UNIT- 1
AIR CONDITIONING:
BASIC REFRIGERATION PRINCIPLES
THERMODYNAMICS
Thermodynamics is derived from thermo, meaning heat, and dynamics, (literally “power”), and is used to describe the move-ment or
change of a process due to heat flow. Heat and temperature are often confused or used interchange-
ably.
Heat may be defined as energy in transit from a high temperature object to a lower temperature object.
Heat is defined in physics as the transfer of thermal energy across a well-defined boundary around
a thermodynamic system.
Heat can move in three ways:
Convection is based on the movement of a fluid to
1. conduction, transfer heat from one object or area to another.
2. convection, and
3. radiation. Weather is a convective process; hot air and warm water
move from the tropics toward the poles.
Conduction is the easiest and most familiar to anyone who
has burned themselves on a stove or a hot pot.
It is the direct transfer of heat from one object to another,
when they are in contact.
Radiation is heat transferred directly from a source to an
object without using a medium like air or water.
Since heat is the flow of energy, and it is driven by the
temperature difference between two objects, an iron at 300˚
Heat radiated from the sun will melt snow on the road,
and a piece of dry ice at -110˚ will both burn a finger because
even on a cold day.
the temperature difference between the finger and the iron
or dry ice is about the same.
The radiant energy warms the road surface without
In one case the flow of heat is into the finger, in the other the directly warming the air.
flow of heat is out of the finger, but in both the heat flow is
high
THREE LAWS OF THERMODYNAMICS
First Law — Energy cannot be created or destroyed, but can change form, and location. For instance, burning wood changes the internal
energy in the wood into heat and light energy.
THREE LAWS OF THERMODYNAMICS
Second Law — The Second Law is the most understandable and useful in real world applications, and makes heating, air conditioning, and
refrigeration possible. Energy must flow from a higher state to a lower state. That is, heat must always flow from the warmer object to a
cooler object and not from the cooler object to the warmer object.
Third Law — As a system approaches Absolute Zero, the entropy of the system approaches a minimum value. Absolute Zero cannot be
attained in a real system; it is only a theoretical limit.
LATENT HEAT is the heat released or absorbed by a body or a thermodynamic system during a constant-temperature process. A typical
example is a change of state of matter, meaning a phase transition such as the melting of ice or the boiling of water.
LATENT HEAT OF FUSION
Latent heat is the energy involved when materials change their phase, meaning they go from one form of a solid, liquid, or gas to
another.
The latent heat of fusion is specifically the amount of energy in the form of heat is absorbed by a material when a solid changes to a
liquid.
In order to melt 1 gram of ice into water, it takes 334 Joules of energy. There is no temperature change during this transition – the 0°C
ice becomes 0°C water after absorbing all of that heat. Keep in mind it takes about 4.2 Joules to heat a gram of water by 1C°. It is pretty
easy to see that it takes nearly 80 times the energy to melt water as it does to then heat it by 1C°. Or to put it another way, the energy
it takes to melt ice could heat a mass equivalent of water by nearly 80C°.
TEMPERATURE IN REFRIGRATION
A refrigerator maintains a temperature a few degrees above the freezing point of water. Optimum temperature range for perishable
food storage is 3 to 5 °C (37 to 41 °F). A similar device that maintains a temperature below the freezing point of water is called a
freezer. The refrigerator replaced the icebox, which was a common household appliance for almost a century and a half prior. For this
reason, a refrigerator is sometimes referred to as an icebox.
Freezer units are used in households and in industry and commerce. Food stored at or below −18 °C (−0 °F) is safe indefinitely. Most
household freezers maintain temperatures from -23 to -18 °C (-9 to -0 °F), although some freezer-only units can achieve −34 °C (−29 °F)
and lower.
