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14. REFRIGERATION

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14. REFRIGERATION Refrigeration Refrigeration A process where heat is transferred from a lower-to a higher- temperature level by doing work on a system. – PowerPoint PPT presentation

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Title: 14. REFRIGERATION


1
14. REFRIGERATION
2
Refrigeration
  • Refrigeration
  • A process where heat is transferred from a
    lower-to a higher-
  • temperature level by doing work on a
    system.

3
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

4
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

5
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

6
Refrigeration
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
7
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
8
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)

9
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

10
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.

11
2. Absorption Refrigeration
solution
heat
heat
generator
high pressure vapor
condenser
throttling valve
expansion valve
heat
heat
evaporator
absorber
low pressure vapor
pump
12
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.

13
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
14
Absorption Refrigeration
15
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

16
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.

17
3. Ejector (Steam Jet) Refrigeration
heat
high pressure vapor
condenser
diffuser
low pressure vapor
evaporator
ejector
boiler
heat
heat
pump
18
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
19
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.

20
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.
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