Unit no 02

Mechanism of a Simple Vapour Compression Refrigeration System:

the schematic diagram of a simple vapour compression refrigeration system It consists of the following five essential
parts:
1. Compressor. The low pressure and temperature vapour refrigerant from evaporator is drawn into the compressor
through the inlet or suction valve A. where it is compressed to a high pressure and temperature. This high pressure and
temperature vapour refrigerant is discharged into the condenser through the delivery or discharge valve B
2. Condenser. The condenser or cooler consists of coils of pipe in which the high pressure and temperature vapour
refrigerant is cooled and condensed. The refrigerant, while passing through the condenser, gives up its latent heat to the
surrounding condensing medium which is normally air or water.

3. Receiver. The condensed liquid refrigerant from the condenser is stored in a vessel known as receiver from where it is
supplied to the evaporator through the expansion valve or refrigerant control valve.
4. Expansion valve. It is also called throttle valve or refrigerant control valve. The function of the expansion valve is to
allow the liquid refrigerant under high pressure and temperature to pass at a controlled rate after reducing its pressure and
temperature. Some of the liquid refrigerant evaporates as it passes through the expansion valve, but the greater portion is
vaporised in the evaporator at the low pressure and temperature.
5. Evaporator. An evaporator consists of coils of pipe in which the liquid-vapour refrigerant at low pressure and
temperature is evaporated and changed into vapour refrigerant at low pressure and temperature. In evaporating, the liquid
vapour refrigerant absorbs its latent heat of vaporisation from the medium (air, water or brine) which is to be cooled.

Pressure-Enthalpy (p-h) Chart:

The most convenient chart for studying the behaviour of a refrigerant is the p-h chart, in which the vertical ordinates
represent pressure and horizontal ordinates represent enthalpy (ie total heat). A typical chart is shown in Fig. 42, in
which a few important lines of the complete chart are drawn. The saturated liquid line and the saturated vapour line
merge into one another at the critical point. A saturated liquid is one which has a temperature equal to the saturation
temperature corresponding to its pressure The space to the left of the saturated liquid line will. therefore, be sub-cooled
liquid region. The space between the liquid and the vapour lines is called wet vapour region and to the right of the
saturated vapour line is a superheated vapour region.
Types of Vapour Compression Cycles:
We have already discussed that vapour compression cycle essentially consists of compression, condensation, throttling
and evaporation. Many scientists have focussed their attention to increase the coefficient of performance of the cycle.
Though there are many cycles, yet the following are important from the subject point of view:
1. Cycle with dry saturated vapour after compression,
2. Cycle with wet vapour after compression.
3. Cycle with superheated vapour after compression
4. Cycle with superheated vapour before compression, and
5. Cycle with undercooling or subcooling of refrigerant.

Theoretical Vapour Compression Cycle with Dry Saturated Vapour after Compression:
A vapour compression cycle with dry saturated vapour after compression is shown on T-S and p-h diagrams in. At point
1, let T1 p1 and s1 be the temperature, pressure and entropy of the vapour refrigerant respectively. The four processes
of the cycle are as follows:
1. Compression process. The vapour refrigerant at low pressure p1 and temperature T1 is compressed isentropically to
dry saturated vapour as shown by the vertical line 1-2 on T-s diagram and by the curve 1-2 on p-h diagram. The pressure
and temperature rises from p1 to p2 and T1 to T2 respectively.
The work done during isentropic compression per kg of refrigerant is given by
W=h2-h1
Condensing process. The high pressure and temperature vapour refrigerant from the compressor is passed through the
condenser where it is completely condensed at constant pressure p2 and temperatureT2 , as shown by the horizontal line
2-3 on T-s and p-h diagrams. The vapour refrigerant is changed into liquid refrigerant. The refrigerant, while passing
through the condenser. gives its latent heat to the surrounding condensing medium.
3. Expansion process. The liquid refrigerant at pressure p3= p2 and temperature T3 = T2 is expanded by throttling
process through the expansion valve to a low pressure p4 = p1 and temperature T4 = T1 as shown by the curve 3-4 on T-
s diagram and by the vertical line 3-4 on p-h diagram. We have already discussed that some of the liquid refrigerant
evaporates as it passes through the expansion valve, but the greater portion is vaporised in the evaporator. We know that
during the throttling process, no heat is absorbed or rejected by the liquid refrigerant.
4. Vaporising process. The liquid-vapour mixture of the refrigerant at pressure p4= p1 and temperature T4 = T1 is
evaporated and changed into vapour refrigerant at constant pressure and temperature, as shown by the horizontal line 4-1
on Tis and p-h diagrams. During evaporation, the liquid-vapour refrigerant absorbs its latent heat of vaporisation from
the medium (air, water or brine) which is to be cooled. This heat which is absorbed by the refrigerant is called
refrigerating effect and it is briefly written as RE The process of vaporisation continues upto point 1 which is the starting
point and thus the cycle is completed.
We know that the refrigerating effect or the heat absorbed or extracted by the liquid-vapour refrigerant during
evaporation per kg of refrigerant is given by