2) Latent heat of vaporization: We have seen the example for boiling water. The amount of heat required by the unit mass of
substance to vaporize from liquid to gaseous state or the amount of heat required to be removed from the unit mass of substance to
condense from the gaseous to liquid phase is called as latent of vaporization
3) Latent heat of sublimation: Some of the substances like naphthalene get directly converted from solid to gaseous state when kept
in the open atmosphere. The amount of heat required by the unit mass of substance to convert directly from solid to gaseous phase
or the amount of heat required to be removed from the gaseous substance to change it to the solid phase is called as latent heat of
sublimation
4) Saturation Temperature: The steam saturation temperature is the temperature at which steam begins to condense (or water
begins to boil) for a given pressure, or more specifically, the temperature at which a liquid-vapor phase change occurs in water
saturation temperature is nothing but the temp at which addition of heat result in change of phase
Pressure / Temperature Relationship
•The boiling point of any liquid is governed by the amount of pressure placed upon its surface.
•If the pressure applied on the liquid refrigerant is increased, then the boiling temperature will also increase.
•If the pressure exerted on the liquid refrigerant is decreased, then the boiling temperature will also decrease.
•By reducing the pressure to a sufficiently low enough value it is possible to drop the boiling temperature to a value that is
cooler than the surrounding ambient air temperature, thus resulting in the process known as refrigeration.
REFRIGRATION & AIRCONDITIONING
Refrigeration and air conditioning is used to cool products or a building environment. The refrigeration or air conditioning system (R)
transfers heat from a cooler low-energy reservoir to a warmer high-energy reservoir
Heat Transfer Loop in Refrigeration System
There are several heat transfer loops in a refrigeration system as shown in Figure , Thermal energy moves from left to right as it is
extracted from the space and expelled into the outdoors through five loops of heat transfer:
Indoor air loop. In the left loop, indoor air is driven by the supply air fan through a cooling coil, where it transfers its heat to chilled water.
The cool air then cools the building space.
Chilled water loop. Driven by the chilled water pump, water returns from the cooling coil to the chiller’s evaporator to be re-cooled.
Refrigerant loop. Using a phase-change refrigerant, the chiller’s compressor pumps heat from the chilled water to the condenser water.
Condenser water loop. Water absorbs heat from the chiller’s condenser, and the condenser water pump sends it to the cooling tower.
Cooling tower loop. The cooling tower’s fan drives air across an open flow of the hot condenser water, transferring the heat to the
outdoors.
Basic Refrigeration Cycle For an air conditioning system to operate with economy, the refrigerant
must be used repeatedly. For this reason, all air conditioners use the
Principles of Refrigeration same cycle of compression, condensation, expansion, and evaporation
in a closed circuit. The same refrigerant is used to move the heat from
Liquids absorb heat when changed from liquid to one area, to cool this area, and to expel this heat in another area.
gas ,
Gases give off heat when changed from gas to
liquid.
The refrigerant comes into the compressor as a low-pressure
gas, it is compressed and then moves out of the compressor as a
high-pressure gas.
The gas then flows to the condenser. Here the gas condenses to
a liquid, and gives off its heat to the outside air.
The liquid then moves to the expansion valve under high
pressure. This valve restricts the flow of the fluid, and lowers its
pressure as it leaves the expansion valve.
The low-pressure liquid then moves to the evaporator, where
heat from the inside air is absorbed and changes it from a liquid
to a gas.
As a hot low-pressure gas, the refrigerant moves to the
compressor where the entire cycle is repeated.
Note that the four-part cycle is divided at the center into a
high side and a low side This refers to the pressures of the
refrigerant in each side of the system
Vapor Compression Refrigeration Cycle
Compression refrigeration cycles take advantage of
the fact that highly compressed fluids at a certain
temperature tend to get colder when they are allowed
to expand.
If the pressure change is high enough, then the
compressed gas will be hotter than our source of
cooling(outside air, for instance) and the expanded gas
will be cooler than our desired cold temperature.
In this case, fluid is used to cool a low temperature
environment and reject the heat to a high temperature
environment.
