temperature of water
droplets in contact with air decides the direction of
heat and mass transfer. As a consequence of the 2nd
law, the heat transfer between air and water droplets
will be in the direction of decreasing temperature
gradient. Similarly, the mass transfer will be in the
direction of decreasing vapor pressure gradient. For
example,
C. Cooling and humidification: tDPT < tw < tWBT.
• Here the sensible heat transfer is from air to water and
latent heat transfer is from water to air, but the total
heat transfer is from air to water, hence, water has to
be cooled externally. This is shown by Process O-C in
Figure (2.11).

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2.8 AIR WASHERS:
• In the air washer, the mean temperature of water droplets
in contact with air decides the direction of heat and mass
transfer. As a consequence of the 2nd law, the heat transfer
between air and water droplets will be in the direction of
decreasing temperature gradient. Similarly, the mass
transfer will be in the direction of decreasing vapor
pressure gradient. For example,
•
D. Cooling and humidification: tWBT < tw < tDBT.
• Here the sensible heat transfer is from air to water and
latent heat transfer is from water to air, but the total heat
transfer is from water to air, hence, water has to be heated
externally. This is shown by Process O-D in Figure (2.11.
This is the process that takes place in a cooling tower. The
air stream extracts heat from the hot water coming from
the condenser, and the cooled water is sent back to the
condenser.

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2.8 AIR WASHERS:
• In the air washer, the mean temperature of water droplets in
contact with air decides the direction of heat and mass transfer. As
a consequence of the 2nd law, the heat transfer between air and
water droplets will be in the direction of decreasing temperature
gradient. Similarly, the mass transfer will be in the direction of
decreasing vapor pressure gradient. For example,
•
E. Heating and humidification: tw > tDBT.
• Here both sensible and latent heat transfers are from water to air,
hence, water has to be heated externally. This is shown by
Process O-E in Figure (2.11).
• Thus, it can be seen that an air washer works as a year-round air
conditioning system. Though air washer is a and extremely useful
simple device, it is not commonly used for comfort air conditioning
applications due to concerns about health resulting from bacterial
or fungal growth on the wetted surfaces. However, it can be used
in industrial applications.

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2.8 AIR WASHERS:
Various Psychrometric Processes that can take
place in an air washer

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2.9 SUMMER AIR
CONDITIONING SYSTEMS:
Figure (2.9) A Simple 100 % re-circulation Type air Condoning System

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2.9 SUMMER AIR CONDITIONING
SYSTEMS:
• 2.9.1 Simple system with 100 % re-circulated air:
• In this simple system, there is no outside air and the same
air is recirculated as shown in Figure (2.9), also shows the
process on a psychrometric chart. It can be seen that cold
and dry air is supplied to the room and the air that leaves
the condition space is assumed to be at the same
conditions as that of the conditioned space. The supply air
condition should be such that as it flows through the
conditioned space it can counteract the sensible and latent
heat transfers taking place from the outside to the
conditioned space, so that the space can be maintained at
required low temperature and humidity. Assuming no heat
gains in the supply and return ducts and no energy addition
due to fans, and applying energy balance across the room;
the Room Sensible Cooling load (Qs,r), Room Latent
Cooling Load (Ql,r) and Room Total Cooling load (Qt,r) are
given by:

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2.9 SUMMER AIR CONDITIONING SYS.:
2.9.1Simple system with 100 % re-circulated air:
The sensible and latent heat transfer rates at the cooling
coil are exactly equal to the sensible and latent heat
transfer rates to the conditioned space:
Assuming no heat gains in the supply and return ducts
and no energy addition due to fans, and applying energy
balance across the room; the Room Sensible Cooling load
(Qs,r), Room Latent Cooling Load (Ql,r) and Room Total
Cooling load (Qt,r) are given by:

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SYSTEMS:
2.9.2 System with outdoor air for ventilation:
Figure (2.9.2A) A Summer Air Conditioning System with Outdoor

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SYSTEMS:
• In actual air conditioning systems, some
amount of outdoor (fresh) air is added to take
care of the ventilation requirements. Normally,
the required outdoor air for ventilation
purposes is known from the occupancy data
and the type of the building (e.g. operation
theatres require 100% outdoor air). Normally
either the quantity of outdoor air required is
specified in absolute values or it is specified as
a fraction of the re-circulated air. See Figure
(2.9.2A), (2.9.2B)
2.9.2 System with outdoor air for ventilation:

