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Refrigeration

Lecture Notes
by

Raed Bourisli

on 18 November 2013

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Transcript of Refrigeration

Refrigeration
ME 424 Lecture Notes
Dr. Raed Bourisli

... refrigeration is "cooling," or the removal of heat.
Innovative Vapor-Compression Refrigeration Systems
Schematic and T-s diagram for a refrigerator–freezer unit with one compressor.
Multipurpose Refrigeration Systems with a Single Compressor
The simple vapor-compression refrigeration cycle is the most widely used refrigeration cycle, and it is adequate for most refrigeration applications; it is simple, inexpensive, reliable, and practically maintenance-free.
However, for large industrial applications efficiency, not simplicity, is the major concern.
Also, for some applications the simple vapor-compression refrigeration cycle is inadequate and needs to be modified.
When there is a source of inexpensive thermal energy at a temperature of 100 to 200°C is absorption refrigeration
.
Ammonia absorption refrigeration cycle.
Absorption Refrigeration Systems
The vapor-compression refrigeration cycle is the best model for refrigeration systems.
The Ideal Vapor-Compression Refrigeration Cycle
ARS Cycle & Pictures
The COP of actual absorption refrigeration systems is usually less than 1.
Air-conditioning systems based on absorption refrigeration, called absorption chillers, perform best when the heat source can supply heat at a high temperature with little temperature drop.

