Refrigeration Cycles

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Refrigeration Cycles

• Chapter 11

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Refrigerators Heat Pump

• Refrigeration The transfer of heat from lower
  temperature regions to higher temperature is
  called refrigeration.
• Refrigerator Devices that produces refrigeration
  are called refrigerators.
• Refrigerant The working fluid used in
  refrigerators are called refrigerant.
• Heat Pump Refrigerator used for the purpose of
  heating a space by transferring heat from a
  cooler medium are called heat pump.

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• COP (coefficient of performance) The performance
  of refrigeration and heat pumps are expressed in
  terms of COP.

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Reversed Carnot Cycle

• All four processes that comprise the Carnot cycle
  can be reversed.
• Reversing the cycle will also reverse the
  directions of any heat and work interactions.
• The result is a cycle that operates in the
  counterclockwise direction, which is called the
  reversed Carnot cycle.
• A refrigerator or heat pump that operates on the
  reversed Carnot cycle is called a Carnot
  refrigerator or a Carnot heat pump.

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• The standard of comparison for refrigeration
  cycle is the reversed Carnot cycle.
• The coefficient of performance e of Carnot
  refrigerators and heat pumps were determined to
  be

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The Ideal Vapor Compression Refrigeration Cycle

• In an ideal vapor compression refrigeration
  cycle, the refrigerant enters the compressor as a
  saturated vapor and is cooled to the saturated
  liquid state in the condenser. It is then
  throttled to the evaporator pressure and
  vaporizes as it absorbs heat from refrigerated
  space.

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• The ideal vapor compression refrigeration cycle
  consists of following four processes.
• 1 2 Isentropic compression in a compressor.
• 2 3 Constant pressure heat rejected in
  condenser.
• 3 4 Throttling in an expansion device (same
  enthalpy remains constant)
• 4 1 Constant pressure heat absorption in an
  evaporator.

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• In a household refrigerator, the freezer
  compartment where heat is absorbed by the
  refrigerant serves as the evaporator. The coils
  behind the refrigerator, where heat is dissipated
  to the kitchen air serve as the condenser.

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• The area under the process curve on a T-s diagram
  represents the heat transfer.
• Another diagram frequently used in the analysis
  of vapor-compression refrigeration cycle is P-h
  diagram.

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Actual Vapor-Compression Refrigeration Cycles

• There are many irreversibilities that occurs in
  various components. Two common sources of
  irreversibilites are fluid friction (causes
  pressure drop) and heat transfer to or from
  surrounding.

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Cascade Refrigeration Systems

• For applications that require large temperature
  and pressure ranges, refrigeration is performed
  in stages(2 or more).
• Large pressure range means poor compressor
  performance.
• Performing refrigeration in stages is achieved by
  Cascade Refrigeration Cycles (that is more than a
  refrigeration cycle operating in series).
• Cascading improves the COP of a refrigeration
  system.
• The refrigerant in both cycles could be the same
  or different.
• Using the following figure ,write expressions for
  mass flow rates ratio and COP?
• See Example 10.3

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Cascade Refrigeration Systems
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Multistage Compression Refrigeration Systems

• The heat exchanger in Cascade Refrigeration
  System can be replaced by a mixing chamber if the
  refrigerant in the two cycles is the same.
• Such system is called Multistage Compression
  Refrigeration System.
• Liquid refrigerant (exit of condenser) expands to
  the mixing (flash) chamber pressure where part of
  it vaporizes ( see Fig.)
• The saturated vapor mixes with the superheated
  vapor (point 3) from the exit of the low
  pressure compressor.
• Hence, two-stage compression with inter-cooling.
• Multistage Compression decreases the work of the
  compressor
• See Example 10.4

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Multistage Compression Refrigeration Systems
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Gas Refrigeration Cycles

• Gas Refrigeration Cycle is reversed Brayton cycle
  (see Fig.).
• Note, the expansion process is performed in a
  turbine rather than a throttling valve as in
  vapor compression refrigeration systems (Why?).
• The heat transfer processes donot take place at
  constant temperatures. Hence, it differs from
  Carnot Cycle.
• Hence, Gas Refrigeration Cycle do have lower COPs
  relative to vaporcompression refrigeration
  cycles. Illustrate by a T-s diag.?
• Gas Refrigeration Cycles involve simple lighter
  components (Aircraft cooling) and can incorporate
  regeneration (suitable for liquidation of gases)
• Multistage Compression decreases the work of the
  compressor
• See Example 10.5

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Gas Refrigeration Cycles
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Gas Refrigeration Cycle with Regeneration
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Absorption Refrigeration Systems

• Refrigeration in which there is a source of
  inexpensive thermal energy at a temperature of
  100 to 200OC is absorption refrigeration
• The refrigerant is absorbed by a transport medium
  and compressed in liquid form.
• The most widely used absorption refrigeration
  system is the ammonia water system where
  ammonia serves as the refrigerant and water as
  the transport medium
• Other absorption refrigeration systems include
  water-Lithium bromide where water serves as a
  refrigerant (limited applications-Why?).

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Absorption Refrigeration Systems

• The basic principles can be discussed by the
  Ammonia absorption refrigeration cycle shown in
  Fig.
• ARS are complex, occupy more space and less
  efficient (hence, expensive compared to vapor
  compression systems).
• In ARS liquid is compressed instead of vapor,
  thus the work input is very small compared to
  vapor compression systems.
• Write an expression for the COP of an ARS?
• Derive an expression for the maximum COP of an
  absorption refrigeration system and comment?

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Absorption chillers

• Absorption chillers are air-conditioning systems
  based on absorption refrigeration.
• Absorption chillers cooling capacity decreases
  sharply with decrease in source temperature.
• The COP is affected less by decrease in source
  temperature.
• Read more about absorption chillers.