1
Introduction
Objectives
After reading this chapter the student should be able to:
• Refresh his knowledge on the engineering basics
• Understand the laws of thermodynamics
1.1
General
Air conditioning for human comfort was considered a luxury a few decades ago, but now it has become
a necessity in life. The air conditioning industry is rapidly developing throughout the world. More than
10 million window installations are being installed each year and residential central cooling
installations are enjoying similar popularity.
Apart from reasons for comfort alone, air conditioning is commonly used nowadays in various
industries such as food, automobiles, hotels, textiles and many more. On Earth, not only pollution from
smoke is on the rise but pollution from dust is also playing havoc with our lives. Air conditioning plays
a vital role in keeping out smoke and dust which could harm our health. Similarly, air conditioning has
an important role to play in the preservation of food.
At present, there is hardly any sector of the economy that is not dependent on this industry. In fact in
most areas of industry, HVAC systems are considered to be a basic necessity.
It is thus important to become part of this industry and this course is targeted at providing you with the
basic knowledge and technology to play a role in designing, installing and commissioning HVAC
systems.
The following gives an overview of the basic principles of thermodynamics, which play a key role in
understanding HVAC systems.
1.2
Principles of Thermodynamics
1.2.1
Force, Newtons
In simple language, force is defined as a push or a pull. It is anything that has a tendency to set a body
into motion, to bring a body to rest or change the direction of any motion.
2 Practical Fundamentals of Heating, Ventilation & Air-conditioning (HVAC) For Engineers and Technicians
1.2.2
Pressure, Pascals
Pressure is the force exerted per unit area. It may be described as the measure of intensity of a force
exerted on any given point on the contact surface. Whenever a force is evenly distributed over a given
area the pressure at any point on the surface is the same. It can be calculated by dividing the total force
exerted by the area (on which the force is exerted).
Atmospheric pressure
The Earth is surrounded by an envelope of air called the atmosphere, which extends upward from the
surface of the earth. Air has mass and due to gravity exerts a force called weight. The force per unit
area is called pressure. This pressure exerted on the Earth’s surface is known as atmospheric pressure.
Gauge pressure
Most pressure measuring instruments measure the difference between the pressure of a fluid and the
atmospheric pressure. This is referred to as gauge pressure.
Absolute pressure
Absolute pressure is the sum of gauge pressure and atmospheric pressure.
Vacuum
If the pressure is lower than the atmospheric pressure, its gauge pressure is negative and the term
vacuum is applied to the magnitude of the gauge pressure when the absolute pressure is zero (i.e. there
is no air present whatsoever).
The relationships among absolute pressure, gauge pressure, atmospheric pressure and vacuum are
shown graphically in the Figure 1.1.
Figure 1.1
Relationship between absolute, gauge and vacuum pressures
In the above figure
Pa is the atmospheric pressure
Pgauge is the gauge pressure
Pab is the absolute pressure
Pvacuum is the vacuum pressure
Introduction 3
1.2.3
Density
It is defined as the mass of a substance divided by its volume or the mass per unit volume.
ρ = mass/volume
Specific Volume is defined as the reciprocal of density or volume per unit mass.
v = V/m
Specific Weight (Ws) is defined as the weight of a substance divided by its volume or the weight per
unit volume.
Ws = m/V
1.2.4
Work
If a system undergoes a displacement under the action of a force, work is said to be done; the amount
of work being equal to the product of force and the component of displacement parallel to the force. If
a system as a whole exerts a force on its surrounding and a displacement takes place, the work that is
done either by or on the system is said to be external work.
1.2.5
Energy
A body is said to possess energy when it is capable of doing work. In more general terms, energy is the
capacity of a body for producing an effect. Energy is classified as
1. Stored Energy; examples are (a) Chemical energy in fuel and (b) Energy stored in dams
2. Energy in Transition: examples are (a) Heat and (b) Work
The following are the various forms of energy.
Potential energy (P.E)
It is the energy stored in the system due to its position in the gravitational force field. If a heavy object
such as a building stone is lifted from the ground to the roof, the energy required to lift the stone is
stored in it as potential energy. This stored potential energy remains unchanged as long as the stone
remains in its position.
P×E = mgH
Where
H = height of the object above the datum
⎛ m⎞
Units ⎜ kg 2 ⎟m = N.m = Joules
⎝ s ⎠
Kinetic Energy (K.E), Joules= Newton meter
If a body weighing one kg is moving with a velocity of v m/sec with respect to the observer, then the
kinetic energy stored in the body is given by:
K.E =
1
mv 2
2
This energy will remain stored in the body as long as it continues in motion at a constant velocity.
When the velocity is zero, the kinetic energy is also zero.
Internal Energy
Molecules possess mass. They possess motion of transactional and rotational nature in liquid and
gaseous states. Owing to the mass and motion these molecules have a large amount of kinetic energy
4 Practical Fundamentals of Heating, Ventilation & Air-conditioning (HVAC) For Engineers and Technicians
stored in them. Any change in the temperature results in the change in the molecular kinetic energy
since molecular velocity is a function of temperature.
Also the molecules are attracted towards each other by forces, which are very large in their solid state
and tend to vanish once they are in a perfect gas state. In the melting of a solid or vaporization of a
liquid it is necessary to overcome these forces. The energy required to bring about this change is stored
in molecules as potential energy.
The internal energy is defined as the total energy of the body - chemical, nuclear, heat, gravitational, or
any other type of energy. This energy is stored within the body which is denoted by the symbol ‘µ’. It
is obvious from the above definition that it is impossible to measure the absolute value of the internal
energy. However, we can measure the changes occurring in the internal energy. Since thermodynamics
deals with the change in the internal energy of the system, it is important to know what causes the
internal energy to change. The change in the internal energy can be caused by either due to absorption
or release of heat in the system or the work done by or on the system., or if any matter enters or leaves
the system.
1.2.6
Heat
Heat is one of the many forms of energy. This is evident from the fact that heat can be converted into
other forms of energy and that other forms of energy can be converted into heat. Heat as molecular
energy is universally accepted and heat as internal energy of the matter is thermodynamics.
Since all other forms of energy may be converted into heat, it is considered to be energy in its lowest
form. The availability of heat energy to do work depends on temperature differential.
1.2.7
Heat capacity
It may be defined as the energy that must be added or removed from one kilogram of a substance to
change its temperature by one degree Centigrade. In refrigeration technology heat capacity is used to
determine how much heat should be removed to refrigerate various products.
1.2.7.1
Sensible heat (QS)
Heat which results in an increase or decrease in the temperature without it changing its phase is called
sensible heat. A change in sensible heat is given by the equation when there is a change in temperature
QS = m× CS (T2 – T1)
Note: CS is the heat capacity at constant pressure
m = mass of the substance in kg
(T2 – T1) = Temperature difference in °C
1.2.7.2
Latent heat (QL)
Latent heat is the heat at which a substance changes its phase without any increase or decrease in the
temperature. It is the amount of heat required to change the state of a substance.
QL = m×Cw(w2 – w1)
Note: Cw is the heat capacity of moisture
m = mass of the substance in kg
(w2 – w1) = change in moisture content in g/kg
Introduction 5
1.2.7.3
Total heat (QT)
Total heat is the sum of sensible heat and latent heat. Heat measurements are taken above a specified
datum. These measurements with water are at zero degrees C, since below this temperature water is
solid. Refrigerant heat measurements are at –400C. For example: The sensible heat, latent heat and
total heat for steam are shown in Figure 1.2 below.
QT = QS + QL
Figure 1. 2
Total Heat Chart Of –400C Ice To Steam at 100 0C
a-b is sensible heat, b-c is latent heat of fusion, c-d is Sensible heat, d-e is latent heat of vaporization, ef is super heat.
1.3
Temperature and its measurement
Temperature is a property of matter. It is the measure of intensity of heat contained in matter and its
relative value. A substance is said to be hot or cold when its temperature is compared with some other
reference temperature. A high temperature indicates a high level of heat intensity or thermal pressure
and a body is said to be hot.
Like other forms of energy heat can be measured because it has quantity and intensity. Heat is not
visible but manifests itself in its effects on various substances either by changing its state or by creating
relative degrees of sensation when in contact with the human body.
Since temperature is a measure of heat content, the temperature can be measured by measuring the
effects of heat on different properties of matter as follows;
• Addition of heat increases the volume of the substance or pressure at constant volume. This
property is used for measuring the temperature with the help of a mercury thermometer.
• With the increase in temperature, the resistivity of metals increases which is utilized in
resistance thermometers
• If two junctions made of two dissimilar metals are maintained at different temperatures, a
current flows in the circuit. This property is used in measuring with a thermocouple.
6 Practical Fundamentals of Heating, Ventilation & Air-conditioning (HVAC) For Engineers and Technicians
• When the temperature of a substance increases, the color also changes. This property is
used for measuring the temperature in radiation pyrometers.
1.4
Pressure and temperature relationship
Water boils at 1000C when the pressure on it is atmospheric at sea level. If the pressure is increased
above the atmospheric pressure, i.e. in a deep mine shaft the boiling point increases and when the
pressure is reduced below atmospheric, i.e. on top of a mountain, it reduces.