Vapour compression refrigeration cycles have two
advantages. First, a large amount of thermal energy is
required to change a liquid to a vapor, and therefore a
lot of heat can be removed from the air-conditioned The refrigeration cycle is shown in Figure and can be broken down into
space. the following stages:
1 – 2. Low-pressure liquid refrigerant in the evaporator absorbs heat from
Second, the isothermal nature of the vaporization
its surroundings, usually air, water or some other process liquid. During
allows extraction of heat without raising the
this process it changes its state from a liquid to a gas, and at the
temperature of the working fluid to the temperature
evaporator exit is slightly superheated.
of whatever is being cooled.
2 – 3. The superheated vapour enters the compressor where its pressure
This means that the heat transfer rate remains high ,
is raised. The temperature will also increase, because a proportion of the
because the closer the working fluid temperature
energy put into the compression process is transferred to the refrigerant.
approaches that of the surroundings, the lower the
rate of heat transfer.
3 – 4. The high pressure superheated gas passes from the compressor into
the condenser. The initial part of the cooling process (3-3a) de-superheats
the gas before it is then turned
Vapor Compression Refrigeration Cycle
The refrigeration cycle is shown in Figure and can be broken down
into the following stages:
4 – 5. The high pressure superheated gas passes from the compressor
into the condenser. The initial part of the cooling process (3-4) de-
superheats the gas before it is then turned back into liquid (4-5).
The cooling for this process is usually achieved by using air or water.
A further reduction in temperature happens in the pipe work and
liquid receiver (4 - 5), so that the refrigerant liquid is sub-cooled as it
enters the expansion device.
5 - 1 The high-pressure sub-cooled liquid passes through the
expansion device, which both reduces its pressure and controls the
flow into the evaporator.
Evaporation
During this stage, the refrigerant travels through a device
called an evaporator that has a large surface area and typically
consists of a coiled tube surrounded by aluminum fins. The
cold fluid is a mixture of liquid and vapor refrigerant as it
begins this stage. While flowing through the evaporator, all the
liquid Evaporates and absorbs heat from the enclosed space.
The energy absorbed is used to change the state of the
refrigerant from liquid to vapor. This lowers the temperature of
the space, along with whatever food or beverages are stored in
it. The refrigerant exits this stage as a saturated vapor.
Vapor Compression Refrigeration Cycle
Compression
The heat that was absorbed in the Evaporation stage must be released into the surroundings, but this will
not happen unless the temperature of the refrigerant is higher than the outside air. This is the purpose of the
Compression stage. A device, predictably called a compressor, raises the pressure of the refrigerant vapor.
Due to basic thermodynamic principles, this causes the temperature of the refrigerant to rise, leaving the
stage as a superheated vapor. Energy is needed to power the compressor, which is why electricity is required
to operate a refrigerator.
Condensation
Increasing the temperature of the refrigerant above that of the surroundings, we can dissipate the heat
necessary to continue the process. This is accomplished with a device very similar to the evaporator. It also
uses a coiled tube with aluminum fins, but may have different dimensions than the evaporator to
accommodate the different state of the refrigerant.
As the hot vapor flows through the condenser, the outside air removes energy And the refrigerant becomes
a saturated liquid. At this point the slightest drop in pressure will initiate evaporation, which is the basis for
the final stage of the process.
Expansion
To begin a new cycle, A lowering of the refrigeration temperature to below that of the enclosure. This is the key
to the entire cycle, can utilize the auto refrigeration effect.
When a saturated liquid experiences a sudden drop in pressure, a small amount of liquid is instantly vaporized
and the temperature of the mixture is drastically reduced.
This cold liquid-vapor mixture can now begin a new cycle. The pressure drop is accomplished by the simplest,
yet most important, part of the system –a simple flow restriction. This part is commonly called a throttle or
expansion valve.
VAPOUR ABSORPTION REFRIGERATION SYSTEM
The vapour absorption refrigeration system consists of:
Absorber: Absorption of refrigerant vapour by a suitable
absorbent or adsorbent, forming a strong or rich solution of
the refrigerant in the absorbent/ adsorbent.
