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iTP330-Prinsip Teknik Pangan-Refrigerasi

8/24/2011

Purwiyatno Hariyadi/ITP/Fateta/IPB -Refrigerasi 1

REFRIGERATION REFRIGERATION Vapor Compression Refrigeration Vapor Compression Refrigeration SystemsSystems

REFRIGERATION REFRIGERATION Vapor Compression Refrigeration Vapor Compression Refrigeration SystemsSystems

Lecture Note Lecture Note Principles of Food EngineeringPrinciples of Food Engineering

DosenDosen : : Prof. Prof. DrDr. . PurwiyatnoPurwiyatno HariyadiHariyadi, , MScMSc

Dept of Food Dept of Food Science & Science & TechnologyTechnologyFaculty of Agricultural TechnologyFaculty of Agricultural TechnologyBogor Agricultural UniversityBogor Agricultural UniversityBOGORBOGOR

VAPOR COMPRESSION VAPOR COMPRESSION REFRIGERATION SYSTEMSREFRIGERATION SYSTEMS

Instructional Objective:Instructional Objective:

•• To learn the basic concepts To learn the basic concepts of a vapor compression of a vapor compression refrigeration systemrefrigeration systemrefrigeration systemrefrigeration system

PurwiyatnoPurwiyatno hariyadihariyadi/ITP//ITP/FatetaFateta/IPB/IPB


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REFRIGERATION REFRIGERATION

•• Provides cool storage of foods Provides cool storage of foods TT > 60> 60°°F (16F (16°°C) t 28C) t 28°°F (F ( 22°°C)C)•• T T ............> 60> 60°°F (16F (16°°C) to 28C) to 28°°F (F (--22°°C) C)

•• Water in the food is not frozen Water in the food is not frozen •• the shelf life of perishable products is extended the shelf life of perishable products is extended

only for days or a few weeksonly for days or a few weeks•• Growth of nearly all pathogenic m.o. is Growth of nearly all pathogenic m.o. is

t dt dprevented prevented •• some spoilage microorganisms (psychrophiles) some spoilage microorganisms (psychrophiles)

may thrivemay thrive


EFFECTS OF REFRIGERATION ON FOODS EFFECTS OF REFRIGERATION ON FOODS

DESIRABLE EFFECTS DESIRABLE EFFECTS a Microbial growth rates decreasea Microbial growth rates decreasea. Microbial growth rates decrease a. Microbial growth rates decrease b. Chemical and biochemical reaction b. Chemical and biochemical reaction

rates decrease rates decrease c. Shelf life increases (2c. Shelf life increases (2--5 fold for every 5 fold for every

1010°°C decrease in temperature) C decrease in temperature) UNDESIRABLE EFFECTS UNDESIRABLE EFFECTS

a. Textural deterioration a. Textural deterioration b. Chilling injury b. Chilling injury



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R l f h t (Q)R l f h t (Q)

ENERGY REMOVAL DURING REFRIGERATION ENERGY REMOVAL DURING REFRIGERATION

Removal of heat (Q) :Removal of heat (Q) :Q = mCQ = mCppΔΔTTm = mass/weight of food m = mass/weight of food CCpp = specific heat of food above freezing= specific heat of food above freezingΔΔT = temperature differenceT = temperature differenceΔΔT = temperature difference T = temperature difference


•• A refrigeration system allows transfer of heatA refrigeration system allows transfer of heat

VAPOR COMPRESSION REFRIGERATION VAPOR COMPRESSION REFRIGERATION SYSTEMS SYSTEMS

•• A refrigeration system allows transfer of heat A refrigeration system allows transfer of heat from a cooling chamber to a location where the from a cooling chamber to a location where the heat can be easily discarded.heat can be easily discarded.

•• The transfer of heat is accomplished by using a The transfer of heat is accomplished by using a refrigerant, which can change its state from refrigerant, which can change its state from liquid to gasliquid to gasliquid to gas.liquid to gas.

