MULTI-STAGE VAPOUR COMPRESSION REFRIGERATION

SYSTEMS(VCR)

WHY MULTI-STAGE ???• The performance of single stage systems shows that

these are adequate as long as the temperaturedifference between evaporator and condenser(temperature lift) is small.

• The temperature lift can become large either due tothe requirement of very low evaporatortemperatures and/or due to the requirement ofvery high condensing temperatures.

• For example, in frozen food industries the requiredevaporator can be as low as –40oC, while inchemical industries temperatures as low as –150oCmay be required for liquefaction of gases.

• On the condenser side, Refrigeration system is usedas a heat pump for heating applications such asprocess heating, drying etc.

As the temperature lift increases thesingle stage systems become inefficientand impractical.

• As evaporator temperaturedecreases:

1. Throttling losses increase

2. Superheat losses increase

3. Compressor discharge temperatureincreases

4. Quality of the vapour at the inlet to theevaporator increases

5. Specific volume at the inlet to thecompressor increases

For fluorocarbon and ammoniabased refrigeration systems:

• Single stage system is used up to anevaporator temperature of –30oC.

• A two-stage system is used up to–60oC and

• A three-stage system is used fortemperatures below –60oC.

• Multi-stage systems are also used inapplications requiring refrigeration atdifferent temperatures.

• For example, in a dairy plant refrigerationmay be required at –30oC for making icecream and at 2oC for chilling milk.

• In such cases it may be advantageous to usea multi-evaporator system

• A multi-stage system is a refrigeration systemwith two or more low-side pressures.

• Multi-stage systems can be classified into:

a) Multi-compression systems

b) Multi-evaporator systems

c) Cascade systems, etc.

• Two concepts which are normally integral tomulti-pressure systems are, i) flash gasremoval, and ii) intercooling.

• Flash gas does not contribute to therefrigeration effect as it is already in theform of vapour, and it increases the pressuredrop in the evaporator.

• It is possible to improve the COP of thesystem if the flash gas is removed as soon asit is formed and recompressed to condenserpressure but it is not practical.

• To improve the COP of the system, removethe flash gas at an intermediate pressureusing a flash tank.

Flash gas removal using flash tank

• A flash tank is a pressure vessel, wherein therefrigerant liquid and vapour are separated at anintermediate pressure. The refrigerant fromcondenser is first expanded to an intermediatepressure corresponding to the pressure of flashtank, Pi using a low side float valve (process 6-7).

• The float valve also maintains a constantliquid level in the flash tank.

• In the flash tank, the refrigerant liquid andvapour are separated.

• The saturated liquid at point 8 is fed to theevaporator after throttling it to the requiredevaporator pressure, Pe (point 9) using anexpansion valve.

• Depending upon the type of the system, thesaturated vapour in the flash tank (point 3) iseither compressed to the condenser pressureor throttled to the evaporator pressure

Intercooling in multi-stage compression

• Specific work input reduces as specificvolume, v1 is reduced.

• At a given pressure, the specific volume canbe reduced by reducing the temperature.

• This is the principle behind intercooling inmulti-stage compression.

• Intercooling of the vapour may be achievedby using either a water-cooled heatexchanger or by the refrigerant in theflash tank.

• With water cooling the refrigerantat the inlet to the high stagecompressor may not be saturated.

• Intercooling not only reduces thework input but also reduces thecompressor discharge temperatureleading to better lubrication andlonger compressor life.

• Intercooling using liquid refrigerant fromcondenser in the flash tank may or may notreduce the power input to the system, as itdepends upon the nature of the refrigerant.

• The heat rejected by the refrigerant duringintercooling generates additional vapour inthe flash tank, which has to be compressedby the high stage compressor.

• Thus the mass flow rate of refrigerantthrough the high stage compressor will bemore than that of the low stage compressor.

• For ammonia, the power input usuallydecreases with intercooling by liquidrefrigerant.

• For refrigerants such as R12, R22, the powerinput marginally increases.

• Thus intercooling using liquid refrigerant isnot effective for R12 and R22.

• Using both water-cooling and flash-tank,the amount of refrigerant vapour handledby the high-stage compressor reducesleading to lower power consumption.

