The document discusses the principles of refrigeration and how it works. It begins by defining refrigeration as the process of removing heat from one substance and transferring it to another. It then explains the key concepts of heat, temperature, methods of heat transfer, latent heat, sensible heat, and the refrigeration cycle. Specifically: - Refrigeration involves removing heat from one substance (the evaporator) and transferring it to another (the condenser) using a refrigerant that changes state from liquid to vapor and back. - As the refrigerant absorbs heat in the evaporator, it changes from liquid to vapor. It then transfers the heat to the condenser, where it condenses back to liquid form after

1. Refrigeration Cycle
ACML Training 2012

2. In Earlier Times, one could hire people to blow cool air off a river using palm leaves.
However, keeping someone by your desk is expensive and impractical, and most
bosses won’t let you work by the river or lake.

3. Luckily, during the industrial revolution, humans discovered the internal combustion
engine, and later a derivative, the fan.
However, unless there is a body of water close by (such as Deep Water Cooling
discussed later), what can you use as a cooling source?
First some definitions…

4. What is Refrigeration?
Refrigeration is the process of removing heat from one
substance and transferring it to another substance.
The term refrigeration is commonly associated
with something cold. A household
refrigerator, for example, keeps food cold. It
accomplishes this task by removing heat from
the food. Therefore, refrigeration involves the
removal of heat. The word cold describes a
state of low heat content.

5. What is Heat?
Heat is energy transferred from one body to another by
thermal interaction. It is not a property of a system or
body, but instead is always associated with a process of
some kind.
Heat intensity is measured by its temperature, commonly in either degrees
Fahrenheit (°F) or degrees Celsius (°C). If all heat were removed from an object, the
temperature of the object would decrease to -459.6°F [-273.2°C]. This temperature
is referred to as “absolute zero” and is the temperature at which all molecular
activity stops.
The quantity of heat contained in an object or substance is not the same as its
intensity of heat. For example, the hot sands of the desert contain a large quantity
of heat, but a single burning candle has a higher intensity of heat.

6. What is Temperature?
Measure of the amount of Thermal Energy in a given area.
These two different masses of water contain the same quantity of heat, yet the
temperature of the water on the left is higher.
Why? The water on the left contains more heat per unit of mass than the water on
the right.
In other words, the heat energy within the water on the left is more concentrated,
or intense, resulting in the higher temperature. Note that the temperature of a
substance does not reveal the quantity of heat that it contains.

7. Measuring Heat Quantity:
1 kg of
1 Ib of
Water
Water
1 Kcal
1 Btu 21oC 22oC
60oF 61oF
In the English system of units, the quantity of heat is measured in terms of
the British Thermal Unit (Btu). The Btu is defined as the quantity of heat
energy required to raise the temperature of 1 lb of water by 1°F.
Similarly, in the metric system of units, the quantity of heat is measured in
terms of the kilocalorie (kilogram-calorie or kcal). The kcal is defined as
the amount of heat energy required to raise the temperature of 1 kg of
water 1°C.

8. Principles of Heat Transfer:
• Heat energy cannot be destroyed
• Heat always flows from a higher temperature substance to a
lower temperature substance
• Heat can be transferred from one substance to another

9. Heat Energy Cannot be Destroyed:
To produce cooling, heat must be removed from the substance by
transferring it to another substance. The first principle to discuss
regarding heat transfer is that heat energy cannot be destroyed; it
can only be transferred to another substance. This is commonly
referred to as the principle of “conservation of energy.”
Ice cubes are typically placed in a beverage to cool it before it is
served. As heat is transferred from the beverage to the ice, the
temperature of the beverage is lowered. The heat removed from
the beverage is not destroyed but instead is absorbed by the ice,
changing the ice from a solid to a liquid.

10. Heat Flows from Hot to Cold:
The second principle is that heat naturally flows from a higher
temperature substance to a lower temperature substance; in other
words, from hot to cold. Heat cannot flow from a cold
substance to a hot substance.
Consider the example of the beverage and the ice cubes. As long
as the temperature of the beverage is higher than the temperature
of the ice cubes, heat will always flow from the beverage to the ice
cubes.

11. Method of Heat Transfer - Conduction:
The third principle is that heat is transferred from one substance
to another by one of three basic processes:
• Conduction
• Convection
• and Radiation.
Conduction is the process of transferring heat through a solid by
way of diffusion and collisions of particles.

12. Method of Heat Transfer - Convection:
Convection is the process of transferring heat as the result of the
movement of a fluid. Convection often occurs as the result of the
natural movement of air caused by temperature (density)
differences.

