Transcript REFRIGERATION, HEAT PUMP CYCLES

REFRIGERATION, HEAT PUMP
CYCLES
REFRIGERATORS
AND HEAT PUMPS
The transfer of heat from a low-temperature
region to a high-temperature one requires
special devices called refrigerators.
Refrigerators and heat pumps are essentially
the same devices; they differ in their
objectives only.
for fixed values of QL and QH
The objective of a refrigerator is to remove heat
(QL) from the cold medium; the objective of a heat
pump is to supply heat (QH) to a warm medium.
The reversed Carnot cycle is the most efficient
refrigeration cycle operating between TL and TH.
However, it is not a suitable model for refrigeration
cycles since processes 2-3 and 4-1 are not practical
because
Process 2-3 involves the compression of a liquid–vapor
mixture, which requires a compressor that will handle
two phases, and process 4-1 involves the expansion of
high-moisture-content refrigerant in a turbine.
THE REVERSED
CARNOT CYCLE
Both COPs increase
as the difference
between the two
temperatures
decreases, that is, as
TL rises or TH falls.
Schematic of a
Carnot refrigerator
and T-s diagram
of the reversed
Carnot cycle.
HISTORY
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In Egypt (2 century) – cooling effect - vaporization water
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1755 - William Cullen – produced ice using vacuum pumps and
phase transformation
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1777 Walther Hermann Nerst – added to water H2SO4
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1834.a. Jacob Perkins – the first prototype as today we use
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1844.a. Jon Corien…air conditions…
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1864.a. absorber effect, Littman
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………………………
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THE IDEAL VAPOR-COMPRESSION REFRIGERATION CYCLE
The vapor-compression refrigeration cycle is the ideal model for refrigeration
systems. Unlike the reversed Carnot cycle, the refrigerant is vaporized completely
before it is compressed and the turbine is replaced with a throttling device.
This is the
most widely
used cycle for
refrigerators,
A-C systems,
and heat
pumps.
Schematic and T-s diagram for the ideal
vapor-compression refrigeration cycle.
The ideal vapor-compression refrigeration cycle involves an irreversible (throttling)
process to make it a more realistic model for the actual systems.
Replacing the expansion valve by a turbine is not practical since the added
benefits cannot justify the added cost and complexity.
Steady-flow
energy balance
An ordinary
household
refrigerator.
The P-h diagram of an ideal vaporcompression refrigeration cycle.
ACTUAL VAPOR-COMPRESSION REFRIGERATION CYCLE
An actual vapor-compression refrigeration cycle differs from the ideal one in
several ways, owing mostly to the irreversibilities that occur in various
components, mainly due to fluid friction (causes pressure drops) and heat transfer
to or from the surroundings. The COP decreases as a result of irreversibilities.
DIFFERENCES
Non-isentropic
compression
Superheated vapor
at evaporator exit
Subcooled liquid at
condenser exit
Pressure drops in
condenser and
evaporator
Schematic and T-s diagram for the actual
vapor-compression refrigeration cycle.
The Compressor
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The compressor is the heart of the
system. The compressor does just
what it’s name is. It compresses
the low pressure refrigerant vapor
from the evaporator and
compresses it into a high pressure
vapor.
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The Condenser
•
The “Discharge Line” leaves the
compressor and runs to the inlet of the
condenser.
•
Because the refrigerant was compressed,
it is a hot high pressure vapor (as
pressure goes up – temperature goes
up).
•
The hot vapor enters the condenser and
starts to flow through the tubes.
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Cool air is blown across the out side of
the finned tubes of the condenser
(usually by a fan or water with a pump).
•
Since the air is cooler than the
refrigerant, heat jumps from the tubing to
the cooler air (energy goes from hot to
cold – “latent heat”).
•
As the heat is removed from the
refrigerant, it reaches it’s “saturated
temperature” and starts to “flash”
(change states), into a high pressure
liquid.
•
The high pressure liquid leaves the
condenser through the “liquid line” and
travels to the “metering device”.
Sometimes running through a filter dryer
first, to remove any dirt or foreign
particles.
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Metering Devices
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Metering devices regulate how much
liquid refrigerant enters the evaporator .
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Common used metering devices are,
small thin copper tubes referred to as
“cap tubes”, thermally controller
diaphragm valves called “TXV’s” (thermal
expansion valves) and single opening
“orifices”.
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The metering device tries to maintain a
preset temperature difference or “super
heat”, between the inlet and outlet
openings of the evaporator.
•
As the metering devices regulates the
amount of refrigerant going into the
evaporator, the device lets small
amounts of refrigerant out into the line
and looses the high pressure it has
behind it.
•
Now we have a low pressure, cooler
liquid refrigerant entering the evaporative
coil (pressure went down – so
temperature goes down).
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Thermal expansion Valves
•
A very common type of metering device is
called a TX Valve (Thermostatic Expansion
Valve). This valve has the capability of
controlling the refrigerant flow. If the load on
the evaporator changes, the valve can
respond to the change and increase or
decrease the flow accordingly.
•
The TXV has a sensing bulb attached to the
outlet of the evaporator. This bulb senses the
suction line temperature and sends a signal
to the TXV allowing it to adjust the flow rate.