•• However, unlike water the refrigerant has a However, unlike water the refrigerant has a much lower boiling point.much lower boiling point.



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REFRIGERANTREFRIGERANT

•• A fluid which through phase changes fromA fluid which through phase changes from•• A fluid which, through phase changes from A fluid which, through phase changes from liquid to gas and back to liquid, facilitates liquid to gas and back to liquid, facilitates heat transfer in a refrigeration system. heat transfer in a refrigeration system.

•• Refrigerants have much lower boiling points Refrigerants have much lower boiling points than water and their boiling points can be than water and their boiling points can be varied by changing the pressure of thevaried by changing the pressure of thevaried by changing the pressure of the varied by changing the pressure of the system. system.

•• A good example of a common refrigerant is A good example of a common refrigerant is ammonia (NHammonia (NH33). ).


•• Ammonia boils at Ammonia boils at --33.3C, compared to 10033.3C, compared to 100ooC for C for

VAPOR COMPRESSION VAPOR COMPRESSION REFRIGERATION SYSTEMSREFRIGERATION SYSTEMS

, p, pwater at atmospheric pressure. water at atmospheric pressure.

•• Similar to water, ammonia needs latent heat of Similar to water, ammonia needs latent heat of vaporization to change from liquid to vapor, and it vaporization to change from liquid to vapor, and it discharges latent heat of condensation to change discharges latent heat of condensation to change from vapor to liquid.from vapor to liquid.Th b ili i t f f i t b i d bTh b ili i t f f i t b i d b•• The boiling point of a refrigerant can be varied by The boiling point of a refrigerant can be varied by changing the pressure. changing the pressure.

•• Thus, to increase boiling point of ammonia to 0Thus, to increase boiling point of ammonia to 0ooC, C, its pressure must be raised to 428.5 kPa (62.1 its pressure must be raised to 428.5 kPa (62.1 psia)psia)



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SELECTION OF A REFRIGERANTSELECTION OF A REFRIGERANTThe following characteristics are important in The following characteristics are important in the selection of a refrigerant:the selection of a refrigerant:

11 A high latent heat of apori ation is preferredA high latent heat of apori ation is preferred1. 1. A high latent heat of vaporization is preferred.A high latent heat of vaporization is preferred.2. 2. Excessively high condensing pressures should be Excessively high condensing pressures should be

avoidedavoided3. 3. The freezing temperature of the refrigerant should be The freezing temperature of the refrigerant should be

below the evaporating temperature.below the evaporating temperature.4. 4. The refrigerant should have a sufficiently high critical The refrigerant should have a sufficiently high critical

temperaturetemperaturetemperature.temperature.5. 5. The refrigerant must nonThe refrigerant must non--toxic, nontoxic, non--corrosive, and corrosive, and

chemically stable.chemically stable.6. 6. It should be easy to detect leaks.It should be easy to detect leaks.7. 7. Low cost refrigerant is preferred in industrial Low cost refrigerant is preferred in industrial

applicationsapplications PurwiyatnoPurwiyatno hariyadihariyadi/ITP//ITP/FatetaFateta/IPB/IPB

•• Ammonia offers exceptionally high latent Ammonia offers exceptionally high latent heat of vaporization among all otherheat of vaporization among all other

SELECTION OF A REFRIGERANTSELECTION OF A REFRIGERANT

heat of vaporization among all other heat of vaporization among all other refrigerants. refrigerants.

•• Other commonly used refrigerants include, Other commonly used refrigerants include, Freon 12 and Freon 22. Freon 12 and Freon 22.

•• Due to the adverse effects of Freon 12 on the Due to the adverse effects of Freon 12 on the ozone layer, the use of this refrigerant is now ozone layer, the use of this refrigerant is now being seriously curtailed.being seriously curtailed.