A vapour compression cycle with wet vapour after compression is shown on T-s and p-h diagrams. In this cycle, the
enthalpy at point 2 is found out with the help of dryness fraction at this point. The dryness fraction at points 1 and 2 may
be obtained by equating entropies at points 1 and 2.
Now the coefficient of performance may be found out as usual from the relation,
A vapour compression cycle with superheated vapour after compression is shown on 7-s and p-h diagrams in Fig. 4.10
(a) and (b) respectively. In this cycle, the enthalpy at point 2 is found out with the help of degree of superheat. The
degree of superheat may be found out by equating the entropies at points 1 and 2.
Now the coefficient of performance may be found out as usual from the relation,
C.O.P. = Refrigerating effect/Work done=h1-h2/h2-h1
A little consideration will show that the superheating increases the refrigerating effect and e amount of work done in the
compressor Since the increase in refrigerating effect is less as mpared to the increase in work done, therefore, the net
effect of superheating is to have low efficient of performance.

A vapour compression cycle with superheated vapour before compression is shown on T-s and p-h diagrams. In this
cycle, the evaporation starts point 4 and continues upto point 1', when it is dry saturated. The vapour is now superheated
before entering the compressor upto the point 1.
The coefficient of performance may be found out as usual from the relation,
C.O.P = Refrigerating effect/Work done=h1-hf3/h2-h1
Sometimes, the refrigerant, after condensation process 2-3, is cooled below the saturation temperature (T3) before
expansion by throttling. Such a process is called undercooling or subcooling of the refrigerant and is generally done
along the liquid line. The ultimate effect of the undercooling is to increase the value of coefficient of performance under
the same set of conditions
The process of undercooling is generallly brought about by circulating more quantity of cooling water through the
condenser or by using water colder than the main circulating water. Sometimes, this process is also brought about by
employing a heat exchanger. In actual practice. the refrigerant is superheated after compression and undercooled before
throttling. A little consideration will show that the refrigerating effect is increased by adopting both the superheating and
undercooling process as compared to a cycle without them which is shown by dotted lines.
In this case, the refrigerating effect or heat absorbed or extracted,
Re= h1 – h4 = h1 - hf 3
w = h2 – h1
C.O.P = Refrigerating effect /work done=h1-hf3/h2-h1