Boiling water does not necessarily have to be hot because if there is vacuum, water boils at a very low
temperature. The same is true when it comes to other liquids, such as various refrigerants. These
refrigerants have the same properties as water except their boiling point ranges are lower.
This pressure temperature relationship is used in most air conditioning and refrigeration systems.
1.5
Laws of Thermodynamics
1.5.1
First law of Thermodynamics and Energy Conservation
It is a fundamental principle that matter can neither be created nor destroyed though it may be made to
take different forms. Similarly, energy cannot be created or destroyed. It can be converted from one
form to another. The first law of thermodynamics states that the total energy in a system always
remains constant.
This law is mainly based on observation and can be best studied with the help of observations.
In the following examples, we can see that heat, work, electricity and chemical energy are various
forms of energy and they are mutually convertible.
•
•
•
•
An electric Iron converts electricity into heat.
An electric fan converts electricity into work.
Water flowing through a turbine converts its potential energy into work.
Churning of water converts work into heat.
The first law of thermodynamics can be represented by the equation:
E1 + Qa – Qt = E2
Where:
E1 is the energy possessed by the system initially
E2 is the energy possessed by the system after the work is done
Qa is the energy added to the system
Qt is the energy taken away from the system.
1.5.2
Second law of Thermodynamics
The second law of thermodynamics can be stated in a number of ways as:
• Heat flows from a body at higher temperature to a body at lower temperature irrespective of
the mass and material of the body participating in the heat transfer. This heat flow is
possible without the addition of external work.
• Work has the tendency to convert into heat but the heat cannot be converted into work.
• Every engine or a refrigerator ejects heat to the surroundings.
Introduction 7
With a brief discussion on the various thermodynamic principles, let us now study the fundamentals of
Heating, Ventilation and Air conditioning in the next chapters.
1.6
Fundamentals of Heat Transfer
1.6.1
Modes of Transferring Heat
Heat is always transferred when a temperature difference exists between two bodies. There are three
basic modes of heat transfer:
• Conduction involves the transfer of heat by the interactions of atoms or molecules of a
material through which the heat is being transferred.
• Convection involves the transfer of heat by the mixing and motion of macroscopic portions
of a fluid.
• Radiation, or radiant heat transfer, involves the transfer of heat by electromagnetic
radiation that arises due to the temperature of a body.
1.6.2
Heat Flux
The rate at which heat is transferred is represented by the symbol. Common units for heat Q transfer
rate is Watts. Sometimes it is important to determine the heat transfer rate per unit area, or heat flux,
which has the symbol. Units for heat flux are W/m2. The heat flux can be Qhf determined by dividing
the heat transfer rate by the area through which the heat is being transferred.
Where:
1.6.3
Q
Qhf = -------A
Qhf = heat flux (W/m2)
Q = heat transfer rate (W)
A = area (m2)
Thermal Conductivity
The heat transfer characteristics of a solid material are measured by a property called the thermal
conductivity (k) measured in W/m.K. It is a measure of a substance’s ability to transfer heat through a
solid by conduction. The thermal conductivity of most liquids and solids varies with temperature. For
vapors, it depends upon pressure.
1 W/(m.K) = 1 W/(m.oC) = 0.85984 kcal/(hr.m.oC)
8 Practical Fundamentals of Heating, Ventilation & Air-conditioning (HVAC) For Engineers and Technicians
Table: 1.1
Thermal conductivity values for various materials at 300 K
Material
Copper
Gold
Aluminum
Iron
Carbon steel
Stainless Steel (18/8)
Glass
Plastics
Wood (shredded/cemented)
Cork
Water
Ethylene glycol
Hydrogen
Benzene
Air
1.6.4
Thermal conductivity
W/m.K
399
317
237
80.2
43
15.1
0.81
0.2 – 0.3
0.087
0.039
0.6
0.26
0.18
0.159
0.026
Log Mean Temperature Difference (LMTD)
In heat exchanger applications, the inlet and outlet temperatures are commonly specified based on the
fluid in the tubes. The temperature change that takes place across the heat exchanger from the entrance
to the exit is not linear. A precise temperature change between two fluids across the heat exchanger is
best represented by the log mean temperature difference (LMTD or ΔTlm).
(ΔT2 - ΔT1)
Δ TLM = ---------------------------In (ΔT2 / ΔT1)
Where:
ΔT2 = the larger temperature difference between the two fluid
streams at either the entrance or the exit to the heat
exchanger
ΔT1 = the smaller temperature difference between the two
fluid streams at either the entrance or the exit to the heat
exchanger
1.6.5
Convective Heat Transfer Coefficient
The convective heat transfer coefficient (hc), defines, in part, the heat transfer due to convection. The
convective heat transfer coefficient is sometimes referred to as a film coefficient and represents the
thermal resistance of a relatively stagnant layer of fluid between a heat transfer surface and the fluid
medium. Common units used to measure the convective heat transfer coefficient are (W/m2K).
Introduction 9
Table 1.2
Typical order-of magnitude values of convective heat transfer coefficients
Type of fluid and flow
Convective heat transfer
coefficient
2
hc (W/m K)
,
Air, free convection
Water, free convection
Air or superheated steam, forced
convection
Oil, forced convection
Water, forced convection
Synthetic refrigerants, boiling
Water, boiling
Synthetic refrigerants, condensing
Steam, condensing
1.6.7
6 – 30
20 – 100
30 – 300
60 – 1800
300 – 18000
500 - 3000
3000 – 60000
1500 - 5000
6000 – 120000
Overall Heat Transfer Coefficient
In the case of combined heat transfer, it is common practice to relate the total rate of heat transfer Q the
overall cross-sectional area for heat transfer (Ao), and the overall temperature difference (ΔTo) using
the overall heat transfer coefficient (Uo). The overall heat transfer coefficient combines the heat
transfer coefficient of the two heat exchanger fluids and the thermal conductivity of the heat exchanger
tubes. Uo is specific to the heat exchanger and the fluids that are used in the heat exchanger.
Q = Uo Ao ΔTo
Where:
1.6.8
Q = The rate of heat transfer (W)
Uo = the overall heat transfer coefficient (W/m2 oK)
Ao = the overall cross-sectional area for heat transfer (m2)
ΔTo = the overall temperature difference (oK)
Bulk Temperature
The fluid temperature (Tb), referred to as the bulk temperature, varies according to the details of the
situation. For flow adjacent to a hot or cold surface, Tb is the temperature of the fluid that is "far" from
the surface, for instance, the center of the flow channel. For boiling or condensation, Tb is equal to the
saturation temperature.
1.7
Fundamentals of Fluid Flow
Fluid flow is an important part of most industrial processes; especially those involving the transfer of
heat. Frequently, when it is desired to remove heat from the point at which it is generated, some type of
fluid is involved in the heat transfer process. Examples of this are the cooling water circulated through
cooling coils in HVAC, the air flow past the heating and cooling coils, from fans and blowers, duct
work, terminal units, packaged air conditioning units etc., Unlike solids, the particles of fluids move
through piping and components at different velocities and are often subjected to different accelerations.
The basic principles of fluid flow include three concepts or principles:
(1)
The first is the principle of momentum (Equations of fluid forces)
(2)
The second is the conservation of energy (First Law of Thermodynamics).
(3)
The third is the conservation of mass (Continuity equation)
10 Practical Fundamentals of Heating, Ventilation & Air-conditioning (HVAC) For Engineers and Technicians
1.7.1
Properties of Fluids
A fluid is any substance which flows because its particles are not rigidly attached to one another. This
includes liquids, gases and even some materials which are normally considered solids, such as glass.
Fluids are materials which have no repeating crystalline structure.
There are several properties, including temperature, pressure, mass, specific volume, density, and
Buoyancy.
• Temperature was defined as the relative measure of how hot or cold a material is. It can be
used to predict the direction that heat will be transferred.
• Pressure was defined as the force per unit area. Common units for pressure are Pascal.
• Mass was defined as the quantity of matter contained in a body and is to be distinguished
from weight, which is measured by the pull of gravity on a body.
• The specific volume of a substance is the volume per unit mass of the are substance.
Typical units are m3/kg .
• Density, on the other hand, is the mass of a substance per unit volume. Typical units are
kg/m3. Density and specific volume are the inverse of one another. Both density and
specific volume is dependant on the temperature and somewhat on the pressure of the fluid.
As the temperature of the fluid increases, the density decreases and the specific volume
increases. Since liquids are considered incompressible, an increase in pressure will result in
no change in density or specific volume of the liquid. In actuality, liquids can be slightly
compressed at high pressures, resulting in a slight increase in density and a slight decrease
in specific volume of the liquid.
• Buoyancy is defined as the tendency of a body to float or rise when submerged in a fluid.
When a body is placed in a fluid, it is buoyed up by a force equal to the weight of the water
that it displaces.
• Compressibility is the measure of the change in volume a substance undergoes when a
pressure is exerted on the substance. Liquids are generally considered to be incompressible.
For instance, a pressure of 1110 kg/ cm 2 will cause a given volume of water to decrease by
only 5% from its volume at atmospheric pressure. Gases on the other hand, are very
compressible. The volume of a gas can be readily changed by exerting an external pressure
on the gas.
