14. REFRIGERATION

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14. REFRIGERATION
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Refrigeration

• Refrigeration
• A process where heat is transferred from a
  lower-to a higher-
• temperature level by doing work on a
  system.

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Carnot Cycle
Temperature-entropy diagram of the Carnot cycle
Q high Q low W
T K
3
2
Thigh
W
Tlow
1
4
Q low Tlow dels
skj/kgK

• Cycle consists of
• (1-2) - adiabatic compression
• (2-3) - isothermal rejection of heat
• (3-4) - adiabatic expansion
• (4-1) - isothermal addition of heat

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Refrigeration

• Basic Refrigeration Methods
• There are many means to obtain refrigeration
  effect, but only the 3 basic methods are
  introduced here
• 1. Mechanical refrigeration (vapor
  compression system)
• 2. Absorption refrigeration system
• 3. Steam-jet (ejector) system
• All 3 methods use similar processes for obtaining
  refrigeration effect
• Evaporation in the evaporator
• Condensation in the condenser where heat is
  rejected to the environment
• Expansion in a flow restrictor

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Refrigeration

• Main differences
• Is in the way compression -method of
  transforming low pressure vapor into high
  pressure vapor in refrigeration system is being
  done
• Mechanical refrigeration (Vapor compression
  system)
• - Compressor is used
• Absorption refrigeration system
• - Absorb vapor in liquid while removing heat
• - Elevate pressure of liquid with pump
• - Release vapor by applying heat
• Steam jet (ejector) system
• - ejector

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Refrigeration
Compression
high pressure vapor
condenser

• 1. Mechanical (Vapor compression)
• - Compressor
• 2. Absorption
• - Absorb vapor in liquid
• while removing heat
• - Elevate pressure of liquid with
• pump
• - Release vapor by applying heat
• 3. Steam jet (ejector)
• - ejector

expansion valve
evaporator
low pressure vapor
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1. Mechanical Refrigeration (vapor compression
system)

• Mechanical Refrigeration (Vapor-Compression
  System)
• Most widely used refrigeration principle.
• Used widely in oil/gas industry and probably
  elsewhere

heat
high pressure vapor
condenser
2
3
expansion valve
compressor
4
evaporator
1
low pressure vapor
work
heat
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Mechanical Refrigeration (vapor compression
system)

• Equipment
• Compressors
• This could be classified by one criteria
  (the way the increase in
• pressure is obtain)
• 1. positive displacement (reciprocating
  and rotary)
• 2. dynamic (centrifugal)
• Using other criteria compressors are
  classified as
• 1. Open
• Characterized by shaft extension
  out of compressor where it is
• coupled to the driver
• 2. Semi-hermetic
• Electric motor is in the same
  housing with compressor mechanism, but
• housing is tightened by screw
• 3. Hermetic
• Same as semi-hermetic but have
  welded housing (enclosure)

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Mechanical Refrigeration (vapor compression
system)

• Equipment (cont)
• 2. Condensers
• These are heat exchangers that convert
  refrigerant vapor to liquid.
• Heat is tranferred in 3 main phases
• 1. desuperheating
• 2. condensing
• 3. subcooling
• The 3 main types of refrigeration
  condensers are
• 1. air cooled
• 2. water cooled
• 3. evaporative with dedicated cooling
  tower

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Mechanical Refrigeration (vapor compression
system)

• Equipment
• 3. Evaporators
• These are heat exchangers where
  refrigerant is
• evaporated while cooling the product,
  fluid or body.
• Refrigeration evaporators could be
  classified according to the method of feed as
  direct (dry expansion) or recirculating ( flooded
• /liquid overflow) system.

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2. Absorption Refrigeration
solution
heat
heat
generator
high pressure vapor
condenser
throttling valve
expansion valve
heat
heat
evaporator
absorber
low pressure vapor
pump
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Absorption Refrigeration System

• Two main absorption systems are used in
  industrial application
• Lithium bromide-water
• - limited to evaporation temperatures
  above freezing because
• water is used as the refrigerant.
• Ammonia-water
• - Because ammonia is the refrigerant it
  can be applied for low
• temperature requirements.

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Ammonia Water Absorption System
ammonia vapor small amt of water
cooling fluid
analyzer
water rich liquid
rectifier
condenser
heat source
generator
ammonia vapor solution
heat exchanger
cooling fluid
COOLED FLUID
absorber
pump
evaporator
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Absorption Refrigeration
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Absorption Refrigeration System

• Ammonia-Water Absorption System
• Refrigerant is ammonia
• Absorbent is dilute aqueous solution of ammonia
• Water (the absorbent) is also volatile, so the
  regeneration of weak water solution to strong
  water solution is fractional distillation
• Ammonia vapor from the evaporator and the weak
  water solution from the generator are producing
  strong water solution in the absorber.
• Strong water solution is separated in the
  rectifier producing
• (1) ammonia with some water vapor content
• (2) very strong water solution at the
  bottom, in the generator

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Absorption Refrigeration System

• Ammonia-Water Absorption System (cont)
• Heat in the in the generator vaporizes ammonia
  and the weak solution returns to absorber.
• Other stream, mostly ammonia vapor but with some
  water vapor content flows to the condenser.
• To remove water as much as possible, stream
  passes through analyzer where it is additionally
  cooled.
• The remaining water escaped from the analyzer
  pass as liquid through the condenser and the
  evaporator to absorber.
• Operation pressure about 1100 2100 kpa
  absolute in condenser.

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3. Ejector (Steam Jet) Refrigeration
heat
high pressure vapor
condenser
diffuser
low pressure vapor
evaporator
ejector
boiler
heat
heat
pump
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Steam Jet (Ejector) Refrigeration System
Steam nozzle
1
Secondary ejectors
Primary ejector
Boiler
Primary condenser
2
Air vent
cw
Make up H2O
3
Heat load
Flash tank-evaporator
Intercondenser
Aftercondenser
drain
Condensate pump
Cold water circ. pump
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Steam (Jet) Ejector System

• This system substitute an ejector for the
  mechanical compressor in a vapor compression
  system.
• Since refrigerant is water, maintaining
  temperatures lower than environment requires that
  the pressure of water in the evaporator must be
  below atmospheric.
• Main components
• 1. Primary steam ejector
• The high velocity steam flowing through
  the nozzle of the
• ejector causes flash vapor from the tank
  to continually aspired into
• the moving steam.
• The mixture of steam and flash vapor
  enters the diffuser section
• where the velocity is gradually reduced
  because increasing cross-
• sectional area.

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Steam (Jet) Ejector System

• 2. Condenser
• The component of the system where the
  vapor condenses and
• where heat is rejected
• 3. Flash tank
• This is the evaporator of the ejector
  system, Since refrigerant is
• water, maintaining temperature lower
  than the environment
• requires that the pressure of water
  in the evaporator must
• below atmospheric. Warm water
  returning from the process is
• sprayed into the flash chamber though
  nozzles.