13. Method of Heat Transfer - Radiation:
Radiation is the process of transferring heat by means of
electromagnetic waves, emitted due to the temperature difference
between two objects. An interesting thing about radiated heat is
that it does not heat the air between the source and the object it
contacts ; it only heats the object itself.

14. Rate of Heat Flow:
In refrigeration, as in heating, emphasis is placed on the rate of heat transfer, that
is, the quantity of heat that flows from one substance to another within a given
period of time. This rate of heat flow is commonly expressed in terms of Btu/hr;
the quantity of heat, in Btus, that flows from one substance to another over a
period of 1 hour.
Similarly, in the SI metric system of units, the rate of heat flow is expressed in
terms of kilowatts (kW), which are equivalent to kJ/sec. Kilowatts describ the
quantity of heat, in kJ, that flows from one substance to another over a period of 1
second.

15. Rate of Heat Flow:
In the English system of units, there is a larger and more convenient measure of
the rate of heat flow. It is called a ton of refrigeration. One ton of refrigeration
produces the same cooling effect as the melting of 2000 lb of ice over a 24-hour
period.
When 1 lb of ice melts, it absorbs 144 Btu. Therefore, when 1 ton (2000 lb) of ice
melts, it absorbs 288,000 Btu (2000 x 144). Consequently, 1 ton of refrigeration
absorbs 288,000 Btu within a 24-hour period or 12,000 Btu/hr (288,000/24).
So, 1 ton of refrigeration is defined as the transfer of heat at the rate of 12,000
Btu/hr.

16. 1 kg of
Water
How Refrigeration Works – Change of State: 1 Kcal
100oC 100oC
1 kg of 1 kg of
Water Water
79 Kcal
21oC 100oC 244 Kcal
100oC 100oC
This question is best answered by examining the effects of heat transfer on water.
Consider 1 kg of 21°C water. By adding or subtracting 1 Kcal of heat energy, the
water temperature is raised or lowered by 1°C.
Therefore, adding 79 Kcal to 1 kg of 21°F water raises its temperature to 100°C.
Although this is the boiling temperature of water at atmospheric pressure, adding
1 more Kcal will not cause all of the water to evaporate.
In fat, 244 kcal (1023 kJ) must be added to 1 kg of 100°C water to completely
transform it to 1 kg of steam at the same temperature.

17. How Refrigeration Works – Change of State:
1 kg of
Steam
-244 Kcal
100oC 100oC
Conversely, when 1 kg of 100°C steam condenses, it gives off 244.5 kcal (1023 kJ) of
heat energy in the process. After the steam condenses completely, the removal of
more heat will begin to lower the temperature of the water below 100°C.
The quantity of heat that must be added to the water in order for it to evaporate
cannot be sensed by an ordinary thermometer. This is because both the water and
steam remain at the same temperature during this phase change.
This kind of heat is called latent heat, which is dormant or concealed heat energy.
Latent heat is the energy involved in changing the phase of a substance—from a
liquid to a vapor in this example.

18. How Refrigeration Works – Change of State:
1 kg of
Steam
-20 Kcal
100oC 80oC
In contrast, sensible heat is heat energy that, when added to or removed from a
substance, results in a measurable change in temperature.
Refrigerants can absorb a significant amount of heat when they change phase;
much more than if they just change temperature. Different substances have
different specific temperatures at which these phase changes occur, and different
quantities of heat are required for this change to take place. They also have
different capacities for absorbing heat. This capacity is a property of the substance
called specific heat.

19. How Refrigeration Works – Change of State:
“Cold” Refrigerant
Airflow
Liquid
“Warm” Refrigerant
Vapour
Evaporator
A rudimentary refrigeration system could be constructed using a drum of liquid
refrigerant at atmospheric pressure, a coil, a collecting drum, and a valve to
regulate the flow of refrigerant into the coil.
Opening the valve allows the liquid refrigerant to flow into the coil (Evaporator) by
gravity. As warm air is blown over the surface of the coil, the liquid refrigerant
inside the coil will absorb heat from the air, eventually causing the refrigerant to
boil while the air is cooled. Adjustment of the valve makes it possible to supply just
enough liquid refrigerant to the coil so that all the refrigerant evaporates before it
reaches the end of the coil.

20. How Refrigeration Works – Change of State:
Evaporator Refrigerant
Liquid
Refrigerant ?
Vapour
One disadvantage of this system is that after the liquid refrigerant passes through
the coil and collects in the drum as a vapor, it cannot be reused. The cost and
environmental impacts of chemical refrigerants require the refrigeration process to
continue without loss of refrigerant.
There needs to be a magic box that converts the refrigerant vapours back to liquid.