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SYSTEMS:
Air for Ventilation and a Zero by –Pass factor
Figure (2.9.2B) A Summer Air Conditioning
System with outdoor -Air for Ventilation and a
non-zero by-pass factor

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SYSTEMS:
• Advantages and disadvantages of reheat coil:
• Refrigeration system can be operated at reasonably
high evaporator temperatures leading to high COP
and low running cost.
• However, mass flow rate of supply air increases due
to reduced temperature rise (ti-ts) across the
conditioned space
• Wasteful use of energy as air is first cooled to a lower
temperature and then heated. Energy is required for
both cooling as well as reheat coils. However, this can
be partially offset by using waste heat such as heat
rejected at the condenser for reheating of air.
• Thus the actual benefit of reheat coil depends may
vary from system.

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2.10 WINTER AIR
Figure (2.10.A) A winter air conditioning sys with a pre-heater

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2.10 WINTER AIR
• In winter the outside conditions are cold and dry. As a
result, there will be a continuous transfer of sensible heat
as well as moisture (latent heat) from the buildings to the
outside. Hence, in order to maintain required comfort
conditions in the occupied space an air conditioning
system is required which can offset the sensible and latent
heat losses from the building. Air supplied to the
conditioned space is heated and humidified in the winter
air conditioning system to the required level of temperature
and moisture content depending upon the sensible and
latent heat losses from the building. In winter the heat
losses from the conditioned space are partially offset by
solar and internal heat gains. Thus in a conservative
design of winter A/C systems, the effects of solar radiation
and internal heat gain are not considered.

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2.10 WINTER AIR CONDITIONING SYS:
Figure (2.10.B) A winter air conditioning sys with a pre-heater

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2.11ALL YEAR (COMPLETE) AIR
• Figure (2.11) shows a complete air conditioning
system that can be used for providing air conditioning
throughout the year, i.e., during summer as well as
winter. As shown in the figure, the system consists of
a filter, a heating coil, a cooling & dehumidifying coil, a
re-heating coil, a humidifier and a blower. In addition
to these, actual systems consist of several other
accessories such as dampers for controlling flow rates
of re-circulated and outdoor (OD) air, control systems
for controlling the space conditions, safety devices
etc. Large air conditioning systems use blowers in the
return air stream also. Generally, during summer the
heating and humidifying coils remain inactive, while
during winter the cooling and dehumidifying coil
remains inactive.

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2.11 ALL YEAR (COMPLETE) AIR
Figure (2.11.A)

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However, in some applications for precise control of
conditions in the conditioned space all the coils may
have to be made active. The blowers will remain active
throughout the year, as air has to be circulated during
summer as well as during winter. When the outdoor
conditions are favorable, it is possible to maintain
comfort conditions by using filtered outdoor air alone, in
which case only the blowers will be running and all the
coils will be inactive leading to significant savings in
energy consumption. A control system is required which
changes-over the system from winter operation to
summer operation or vice versa depending upon the
outdoor conditions.

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CONDITIONING UNIT:

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2.11 ALL YEAR (COMPLETE) AIR

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2.12 GUIDELINES FOR SELECTION OF
SUPPLY STATE AND COOLING COIL:
As much as possible the supply air
quantity should be minimized so that
smaller ducts and fans can be used
leading savings in cost of space, material
and power.

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However, the minimum amount should be
sufficient to prevent the feeling of stagnation. If
the required air flow rate through the cooling coil
is insufficient, then it is possible to mix some
amount of re-circulated air with this air so that
amount of air supplied to the conditioned space
increases. This merely increases the supply air
flow rate, but does not affect sensible and
cooling loads on the conditioned space.
Generally, the temperature rise (ti-ts) will be in
the range of 8 to 15oC.

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The cooling coil should have 2 to 6 rows for
moderate climate and 6 to 8 rows in hot and
humid climate. The by-pass factor of the coil
varies from 0.05 to 0.2. The by-pass factor
decreases as the number of rows increases and
vice versa. The fin pitch and air velocity should
be suitable.

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• If chilled water is used for cooling and
dehumidification, then the coil ADP will be
higher than about 4oC.

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Where Our Ambient Location?
O
i

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Comfort Zone as per ASHRAE
Recommendation:

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More Cooling … Means Dehumidification

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Summary of All Procceses:

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By Eng. Juma Yousef Juma
jumayjuma@gmail.com +971-50-4948385
Start Smart–Your Skills Solution

Module (1.2) psychometrics-air parameters-hvac_by ss-eng. juma

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Module (1.2) psychometrics-air parameters-hvac_by ss-eng. juma