A two-stage compression refrigeration system with a flash chamber.
Multistage Compression Refrigeration Systems
Some applications require refrigeration at more than one temperature. A practical and economical approach is to route all the exit streams from the evaporators to a single compressor and let it handle the compression process for the entire system.
When the fluid used throughout the cascade refrigeration system is the same, the heat exchanger between the stages can be replaced by a mixing chamber (called a flash chamber) since it has better heat transfer characteristics.
Linde-Hampson system for liquefying gases.
Many important scientific and engineering processes at cryogenic temperatures (below about 100°C) depend on liquefied gases including the separation of oxygen and nitrogen from air, preparation of liquid propellants for rockets, the study of material properties at low temperatures, and the study of superconductivity.
The storage (i.e., hydrogen) and transportation of some gases (i.e., natural gas) are done after they are liquefied at very low temperatures. Several innovative cycles are used for the liquefaction of gases.
Liquefaction of Gases
Cascading improves the COP of a refrigeration system.
Some systems use three or four stages of cascading.
A two-stage cascade refrigeration system with the same refrigerant in both stages.
Some industrial applications require moderately low temperatures, and the temperature range they involve may be too large for a single vapor-compression
refrigeration cycle to be practical. The solution is
cascading
.
Schematic and T-s diagram for the actual vapor-compression refrigeration cycle.
It differs from the ideal one in several ways, because of the irreversibilities that occur in various components (mainly due to fluid friction, which causes pressure drops) and the heat transfer to and from the surroundings.
Actual Vapor-Compression Refrigeration Cycles
COP for HP = COP for R + 1 for fixed values of QL and QH
The Coefficients of Performance are:
The objective of a refrigerator
is to remove heat (QL)
from the cold medium;
while the objective of a heat pump
is to supply heat (QH)
to a warm medium.
REFRIGERATORS AND HEAT PUMPS
The performance of any refrigerating system when used for cooling is given by the coefficient of performance, or COP ,
Performance: COP
Typical Operation of The Ideal Cycle
This is the most widely used cycle in refrigerators, AC systems, and heat pumps.
An ordinary household refrigerator.
The P-h diagram of an ideal vapor-compression refrigeration cycle.
Steady-flow energy balance
in simplest terms,
... it is the transfer of heat
"from a place where it is not wanted
to a place where it is less objectionable."
in terms of heat transfer,
Example:
The old-fashioned icebox.
The icebox was not mechanically refrigerated.
The substance doing the cooling is open to its environment.
The thing doing the cooling cannot be easily reused.
At the heart of
every “air-conditioner,”
there is a refrigeration cycle.
It can be either open or closed.
The refrigerant is isolated from the environment.
Examples of those include:
Open refrigeration cycle:
Closed refrigeration cycle:
Mechanical refrigeration systems
Absorption units
Electric cooling
Mechanical
Refrigeration
Systems
Evaporator (boiler)
Condenser
Compressor
Throttling valve (expansion tube)
have four main components
Condensers
Evaporators
Compressors
Throttling valve
Refrigerators and heat pumps are essentially the same devices;
they differ only in their "objectives."
How to measure the "performance" of
via the
Coefficient
Of
Performance
COP
Differences:
Non-isentropic compression
Superheated vapor at evaporator exit
Subcooled liquid at condenser exit
Pressure drops in condenser and evaporator
The COP decreases as a result of irreversibilities.
Refrigerants
Adding 90 to the number gives three digits which stands for the number of carbon, hydrogen and fluorine atoms, respectively.
The remaining bonds not accounted for are occupied by chlorine atoms.
A suffix of a lower-case letter a, b, or c indicates increasingly unsymmetrical isomers.
As a special case, the R-400 series is made up of zeotropic blends (those where the boiling point of constituent compounds differs enough to lead to changes in relative concentration because of fractional distillation) and the R-500 series is made up of so-called azeotropic blends. The rightmost digit is assigned arbitrarily by ASHRAE.
They are used in air-conditioning systems for buildings, in sport and leisure facilities, in the chemical/pharmaceutical industry, in the automotive industry and above all in the food industry (production, storage, retailing).
Several refrigerants may be used in refrigeration systems such as:
chlorofluorocarbons (CFCs)
ammonia,
hydrocarbons (propane, ethane, ethylene, etc.)
carbon dioxide,
air (in the air-conditioning of aircraft)
even water (in applications above the freezing point).
Refrigerants by class
Divided into three classes according to their manner of absorption or extraction of heat from the substances to be refrigerated:
Class 1: This class includes refrigerants that cool by phase change (typically boiling), using the refrigerant's latent heat.
Class 2: These refrigerants cool by temperature change or 'sensible heat', the quantity of heat being the specific heat capacity x the temperature change. They are air, calcium chloride brine, sodium chloride brine, alcohol, and similar nonfreezing solutions. The purpose of Class 2 refrigerants is to receive a reduction of temperature from Class 1 refrigerants and convey this lower temperature to the area to be air-conditioned.
Class 3: This group consists of solutions that contain absorbed vapors of liquefiable agents or refrigerating media. These solutions function by nature of their ability to carry liquefiable vapors, which produce a cooling effect by the absorption of their heat of solution. They can also be classified into many categories.
Refrigerants by the R-Number
The R-# numbering system was developed by
DuPont corporation
(which owns the Freon trademark) and systematically identifies the molecular structure of refrigerants made with a single halogenated hydrocarbon. The meaning of the codes is as follows:
For example, R-134a has 2 carbon atoms, 2 hydrogen atoms, and 4 fluorine atoms, an empirical formula of
tetrafluoroethane
. The "a" suffix indicates that the isomer is unbalanced by one atom, giving 1,1,1,2-Tetrafluoroethane. R-134 (without the "a" suffix) would have a molecular structure of 1,1,2,2-Tetrafluoroethane—a compound not especially effective as a refrigerant.
R-11, R-12, R-22, R-134a, and R-502 account for over 90% of the market.
The industrial and heavy-commercial sectors use ammonia (toxic).
R-11 is used in large-capacity water chillers serving systems in buildings.
R-134a (replaced R-12, which is ozone-eating) is used in domestic refrigerators and freezers, as well as automotive air conditioners.
When refrigerants are removed they should be recycled to clean out any contaminants and return them to a usable condition.
Refrigerants should never be mixed together outside of facilities licensed to do so for the purpose of producing blends.
Some refrigerants must be managed as hazardous waste even if recycled, and special precautions are required for their transport, depending on the legislation of the country's government.
Available Refrigerants
CFCs (chlorofluorocarbons) allow more ultraviolet radiation into the earth’s atmosphere by destroying the protective ozone layer and thus contributing to the greenhouse effect that causes global warming.
Fully halogenated CFCs (such as R-11, R-12, and R-115) damage the ozone layer the most.
Many of the refrigerants developed in recent years are friendly to the ozone layer.
Two selection considerations:
The temperatures of the two media with which the refrigerant exchanges heat (i.e., the refrigerated space and the surroundings).
The (well-being of the) environment.
R-11, R-12, R-22, R-134a, and R-502 account for over 90% of the market.
The industrial and heavy-commercial sectors use ammonia (toxic).
R-11 is used in large-capacity water chillers serving systems in buildings.
R-134a (replaced R-12, which is ozone-eating) is used in domestic refrigerators and freezers, as well as automotive air conditioners.
R-22 is used in window air conditioners, heat pumps, air conditioners of commercial buildings, and large industrial refrigeration systems, and offers strong competition to ammonia.
The RIGHT Refrigerant for the job
All R-400 (R-4xx) and R-500 (R-5xx) hydroflurocarbons are blends.
R-502 (a blend of R-115 and R-22) is the dominant refrigerant used in commercial refrigeration systems such as those in supermarkets.
The RIGHT Refrigerant for the environment
All R-400 (R-4xx) and R-500 (R-5xx) hydroflurocarbons are blends.
R-502 (a blend of R-115 and R-22) is the dominant refrigerant used in commercial refrigeration systems such as those in supermarkets.
R-22 is used in window air conditioners, heat pumps, air conditioners of commercial buildings, and large industrial refrigeration systems, and offers strong competition to ammonia.
Disposal of Refrigerants
As of July 1, 1992 it is illegal in the United States to release refrigerants into the atmosphere (intentional or accidental).
Cascade refrigeration systems
Multistage compression refrigeration systems
Multipurpose refrigeration systems with a single compressor
Liquefaction of gases
For moderate and very low temperature applications, some innovative refrigeration systems are used. The following are some innovative cycles:
Cascade Refrigeration Systems
Some examples include geothermal energy, solar energy, and waste heat from cogeneration or process steam plants, and even natural gas when it is at a relatively low price.
ARS involve the absorption of a refrigerant by a transport medium.
The most widely used system is the ammonia–water system, where ammonia (NH3) serves as the refrigerant and water (H2O) as the transport medium.
Other systems include water–lithium bromide and water–lithium chloride systems, where water serves as the refrigerant. These systems are limited to applications such as AC where the minimum temperature is above the freezing point of water.
Compared with vapor-compression systems, ARS have one major advantage: A liquid is compressed instead of a vapor and as a result the work input is very small (on the order of one percent of the heat supplied to the generator) and often neglected in the cycle analysis.
ARS are often classified as heat-driven systems.
ARS are much more expensive than the vapor-compression refrigeration systems. They are more complex and occupy more space, they are much less efficient, requiring much larger cooling towers to reject waste heat, and they are more difficult to service since they are less common and less safe.