Pump: Pumping of the rich solution and raising its pressure to
the pressure of the condenser.
Generator: Distillation of the vapour from the rich solution
leaving the poor solution for
recycling
The absorption chiller is a machine, which produces chilled water by using heat such as steam, hot water, gas, oil etc. Chilled water
is produced based on the principle that liquid (i.e. refrigerant , which evaporates at a low temperature) absorbs heat from its
surroundings when it evaporates. Pure water is used as refrigerant and lithium bromide solution is used as absorbent.
Heat for the vapour absorption refrigeration system can be provided by waste heat extracted from the process, diesel generator sets
etc. In that case absorption systems require electricity for running pumps only. Depending on the temperature required and the
power cost, it may even be economical to generate heat / steam to operate the absorption system.
VAPOUR ABSORPTION REFRIGERATION SYSTEM
Evaporator
The refrigerant (water) evaporates at around 4C under a high vacuum
condition of 754 mm Hg in the evaporator.
Chilled water goes through heat exchanger tubes in the evaporator
and transfers heat to the evaporated refrigerant.
The evaporated refrigerant (vapor)turns into liquid again, while the
latent heat from this vaporization process cools the chilled water (in
the diagram from 12C to 7C). The chilled water is then used for cooling purposes.
Absorber
In order to keep evaporating, the refrigerant vapor must be discharged from the
evaporator and refrigerant (water) must be supplied.
The refrigerant vapor is absorbed into lithium bromide solution, which is
convenient to absorb the refrigerant vapor in the absorber.
The heat generated in the absorption process is continuously removed from the
system by cooling water. The absorption also maintains the vacuum inside the
evaporator.
VAPOUR ABSORPTION REFRIGERATION SYSTEM
High Pressure Generator
As lithium bromide solution is diluted, the ability to absorb the refrigerant vapor
reduces. In order to keep the absorption process going, the diluted lithium bromide
solution must be concentrated again.
An absorption chiller is provided with a solution concentrating system,
called a generator. Heating media such as steam, hot water, gas or oil
perform the function of concentrating solutions.
The concentrated solution is returned to the absorber to absorb refrigerant
vapor again.
Condenser
To complete the refrigeration cycle, and thereby ensuring the refrigeration takes
place continuously, the following two functions are required
1. To concentrate and liquefy the evaporated refrigerant vapor,
which is generated in the high pressure generator.
2. To supply the condensed water to the evaporator as refrigerant
(water)For these two functions a condenser is installed.
Absorption refrigeration systems that use Li-Br-water as a refrigerant have a Coefficient of Performance (COP) in the range of 0.65 - 0.70
and can provide chilled water at 6.7C with a cooling water temperature of 30C.
Systems capable of providing chilled water at 3C are also available. Ammonia based systems operate at above atmospheric pressures
and are capable of low temperature operation (below 0C).
Absorption machines are available with capacities in the range of 10-1500 tons. Although the initial cost of an absorption system is
higher than that of a compression system, operational costs are much lower if waste heat is used.
AIR HANDLING UNITS ( AHU)
An air handler, or air handling unit (often abbreviated to
AHU), is a device used to condition and circulate air as part
of a heating, ventilating, and air-conditioning (HVAC)
system.
controls temperature,
controls humidity,
controls pressure & air exchange
Air handling components
An air handling unit; air flow is from the right to
left in this case. Some AHU components shown
are
1 – Supply duct
2 – Fan compartment
3 – Vibration isolator ('flex joint')
4 – Heating and/or cooling coil
5 – Filter compartment
6 – Mixed (recirculated + outside) air duct
Controls
Controls are necessary to regulate every aspect of an air handler, such as: flow rate of air, supply air temperature, mixed air temperature,
humidity, air quality. They may be as simple as an off/on thermostat or as complex as a building automation system using BACnet or
LonWorks, for example.