21. How Refrigeration Works – Change of State:
Refrigerant - Heat Refrigerant
Vapour Liquid
Heat flows from a higher temperature substance to a lower temperature
substance. In order to remove heat from the refrigerant vapor, it must transfer this
heat to a substance that is at a lower temperature. Assume that the refrigerant
evaporated at -41.4°F [-40.8°C]. To condense back into liquid, the refrigerant vapor
must transfer heat to a substance that has a temperature less than -41.4°F [-
40.8°C].

22. How Refrigeration Works – Lowering Temperature:
One way to change the temperature is to adjust the pressure on the vapour.
Gay-Lussac's_law holds that the pressure of a gas of fixed mass and fixed volume is
directly proportional to the gas' absolute temperature. Simply put, if a gas'
temperature increases then so does its pressure, if the mass and volume of the gas
are held constant. The law has a particularly simple mathematical form if the
temperature is measured on an absolute scale, such as in kelvins. The law can then
be expressed mathematically as:
This law holds true because temperature is a measure
of the average kinetic energy of a substance; as the
kinetic energy of a gas increases, its particles collide
with the container walls more rapidly, thereby exerting
increased pressure.

23. How Refrigeration Works – Lowering Temperature:
Example:
If the refrigerant is at -40oC (233 Kelvin) and
we wish to increase the temperature to -10oC
(263 Kelvin) and the current pressure is 14psi;
P1T2 = P2T1
P2 = (14psi)*(263K)/ (233K)
P2 = 15.8psi
To raise the temperature 30C, raise the
pressure by 1.81psi (assuming the volume and
mass remain the same).

24. How Refrigeration Works – Compressor:
Evaporator Refrigerant
Liquid
Refrigerant
Vapour
Compressor
Once the refrigerant leaves the Evaporator as vapour, it
flows through the Compressor which compress it to a
higher pressure (and therefore also a higher temperature).

25. How Refrigeration Works – Condenser:
Evaporator Refrigerant
Liquid
Refrigerant
Vapour
Condenser
Compressor
After the Refrigerant leaves the compressor as a high-
temperature-high-pressure vapour, it is passed through a
series of coils called the Condenser located outside the
cooling area, where outside air is blown past to cool the
refrigerate enough to return it back to its liquid state.

26. How Refrigeration Works – Expansion Device:
Expansion Device
Evaporator Refrigerant
Liquid
Refrigerant
Vapour
Condenser
Compressor
Finally, an expansion device is used to create a large
pressure drop that lowers the pressure, and
correspondingly the temperature, of the liquid refrigerant.
The temperature is lowered to a point where it is again cool
enough to absorb heat in the evaporator

27. How Refrigeration Works – Recap:
0. Refrigerant starts as cool liquid at low pressure.
1. Refrigerant runs through Evaporator coil to cool area, gains heat, boils into vapour.
2. Refrigerant runs through Compressor, becomes high press./high temp vapour.
3. Refrigerant runs through Condenser coil, Heat transfer to outside air.
4. Refrigerant runs through Expansion Device, becomes low press./low temp liquid.

28. Deep Water Cooling
“Enwave's three intake pipes draw water (4 degrees Celsius) from 5 kilometres off the
shore of Lake Ontario at a depth of 83 metres below the surface. Naturally cold water
makes its way to the City's John Street Pumping Station. There, heat exchangers
facilitate the energy transfer between the icy cold lake water and the Enwave closed
chilled water supply loop.
The water drawn from the lake continues on its regular route through the John Street
Pumping Station for normal distribution into the City water supply. Enwave uses only
the coldness from the lake water, not the actual water, to provide the alternative to
conventional air-conditioning.”

29. Questions
• 1. If one was to increase the pressure in a closed refrigeration cycle,
what happens to the temperature?
• 2. When the refrigerant passed through the Evaporator, the
temperature of the refrigerant goes down and the surrounding area
is cooled (T/F)
• 3. There are 12,000 Btu in a ton of cooling (T/F)
• 4. The SI unit of pressure is “Pressure per square inch (psi)” (T/F)
• 5. Convection is heat transfer through liquids (T/F)
• 6. The Evaporator comes before the expansion device, and after the
compressor. (T/F)
• 7. The relationship between pressure & temperature is described
by: a) Guy-Lussac’s Law b)Gay-Lussac’s Law c) Ideal Gas Law d)
Boyle’s Law e) none of the above