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 + WT [K]32ThighWTlow41Q low = Tlow+ delss[kj/kgKCycle 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 system3. Steam-jet (ejector) systemAll 3 methods use similar processes for obtaining refrigeration effect:Evaporation in the evaporatorCondensation in the condenser where heat is rejected to the environmentExpansion 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 usedAbsorption refrigeration system- Absorb vapor in liquid while removing heat- Elevate pressure of liquid with pump- Release vapor by applying heatSteam jet (ejector) system- ejector
6 Refrigeration Compression 1. Mechanical (Vapor compression) high pressure vaporcondenser1. Mechanical (Vapor compression)- Compressor2. Absorption- Absorb vapor in liquidwhile removing heat- Elevate pressure of liquid withpump- Release vapor by applying heat3. Steam jet (ejector)- ejectorexpansion valveevaporatorlow pressure vapor
7 1. Mechanical Refrigeration (vapor compression system) Most widely used refrigeration principle.Used widely in oil/gas industry [and probably elsewhere]heathigh pressure vaporcondenser23expansion valvecompressor41evaporatorlow pressure vaporworkheat
8 Mechanical Refrigeration – (vapor compression system) EquipmentCompressorsThis could be classified by one criteria (the way the increase inpressure is obtain):1. positive displacement (reciprocating and rotary)2. dynamic (centrifugal)Using other criteria compressors are classified as:1. OpenCharacterized by shaft extension out of compressor where it iscoupled to the driver2. Semi-hermeticElectric motor is in the same housing with compressor mechanism, buthousing is tightened by screw3. HermeticSame as semi-hermetic but have welded housing (enclosure)
9 Mechanical Refrigeration – (vapor compression system) Equipment (con’t)CondensersThese are heat exchangers that convert refrigerant vapor to liquid.Heat is tranferred in 3 main phases:1. desuperheating2. condensing3. subcoolingThe 3 main types of refrigeration condensers are:1. air cooled2. water cooled3. evaporative – with dedicated cooling tower
10 Mechanical Refrigeration – (vapor compression system) EquipmentEvaporatorsThese are heat exchangers where refrigerant isevaporated 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.
12 Absorption Refrigeration System Two main absorption systems are used in industrial application:Lithium bromide-water- limited to evaporation temperatures above freezing becausewater is used as the refrigerant.Ammonia-water- Because ammonia is the refrigerant it can be applied for lowtemperature requirements.
13 Ammonia – Water Absorption System ammonia vapor + small amt of watercooling fluidanalyzerwater rich liquidrectifiercondenserheat sourcegeneratorammonia vapor + solutionheat exchangercooling fluidCOOLED FLUIDpumpabsorberevaporator
15 Absorption Refrigeration System Ammonia-Water Absorption SystemRefrigerant is ammoniaAbsorbent is dilute aqueous solution of ammoniaWater (the absorbent) is also volatile, so the regeneration of weak water solution to strong water solution is fractional distillationAmmonia 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 (con’t)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.
18 Steam Jet (Ejector) Refrigeration System Steam nozzle1Secondary ejectorsPrimary ejectorBoilerPrimary condenser2Air ventcwMake up H2O3Heat loadFlash tank-evaporatorIntercondenserAftercondenserdrainCondensate pumpCold 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 ejectorThe high velocity steam flowing through the nozzle of theejector causes flash vapor from the tank to continually aspired intothe moving steam.The mixture of steam and flash vapor enters the diffuser sectionwhere the velocity is gradually reduced because increasing cross-sectional area.
20 Steam (Jet) Ejector System 2. CondenserThe component of the system where the vapor condenses andwhere heat is rejected3. Flash tankThis is the evaporator of the ejector system, Since refrigerant iswater, maintaining temperature lower than the environmentrequires that the pressure of water in the evaporator mustbelow atmospheric. Warm water returning from the process issprayed into the flash chamber though nozzles.