Selection of suitable intermediate pressure

• For air compressors with intercooling tothe initial temperature, the theoreticalwork input to the system will be minimumwhen the pressure ratios are equal for allstages.

• For refrigerants, correction factors to theabove equation are suggested.

Multi-stage system with flash gas removal and intercooling

• The above system offers several advantages,a) Quality of refrigerant entering the evaporatorreduces thus giving rise to higher refrigerating effect,lower pressure drop and better heat transfer in theevaporatorb) Throttling losses are reduced as vapour generatedduring throttling from Pc to Pi is separated in the flash tankand recompressed by Compressor-II.c) Volumetric efficiency of compressors will be highdue to reduced pressure ratiosd) Compressor discharge temperature is reducedconsiderably.• One disadvantage of the above system is that since

refrigerant liquid in the flash tank is saturated, thereis a possibility of liquid flashing ahead of theexpansion valve due to pressure drop or heat transferin the pipelines connecting the flash tank to theexpansion device.

Refrigeration system with liquid sub cooler

Use of flash tank for flash gas removal

Use of flash tank for intercooling only

Problem 2The required refrigeration capacity of a vapourcompression refrigeration system (with R-22 asrefrigerant) is 100 kW at –30oC evaporatortemperature. Initially the system was single-stage witha single compressor compressing the refrigerantvapour from evaporator to a condenser operating at1500 kPa pressure.Later the system was modified to a two-stage systemoperating on the cycle shown below. At theintermediate pressure of 600 kPa there is intercoolingbut no removal of flash gas. Finda) Power requirement of the original single-stage

system;b) Total power requirement of the two compressors in

the revised two-stage system. Assume that thestate of refrigerant at the exit of evaporator,condenser and intercooler is saturated, and thecompression processes are isentropic.

Multi-Evaporator Systems

Individual evaporators and a singlecompressor with a pressure-reducing valve

1. Individual expansion valves

Problem 3:The high temperature evaporator (Refrigerationcapacity 5 TR) of a multi-evaporator VCR system,working with ammonia, is operating at –6oC andthe low temperature evaporator (Refrigerationcapacity 10 TR) is operating at –34oC. Thecondenser pressure is 10.99 bar. The systemusing individual expansion valves for eachevaporator. Assuming saturated conditions at theexit of evaporators and condenser and isentropiccompression:a) Find the required power input and COP if a

single compressor is used.b) If individual compressors are used for both

stage find the power input and COP.c) Compare both cases.

SOLUTION

Single Compressor Individual Compressors

2. Multiple expansion valves

Multi-evaporator system with individualcompressors and multiple expansionvalves

Multi-evaporator system with multi-compression, intercooling and flash gasremoval

• The mass flow rate of refrigerant through thehigh-stage compressor which can be obtainedby taking a control volume which includes theflash tank and high temperature evaporator(as shown by dashed line in the schematic)and applying mass and energy balance:

m5 + m2 = m7 + m3; m5 = m3 & m2 = m7 = mI

Problem 4

In an ammonia system one evaporator is toprovide 180 kW of refrigeration at -300C andanother evaporator is to provide 200 kW at50C. The system uses two stage compressionwith intercooling and flash gas removal asshown below. The condensing temperature is400C. Calculate the power requirement andCOP of the system.

Limitations of multi-stage systems

• The refrigerant used should have high criticaltemperature and low freezing point.

• Generally only R12, R22 and NH3 systems havebeen used in multi-stage systems as otherconventional working fluids may operate invacuum at very low evaporator temperatures-leads to leakages into the system.

• Possibility of migration of lubricating oil fromone compressor to other leading to compressorbreak-down.

Cascade Systems• In a cascade system a series of refrigerants

with progressively lower boiling points areused in a series of single stage units.

• The condenser of lower stage system iscoupled to the evaporator of the next higherstage system and so on.

• The component where heat of condensationof lower stage refrigerant is supplied forvaporization of next level refrigerant iscalled as cascade condenser.

• Two different refrigerants operating in twoindividual cycles.

• They are thermally coupled in the cascadecondenser.

