1. The Refrigeration Cycle
Kaitlin Keegan
English 202C
10/24/2012

2. Audience and Scope
The purpose of this description is to inform someone who has not had a classical
Thermodynamics course on how the refrigeration cycle works. This description
assumes no prior knowledge, and aims to be understandable on face-value. This
information is important because refrigeration is something that Americans deal with
on a daily basis, whether they are grabbing a glass of milk in the morning, cranking up
the AC in the summer, or working at an industrial plant. However, the process by
which all these common appliances work is rarely appreciated and examined, except
perhaps by a struggling Engineering student in a Thermodynamics class. The
refrigeration cycle revolutionized how modern society operates; without refrigeration,
our current standard of living would be unattainable. This type of description could be
found in a science magazine, or as an intro to an entry level Thermodynamics course.
Introduction
The refrigeration cycle is the process by which heat is removed from a system, and a cool
temperature in the surroundings is achieved. This is done by a series of phase changes of a
special fluid, and the heat required to power them. Refrigeration has been explored
since the 1750’s, starting with a scientist in Scotland named William Cullen. Over the
centuries, the process has been extensively developed, and used in producing the
appliances that provide us with modern comforts such as refrigerators, freezers, and air
conditioners.
The refrigeration cycle can be described by the science of Thermodynamics.
Thermodynamics is the study of the relationships between heat, energy, and work. The
refrigeration cycle utilizes these principles in a four step process that continuously cools
the surroundings inside the refrigerator.
The four steps are as follows: throttling, evaporation, compression and condensation. These
steps are repeated in a continuous loop through a refrigerating unit, which requires a
special type of coolant to operate. To fully understand and appreciate the refrigeration
cycle, we will examine each step of the process, the coolants involved, and the
relationship between the heat loss and energy of the system.

3. Figure 1: Diagram of a refrigerator
Above is a diagram of the mechanical components of a refrigerator. Each component: the compressor,
condenser coils, evaporator coils, and capillary tube all contribute to a step in the cycle. QL represents
the heat being lost from the freezer compartment when the heat is used as energy to power the
evaporation. QH represents the heat being dispelled by the condenser. These concepts will be
explored in more detail in the following steps.
Refrigerants
The first step requirement for cooling a system is to start with a coolant, which is a
compressed liquid. A compressed liquid is a substance that is naturally a vapor at a
specific temperature of the system, but enough pressure has been applied that the
substance is now in the liquid phase. Common coolants used in industry are shown
below in Table 1. These coolants are used because they work well as compressed
liquids at temperatures and pressures achievable in industry. Coolants, or refrigerants,
are organized into different classes, based on how a certain thermodynamic property is
used. For this discussion on the refrigeration cycle, we will be concerned with coolants
that operate as vapors at ambient temperature.

4. Chemical Name Normal Boiling Point (⁰C) Unit
Isobutane -12 Refrigerator
R-22 (a type of Freon) -41.44 Air Conditioner
Ethylene Glycol 135-139 Car Engine
Table 1: Displays common coolants and where they are used.
Figure 2: A picture of a pressurized tank of Freon-22
Stage One: Throttling
The process can be examined starting with the compressed liquid about to enter a
throttle. A throttle can be viewed as an “expansion valve”; it is a valve that controls the
area of the pipe line. Entering the expansion valve, the coolant is a compressed liquid
with a high pressure, and a relatively low temperature. Once the refrigerant travels
through the throttle, the cross sectional area is increased, and the pressure will drop.
This drop in pressure allows the refrigerant to leave the compressed liquid state to a
state of mixed liquid and vapor. This mixture is at the saturation temperature and pressure
for the substance. Saturation is when a substance is at its boiling point. Saturation
pressure is the pressure that corresponds to this temperature. This vapor/liquid
mixture then travels to the evaporator.

5. Stage Two: Evaporation
The second stage in the refrigeration cycle is the evaporation of the coolant. This is the
most critical stage and the part of the process where the cooling effect is produced. The
rest of the cycle is just a recycle of the coolant to allow it to re-reach this stage of the
cycle. What happens is that when the vapor/liquid mixture reaches the evaporator, the
mixture wants to complete the change of state to the pure vapor phase. The coolant
wants to reach the vapor phase, because as a fluid, it desires to totally fill the volume of
the coil that it flows through. When the volume is increased to the size of the
evaporator, the refrigerant tries to reach the vapor phase to fully fill the coil. This
requires work, or the use of energy. To drive this process, or to do the work necessary,
the refrigerant removes heat from the surroundings to power this phase change from
vapor/liquid to pure vapor. This loss of heat is felt by the surroundings as coldness, or
a drop in temperature. This evaporation process occurs at constant pressure.
An Every Day Example
This loss of heat in evaporation can easily be seen on a small scale, to truly
understand why this works. If you have ever used nail polish remover,
you may have noticed that your fingers become cold. This is because the
active ingredient in nail polish remover is acetone, and acetone likes to
evaporate at room temperature. This evaporation requires energy to drive
it, so the evaporation removes heat from your fingers to power the process.
This is why your fingers become cold. This process happens also when
you use hand sanitizer. The active ingredient is alcohol, and alcohol also
likes to evaporate at room temperature. This evaporation process works
the same way, by removing heat from your fingers to power the process.