1.7.2
Pascal’s Law
Pascal's law, or the Principle of transmission of fluid-pressure, states that "pressure exerted anywhere
in a confined incompressible fluid is transmitted equally in all directions throughout the fluid such that
the pressure ratio (initial difference) remains the same."
where
ΔP is the hydrostatic pressure (given in pascals in the SI system), or the difference in pressure at two
points within a fluid column, due to the weight of the fluid;
ρ is the fluid density (in kilograms per cubic meter in the SI system);
g is acceleration due to gravity (normally using the sea level acceleration due to Earth’s gravity in
metres per second squared);
Δh is the height of fluid above the point of measurement, or the difference in elevation between the
two points within the fluid column (in metres in SI).
Introduction 11
1.7.3
Control Volume
In thermodynamics, a control volume was defined as a fixed region in space where one studies the
masses and energies crossing the boundaries of the region. This concept of a control volume is also
very useful in analyzing fluid flow problems. The boundary of a control volume for fluid flow is
usually taken as the physical boundary of the part through which the flow is occurring.
The control volume concept is used in fluid dynamics applications, utilizing the continuity,
momentum, and energy principles
1.7.4
Volumetric Flow Rate
The volumetric flow rate V of a system is a measure of the volume of fluid passing a point in the
system per unit time. The volumetric flow rate can be calculated as the product of the cross sectional
area (A) for flow and the average flow velocity (v).
˙V = A x v
The area is measured in square meter and velocity in meters per second, results in volumetric flow rate
measured in cubic meter per second. Other common units for volumetric flow is liters per minute.
1.7.5
Mass Flow Rate
The mass flow rate (m ) of a system is a measure of the mass of fluid passing a point in the system per
unit time. The mass flow rate is related to the volumetric flow rate.
Mass flowrate = Density x Volumetric flowrate
m=ρxV
The volumetric flow rate is in m 3 /s and the density is kg/m 3 results in mass flow rate measured in
kilograms per second
1.7.8
Conservation of Mass
In thermodynamics, we know that the energy can neither be created nor destroyed, only changed from
one form to another form. The same is true for mass. Conservation of mass is a principle of
engineering that states that all mass flow rates into a control volume are equal to all mass flow rates out
of the control volume plus the rate of change of mass within the control volume.
Δm
mIN = mOUT + ------Δt
1.7.9
Steady-State Flow
Steady-state flow refers to the condition where the fluid properties at any single point in the system do
not change over time. These fluid properties include temperature, pressure, and velocity. One of the
most significant properties that is constant in a steady-state flow system is the system mass flow rate.
This means that there is no accumulation of mass within any component in the system.
12 Practical Fundamentals of Heating, Ventilation & Air-conditioning (HVAC) For Engineers and Technicians
1.7.10
Continuity Equation
The continuity equation is simply a mathematical expression of the principle of conservation of mass.
For a control volume that has a single inlet and a single outlet, the principle of conservation of mass
states that, for steady-state flow, the mass flow rate into the volume must equal the mass flow rate out.
The continuity equation for this situation is expressed by the following equation:
mIN = mOUT
ρ x A x v (inlet) = ρ x A x v (Outlet)
1.7.11
Head Loss
Head loss is a measure of the reduction in the total head (sum of elevation head, velocity head and
pressure head) of the fluid as it moves through a fluid system. Head loss is unavoidable in real fluids. It
is present because of: the friction between the fluid and the walls of the pipe; the friction between
adjacent fluid particles as they move relative to one another; and the turbulence caused whenever the
flow is redirected or affected in any way by such components as piping entrances and exits, pumps,
valves, flow reducers, and fittings.
1.7.12
Frictional Loss
Frictional loss is that part of the total head loss that occurs as the fluid flows through straight pipes.
The head loss for fluid flow is directly proportional to the length of pipe, the square of the fluid
velocity, and a term accounting for fluid friction called the friction factor. The head loss is inversely
proportional to the diameter of the pipe.
Lv2
Heat loss ∝ f ------------D
1.7.13
Frictional Factor
The friction factor has been determined to depend on the Reynolds number for the flow and the degree
of roughness of the pipe’s inner surface. The quantity used to measure the roughness of the pipe is
called the relative roughness, which equals the average height of surface irregularities “∈”divided by
the pipe diameter “D”
∈
Relative Roughness = ---------------D
The value of the friction factor is usually obtained from the Moody Chart.
Introduction 13
1.7.14
Darcy’s Equation
The frictional head loss can be calculated using a mathematical relationship that is known as Darcy’s
equation for head loss. The equation takes two distinct forms. The first form of Darcy’s equation
determines the losses in the system associated with the length of the pipe.
L
v2
Hf = f ----- x -----D
2g
Where:
1.7.15
f = friction factor (unitless)
L = length of pipe (meters)
D = diameter of pipe (meters)
v = fluid velocity (m/sec)
g = gravitational acceleration (m/sec2)
Minor Losses
The losses that occur in pipelines due to bends, elbows, joints, valves, etc. are sometimes called minor
losses. This is a misnomer because in many cases these losses are more important than the losses due to
pipe friction, considered in the preceding section. For all minor losses in turbulent flow, the head loss
varies as the square of the velocity. Thus a convenient method of expressing the minor losses in flow is
by means of a loss coefficient (k). Values of the loss coefficient (k) for typical situations and fittings is
found in standard handbooks. The form of Darcy’s equation used to calculate minor losses of
individual fluid system components is expressed by Equation:
v2
Hf
1.7.16
= k -----------2g
Equivalent Piping Length
Minor losses may be expressed in terms of the equivalent length (Leq) of pipe that would have the same
head loss for the same discharge flow rate. This relationship can be found by setting the two forms of
Darcy’s equation equal to each other.
L
v2
Hf = f ----- x -----D
2g
v2
= k ------------2g
This yields two relationships that are useful.
D
Leq
Leq = k --------- and k = f -------------f
D
14 Practical Fundamentals of Heating, Ventilation & Air-conditioning (HVAC) For Engineers and Technicians
2
Psychrometry
Objectives
At the conclusion of this chapter, students should be able to:
• Understand psychrometry and read a psychrometric chart.
• Construct a psychrometric chart
• Describe various psychrometric processes
• Understand various air-conditioning systems
2.1
Introduction to psychrometry
Psychrometry is a science that involves the property of moist air (a mixture of dry air and water vapor)
and the process in which the temperature and/or the water vapor content of the mixture are changed.
As per ASHRAE definition, the psychrometry as that branch of physics concerned with the
measurement or determination of atmospheric conditions, particularly the moisture in the air.
The Psychrometric chart is a convenient tool for determining the moist air psychrometric properties
and visualizing the changes of moist air properties in various sequences of psychrometric processes.
These charts are also drawn on the basis of specified barometric pressure or elevation with respect to
the sea level.
The Psychrometric tables exhibit more accurate changes occurring in air and moisture mixtures in the
air conditioning processes, but the psychrometric charts are more convenient to use in all practical
purposes.
2.2
The properties of air
The atmospheric air is a mixture of dry air and water vapor (moisture). The air in natural state; always
contain certain amount (3.5%) of water vapor. The dry air and water vapor, do not react chemically
with one another. Although they are present as mixture, each acts independent of the other.
2.2.1
Dalton’s law
Dalton’s law states that two gases can occupy the same space (Volume) at the same time, but each acts
independently of the other and each exerts its own pressure.
16 Practical Fundamentals of Heating, Ventilation & Air-conditioning (HVAC) For Engineers and Technicians
Total pressure = Partial pressure of dry air + partial pressure of water vapor.
In common usage, total pressure is referred to as “Barometric Pressure” or “Atmospheric Pressure”.
2.2.2
Air density and specific volume
Air has its own weight.
The density of standard air is 1.2 kg/m3 and specific volume 0.83 m3/kg
For example, a fan in an air conditioning system is 300 m3/min,
Then the weight of the air handled will be 300 x 1.2 = 360 kg/min.
2.2.3
Dry air
The dry air in the atmosphere is mixture of oxygen (21%) and nitrogen (78%). The balance (1%)
consists of other gases, such as argon, carbon dioxide, hydrogen etc. Both oxygen and nitrogen are in
highly superheated state and therefore, the dry air is also in super heated state. Due to this state, the air
conditioning processes make only slight changes in the density/ volume of dry air.
When dry air is heated or cooled, only the sensible heat is added or deleted, without any effect on the
latent heat.
The specific heat of dry air = 0.133 kcal/kg
2.2.4
Moist air
It is a mixture of dry air and water vapor. The content of water vapor depends upon the temperature of
air and its quantity may change from zero to maximum, i.e the saturation capacity of air.
The mass of water vapor associated with the dry air is not constant. But how the water vapor is added
to the dry air? The following points will illustrate how this is being carried out:
(a) The water vapor constantly evaporating from the lake, sea and oceans into the earth’s
atmosphere and returns as precipitation to the earth.
(b) Water vapor is added to the air from our homes, buildings by infiltration, perspiration,
respiration, cooking, cloth washing, plants and trees from residential areas and forest.
(c) Water vapor is added to the air from the building materials and furnishings
(d) Water vapor is added to the air by humidification or evaporative cooling processes.
The table below shows the composition of the water vapor for calculating the molecular mass.