• The refrigerants selected should havesuitable pressure-temperaturecharacteristics.

• It is possible to use more than two cascadestages, and it is also possible to combinemulti-stage systems with cascade systems.

Applications of cascade systems

• Liquefaction of petroleum vapors

• Liquefaction of industrial gases

• Manufacturing of dry ice

• Deep freezing etc.

Advantages of cascade systems

• Since each cascade uses a differentrefrigerant, it is possible to select arefrigerant that is best suited for thatparticular temperature range. Very high orvery low pressures can be avoided

• Migration of lubricating oil from onecompressor to the other is prevented

Optimum cascade(coupling) temperature

Where Te and Tc are the evaporatortemperature of low temperature cascade andcondenser temperature of high temperaturecascade, respectively.

Problem 5A cascade system is to be designed to attain atemp. of 213K in an environment of 313K. Therefrigerant for the high temp. side is R-12 andthat for low side is R-13. The condensing temp. ofthe low side is 5K above the evaporation temp. inthe cascade. Compute using appropriate tablesand charts

a) COP

b) Pressure ratios

c) Mass flow rates of each refrigerants

d) Volume flow rate per TR.

Problem 6The refrigeration system using R-22 asrefrigerant consists of 3 evaporators of capacities30 TR, 20 TR and 10 TR with individualexpansion valves and compressors. Thetemperature in the 3 evaporators is to bemaintained at -100C, 50C and 100C respectively.The vapor leaving the evaporator is dry andsaturated. The condenser temperature is 400Cand the liquid refrigerant leaving the condenseris sub cooled to 300C. Assuming isentropiccompression, find:a) Mass of refrigerant flowing through each

evaporator,b) Power required to drive the system andc) COP of the system.

VAPOR ABSORPTION REFRIGERATION SYSTEMS(VARS)

OR THERMAL ENERGY DRIVEN SYSTEMS

• The required input to absorptionsystems is in the form of heat so alsocalled as heat operated or thermalenergy driven systems.

• Since conventional absorption systemsuse liquids for absorption ofrefrigerant, these are also sometimescalled as wet absorption systems.

• Since conventional absorption systemsuse natural refrigerants such as wateror ammonia they are environmentfriendly.

BASIC PRINCIPLE

SIMPLE VARS

Continuous refrigeration is produced atevaporator, while heat at high temperature iscontinuously supplied to the generator.

Heat rejection to the external heat sink takesplace at absorber and condenser.

A small amount of mechanical energy isrequired to run the solution pump.

If we neglect pressure drops, then theabsorption system operates between thecondenser and evaporator pressures.

Pressure in absorber is same as the pressure inevaporator and pressure in generator is sameas the pressure in condenser.

In VCRS - the vapour is compressedmechanically using the compressor.

In VARS - the vapour is first convertedinto a liquid and then the liquid ispumped to condenser pressure usingthe solution pump.

For the same pressure difference, themechanical energy required to operateVARS is much less than that required tooperate a VCRS.

Maximum COP of Ideal VARS

From first law of thermodynamics, if weneglect the pump work,

𝑄𝐶+𝐴 = 𝑄𝐸 + 𝑄𝐺From second low of thermodynamics,

ie,𝑄𝐶+𝐴

𝑇0=

𝑄𝐸

𝑇𝐸+𝑄𝐺

𝑇𝐺

𝑄𝐸+𝑄𝐺

𝑇0=

𝑄𝐸

𝑇𝐸+𝑄𝐺

𝑇𝐺

Refrigerant-absorbent combinations for VARS

The desirable properties of refrigerant-absorbentmixtures for VARS are:

• The refrigerant should exhibit high solubilitywith solution in the absorber.

• There should be large difference in the boilingpoints of refrigerant and absorbent (greaterthan 200oC), so that only refrigerant is boiled-off in the generator - Only pure refrigerantcirculates through refrigerant circuit(condenser-expansion valve-evaporator)leading to isothermal heat transfer inevaporator and condenser.

• It should exhibit small heat of mixing so thata high COP can be achieved

• The refrigerant-absorbent mixture shouldhave high thermal conductivity and lowviscosity for high performance.