Therefore, ARS should be considered only when the unit cost of thermal energy is low and is projected to remain low relative to electricity.
ARS are primarily used in large commercial and industrial installations.
"Einstein Refrigerator" op
erates on such principle.
ARS Facts
Evaporators absorb heat from the indoor air in order to keep it cooler.
(e.g., for R-22, Tsat@69PSIG = 40 F)

Condensers reject heat from the refrigerant to the outdoor air
to make it reusable.
Compressors allow us increase the pressure of the refrigerant
(and therefore Tsat.)
Throttling valves allow us to decrease the pressure
(and therefore Tsat.)
R
Refrigeration efficiency (2nd-law efficiency) is,
Performance: SEER
The SEER rating of a unit is the
cooling output
during a
typical cooling-season
divided by the
total electric energy input
during the same period.
The efficiency of air conditioners is often rated by the Seasonal Energy Efficiency Ratio (SEER) which is defined by the
Air Conditioning, Heating and Refrigeration Institute
in its standard
ARI 210/240
, "
Performance Rating of Unitary Air-Conditioning and Air-Source Heat Pump Equipment
." In the U.S., the SEER is the ratio of cooling in British thermal unit (BTU) to the energy consumed in watt-hours.
The higher the unit's SEER rating the more energy efficient it is.
The annual total cooling output would be,
As an example, consider a 5-ton (60,000 Btu/hr or 17,6 kW) AC unit, with a SEER of 10 Btu/W·hr, operating for a total of 8,760 hours (one year).
With a SEER of 10 Btu/W·h, the annual electrical energy usage would be,
Performance: SEER Example
(Seasonal Energy Efficiency Ratio)
60000 Btu/hr × 24 hr/day × 365 days/year = 5.256 x 10^8 Btu/year
5.256 x 10^8 Btu/year / 10 Btu/W·hr = 52,560 kW·hr/year
The average power usage may also be calculated more simply by,
Average power = (Btu/hr) / (SEER) = 5,2560 / 10 = 5,256 kW
If your electricity cost is 57 fils/kW·hr, then your cost per operating hour is,
0.5 kW x 20 fils/kW·hr = 10 fils/hr
COP of ARS
The COP of actual absorption refrigeration systems is usually less than 1.
AC systems based on absorption refrigeration (called absorption chillers) perform best when the heat source can supply heat at a high temperature with little temperature drop.
<--- Maximum COP of an absorption refrigeration system
to be completed ....
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