This is important because, if not all, the
refrigerant in the evaporator changes state
into a gas, there could be liquid refrigerant
content returning to the compressor. This
can be fatal to the compressor. Liquid can not
be compressed and when a compressor tries
to compress a liquid, mechanical failing can
happen. The compressor can suffer
mechanical damage in the valves and
bearings. This is called” liquid slugging”.
•
Normally TXV's are set to maintain 10
degrees of superheat. That means that the
gas returning to the compressor is at least 10
degrees away from the risk of having any
liquid.
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The Evaporator
•
The evaporator is where the heat is
removed from your house , business or
refrigeration box.
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Low pressure liquid leaves the metering
device and enters the evaporator.
•
Usually, a fan will move warm air from
the conditioned space across the
evaporator finned coils.
•
The cooler refrigerant in the evaporator
tubes, absorb the warm room air. The
change of temperature causes the
refrigerant to “flash” or “boil”, and
changes from a low pressure liquid to a
low pressure cold vapor.
•
The low pressure vapor is pulled into the
compressor and the cycle starts over.
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The amount of heat added to the liquid
to make it saturated and change states is
called “Super Heat”.
•
One way to charge a system with
refrigerant is by super heat.
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Refrigerant
A liquid
that has a low boiling point.
´There are several refrigerant manufacturers.
Heat pumps still use R22 refrigerants. R22 performs well over
the range of temperatures that heat pumps operate at.
R22 is known as a hydrochlorofluorocarbon (HCFC) refrigerant
and has an ozone depletion (ODP) factor of 0.05.
Many heat pumps today use R-407C or R-410A, which are hydrofluorocarbons (HFC).
Both R-407C and R-410A have zero ozone depletion potential (ODP), and slightly
lower global warming potential (GWP) in the case of R-407C, than R-22. R410A
has a slightly higher GWP than R22.
Performance (heating capacity and efficiency) is about the same with R-407C and
about 4% better with R410A compared to R-22.
R-22 will be phased out for new equipment by January 1, 2010.
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SELECTING THE RIGHT REFRIGERANT
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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), and even water (in
applications above the freezing point).
R-11, R-12, R-22, R-134a, and R-502 account for over 90 percent of the market.
The industrial and heavy-commercial sectors use ammonia (it is toxic).
R-11 is used in large-capacity water chillers serving A-C systems in buildings.
R-134a (replaced R-12, which damages ozone layer) 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.
R-502 (a blend of R-115 and R-22) is the dominant refrigerant used in commercial
refrigeration systems such as those in supermarkets.
CFCs 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) do the
most damage to the ozone layer. Refrigerants that are friendly to the ozone layer
have been developed.
Two important parameters that need to be considered in the selection of a
refrigerant are the temperatures of the two media (the refrigerated space and the
environment) with which the refrigerant exchanges heat.
HEAT PUMP SYSTEMS
A heat pump can be
used to heat a house
in winter and to cool
it in summer.
The most common energy source for
heat pumps is atmospheric air (air-toair systems).
Water-source systems usually use
well water and ground-source
(geothermal) heat pumps use earth as
the energy source. They typically have
higher COPs but are more complex
and more expensive to install.
Both the capacity and the efficiency of
a heat pump fall significantly at low
temperatures. Therefore, most airsource heat pumps require a
supplementary heating system such
as electric resistance heaters or a gas
furnace.
Heat pumps are most competitive in
areas that have a large cooling load
during the cooling season and a
relatively small heating load during the
heating season. In these areas, the
heat pump can meet the entire cooling
and heating needs of residential or
commercial buildings.
INNOVATIVE VAPOR-COMPRESSION
REFRIGERATION SYSTEMS
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The simple vapor-compression refrigeration cycle is the most widely
used refrigeration cycle, and it is adequate for most refrigeration
applications.
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The ordinary vapor-compression refrigeration systems are simple,
inexpensive, reliable, and practically maintenance-free.
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However, for large industrial applications efficiency, not simplicity, is
the major concern.
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Also, for some applications the simple vapor-compression
refrigeration cycle is inadequate and needs to be modified.
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For moderately and very low temperature applications some
innovative refrigeration systems are used. The following cycles are
generally employed:
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Cascade refrigeration systems
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Multistage compression refrigeration systems
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Multipurpose refrigeration systems with a single compressor
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Liquefaction of gases
Gas for Heat Pumps
• Heat pumps fired by natural gas have been commercially
developed.
• One type uses the absorption cycle, where the energy for refrigerant
compression is provided by a gas burner.
• Another variation is the engine-driven heat pump cycle. Here a
natural gas engine is used to drive the compressor. During
operation, heat is recovered from the engine jacket cooling water
and engine exhaust.
Gas heat pumps are less common than electric heat pumps.
• Performance compared to electric heat pumps is lower, with lower
COPs for both absorption and engine-driven units than for
conventional electric heat pumps.
• They promise to reduce global warming through more efficient
conversion of natural gas and reduced emissions from electric
power plants as they do not use electricity to drive the heat pump.
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Gas engine driver
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District heating and heat pump
Oslo
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