• It should not undergo crystallization orsolidification inside the system

• The mixture should be safe, chemicallystable, non-corrosive, inexpensive andshould be available easily.

Commonly used refrigerant-absorbentpairs are:

1. Water-Lithium Bromide (H2O-LiBr)system for above 0oC applicationssuch as air conditioning. Here wateris the refrigerant and lithium bromideis the absorbent.

2. Ammonia-Water (NH3-H2O) systemfor refrigeration applications withammonia as refrigerant and water asabsorbent

Problem

The operating temperatures of a single stage vapourabsorption refrigeration system are: generator: 90oC;condenser and absorber: 40oC; evaporator: 0oC. Thesystem has a refrigeration capacity of 100 kW and theheat input to the system is 160 kW. The solutionpump work is negligible.

a) Find the COP of the system and the total heatrejection rate from the system.

b) An inventor claims that by improving the design ofall the components of the system he could reducethe heat input to the system to 80 kW whilekeeping the refrigeration capacity and operatingtemperatures same as before. Examine the validityof the claim.

PRACTICAL AMMONIA-WATER VARS

• Ammonia is the refrigerant and water is theabsorbent.

• More complex in design and operation dueto the smaller boiling point temperaturedifference between the refrigerant andabsorbent (about 133oC).

• Due to smaller BP temp difference, thevapour generated in the generator consistsof both ammonia as well as water.

If water is allowed to circulate with ammoniain the refrigerant circuit, then:

i. Heat transfer in condenser and evaporatorbecomes non-isothermal-pressure drop.

ii. Evaporator temperature increases.

iii. Evaporation will not be complete.

iv. Water may get accumulated in theevaporator leading to malfunctioning of theplant.

iv. Circulation ratio increases.

Analyzer• It consists of a series of trays mounted

above the generator.

• The strong solution from the absorber andthe aqua from the rectifier are introduced atthe top of the analyzer and flow downwardover the trays into the generator.

• In this way, considerable liquid surface areais exposed to the vapour rising from thegenerator.

• The vapour is cooled and most of the watervapour condenses.

Rectifier• Is a closed type vapour cooler also known as

dehydrator.

• It is generally water cooled and may be of thedouble pipe, shell and coil or shell and tubetype.

• Its function is to cool further the ammoniavapors leaving the analyzer so that theremaining water vapors are condensed.

• Thus, only dry or anhydrous ammonia vaporsflow to the condenser.

• The condensate from the rectifier is returnedto the top of the analyzer by a drip return pipe.

Heat exchangers

1. The HXr between the pump and thegenerator - cool the weak hot solutionfrom the generator which raises thetemperature of the strong solution. Thisreduces the heat supplied to the generatorand the amount of cooling required for theabsorber.

2. The HXr b/w the condenser and theevaporator also called liquid sub-cooler -the liquid refrigerant leaving the condenseris sub- cooled by the low temperatureammonia vapour from the evaporator.

ELECTROLUX REFRIGERATOR

• Invented by two Swedish engineers CarlMunters and Baltzer Von Platan in 1925.

• The idea was first developed by the ‘ElectroluxCompany’ of Luton, England.

• Also called three-fluids absorption system.

• The main purpose of this system is toeliminate the pump so the machine becomesnoise-less.

• Ammonia is used as the refrigerant.

• The hydrogen is used to increase the rate ofevaporation of the liquid ammonia passingthrough the evaporator and it is insoluble inwater(solvent or absorbent).

• The hydrogen gas only circulates fromthe absorber to the evaporator andback.

• The whole cycle is carried out entirelyby gravity flow of the refrigerant.

• It can not be used for industrialpurposes as the C.O.P. of the system isvery low.

LITHIUM BROMIDE VARS

• The lithium bromide solution has a strongaffinity for water vapour because of its verylow vapour pressure.

• Lithium bromide solution is corrosive,therefore inhibitors should be added.

• Lithium chromate is often used as acorrosion inhibitor.

• The absorber and the evaporator are placedin one shell - low pressure of the system.

• The generator and condenser are placed inanother shell - high pressure of the system.