The actual vapour compression cycle differs from the theoretical vapour compression cycle in many ways, some of
which are unavoidable and cause losses. The main deviations between the theoretical cycle and actual cycle are as
follows:
1. The vapour refrigerant leaving the evaporator is in superheated state.
2. The compression of refrigerant is neither isentropic nor polytropic.
3. The liquid refrigerant before entering the expansion valve is sub-cooled in the condenser.
4. The pressure drops in the evaporator and condenser
The actual vapour compression cycle on 7-s diagram is The variou processes are discussed below:
(a) Process 1-2-3. This process shows the flow of refrigerant in the evaporator. The point represents the entry of refrigerant
into the evaporator and the point 3 represents the exit of refrigerant from evaporator in a superheated state. The point 3
also represents the entry of refrigerant into the compressor in a superheated condition. The superheating of vapour
refrigerant from point 2 to point 3 may be due to
(1) automatic control of expansion valve so that the refrigerant leaves the evaporator as the superheated vapour.
(ii) picking up of larger amount of heat from the evaporator through pipes located within th cooled space.
(iii) picking up of heat from the suction pipe, te. the pipe connecting the evaporator delivery and the compressor suction
valve.
In the first and second case of superheating the vapour refrigerant, the refrigerating effect as well as the compressor work
is increased. The coefficient of performance, as compared to saturation cycle at the same suction pressure may be greater,
less or unchanged.
The superheating also causes increase in the required displacement of compressor and load on the compressor and
condenser. This is indicated by 2-3 on T-s diagram
(b) Process 3-4-5-6-7-8. This process represents the flow of refrigerant through the compressor. When the refrigerant
enters the compressor through the suction valve at point 3, the pressure falls to point 4 due to frictional resistance to flow.
Thus the actual suction pressure (ps) is lower than the evaporator pressure (Pe). During suction and prior to compression,
the temperature of the cold refrigerant vapour rises to point 5 when it comes in contact with the compressor cylinder walls.
The actual compression of the refrigerant is shown by 5-6 which is neither isentropic nor polytropic.
This is due to the heat transfer between the cylinder walls and the vapour refrigerant. The temperature of the cylinder
walls is some-what in between the temperatures of cold suction vapour refrigerant and hot discharge vapour refrigerant.
It may be assumed that the heat absorbed by the vapour refrigerant from the cylinder walls during the first part of the
compression stroke is equal to heat rejected by the vapour refrigerant to the cylinder walls. Like the heating effect at
suction given by 4-5 in, there is a cooling effect al discharge as given by 6-7. These heating and cooling effects take place
at constant pressure. Dur to the frictional resistance of flow, there is a pressure drop ie. the actual discharge pressure (P)
is more than the condenser pressure (pc).
(c) Process 8-9-10-11. This process represents the flow of refrigerant through the condenser The process 8-9 represents
the cooling of superheated vapour refrigerant to the dry saturated state The process 9-10 shows the removal of latent heat
which changes the dry saturated refrigerant into Hquid refrigerant. The process pans represents the sub-cooling of liquid
refrigerant in the condenser before passing through the expansion valve. This is desirable as it increases the refrigerating
effect per kg of the refrigerantile patting through the volume of the refrigerant partially frigerating effe the liquid
refrigerant while pasting thrust the expansion valve. The increase in refrigerating effect the be obtained by large quantities
of circulating cooling water which should be at a temperature.

Compressors:
Compressors are used to increase the pressure of a gas by reducing its volume. They are essential components in
refrigeration and air conditioning systems. They can be classified based on their mechanism and design:
1.Positive Displacement Compressors:
 Reciprocating Compressors: Utilize a piston in a cylinder to compress the refrigerant. Common in smaller systems.
 Rotary Compressors: Include rolling piston and vane types, used in residential and commercial air conditioners.
 Scroll Compressors: Use two interleaved spiral scrolls, known for higher efficiency and reliability.
 Screw Compressors: Use two interlocking helical rotors, ideal for large systems due to high efficiency.
2.Dynamic Compressors:
 Centrifugal Compressors: Use a rotating impeller to impart velocity to the refrigerant, converting kinetic
energy into pressure energy. Suitable for large systems.
 Axial Compressors: Utilize a series of fan-like airfoils to compress the refrigerant, mostly used in industrial

Condensers
Condensers are heat exchangers that condense refrigerant from its gaseous to liquid state. They are classified based on
their cooling method:
Air-Cooled Condensers:
 Natural Convection: Utilize natural airflow to cool the refrigerant.
 Forced Convection: Use fans to increase airflow and improve heat dissipation.
 Water-Cooled CondensersShell and Tube: Water flows through tubes surrounded by refrigerant, commonly used
in industrial applications.
 Evaporative CondensersCombine air and water cooling, where refrigerant is condensed by the evaporation of water
sprayed over coils containing refrigerant.