Table 2.1
Composition of water vapor
Substance
Hydrogen (H2)
Oxygen (O)
Total
Atoms
2
1
Atomic mass
1.00794
15.9994
Molecular mass
2.01588
15.99940
18.01528
The pressure exerted by the water vapor in a mixture of air, will depend upon the amount of vapor
present or the percentage of saturation. It is a known fact that the saturation pressure will be achieved
only if the water and vapor formed are inside a container. Therefore, it is obvious that the pressure of
water vapor present in atmosphere need not be the saturation pressure at the corresponding
temperature.
Psychrometry 17
The density of water vapor is very low and it is 0.0253 kg/m3. So the smaller units of grams (g) or
grains (gr) are used to express its density.
(1 Lb = 7000 grains) (1 grain=0.06g).
The following table shows the saturated water vapor and density at different temperatures.
Table 2.2
Saturated vapor pressure
2.2.5
Dry Bulb Temperature, t db
Dry-bulb temperature is the temperature of the air measured by an ordinary thermometer or a
temperature sensor like thermocouple, thermister, RTD, bi-metal and mercury bulb
It is the true temperature of moist air at rest, and not subjected to evaporation, condensation or
radiation.
Since air is a mixture of dry air and water vapor, the dry-bulb temperature is the temperature of not
only the dry air component but also the temperature of the water –vapor component.
18 Practical Fundamentals of Heating, Ventilation & Air-conditioning (HVAC) For Engineers and Technicians
The usage of dry-bulb temperature measurement;
(a)
In calculating the sensible energy knowing the beginning and ending points, the mass
flow of air, and the specific heat capacity of the moist air.
q = m Cp (t2 – t1)
Where:
(b)
(c)
2.2.6
q = Sensible heat
m = Mass of dry-air)
Cp = Specific heat of water vapor
t2 = Entry temperature
t1 = Exit temperature
In psychrometric charts as bottom X-axis coordinate, to calculate other properties of
moist air
To calculate Enthalpy of mixed air (dry air + Water vapor) at a particular temperature
measured.
Wet Bulb Temperature, twb
The temperature measured by the thermometer with its bulb covered with a wet cloth and exposed to a
current of moving air at 3 to 4 m/s is known as wet bulb temperature (WBT).
As the air passes over the wet wick of the thermometer the water of the wick tends to evaporate. The
cooling effect of evaporation lowers the temperature measured by the wet bulb thermometer
corresponding to the rate of evaporation. When the temperature measured by the WBT reaches a steady
state, then the heat absorbed by the bulb for evaporation of water vapor is equal to the heat given by air
(by convection) to the thermometer. This means that the total heat of air leaving the thermometer
remains constant.
The heat necessary to cause evaporation in the manner stated above is present in air in the form of
sensible heat. During the process of evaporation, sensible heat is converted into latent heat of
vaporization maintaining the total heat of air constant. This conversion to latent heat is accomplished
without change in total heat.
The evaporation rate from the wet wick depends on the condition of the air passing over it. If the
surrounding air is dry then the evaporation rate will be more rapid and the drop in temperature
(difference between temp. measured by WBT and DBT) will be appreciable. When the surrounding air
is moist, then the evaporation rate will be slower; so will be the drop in temperature. This shows that
the wet bulb temperature is a measure of degree of saturation or the relative humidity. Air with high
relative humidity will have lesser drop in temperature compared to air with low relative humidity. Air
with 100% relative humidity will have no drop in temperature.
The equipment used for measuring dry bulb temperature and wet bulb temperature simultaneously is
called a psychrometer. There are different types of psychrometers, as listed below.
(a) Laboratory Psychrometer
This is a simple instrument, which houses both the dry bulb thermometer and the wet bulb
thermometer. This is generally used in college laboratories.(Figure 2.1)
(b) Sling Psychrometer
This psychrometer consists of two mercury thermometers mounted on a frame, which has a handle.
The handle of the frame helps in the rotating of the psychrometer to produce the necessary air
motion. One bulb of the two thermometers is covered with a wet wick to measure the WBT. The
Psychrometry 19
rotating motion of the sling provides necessary air velocity over the thermometers. This air
movement passing the wick helps to bring the air at temperature (WBT) in immediate contact with
the wet wick.(Figure 2.2)
(c) Aspirating Psychrometer
This is similar to the other psychrometers with the exception of the blower, which provides a rapid
motion of air over the thermometers. These types are used to measure the temperatures after a
particular period of time mostly to measure the atmospheric conditions of cities throughout the day
and year. The motor is connected to the time switch as per the interval required for the
measurement of temperature.(Figure 2.3)
Figure 2.1
Laboratory Psychrometer
Figure 2.2
Sling Psychrometer
20 Practical Fundamentals of Heating, Ventilation & Air-conditioning (HVAC) For Engineers and Technicians
Figure 2.3
Aspirating Psychrometer
2.2.7
Relative Humidity
Relative Humidity can be defined in two ways:
“The ratio of the actual amount of moisture content in one unit volume of dry air at a certain
temperature to the amount of moisture needed to saturate it at that temperature”
“The ratio of the actual pressure of water vapor of a certain unsaturated moist air at a given
temperature to the vapor pressure when saturated at the same temperature”.
Relative humidity signifies the absorption capacity of air. More moisture will be absorbed by air if the
initial relative humidity is less. It is derived by the equation:
PV
φ = ----------------PVS
Where
Pv is vapor pressure;
Pvs is saturated vapor pressure.
Referring to the table on saturated vapor pressure shown;
At 21.1ºC (70ºF), the air is holding 0.072 g/cc of moisture and that is saturated
At 26.7 ºC (80ºF), the air is holding 0.098 g/cc of moisture and that is saturated
For the above two conditions, the relative humidity is 100%
Psychrometry 21
Here, if we need to increase the temperature only from 21.1ºC to 26.7ºF without increasing the
moisture content, then the relative humidity will be:
Table 2.3
Relationship between temperature, density and RH
Temperature ºC
(ºF)
Actual Density
g/cc
21.1 (70)
26.7 (80)
32.2 (90)
37.8 (100)
0.072
0.072
0.072
0.072
Saturated
Density
g/cc
0.072
0.098
0.133
0.177
Relative
Humidity %
100%
73.4%
54.1%
40.7%
The above chart indicate that if we increase the temperature of the air without increasing the moisture
content, the relative humidity comes down and actually the air being dried. Drying is really means the
removal or reduction of water vapor content
As the temperature increases, the amount of water vapor needed for saturation also increases. When we
push more water vapor into the same volume of air, the pressure exerted by the water vapor increases.
This is seen from the table also. But increasing the temperature without adding water vapor, the
pressure increase is appreciably low.
2.2.8
Dew point
The temperature at which the water vapor contained in an air sample just starts to condense is called its
“Dew Point”. Another defining statement is that the dew point is the temperature at which the moisture
contained in the air at a particular temperature becomes saturated.
When the R.H value reaches 100% with respect to the temperature and the moisture content, for
example, at 21.1ºC, the moisture content is 0.072 g/cc, the relative humidity is 100% and any further
cooling of air below 21.1ºC, some of the moisture will condense into water.
Any object at a temperature below the dew point of the surrounding air, it will condense some moisture
out of the air. the sweating observed on the outside of a glass of ice water is due to the condensation of
moisture from air on to the cold surface of the glass.
When we need to remove moisture from the air and condense it to liquid water, only the latent heat of
the amount of water vapor to be condensed has also to be removed.. Since the latent heat of water
being high,-an average of 555 kacl/kg of moisture-the load on the cooling system increases.
2.2.9
Humidity ratio
Humidity ratio is the ratio of the mass of water vapor to the mass of dry air, in a sample or volume of
moist air.
mwv
W = ----------mda
Where;
W = Humidity ratio in kgwv/kgda
mwv = mass of water vapor in the space or sample of moist air
mda = mass of dry air in the space or sample of moist air.
The measurement of humidity ratio can be done by utilizing “Gravimetric Hygrometer”
22 Practical Fundamentals of Heating, Ventilation & Air-conditioning (HVAC) For Engineers and Technicians
The following equations are derived from the humidity ratio, water vapor pressure and relative
humidity.
The Ideal gas equation:
pV = mRT
Humidity ratio:
W = mwv/mda
Rearranging the ideal gas equation:
m = pV/RT
As “m” mass represents the mass of air, which is the sum of mwv & mda, we can
individually write the equation as:
Mass of water vapor
mwv = pwv Vwv / Rwv Twv
mda = pda Vda / Rda Tda
As per Dalton’s Law, the water vapor and dry air occupy the same volume and are at the same
temperature. Therefore eliminating the volume and temperatures terms:
mwv = pwv / Rwv
mda = pda / Rda
Here again, Rwv and Rda represent the Specific Gas Constant for water vapor and dry air.