Comparison between VC and VA systems

VAPOR COMPRESSION SYSTEM

• Compressor work operated • High COP because of using high grade

energy(work)• Performance is very sensitive to

evaporator temperature• COP reduces considerably at part

loads• Presence of liquid at the exit of the

evaporator may damage compressor.• Superheating at the evaporator exit

increases compressor work• Many moving parts• Regular maintenance required• Higher noise and vibrations• Small systems are compact and large

systems are bulky (e.g. house hold)• Economical when electricity is

available (house, malls etc)

VAPOR ABSORPTION SYSTEM• Heat operated• Low COP because of using low grade

energy(heat)• Performance not very sensitive to

evaporator temperature• COP doesn’t reduce considerably at part

loads• Presence of liquid at the exit of the

evaporator is not a problem• Superheating at the evaporator exit is

not a problem• Few moving parts• low maintenance required• Less noise and vibrations• Small systems are bulky and large

systems are compact (e.g. ice plants)• Economical when waste heat is

available in large quantity (industries)

STEAM JET REFRIGERATION SYSTEM

• This system uses the principle of boiling thewater below 1000C.

• If the pressure on the surface of the water isreduced below atmospheric pressure, watercan be made boil at low temperatures.

• High vacuum on the surface of the water canbe maintained by throttling the steamthrough jets or nozzles.

• Water is the refrigerant.

• High pressure steam from the boiler isexpanded in the nozzle.

• The water vapor originated from the flashchamber is entrained with the high velocitysteam jet and it is further compressed in thethermo compressor.

• The kinetic energy of the mixture is convertedinto static pressure in the ejector and isdischarged to the condenser.

• The condensate is usually returned to theboiler.

• Generally, 1% evaporation of water in the flashchamber is sufficient to decrease thetemperature of chilled water to 60C.

• The chilled water in the flash chamber iscirculated by a pump to the point ofapplication.

• The warm water from the refrigerated spaceis returned to the flash chamber.

• The water is sprayed through the nozzles toprovide maximum surface area for cooling.

• The water, which is splashed in the chamberand any loss of cold water at the application,must be replaced by makeup water added tothe cold water circulating system.

REFRIGERANTS

Why Selection of Refrigerant is important?

• Important practical issues such as thesystem design, size, initial and operatingcosts, safety, reliability, andserviceability etc.

• Due to several environmental issuessuch as ozone layer depletion and globalwarming and their relation to the variousrefrigerants used.

Types:• Primary refrigerants: Primary refrigerants

are those fluids, which are used directly asworking fluids. These fluids provide refrigerationby undergoing a phase change process in theevaporator.

• Secondary refrigerants: Liquids which areused for transporting thermal energy from onelocation to other. The secondary refrigerants donot undergo phase change as they transportenergy from one location to other. Eg:- Brinessuch as solutions of water and ethylene glycol,propylene glycol or calcium chloride.

Refrigerant selection criteria

• Thermodynamic and thermo-physical properties.

• Environmental and safetyproperties, and

• Economics

Thermodynamic and thermo-physical properties

1. Suction pressure : At a given evaporatortemperature, (a) The saturation pressure -aboveatmospheric - for prevention of air or moistureingress and ease of leak detection. (b)Suctionpressure – higher - it leads to smallercompressor displacement.

2. Discharge pressure: At a given condensertemperature, the discharge pressure - small -allows light-weight construction of compressor,condenser etc.

3. Pressure ratio: small - high volumetric efficiencyand low power consumption

4. Latent heat of vaporization: large - therequired mass flow rate per unit coolingcapacity will be small.

From Clausius-Clapeyron Equation:

For given condenser and evaporatortemperatures as the latent heat of vaporizationincreases, the pressure ratio also increases.Hence a trade-off is required.

5. Isentropic index of compression: Small -Temperature rise during compression will besmall.

6. Liquid specific heat: Small - Degree ofsub cooling will be large - leading tosmaller amount of flash gas at evaporatorinlet.

7. Vapour specific heat: Large - Degree ofsuperheating will be small.

8. Thermal conductivity: In both liquid aswell as vapour phase should be high forhigher heat transfer coefficients.

9. Viscosity: Small in both liquid and vapourphases for smaller frictional pressuredrops.

• These properties are interrelated andmainly depend on normal boiling point,critical temperature, molecular weight .