ExpansionDevices:
Expansion devices reduce the pressure of the refrigerant, causing it to cool before entering the evaporator. They can be
classified as:
 Capillary Tubes Simple, fixed-size tubes providing constant throttling, used in small systems.
 Thermostatic Expansion Valves (TXVs)Modulate refrigerant flow based on evaporator temperature, maintaining
desired superheat.
 Automatic or Constant Pressure Expansion Valves (AXVs)Maintain constant evaporator pressure, suitable for
systems with varying loads.
 Electronic Expansion Valves (EEVs)Use electronic control to precisely regulate refrigerant flow, offering high
efficiency.
 Float Valves Control refrigerant flow based on liquid level in the evaporator, used in large systems.

Evaporators
Evaporators are heat exchangers where the refrigerant absorbs heat and evaporates. They can be classified based on their
design and application:
 Dry Expansion Evaporators Refrigerant enters as a liquid and leaves as a vapor, with minimal liquid remaining.
 Flooded Evaporators Entire surface is covered with liquid refrigerant, offering better heat transfer.
 Shell and Tube Evaporators Refrigerant flows through tubes while water or air flows over them, used in industrial
applications.
 Finned Tube Evaporators Tubes with external fins to increase surface area, common in air conditioning systems.
 Plate Type Evaporators Comprise multiple plates for efficient heat transfer, used in compact systems.
 Effect of Superheating and Subcooling:
Superheating and subcooling are important concepts in thermodynamics, particularly in the context of refrigeration, air
conditioning, and various heat engine cycles. Here's an overview of their effects:
Superheating Definition: Superheating occurs when a vapor is heated beyond its boiling point at a given pressure. For
example, in a refrigeration cycle, the refrigerant vapor is superheated after it leaves the evaporator and before it enters the
compressor.
Effects:Efficiency of Compression:
Positive: Superheating ensures that only vapor enters the compressor, which prevents damage that can occur from liquid
entering the compressor (a condition known as liquid slugging).
Negative: Excessive superheating can lead to higher temperatures and pressures at the compressor outlet, which can
decrease the overall efficiency of the cycle and increase wear and tear on the compressor.
Heat Transfer:Superheating can reduce the effectiveness of heat exchangers since the temperature difference driving heat
transfer in the evaporator is reduced.Refrigeration Capacity:While a small amount of superheating is necessary, too much
can reduce the refrigeration effect, as more energy is needed to superheat the vapor instead of producing useful cooling.

Subcooling:
Definition: Subcooling occurs when a liquid is cooled below its condensing temperature at a given pressure. In a
refrigeration cycle, this typically happens to the refrigerant after it leaves the condenser and before it enters the expansion
valve.
Effects:Efficiency of Expansion:
Positive: Subcooling increases the refrigerant's capacity to absorb heat in the evaporator since more of the refrigerant
enters the evaporator as a liquid, increasing the refrigerating effect.
System Capacity:
Positive: Subcooling can increase the refrigeration capacity of the system without increasing the amount of refrigerant
flow, as it allows more heat absorption per unit of refrigerant.
Heat Transfer:
Positive: It ensures that the refrigerant is fully condensed before it enters the expansion valve, which improves the
efficiency of the heat exchangers.

Use of Refrigeration Table and Chart:

Refrigeration tables and charts are essential tools in the field of HVAC (Heating, Ventilation, and Air Conditioning) and
refrigeration engineering. They provide critical data for designing, analyzing, and troubleshooting refrigeration systems.
Here’s a detailed guide on their use:Refrigeration Tables Refrigeration tables list the thermodynamic properties of
refrigerants at various temperatures and pressures. Common properties include:
Saturated Pressure and Temperature: Indicates the pressure at which a refrigerant will be saturated (i.e., begin to boil
or condense) at a given temperature, and vice versa. Specific Volume: The volume occupied by a unit mass of refrigerant.
Enthalpy (h): The heat content per unit mass. Entropy (s): A measure of disorder or energy dispersal per unit mass.Internal
Energy (u): The total energy contained within the refrigerant.How to Use Refrigeration TablesIdentify the Refrigerant:
Determine which refrigerant is used in your system (e.g., R-134a, R-22).Determine the State: Identify the state of the
refrigerant (saturated liquid, saturated vapor, or superheated vapor).Locate the Relevant Table: Use the refrigerant-specific
table to find the desired properties.
Interpolation: If the exact value is not available, use linear interpolation to estimate between two known
values.Refrigeration ChartsRefrigeration charts, like Pressure-Enthalpy (P-h) and Temperature-Entropy (T-s) diagrams,
graphically represent the properties of refrigerants. They provide a visual method to understand the refrigeration cycle and
locate various states of the refrigerant.Pressure-Enthalpy (P-h) ChartIdentify Points on the Cycle: Mark key points like
the evaporator inlet, evaporator outlet, condenser inlet, and condenser outlet on the P-h chart.Isothermal Lines: Use the
isothermal lines to understand the changes in temperature throughout the cycle.Enthalpy Change: Measure the change in
enthalpy (horizontal movement) to determine heat transfer in components.Temperature-Entropy (T-s) Chart
Identify Processes: Locate processes such as isentropic compression and isentropic expansion.Isochoric Lines:
Understand the constant volume processes.Temperature Changes: Visualize temperature changes during compression and
expansion.Practical ApplicationsSystem Design: Use tables and charts to select appropriate components (compressors,
evaporators, condensers) based on desired capacities and operating conditions.Performance Analysis: Analyze the
efficiency and performance of existing systems by comparing actual operating conditions to theoretical cycles.
Troubleshooting: Diagnose issues by comparing actual system pressures and temperatures to expected values from tables
and charts.

The various points on the p-h diagram are plotted as discussed below:
1. First of all, draw a horizontal pressure line representing the evaporator pressure p, tot suction pressure of low pressure
compressor) which intersects the saturated vapour line at point 1. At this point, the saturated vapour is supplied to the low
pressure compressor. Let, at point 1. the enthalpy of the saturated vapour is h1 and s1 entropy
2 The saturated vapour refrigerant admitted at point I is compressed isentropically in the low pressure compressor and
delivers the refrigerant in a superheated state. The pressure rises from pE to p2 The curve 1-2 represents the isentropic
compression in the low pressure compressor. In order to obtain point 2, draw a line from point 1. with entropy equal to s1
along the constant entropy line intersecting the intermediate pressure line p2 at point 2. Let enthalpy at this point is h2
3. The superheated vapour refrigerant leaving the low pressure compressor at point 2 is cooled (or desuperheated) at
constant pressure p2=p3 in a liquid intercooler by the liquid refrigerant from the condenser. The refrigerant leaving the
liquid intercooler is in saturated vapour state. The line 2-3 represents the cooling or desuperheating process. Let the
enthalpy and entropy at point 3 is h3 and s3 respectively.
4 The dry saturated vapour refrigerant is now supplied to high pressure compressor where it is compressed isentropically
from intermediate or intercooler pressure p2 to condensor pressure pe The curve 3-4 represents the isentropic compression
in the high pressure compressor. The point 4 on the p-h diagram is obtained by drawing a line of entropy equal to s3 along
the constant entropy line. Let the enthalpy of superheated vapour refrigerant at point 4 is h
5. The superheated vapour refrigerant leaving the high pressure compressor at point 4 is now passed through the condenser
at constant pressure pc as shown by a horizontal line 4-5. The condensing process 4-5 changes the state of refrigerant from
superheated vapour to saturated liquid.
6. The high pressure saturated liquid refrigerant from the condenser is passed to the intercooler where some of liquid
refrigerant evaporates in desuperheating the superheated vapour refrigerant from the low pressure compressor. In order to
make up for the liquid evaporated, te. to maintain a constant liquid level, an expansion valve E1 which acts as a float
valve, is provided.
7. The liquid refrigerant from the intercooler is first expanded in an expansion valve E and then evaporated in the
evaporator to saturated vapour condition.