Therefore,
Rwv = R / M,
Where,
R = Molecular Gas Constant
Mda= Molecular mass of dry air
Mwv= Molecular mass of water vapor
Therefore,
Rda
Rwv
Therefore,
= 8314.1
= 28.9645
= 18.01528
= 8314.1 / 28.9645 = 287.04 J/kg.K
= 8314.1 / 18.0152 = 461.520 J/kg.K
mwv
pwv / Rwv
W = ----------- = ------------------------ =
mda
pda / Rda
pwv
W = ----------- X
pda
Rda
287.04 x pwv
----------- = ---------------------------Rwv
461.52 x pda
pwv
W = 0.62198 x ---------------pda
Total pressure, pbar = pwv + pda (or) pda = pbar – pwv
0.62198 x pwv
The equation for humidity ratio can be written as W = ---------------------pbar - pwv
Psychrometry 23
Since, RH equation, 100 x pwv = RH x pwvs,
0.62198 x RH x pwvs, / 100
Then W =-----------------------------------------pbar – RH x pwvs, / 100
Where: pwv
pda
pbar
pwvs
R.H
= Partial pressure of the water-vgapor component of moist air mixture
= Partial pressure of the dry air component of the moist air mixture
= Total pressure, i.e., atmospheric or barometric pressure
= Partial pressure of saturated water vapor at dry-bulb temperature
= Relative humidity expressed in percentage
Humidity ratio is sometimes incorrectly called “Specific Humidity” or “Absolute Humidity”
To avoid this confusion, both specific humidity and absolute humidity is defined as follows:
“Specific humidity is the weight of water vapor in unit mass of dry air (g/kg)”
“Absolute humidity is the weight of moisture per unit volume of dry air (g/cc)”
2.2.10
Sensible heat flow
Sensible heat is dry heat causing change in temperature but not in the moisture content. The sensible
heat flow can be expressed as
Qs = cp ρ q Δt / 3600
Where:
2.2.11
Qs = sensible heat flow (kW)
cp = specific heat of air (kJ/kg K) = 1.0 kJ/kg.k
ρ = air density at standard conditions = 1.202kg/m3
q = air flow (m3/hr)
Δt = temperature (oC)
Latent heat flow
Latent heat is the heat, when supplied to or removed from air, results in a change of moisture content the temperature of the air is not changed
The latent heat flow can be expressed as:
Ql = hwe ρ q Δx / 3600
Where
2.2.12
Ql = latent heat flow (kW)
hwe = 2465.56 - latent heat of vaporization of water (kJ/kg)
ρ = 1.202 - air density at standard conditions (kg/m3)
q = air flow (m3/hr)
Δx = humidity ratio difference (kg water/kg dry air)
Specific Volume
The Specific volume in psychrometrics is the volume per unit mass of the dry air component and
expressed as m3 / kgda
The specific volume is used in process calculations in converting between moist air volumetric flow
(m3/s) and the mass flow (kgda/s) of the dry air component.
24 Practical Fundamentals of Heating, Ventilation & Air-conditioning (HVAC) For Engineers and Technicians
The equation for specific volume , applying Ideal Gas Equation is as follows:
Rda x T
287.055 J/ (kg.k) x (Tc + 273.15)
v = ----------------- = -------------------------------------------- m3/kgda
pbar - pwv
(pbar - pwv)
2.2.13
Enthalpy or Heat content of air
“The enthalpy of moist air is the sum of the enthalpy of the dry air and the enthalpy of the water
vapour. In atmospheric air, water vapor content varies from 0 to 3% by mass. The enthalpy of moist air
includes the:
1. Enthalpy of the dry air – The sensible heat
2. Enthalpy of water vapor – The latent heat
For moist air, the enthalpy of dry air is given a zero value at 0°C, and for water vapour the enthalpy of
saturated water is taken as zero at 0°C.
The enthalpy of moist air is given by (h)
h = ha + W hw
Where:
h = specific enthalpy of moist air (kJ/kg)
ha = specific enthalpy of dry air (kJ/kg)
W = humidity ratio ( kgwv / kgda)
hw = specific enthalpy of water vapor (kJ/kg)
Specific Enthalpy of Dry Air - Sensible Heat (ha)
Assuming constant pressure conditions the specific enthalpy of dry air can be expressed as:
ha = cpa t
Where:
cpa = specific heat capacity of air at constant pressure (kJ/kg°C)
(For air temperature between -100°C and 100°C the specific heat capacity can be set to
cpa = 1.006 (kJ/kg°C)
t = air temperature (°C)
Specific Enthalpy of Water Vapor - Latent Heat (hw)
Assuming constant pressure conditions the specific enthalpy of water vapor can be expressed as:
hw = cpw t + hwe
Where
cpw = specific heat of water vapor at constant pressure (kJ/kg°C)
t = water vapor temperature (°C)
hwe = evaporation heat of water at °C (kJ/kg)
For water vapor the specific heat capacity can be set to cpw = 1.84 kJ/kg°C
The heat of evaporation (water at °C) can be set to hwe = 2501 kJ/kg
Psychrometry 25
Therefore, the enthalpy of moist air is summed up as:
h = cpa t + W [cpw t + hwe]
Where
cpa= specific heat of dry air at constant pressure, kJ/kg°C, 1.006 kJ/kg°C
cpw= specific heat of water vapor, kJ/kg°C, 1.84 kJ/Kg°C
t = Dry-bulb temperature of air-vapor mixture, °C
W = Humidity ratio, kg of water vapor/kg of dry air
hwe = enthalpy of water vapor at temperature t, kJ/kg
The unit of h is kJ/kg of dry air. Substituting the approximate values of cpa and cpw ,we obtain:
h = 1.006 t + W (1.84 t + 2501)
2.3
Understanding the psychrometric charts
2.3.1
Dry Bulb Temperature – Tdb
The Dry Bulb temperature, usually referred to as air temperature, is the air property that is most
common used. When people refer to the temperature of the air, they are normally referring to its dry
bulb temperature.
Figure 2.4
Dry-bulb Temperature
The Dry Bulb Temperature refers basically to the ambient air temperature. It is called "Dry Bulb"
because the air temperature is indicated by a thermometer not affected by the moisture of the air.
Dry-bulb temperature - Tdb, can be measured using a normal thermometer freely exposed to the air but
shielded from radiation and moisture. The temperature is usually given in degrees Celsius (oC) or
degrees Fahrenheit (oF). The SI unit is Kelvin (K). Zero Kelvin equals to - 273oC.
The dry-bulb temperature is an indicator of heat content and is shown along the bottom axis of the
psychrometric chart. Constant dry bulb temperatures appear as vertical lines in the psychrometric chart.
26 Practical Fundamentals of Heating, Ventilation & Air-conditioning (HVAC) For Engineers and Technicians
2.3.2
Wet Bulb Temperature - Twb
The Wet Bulb temperature is the temperature of adiabatic saturation. This is the temperature indicated
by a moistened thermometer bulb exposed to the air flow.
Wet Bulb temperature can be measured by using a thermometer with the bulb wrapped in wet muslin.
The adiabatic evaporation of water from the thermometer and the cooling effect is indicated by a "wet
bulb temperature" lower than the "dry bulb temperature" in the air.
Figure 2.5
Wet-Bulb Temperature
The rate of evaporation from the wet bandage on the bulb, and the temperature difference between the
dry bulb and wet bulb, depends on the humidity of the air. The evaporation is reduced when the air
contains more water vapor.
The wet bulb temperature is always lower than the dry bulb temperature but will be identical with
100% relative humidity (the air is at the saturation line).
Combining the dry bulb and wet bulb temperature in a psychrometric diagram or Mollier chart, gives
the state of the humid air. Lines of constant wet bulb temperatures run diagonally from the upper left to
the lower right in the Psychrometric Chart.
2.3.3
Dew Point Temperature - Tdp
The Dew Point is the temperature at which water vapor starts to condense out of the air, the
temperature at which air becomes completely saturated. Above this temperature the moisture will stay
in the air.
Psychrometry 27
Figure 2.6
Dew Point Temperature
If the dew-point temperature is close to the air temperature, the relative humidity is high, and if the
dew point is well below the air temperature, the relative humidity is low.
If moisture condensates on a cold bottle from the refrigerator, the dew-point temperature of the air is
above the temperature in the refrigerator.
The Dew Point temperature can be measured by filling a metal can with water and ice cubes. Stir by a
thermometer and watch the outside of the can. When the vapor in the air starts to condensate on the
outside of the can, the temperature on the thermometer is pretty close to the dew point of the actual
air.
The Dew Point is given by the saturation line in the psychrometric chart.
2.3.4
Humidity Ratio or Moisture content
Specific Humidity is the water vapor content of air, given in grams of water vapor per kg of dry air
(i.e., kg of moisture/kg of dry air). It is also known as moisture content or humidity ratio. Air at a given
temperature can support only a certain amount of moisture and no more. This is referred to as the
saturation humidity.
Humidity ratio is represented on the chart by lines that run horizontally and the values are on the right
hand side (Y-axis) of the chart increasing from bottom to top.
28 Practical Fundamentals of Heating, Ventilation & Air-conditioning (HVAC) For Engineers and Technicians
Figure 2.7
Humidity Ratio or Moisture content
2.3.5
Specific Air Volume
Specific Volume is the volume that a certain weight of air occupies at a specific set of conditions. The
specific volume of air is basically the reciprocal of air density.
As the temperature of the air increases, its density will decrease as its molecules vibrate more and take
up more space (as per Boyle’s law). Thus the specific volume will increase with increasing
temperature.
Since warm air is less dense than cool air which causes warmed air to rise. This phenomenon is known
as thermal buoyancy. By similar reasoning, warmer air has greater specific volume and is hence lighter
than cool air.
