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Refrigeration • A process where heat is transferred from a lower-to a highertemperature level by doing work on a system.
Carnot Cycle Temperature-entropy diagram of the Carnot cycle Q high= Q low + W T [K] Thigh 3 2 W Tlow 4 Q low = Tlow+ dels 1 s[kj/kg. K Cycle consists of: • (1 -2) - adiabatic compression • (2 -3) - isothermal rejection of heat • (3 -4) - adiabatic expansion • (4 -1) - isothermal addition of heat
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
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
Refrigeration Compression 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 high pressure vapor condenser expansion valve evaporator low pressure vapor
1. Mechanical Refrigeration (vapor compression system) 1. • • 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 1 evaporator low pressure vapor work heat
Mechanical Refrigeration – (vapor compression system) Equipment 1. 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)
Mechanical Refrigeration – (vapor compression system) Equipment (con’t) 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
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.
2. Absorption Refrigeration solution heat high pressure vapor generator condenser throttling valve expansion valve heat evaporator absorber low pressure vapor pump
Absorption Refrigeration System Two main absorption systems are used in industrial application: 1. Lithium bromide-water - limited to evaporation temperatures above freezing because water is used as the refrigerant. 2. Ammonia-water - Because ammonia is the refrigerant it can be applied for low temperature requirements.
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 pump absorber evaporator
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
Absorption Refrigeration System Ammonia-Water Absorption System (con’t) • Heat 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.
3. Ejector (Steam Jet) Refrigeration heat high pressure vapor condenser diffuser low pressure vapor ejector evaporator boiler heat pump heat
Steam Jet (Ejector) Refrigeration System Steam nozzle 1 Secondary ejectors Primary ejector Boiler Primary condenser 2 Air vent cw Make up H 2 O Heat load 3 Flash tank-evaporator Intercondenser Aftercondenser Cold water circ. pump Condensate pump drain
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 crosssectional area.
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.