Common control components include temperature sensors, humidity sensors, sail switches, actuators, motors, and controllers.
Filters
Air filtration is almost always present in order to provide clean dust-free air to the building occupants
Air handling components
Supply duct
Supply Ducts are used in heating, ventilation, and
air conditioning (HVAC) to deliver and remove air.
These needed airflows include, for example, supply
air, return air, and exhaust air.
Ducts also deliver, most commonly as part of the
supply air, ventilation air.
As such, air ducts are one method of ensuring
acceptable indoor air quality as well as thermal
comfort.
Vibration isolators
The blowers in an air handler can create substantial vibration and the large area of the duct system would transmit this noise and
vibration to the occupants of the building.
To avoid this, vibration isolators (flexible sections) are normally inserted into the duct immediately before and after the air handler and
often also between the fan compartment and the rest of the AHU.
The rubberized canvas-like material of these sections allows the air handler components to vibrate without transmitting this motion to
the attached ducts.
Air handling components
Mixed ( recirculated + outside) air duct
The outside air is drawn through a duct which is usually located on the roof of the building, and the return air is delivered to the supply
fan by the return or exhaust fan.
The return air fan exhausts air from the inside room through return air vents into the space above the drop ceiling (called the return air
plenum). This return air then goes through a return duct to the fan room. The return air can then be exhausted to the outside and/or
recirculated.
The return air moving through the plenum above the drop ceiling often comes in contact with pollutant sources, including fibrous glass
insulation on the ducts or in sprayed on fire proofing, asbestos insulation on pipes or in fire proofing, and chemicals off-gassing from the
ceiling tiles. ( " Indoor Air Quality".)
The amounts of return air and outside air mixed together is controlled by dampers in the return and outside air ducts. Each damper can
move to any position from fully closed to fully open.
The return air, exhaust air, and outside air dampers all work together so that when the outside air damper is fully open, the return air
damper is closed and the exhaust air damper is open. On the other hand, when the outside air damper is closed, the return air damper
is fully open and the exhaust damper is closed. In this mode the ventilation system would be entirely recirculating office air.
Indoor air quality problems usually occur when the outside air dampers are not open enough to provide adequate amounts of outside
air. The American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) normally recommends that a ventilation
system should deliver at least 15 to 20 cubic feet per minute (cfm) of outside air per occupant.
Heating and/or cooling elements
Air handlers may need to provide heating, cooling, or both to change the supply air temperature, and humidity level depending on the
location and the application. Such conditioning is provided by heat exchanger coil(s) within the air handling unit air stream, such coils
may be direct or indirect in relation to the medium providing the heating or cooling effect.
Direct heat exchangers include those for gas-fired fuel-burning heaters or a refrigeration evaporator, placed directly in the air stream.
Electric resistance heaters and heat pumps can be used as well. Evaporative cooling is possible in dry climates.
Indirect coils use hot water or steam for heating, and chilled water for cooling (prime energy for heating and cooling is provided by
central plant elsewhere in the building). Coils are typically manufactured from copper for the tubes, with copper or aluminium fins to aid
heat transfer.
Cooling coils will also employ eliminator plates to remove and drain condensate. The hot water or steam is provided by a central boiler,
and the chilled water is provided by a central chiller. Downstream temperature sensors are typically used to monitor and control "off
coil" temperatures, in conjunction with an appropriate motorized control valve prior to the coil.
If dehumidification is required, then the cooling coil is employed to over-cool so that the dew point is reached and condensation occurs.
A heater coil placed after the cooling coil re-heats the air (therefore known as a re-heat coil) to the desired supply temperature. This has
the effect of reducing the relative humidity level of the supply air.
In colder climates, where winter temperatures regularly drop below freezing, then frost coils or pre-heat coils are often employed as a
first stage of air treatment to ensure that downstream filters or chilled water coils are protected against freezing. The control of the frost
coil is such that if a certain off-coil air temperature is not reached then the entire air handler is shut down for protection.