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Table. Properties of refrigerants used in warehouse Table. Properties of refrigerants used in warehouse refrigeration at refrigeration at --1515ooC evaporator temperature and C evaporator temperature and 3030ooC condenser pressureC condenser pressure

RefrigerantRefrigerant AmmoniaAmmonia Freon 12Freon 12RefrigerantRefrigerant

Evaporator pressure, kPaEvaporator pressure, kPaCondenser pressure, kPaCondenser pressure, kPaLatent heat of vaporization @ Latent heat of vaporization @

--15 C, kj/kg15 C, kj/kgLiquid refrigeration circulated Liquid refrigeration circulated

per ton of refrigeration kg/sper ton of refrigeration kg/s

AmmoniaAmmonia

236.5236.51166.51166.5

1314.21314.2

31 x 1031 x 10--22

Freon 12Freon 12

182.7182.7744.6744.6

161.7161.7

2 8 x 102 8 x 10--22per ton of refrigeration, kg/sper ton of refrigeration, kg/sStability (Toxic products)Stability (Toxic products)FlammabilityFlammabilityOdorOdorEvaporator temperature rangeEvaporator temperature range

31 x 1031 x 10nonoyesyes

acridacrid--68 to 68 to --77

2.8 x 102.8 x 10yesyesnono

etherealethereal--73 to 1073 to 10


COMPONENT OF A COMPONENT OF A REFRIGERATIONREFRIGERATION SYSTEMSYSTEM

Major component of a vaporMajor component of a vapor--compression compression refrigeration system are shown in the following refrigeration system are shown in the following diagramdiagramdiagramdiagram

CONDENSORCONDENSOR

COMPRESSORCOMPRESSOREXPANSION EXPANSION VALVEVALVE

bb

cc

dd

EVAPORATOREVAPORATOR

VALVEVALVE aa

eePurwiyatnoPurwiyatno hariyadihariyadi/ITP//ITP/FatetaFateta/IPB/IPB


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CONDENSORCONDENSOR

COMPRESSORCOMPRESSOREXPANSIONEXPANSIONbb

cc

dd

MECHANISMMECHANISM


COMPRESSORCOMPRESSOREXPANSION EXPANSION VALVEVALVE aa

ee

P (k

Pa)

P (k

Pa)

bbccddPP11PP

Enthalpy (H; kJ/kg)Enthalpy (H; kJ/kg)

aaeePP22

HH11 HH22 HH33

Diagram PDiagram P--HH

CONDENSORCONDENSOR


cc

dd

COMPONENT OF A COMPONENT OF A REFRIGERATIONREFRIGERATION SYSTEMSYSTEM



ee

Function as Function as heat pumpsheat pumps and contain four essential and contain four essential mechanical componentsmechanical components

A. Evaporator A. Evaporator (1) Where the liquid (1) Where the liquid refrigerantrefrigerant vaporizes into a gas vaporizes into a gas (2) When this happens, heat from the stored food is (2) When this happens, heat from the stored food is "extracted" "extracted"

pp



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COMPONENT OF A REFRIGERATION SYSTEMCOMPONENT OF A REFRIGERATION SYSTEMCONDENSORCONDENSOR


cc

dd

C. Condenser C. Condenser (1) Where the heat is transferred (1) Where the heat is transferred

B. Compressor B. Compressor (1) Where the T and P (1) Where the T and P

of the refrigerant of the refrigerant



ee

( )( )from the refrigerant to another from the refrigerant to another medium (air or water) medium (air or water)