The specific volume of air is also affected by humidity levels and overall atmospheric pressure. The
more the moisture vapor present in the air, the greater shall be the specific volume. With increased
atmospheric pressure, the greater the density of the air - so the lower its specific volume. The unit of
measure used for specific volume is cubic meter / kg of dry air.
Specific volume is represented on Psychrometric Chart by lines that slant from the lower right hand
corner towards the upper left hand corner at a steeper angle than the lines of wet bulb temperature and
enthalpy.
Psychrometry 29
Figure 2.8
Specific Air Volume
2.3.6
Sensible Heat Ratio (SHF)
Figure 2.9
Sensible Heat Ratio
Sensible Heat, Qs
Sensible Heat Ratio = ---------------------------------------------Sensible Heat, Qs + Latent Heat, QL
30 Practical Fundamentals of Heating, Ventilation & Air-conditioning (HVAC) For Engineers and Technicians
The sensible heat ratio helps to determine the percentage of sensible heat and latent heat contribution to
the total cooling load. ASHRAE psychrometric chart uses a protractor to plot the slope of the line
representing the sensible heat ratio.
Figure 2.10
Sensible Heat RatioProtractor
2.3.7
Relative Humidity (RH)
Relative humidity (RH) is a measure of the amount of water air can hold at a certain temperature.
temperature (dry-bulb) is important because warmer air can hold more moisture than cold air.
Air
Lines of constant relative humidity are represented by the curved lines running from the bottom left
and sweeping up through to the top right of the chart. The line for 100 percent relative humidity, or
saturation, is the upper, left boundary of the chart.
Figure 2.11
Relative humidity
Psychrometry 31
2.3.8
Enthalpy
Enthalpy is the measure of heat energy in the air due to sensible heat or latent heat. Sensible heat is the
heat (energy) in the air due to the temperature of the air and the latent heat is the heat (energy) in the
air due to the moisture of the air.
The sum of the latent energy and the sensible energy is called the air enthalpy. Enthalpy is expressed in
Btu per pound of dry air (kilojoules per kilogram (kJ/kg).
Enthalpy is useful in air heating and cooling applications. Air with same amount of energy may either
be dry hot air (high sensible heat) or cool moist air (high latent heat).The enthalpy scale is located
above the saturation, upper boundary of the chart. Lines of constant enthalpy run diagonally downward
from left to right across the chart; follow almost exactly the line of constant wet bulb temperature.
The enthalpy of moist air, in kJ/kg, is therefore:
h = (1.007*t - 0.026) + g*(2501 + 1.84*t)
Where g is the water content in kg/kg of dry air
Figure 2.12
Enthalpy
32 Practical Fundamentals of Heating, Ventilation & Air-conditioning (HVAC) For Engineers and Technicians
2.3.9
Combination of properties
The chart below is the complete chart combining most of the lines and other parameters so far
discussed:
Figure 2.13
Combination of properties
1. Represents Sensible Heating
2. Sensible Heating and Humidification
3. Chemical Dehydration
4. Sensible Cooling
2.4
5. Cooling & dehumidification
6. Evaporative cooling
7. Latent heat addition-Humidification
8. Latent heat removal-Dehumidification
Psychrometric processes
The psychrometric process happens when the air at an initial state transforms and changes to final
state. The transformation of air undergoes four basic processes
(a)
Sensible heating only (Heat addition into the air takes place without altering the
moisture content)
(b)
Sensible cooing only (Heat removal from the air takes place without altering the
moisture content)
(c)
Humidification only (Latent energy addition-Latent heating only-No change in dry-bulb
temperature)
(d)
Dehumidification only (Latent energy removal-Latent cooling only-No change in dry
bulb temperature)
Figure 2.14
Four Basic Processes
Psychrometry 33
In general the above four process involves the phase changes in water content, which are represented
by the following figure:
Figure 2.15
The Phase Change of water
There are other processes involving both heat and water vapor transformation too, and they are
classified as:
Single processes
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
Cooling and Dehumidification process (involving coils in air washer with chilled spray
of water)
Evaporative cooling process (Involving adiabatic process called Sensible cooling and
humidification-Constant wet-bulb temperature)
Water spray process
Chemical Dehumidification process (Involving chemical or sorbent materials in
adiabatic dehumidification-Constant wet-bulb temperature)
Mixing of two air stream (Involving adiabatic process with no heat transfer)
Room effect (Changes to supply air due to sensible and latent heat gains in the room)
Fan Heat (Including fan, motor and drive(Similar to a sensible heating process-No
change in water vapor)
Enthalpy Wheel (Mixing process)
Two or more processes in sequence
(a)
(b)
(c)
(d)
(e)
(f)
Face & Bypass of mixing air-2 process in sequence
Return Air Face & Bypass
Reheat with cool and Dehumidification
Sensible Precooling followed by Evaporative cooling
Sensible Heating followed by Humidification
Typical air conditioning cycle
Now let us consider the first four basic processes
34 Practical Fundamentals of Heating, Ventilation & Air-conditioning (HVAC) For Engineers and Technicians
2.4.1
Sensible Heating only
Figure 2.16
Sensible Heating only
This is the process where the temperature of the air stream is increased without any change in moisture
content or specific humidity. The transfer heat into the air stream is done by one of the following
devices:
(a)
Steam coil
(b)
Hot water coil
(c)
Heat pipe
(d)
Air-to-air heat exchanger
(e)
Sensible only rotary heat wheel.
(f)
Electrical heating coil
(g)
Furnace
2.4.2
Latent Heating Process
A latent heating process occurs when water is evaporated without changing the dry bulb temperature.
This is shown as vertical line in psychrometric chart.
Figure 2.17
Latent Heating only
Psychrometry 35
2.4.3
Sensible Cooling only process (Cooling without change in water vapor
content)
The transfer of heat from air using one of the following devices:
(a)
Chilled water
(b)
Refrigerant cooling coil
(c)
Indirect evaporative cooler
(d)
Heat pipe
(e)
Air-to-air heat exchanger
(f)
Sensible only rotary heat wheel
(g)
Air washers
On a psychrometric chart, the sensible cooling process proceeds horizontally to the left along a line of
constant humidity ratio towards the saturation line. In this process, there is no change in dew-point
temperature, water vapor pressure, or humidity ratio.
The heating and cooling explained above are represented on psychrometric charts as shown in the
following figures.
2.4.4
Figure 2.18
Figure 2.19
Heating
Cooling
Heating and Humidification
In this process, the air first passes through a heating coil and then through the humidifier where steam
at a mass flow rate of required value and specific enthalpy hx is sprayed into the air stream.
The heating and humidification of the air can be considered as two separate processes in sequence.
Figure 2.20
Heating and Humidification Psychrometric chart
36 Practical Fundamentals of Heating, Ventilation & Air-conditioning (HVAC) For Engineers and Technicians
Referring to the psychrometric chart above, from point 1 to 2, the air passes through the heating coil
and the sensible heat transfer takes place without altering the moisture content. From point 2 to 3, the
moisture is added and the humidification of air occurs.
During the sensible heating process of the moist air, the energy added is calculated by the following
equation:
Q = ma (h2 – h1)
Where:
Q = Rate of energy added, KJ/hr
ma = mass flow rate of dry air through the process
h2 = Specific enthalpy of moist air downstream of heating coil
h1 = Specific enthalpy of moist air upstream of heating coil
During the humidification process, the energy equation is;
ma (h3 – h2) = mw hw
Where:
h3 = The specific enthalpy of the moist air downstream of the humidifier
h2 = Specific enthalpy of moist air upstream of the humidifier
hw = Specific enthalpy of the steam
mw = Mass flow rate of the steam
The rate of moisture addition to the air, mw, is determined by a water vapor mass balance
mw = ma (w3 – w2)
Where:
w2 = Humidity ratio of the moist air upstream of the humidifier
w3 = Humidity ratio of the moist air downstream of the humidifier
Combining the equations,
ma (h3 – h2) = mw hw and mw = ma (w3 – w2)
ma (h3 – h2) = ma (w3 – w2) x hw
h3 – h2
--------- = hw
w3 – w2
Where the left hand side of the equation represents the slope of the humidification process on a
psychrometric chart. Thus the direction of the process can be determined from the enthalpy of the
steam added to the air stream and the enthalpy-Moisture protractor on a psychrometric chart.
The specific humidity of air can also be increased by the injection of a predetermined quantity of steam
into the air. It is important here that the steam is dry and saturated and there is no condensation at all.
It is not possible to spray steam below 100ºC (at atmospheric pressure) as it is necessary to spray steam
though the nozzles, which require higher pressure than atmospheric. Hence the lowest possible
enthalpy carried with steam is the total heat of steam at 100ºC when the steam is fully dry and
saturated.
The amount of steam sprayed per kg of air is given by (W2 – W1).
Psychrometry 37
2.4.5
Cooling and Dehumidification
The removal of water vapor from air is termed dehumidification. It is only possible when the air is
cooled below its dew point temperature. For effective dehumidification, it is necessary to maintain the
cooling coil surface below the dew point temperature of air.