• The normal boiling point indicates theuseful temperature levels as it is directlyrelated to the operating pressures.

• A high critical temperature yields higherCOP due to smaller compressor superheatand smaller flash gas losses.

• For most of the refrigerants the ratio ofnormal boiling point to criticaltemperature is in the range of 0.6 to 0.7.

• The latent heat of vaporization will be high forrefrigerants having lower molecular weight.

• The specific heat of refrigerant is related to thestructure of the molecule.

• If specific heat of refrigerant vapour is highthen compression process will be dry.

• But a large value of vapour specific heat resultsin a higher value of liquid specific heat, leadingto higher flash gas losses.

• The optimum value of molar vapour specificheat lies in the range of 40 to 100 kJ/kmol.K.

• The freezing point - lower than the lowestoperating temperature of the cycle to preventblockage of refrigerant pipelines.

Environmental and safety properties

a) Ozone Depletion Potential (ODP): Accordingto the Montreal protocol, the ODP should bezero. ODP depends mainly on the presenceof chlorine or bromine in the molecules. Sorefrigerants like R 11, R 12 are not usednowadays.

b) Global Warming Potential (GWP): Shouldhave low GWP value. Refrigerants with zeroODP but a high value of GWP (e.g. R134a) arelikely to be regulated in future.

c) Total Equivalent Warming Index (TEWI): Thefactor TEWI considers both direct (due torelease into atmosphere) and indirect (throughenergy consumption) contributions ofrefrigerants to global warming.

d) Refrigerants should be non toxic and it shouldnot become toxic when mixed with othersubstances.

e) Flammability: Should be Non-flammable andnon-explosive. ASHRAE has divided refrigerantsinto six safety groups (A1 to A3 and B1 to B3).Refrigerants belonging to Group A1 (e.g. R11,R12, R22, R134a, R744, R718) are leasthazardous, while refrigerants belonging toGroup B3 (e.g. R1140) are most hazardous.

f) Refrigerants should be chemicallystable. It should not decompose underoperating conditions.

g) Refrigerant should be non corrosive. Itincreases life of the system.

h) Refrigerant should be miscible withlubricating oils and should not reactwith the lubricating oils.

i) Refrigerant should be odorless. Itmaintains the taste of food stuffspreserved.

Designation of refrigerants • Since a large number of refrigerants have

been developed over the years for a widevariety of applications, a numbering systemhas been adopted to designate variousrefrigerants.

• From the number one can get some usefulinformation about the type of refrigerant, itschemical composition, molecular weight etc.

• All the refrigerants are designated by Rfollowed by a unique number.

• Refrigerants are either Organic orInorganic.

• Organic compounds – either Hydrocarbonor Halocarbons.

• Halocarbons - contains chlorine andbromine cause ozone depletion.

• CFC – contains chorine and Fluorine atoms.

• Fluorine does not cause ozone depletion.

• Hydrogen atom decreases ozone depletion.

• So new classification – HC, FC, HFC, HCFCand CFC.

Designation of Refrigerants1. Organic Refrigerants

a) Saturated compounds:

R(c-1)(h+1)(f)

b) Unsaturated compounds

R1(c-1)(h+1)(f)

Where, c: No. of Carbon atoms

h: No. of Hydrogen atoms

f : No. of Fluorine(or Chlorine orIodine) atoms

Note: If the naming is two digits, then c-1=0

Chemical Formula for refrigerant:

Cc Hh Ff Clcl

• For saturated compounds, h+f+cl = 2c+2

Eg:- R11(CCl3F), R12(CCl2F2), R40, R160etc.

• For unsaturated compounds, h+f+cl = 2c

Eg:- R1150(C2H4), R1270 (C3H6)

2. Inorganic Refrigerants –

R(700+Molecular Wt.)

Eg:- R717(NH3), R744(CO2) R718(H2O)

3. Mixtures: Azeotropic mixtures aredesignated by 500 series, where aszeotropic refrigerants are designated by400 series.

Applications and Substitutes