6. Step Three: Compression
The third step of the refrigeration cycle is the compression stage. This is to begin re-
prepping the coolant for the evaporation. When the refrigerant left the evaporator, it
exited as cold vapor. The compressor then takes in this cold vapor and pressurizes it, so
that it will reach the superheated vapor phase. As a consequence, the temperature of
the vapor rises past the boiling point. A superheated vapor is a vapor that is
continuously heated to a temperature higher than its boiling point. This superheated
vapor has a temperature and pressure higher than the saturated pressure and
temperature of the coolant. To compress this vapor, the compressor does work on the
system. The energy to power this work is supplied by an electrical connection.
Step Four: Condensation
The final stage of the refrigeration cycle is the condensation step. This is where the fluid
is returned to its initial compressed liquid state. The refrigerant enters a condenser after
it exits the compressor. The purpose of the condenser is to dump the excess heat
generated through the compression to the surroundings. This loss of heat allows the
superheated vapor to condense back to the compressed liquid phase. The refrigerant
condenses to a compressed liquid because it maintains the high pressure generated
from the compression, but loses the high temperature, allowing the substance to reach
the compressed liquid region. The refrigerant is now ready to repeat the cycle.

7. Figure3: image of a piston powered compressor
Graphical Representation of Process
Below is a diagram displaying all four stages and components required for the
refrigeration cycle. It is a cyclical process, so there is no true beginning or end. One can
start as we did, with the expansion vale. This diagram shows that as the refrigerant
exits the expansion valve, it is a mixture of liquid and vapor. As it passes through the
evaporator, cold air is felt as the coolant removes heat from its surroundings so that it
may change phase to pure vapor. This pure vapor then travels through the compressor,
where while the diagram does not show, it exits as a hotter, higher pressurized vapor.
This pressurized, or superheated vapor then travels through the condenser which
removes the excess heat, and allowing the refrigerant to undergo another phase change,
back into a compressed liquid.
Figure 4: Circular diagram of refrigeration cycle

8. Conclusion:
In summary, the refrigeration cycle can be broken down into four basic stages:
1. Throttle : pressure is decreased, allowing the coolant to expand into a
liquid/vapor mixture
2. Evaporation : cross sectional area is increased so that the coolant expands into a
vapor, removing heat from the surroundings and producing a cooling effect on
the desired area
3. Compression : the refrigerant is compressed into a superheated vapor, which
results in an increase in pressure and temperature
4. Condensation : the condenser dumps the excess heat generated from
compression, allowing the temperature to drop, and for the refrigerant to return
back to the compressed liquid phase, where it is under high pressure, but a low
temperature
Special chemicals called refrigerants are used as the substance that enables the process
to function. The cycle involves the changes of states, and the energy consumption and
heat loss associated with these phase changes is the key to the process. This cycle has
revolutionized how modern society operates. Without this process, modern grocery
stores would be unable to sell perishable goods such as meat, frozen vegetables, and
dairy products. Our homes would not be cooled with air conditioning, and buying ice
at the local convenience store would be impossible. Even many large scale production
processes of goods and services would be impossible without an understanding of this
cycle. The refrigeration cycle has quietly influenced and shaped modern society, and
without it, our progress would be halted.

9. Glossary of Terms
Ambient Temperature: Temperature of surroundings
Compression: The change of volume by applied pressure, greater compression, smaller
volume, higher pressure
Compressor: A mechanical unit that applies compression, powered by electricity
Compressed Liquid: A liquid that is under high pressure, but kept at low temperature, higher
than saturation pressure
Condensation: The phase change by which a vapor is changed back into a liquid
System: Refers to what is undergoing the thermodynamic change; example: the fluid traveling
through the pipe is a “system”
Surroundings: The universe outside of a system
Energy: The potential to do work, the “fuel” that powers a process
Evaporator: A unit that allows evaporation, usually a coil of pipe with a higher cross sectional
area than the piping that was upstream from the expansion valve
Evaporation: The phase change by which a liquid is changed to a vapor
Expansion Valve: A valve that separates a section of piping with different cross sectional areas,
creates a pressure drop from the high pressure upstream, to the low pressure downstream of
the valve
Heat: The thermal equivalent of energy, related to temperature
Phase Changes: A change that a fluid undergoes, from either a liquid to a vapor or from a
vapor to a liquid
Saturation Temperature and Pressure: The boiling temperature of a chemical, and the
corresponding pressure. When a fluid is in a vapor/liquid mixture, it is at saturation
Superheated Vapor: Vapor heated past its boiling point, high temperature, high pressure
Throttling: The way a pressure drop is achieved, by a change in cross sectional area of piping
Work: The transfer of energy required for a process to be completed

10. Works Cited
General Information:
1. Brain, Marshall, and Sara Elliott. "How Refrigerators Work" 29 November
2006. HowStuffWorks.com.
<http://www.howstuffworks.com/refrigerator.htm> 24 October 2012.
2. Matsoukas, Themis. "Chapter 6 Balances in Open Systems." Fundamentals of
Chemical Engineering Thermodynamics. 1st ed. Vol. 1. Upper Saddle River:
Pearson Education, 2013. 287-309. Print.
Figures:
1. Figure 1:
http://www.google.com/imgres?q=refrigeration+cycle&start
2. Figure 2:
http://www.google.com/imgres?q=freon+r22&num
3. Figure 3:
http://www.central-air-conditioner-and-
refrigeration.com/Air_Conditioner_Compressors.htm
4. Figure 4:
http://www.google.com/imgres?q=refrigeration+cycle+diagram&um=1&hl=en
&sa=X&biw=
5. Cover Picture
http://www.google.com/imgres?q=refrigeration&start=89&hl=en&biw=1304&
bih=664&
Empirical Data:
1. http://www.sigmaaldrich.com/catalog/product/sial/324558?lang=en&region=US
2. http://www.sigmaaldrich.com/catalog/product/aldrich/295450?lang=en&region=
US
3. http://www.engineeringtoolbox.com/r22-properties-d_365.html