Figure 2.21
Cooling and Dehumidification
Let us take an example:
Air is to be cooled from 35ºC DB and 24ºC WB to 20ºC DB and 17.6ºC WB. Take a psychrometric
chart and mark these values. If we join these two points and draw a parallel line from the reference
point to intersect the sensible heat factor line, we will notice that it intersects at 0.74 indicating that
there is 26% latent heat removal and 74% sensible heat removal.
The process of cooling and dehumidification is represented in the chart as follows:
The cooling and dehumidifying process is shown in the psychrometric chart below. It begins at point 1
and ends at point 2.
The refrigeration capacity required to accomplish this QR, is obtained from the energy balance.
Energy Balance
ma h1 = QR + ma h2 + mw hw
Mass flow value for the water in the air
ma W1 = mw + ma W2
Combining the above two equations
QR = ma (h1-h2) – ma (W1-W2) hw
38 Practical Fundamentals of Heating, Ventilation & Air-conditioning (HVAC) For Engineers and Technicians
Figure2.22
Cooling and Dehumidification Psychrometric chart
Where hw is the enthalpy of saturated liquid at temperature t2.
The second term in the square bracket is the enthalpy associated with the liquid condensate as it runs
out of the cooling coil. This term is small compared to (h1-h2) which is the enthalpy difference to cool
the air and condense the water.
The approximation is often made where the process is divided into sensible (S) and latent (L),
components.
QRS = ma (h2 – ha)
and
QRL = ma (ha – h1)
Then QR = QRS + QRL
QRS
The sensible heat ratio for the process is then, SHR = --------------------------QRS + QRL
The rate at which the moisture removed from the air is:
mw = ma (W1 – W2)
2.4.6
Cooling with adiabatic humidification of air
In this process, air is passed over a spray chamber. A spray chamber is a chamber with nozzles, which
spray water. The temperature of the spraying water is more than the WBT of entering air and below the
temperature of air. When air passes over this chamber, part of the water evaporates and is carried away
by the air, increasing the specific humidity of air as shown in the figure below.
Psychrometry 39
Figure 2.23
Cooling with Adiabatic saturation
Air provides the heat required for the evaporation of water. During this time, the temperature of air
decreases keeping the total enthalpy constant.
Generally, complete humidification of the air in not possible thus the effectiveness of the spray
chamber can be defined as:
E = T1 – T3/(T1 – T2)
Where: (T1 – T3) is the actual drop in the DBT
(T1 – T2) is the ideal drop in DBT
The humidifying efficiency is given by; efficiency = 100 × E.
2.4.7
Adiabatic chemical dehumidification
When the high humid air is passed over a solid absorbent bed or a liquid absorbent spray, part of the
water vapor will be absorbed reducing the water content in the air. The latent heat released is absorbed
by air increasing its DBT and the total enthalpy remains constant. Thus the chemical
dehumidification process follows the path along the constant enthalpy line.
The effectiveness of the dehumidifier is defined as:
E = (T3 – T1)/(T2 – T1)
2.4.8
Evaporative Cooling Systems
The evaporative cooling can be classified as:
1. Direct evaporative system
2. Indirect evaporative system
3. Multi-stage evaporative systems
2.4.8.1
Direct evaporative system
The figure below shows the schematic of an elementary direct, evaporative cooling system and the
process on a psychrometric chart
40 Practical Fundamentals of Heating, Ventilation & Air-conditioning (HVAC) For Engineers and Technicians
Figure 2.24A
Direct Evaporative Cooling system
As shown in the figure, the direct evaporative cooling, the conditioned air comes in direct contact with
the wetted surface, and gets cooled and humidified.
In this process:
a. The hot and dry outdoor air is first filtered and then is brought in contact with the wetted
surface or spray of water droplets in the air washer.
b. The air gets cooled and dehumidified due to simultaneous transfer of sensible and latent
heats between air and water (process o-s).
c. The cooled and humidified air is supplied to the conditioned space, where it extracts the
sensible and latent heat from the conditioned space (process s-i).
d. Finally the air is exhausted at state i.
In an ideal case when the air washer is perfectly insulated and an infinite amount of contact area is
available between air and the wetted surface, then the cooling and humidification process follows the
constant wet bulb temperature line and the temperature at the exit of the air washer is equal to the wet
bulb temperature of the entering air (to,WBT), i.e., the process becomes an adiabatic saturation process.
However, in an actual system the temperature at the exit of the air washer will be higher than the inlet
wet bulb temperature due to heat leaks from the surroundings and also due to finite contact area.
Psychrometry 41
One can define the saturation efficiency or effectiveness of the evaporative cooling system ε as:
Where,
ε
to
tS
to,WBT
= Saturation efficiency
= Outside air entering temperature
= Supply air to conditioned space after evaporative cooling
= Saturated wet-bulb temperature
The amount of supply air required can be obtained by writing energy balance equation for the
conditioned space, i.e.
Where,
mS
Qt
hi
hS
= The amount of supply air
= The Total heat transfer rate (QS+Ql)
= Enthalpy of return or exhaust air
= Enthalpy of supply air
Advantages
(a) Compared to the conventional refrigeration based air conditioning systems, the amount of
airflow rate required for a given amount of cooling is much larger in case of evaporative
cooling systems.
(b) The evaporative coolers are very useful essentially in dry climates
Disadvantages
(a) The evaporative coolers cannot provide comfort as the cooling and humidification line lies
above the conditioned space condition ‘i’.
(b) For a given outdoor dry bulb temperature, as the moisture content of outdoor air increases,
the required amount of supply air flow rate increases rapidly
(c) The conventional refrigeration based air conditioning systems can be used in any type of
climate.
2.4.8.2
Indirect evaporative cooling system:
The figure below shows the schematic of a basic, indirect evaporative cooling system and the process
on a psychrometric chart.
As shown in the figure, in an indirect evaporative cooling process, two streams of air - primary and
secondary are used.
Stream-1-The primary air stream becomes cooled and humidified by coming in direct contact with the
wetted surface (o-o’),
Stream-2-The secondary stream which is used as supply air to the conditioned space, decreases its
temperature by exchanging only sensible heat with the cooled and humidified air stream (o-s).
The moisture content of the supply air remains constant in an indirect evaporative cooling system,
while its temperature drops. Obviously, everything else remaining constant, the temperature drop
obtained in a direct evaporative cooling system is larger compared to that obtained in an indirect
system, in addition the direct evaporative cooling system is also simpler and hence, relatively
inexpensive. However, since the moisture content of supply air remains constant in an indirect
42 Practical Fundamentals of Heating, Ventilation & Air-conditioning (HVAC) For Engineers and Technicians
evaporation process, this may provide greater degree of comfort in regions with higher humidity ratio.
The commercially available indirect evaporative coolers have saturation efficiency as high as 80%.
Figure 2.24B
Indirect Evaporative Cooling system
2.4.8.3
Multi-stage evaporative cooling systems:
Figure below shows a typical two-stage evaporative cooling system and the process on a psychrometric
chart. As shown in the figure, in the first stage the primary air cooled and humidified (o -o’) due to
direct contact with a wet surface cools the secondary air sensibly (o -1) in a heat exchanger. In the
second stage, the secondary air stream is further cooled by a direct evaporation process (1-2). Thus in
an ideal case, the final exit temperature of the supply air (t2) is several degrees lower than the wet bulb
temperature of the inlet air to the system (to).
Psychrometry 43
Figure 2.25
Two-stage Evaporative Cooling system
To improve efficiency of the evaporative cooling systems first sensibly cool the outdoor air before
sending it to the evaporative cooler by exchanging heat with the exhaust air from the conditioned
space. This is possible since the temperature of the outdoor air will be much higher than the exhaust
air. It is also possible to mix outdoor and return air in some proportion so that the temperature at the
inlet to the evaporative cooler can be reduced, thereby improving the performance. For example, one
can use multistage evaporative cooling systems and obtain supply air temperatures lower than the wet
bulb temperature of the outdoor air. Thus multistage systems can be used even in locations where the
humidity levels are high.
2.5
Air-conditioning systems- Summer and Winter
There are two basic systems in air-conditioning:
• Summer air-conditioning systems
• Winter air-conditioning systems
Lets us now briefly study the various methods used for the above air-conditioning systems.
44 Practical Fundamentals of Heating, Ventilation & Air-conditioning (HVAC) For Engineers and Technicians
2.5.1
Summer air-conditioning systems
Summer air-conditioning system for hot and dry outdoor conditions
As the name suggests these systems are used for hot and dry atmospheric conditions like
temperature of 38–42°C and relative humidity of about 20–25%.
In this process our purpose would be to reduce the air temperature and increase its relative
humidity where the required comfort conditions are 24ºC and 60% RH. The general arrangement
of the equipment and the psychrometric process are represented in the figures below.
Figure 2.26
Summer air conditioning system for hot and dry outdoor conditions
Figure 2.27
Representation of psychrometric process
Atmospheric air is passed through the dampers and gets filtered before passing over the cooling coil.
When the air is passed over the cooling coil, its temperature is reduced by sensible cooling as
represented by point 2 on the psychrometric process chart.
Psychrometry 45
The air coming out from the cooling coil at point 2 is passed into an adiabatic humidifier where the
water vapor increases the humidity of air and the conditioned air leaves the humidifier at point 3.