Two Stage Compression with Water Intercooler and Liquid Sub-cooler:

The arrangement of a two-stage compression with water intercooler and liquid sub-cooler. The corresponding p-h
diagram is. The various processes in this system are as follows:
1. The saturated vapour refrigerant at the evaporator pressure pE is admitted to low pressure compressor at point 1. In this
compressor, the refrigerant is compressed isentropically from the evaporator pressure pe to the water intercooler pressure
p2 as shown by the curve 1-2.
2. The refrigerant leaving the low pressure compressor at point 2 is in superheated state. This superheated vapour
refrigerant is now passed through the water intercooler at constant pressure, in order to reduce the degree of superheat.
The line 2-3 represents the water intercooling or desuperheating process.
3. The refrigerant leaving the water intercooler at point 3 (which is still in the superheated state) is compressed
isentropically in the high pressure compressor to the condenser pressure PC The curve 3-4 shows the isentropic
compression in high pressur compressor
4. The discharge from the high pressure compressor is now passed through the condenser which changes the state of
refrigerant from superheated vapour to saturated liquid a shown by process 4-5.
5. The temperature of the saturated liquid refrigerant is further reduced by passing through a liquid sub-cooler as shown
by process 5-6.
6. The liquid refrigerant from the sub-cooler is now expanded in an expansion vahe (process 6-7) before being sent to the
evaporator for evaporation (process 7-1)
Multi Evaporators Systems.
Multi-evaporator systems are refrigeration setups that include multiple evaporators connected to a common compressor
and condenser. These systems are often used in large commercial or industrial refrigeration applications where cooling
needs vary across different zones or areas. Here are some key aspects of multi-evaporator systems:
Components and ConfigurationEvaporators: Multiple units where refrigerant absorbs heat, causing the refrigerant to
evaporate. Each evaporator can serve a different area or process, maintaining distinct temperature zones.
Compressor: A single or multiple compressors that compress the refrigerant vapor from all evaporators and send it to the
condenser.
Condenser: Releases heat from the refrigerant to the outside environment, turning it back into a liquid.
Expansion Devices: Control the flow of refrigerant into each evaporator. This can be a thermostatic expansion valve
(TXV) or an electronic expansion valve (EXV).

Ozone Depletion and Global Warming Issues.

Ozone depletion and global warming are two significant environmental issues, often confused but distinct in their causes
and impacts.

Ozone Depletion,
Cause:
1.Ozone depletion refers to the thinning of the ozone layer in the Earth’s stratosphere, primarily due to human-made
chemicals like chlorofluorocarbons (CFCs), halons, and other ozone-depleting substances (ODS).
2.When these substances reach the stratosphere, they are broken down by ultraviolet (UV) light, releasing chlorine and
bromine atoms, which then react with ozone (O3) molecules, causing the ozone to break apart.

Impact:
1.Increased UV radiation reaching the Earth’s surface, leading to higher incidences of skin cancer, cataracts, and other
health issues.
2.Negative effects on ecosystems, particularly affecting phytoplankton in oceans and terrestrial plant life.Impaired
immune systems in humans and wildlife.

Mitigation:
1.The Montreal Protocol, an international treaty signed in 1987, has been effective in reducing the production and release
of ozone-depleting substances.
2.Continued efforts to phase out ODS and replace them with safer alternatives.
Global Warming,

Cause:
1.Global warming refers to the long-term increase in Earth’s average surface temperature due to human activities,
primarily the emission of greenhouse gases (GHGs) like carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O).
2.Burning fossil fuels (coal, oil, and natural gas), deforestation, industrial processes, and agricultural practices are major
contributors to greenhouse gas emissions.

Impact:
1.Rising global temperatures, leading to more frequent and severe heatwaves.
2.Melting polar ice caps and glaciers, contributing to sea-level rise and loss of habitat for species like polar bears and
penguins.Changes in weather patterns, resulting in more intense storms, droughts, and flooding.
3.Ocean acidification, affecting marine life and coral reefs.Impacts on agriculture, water resources, and human health.

Mitigation:
1.The Paris Agreement, adopted in 2015, aims to limit global warming to well below 2°C above pre-industrial levels, with
efforts to limit the increase to 1.5°C.
2.Transitioning to renewable energy sources (wind, solar, hydro, and geothermal).
3.Enhancing energy efficiency and conservation.
4.Reforestation and afforestation to absorb CO2.
5.Developing and deploying carbon capture and storage technologies.

Interconnection:
While ozone depletion and global warming are driven by different mechanisms, they are interconnected:
1.Some substances that deplete the ozone layer are also potent greenhouse gases.
2.Climate change can influence the ozone layer, with warmer temperatures in the lower atmosphere potentially affecting
ozone chemistry in the stratosphere.