The efficiency of the humidifier is given by the equation:
Efficiency = [(T2 – T3) /(T2 – T5 )] × 100
If the quantity of atmospheric air supplied is V L/sec, then the capacities of the cooling coil and
the humidifier are given by:
Total capacity of cooling coil = (V / Hf) × [( h3 – h1)/1000] KW of refrigeration
Where:
V is the volume of handled air in L/sec
Hf is the density of moist air Kg/m3.
Capacity of humidifier = (V / Vs) × [(w3 – w2) / 1000] kg/sec
Summer air-conditioning system for hot and humid outdoor conditions
As the name suggests these systems are used for hot and humid atmospheric conditions like;
temperature of 32–38°C and relative humidity of about 70–75 %.
Figure 2.28
Summer air conditioning system for hot and wet weather
In this process our purpose would be to reduce the air temperature and its relative humidity where the
required comfort conditions are the same: 24ºC and 60 % RH. The general arrangement of the
equipment and the psychrometric process are represented in the following figures.
Figure 2.29
Representation of psychrometric process
46 Practical Fundamentals of Heating, Ventilation & Air-conditioning (HVAC) For Engineers and Technicians
Here the air is filtered and then passed over the cooling coil for dehumidification. As air is passed over
the cooling coil whose temperature is below the dew point temperature of incoming air, the
temperature and humidity of air is reduced and it comes out at point 3.
The capacity of the cooling coil is given by the equation;
Total capacity of cooling coil = Hf×(V) × [( h1 – h3 ) / 1000] KW of refrigeration
The air then enters the heating coil condition 3 and leaves at condition 5
Capacity of heating coil = Hf × (V) × (h5 – h3)/1000 KW
Summer air-conditioning system with single cooling coil and mixing
This type of system is used to reduce the load on the cooling coil as part of the air going out of the
room, which at a lower temperature than the outdoor condition, is mixed with fresh air. The
arrangement of the system is shown in figure and the corresponding processes are represented on the
psychometric chart
Figure 2.30
Arrangement for the components for the given air-conditioning system
Figure 2.31
Psychrometric process for the given system
Psychrometry 47
Condition (4) is the mixing of air at conditions (2) and (3). Condition (5) is the condition of air leaving
the cooling coil and 5–1 represents the heating of air passing through the blower due to friction. The
process 1–2 represents the condition of air passing through the air-conditioned room taking the load in
the room.
The details of the cooling system (refrigeration) used in single coil direct expansion system are shown
in following Figure.
Figure 2.32
Direct expansion refrigeration system for cooling and dehumidifying of hot and moist air
This system is known as a direct expansion system as the refrigerant is directly used for cooling the air
in the evaporator. But in large systems, used for comfort air-conditioning and having several cooling
coils, a centrifugal refrigeration plant processes chilled water and chilled water is further circulated to
the various cooling coils.
This system is known as an indirect cooling system. A centrifugal compressor using would be used for
producing chilling water, as it has to handle a large quantity of refrigerant.
Summer air-conditioning with single coil and bypass mixing
This system is used to control DBT in the air-conditioned room as per the load in the room. The
arrangement of the system is shown in Figure.
48 Practical Fundamentals of Heating, Ventilation & Air-conditioning (HVAC) For Engineers and Technicians
Figure 2.33
Arrangement for the components for the given air-conditioning system
Condition 4 is the mixing of air at conditions 2 and 3. Condition 5 is the condition of air coming out of
the cooling air. Condition 6 is the mixing of air at conditions 5 and 2. Process 4–5 represents the
cooling and dehumidifying of air passing through the cooling coil. Process 6-1 is heat generated by fan
and motor. Process 1–2 represents the condition of air passing through the room as it takes the load in
the room. The re-heating of air passing through the blower due to friction is neglected for plotting on
the psychrometric chart.
The previous system used has some limitations, as the temperature in the air conditioned room cannot
be controlled according to the load in the room. The control of DBT is more important than humidity
control as long as humidity is not excessive.
The present system is used during partial load operation. The face dampers on the cooling coil and
bypass dampers are controlled by a motor, which positions them so as to maintain a constant DBT. As
the sensible heat gain of the air-conditioned space decreases, more re-circulated air is bypassed.
However with direct expansion cooling coil, the air, which passes across the coil, may be more
thoroughly dehumidified than when the full air quantity is handled. Thus satisfactory space humidity
conditions may be maintained during some partial load conditions without the need for re-heating.
Summer air-conditioning with single cooling coil and absorbent dehumidifier:
The cooling coil discussed in the above methods for cooling the air, also produces some
dehumidification in conjunction with the cooling process. The dehumidification of air by a refrigerant
cooling coil has limitations. If the coil surface temperature is less than 0°C, frost
forms on the coil
and the heat transfer rate reduces. A defrosting system is required and reheating of the air is needed
before passing into the air-conditioned space. This refrigeration system becomes more complicated and
more expansive to own and operate as the required air dew point temperature is reduced.
The absorbent system shown in the following figure can reduce the required surface temperature of the
cooling coil and completely avoids the possibility of frosting the coil as the required coil temperature is
always above 0°C. Therefore this method produces extremely low air dew-point temperatures, more
reliably and more economically than the refrigeration method. The psychrometric processes for the
above-described system are shown in the following Figure.
Psychrometry 49
Figure 2.34
Arrangement for the components for the given air-conditioning system
Figure 2.35
Psychrometric Processes For Given System
Condition 4 is the mixing of airs at conditions 2 and 3. Process 4–5 represents the adiabatic
dehumidification of air passing through the absorbent dehumidifier. Process 5–6 is the sensible cooling
of air passing through the cooling coil whose surface temperature is considerably above the
temperature required for frosting. Process 6–1 is heat generated by fan and fan motor.
Process 1–2 is the condition of air passing through the air-conditioned room, taking the existing load.
Summer air-conditioning with evaporating cooling:
Comfort air-conditioning systems capable of maintaining optimum thermal conditions may be
expensive to own and operate. Partially effective systems, which involve much less costs, may be
attractive where finances preclude the installation of a completely effective system. In hot dry regions
evaporating cooling systems may be capable of providing considerable relief in enclosed spaces.
The evaporative cooling system commonly used is shown in Figure below and the corresponding
processes.
50 Practical Fundamentals of Heating, Ventilation & Air-conditioning (HVAC) For Engineers and Technicians
Figure 2.36
Arrangement for the components for the given air-conditioning system
Process 3–1 represents evaporative cooling and process 1–2 represents room load taken by the air
passing through the room. State 2 is an acceptable space condition although not necessarily an
optimum one. State 3 of the outdoor air is at a much higher temperature but lower RH than state 2. As
the air-washer is the only processing device in the system, the cost of the system is considerably lower
than the system used for optimum comfortable conditions.
Generally, a much higher flow rate of air is used with an evaporative cooling system (2 to 3 times of
conventional) than with conventional systems. A high rate of air movement past a person allows the
same degree of comfort but with higher effective temperatures as compared to the situations where airmovement is low.
2.5.2
Winter air-conditioning systems
The required comfort air conditions are the same as in summer. The typical arrangement of the
required equipment and its representation on the psychrometric chart are shown in the following Figure
below.
Figure 2.37
Winter air conditioning system
Psychrometry 51
Air is passed through the resistance heater known as the preheating coil, and then through the
humidifier. It is then passed through the second preheating coil.
Winter air-conditioning with double reheat coils and air washer
During severe winter conditions it is always necessary to increase the DBT and RH of the air. The
arrangement of the components in this system and its representation on the psychrometric chart are
shown in the following figures.
Figure 2.38
Arrangement for the components for the given air-conditioning system
Condition 4 is the mixing of air at conditions 2 and 3. Process 4–5 is the sensible heating in the preheat
coil. Process 5–6 is the adiabatic cooling of the air passing through the air washers and process 6–1 is
the sensible heating in the reheat coil. Process 1–2 is the cooling and dehumidifying of the air passing
through the conditioned room. This compensates for the heat and the vapor loss of the air in the
conditioned room. In large systems it is a common practice to use re-circulating air fans as well as
supply air fans. However this condition does not affect the process represented in the above
psychrometric chart.
Winter air-conditioning using 100% outdoor air with pre-heating (by waste heat of
the exhaust)
In designing any air-conditioning system every effort has to be made to utilize internal heat emission
wherever economically feasible. Such a system is shown in the following figure where the waste heat
from the exhaust is used for preheating fresh air.
Figure 2.39
Arrangement for the components for the given air-conditioning system
52 Practical Fundamentals of Heating, Ventilation & Air-conditioning (HVAC) For Engineers and Technicians
The air washers serve as humidifying devices to offset the moisture losses in the air conditioned space
and in addition to this, it cleans the air. The reheat coil regulates the heat supply thus controlling the
DBT of the air-conditioned space as required.
Process 4–5 is the preheating of fresh air by using the waste heat in the exhaust air. Here process 2–3
shows the cooling of exhaust air. Process 5–6 is the humidification of air by using steam and process
6–1 is the sensible heating in the reheat coil.
Process 1–2 is the cooling and dehumidification of air to compensate for the heat and vapor loss in the
conditioned space. In winter air-conditioning systems where heating is required, the use of outdoor air
should be kept to a minimum.