(2) When this happens, the (2) When this happens, the refrigerant decreases in T and refrigerant decreases in T and condenses condenses

vapor is increased vapor is increased (2) When this happens, (2) When this happens,

the heat in the the heat in the refrigerant is refrigerant is releasedreleased


COMPONENT OF A REFRIGERATION SYSTEMCOMPONENT OF A REFRIGERATION SYSTEMCONDENSORCONDENSOR


cc

dd

D. Expansion valveD. Expansion valve



ee

D. Expansion valve D. Expansion valve (1) Where the flow of liquid refrigerant is (1) Where the flow of liquid refrigerant is

controlled controlled (2) When this happens, the evaporator receives a (2) When this happens, the evaporator receives a

constant supply of refrigerantconstant supply of refrigerant



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MECHANISMMECHANISMCONDENSORCONDENSOR


cc

dd

Location a :Location a : -- refrigerant gas enters compressorrefrigerant gas enters compressor



ee

Location a :Location a : -- refrigerant gas enters compressor refrigerant gas enters compressor and compressed to a high and compressed to a high pressurepressure

Location b :Location b : -- superheated compressed gas superheated compressed gas exits the compressorexits the compressor




cc

dd

Location c :Location c :-- compressed gas enters the condensercompressed gas enters the condenser



ee

-- the condensing temperature must be higher than the condensing temperature must be higher than that of an easily available heat sink, e.g., ambient that of an easily available heat sink, e.g., ambient air, water, etc.air, water, etc.

-- the refrigerant gas discharges latent heat of the refrigerant gas discharges latent heat of condensation the heat sink and changes phase condensation the heat sink and changes phase to liquidto liquid PurwiyatnoPurwiyatno hariyadihariyadi/ITP//ITP/FatetaFateta/IPB/IPB


8/24/2011




cc

dd

Location d :Location d :-- refrigerant in a saturated liquid staterefrigerant in a saturated liquid state



ee

refrigerant in a saturated liquid staterefrigerant in a saturated liquid state-- expansion valve separates high as refrigerant expansion valve separates high as refrigerant

passes through the expansion valve the sudden passes through the expansion valve the sudden decrease in pressure causes some of the decrease in pressure causes some of the refrigerant to change into gasrefrigerant to change into gas




cc

dd

Location e :Location e : -- the refrigerant absorbs heatthe refrigerant absorbs heat



ee

Location e :Location e : -- the refrigerant absorbs heat, the refrigerant absorbs heat, equivalent to its latent heat of equivalent to its latent heat of vaporization, and completely vaporization, and completely converts into gasconverts into gas



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MATHEMATICAL EXPRESSIONS USEFUL IN THE MATHEMATICAL EXPRESSIONS USEFUL IN THE ANALYSIS OF VAPORANALYSIS OF VAPOR--COMPRESSION COMPRESSION REFRIGERATIONREFRIGERATION

COOLING LOAD:COOLING LOAD:COOLING LOAD:COOLING LOAD:

•• The cooling load is total heat energy that The cooling load is total heat energy that must be removed from a given space in order must be removed from a given space in order to lower the temperature to a desired level. to lower the temperature to a desired level.

•• A common unit of cooling load is “ton of A common unit of cooling load is “ton of ggrefrigeration”refrigeration”

1 ton of refrigeration1 ton of refrigeration = 288,000 Btu/24 hr= 288,000 Btu/24 hr= 303,852 kJ/24 hr= 303,852 kJ/24 hr


REFRIGERANT FLOW RATEREFRIGERANT FLOW RATE


REFRIGERANT FLOW RATEREFRIGERANT FLOW RATE

•• The refrigerant flow rate depends upon the The refrigerant flow rate depends upon the total cooling load on the system and the total cooling load on the system and the amount of heat that refrigerant can absorbamount of heat that refrigerant can absorb

Refrigerant flow rate Refrigerant flow rate = (Cooling Load) / (H= (Cooling Load) / (H22 -- HH11))



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COMPRESSORCOMPRESSOR


COMPRESSORCOMPRESSOR

•• The work done on the refrigerant during the The work done on the refrigerant during the compression step is the product on the compression step is the product on the enthalpy increase of the refrigerant inside the enthalpy increase of the refrigerant inside the

d th f i t fl td th f i t fl tcompressor and the refrigerant flow ratecompressor and the refrigerant flow rate

rate of work done on the compressor rate of work done on the compressor = (refrigerant flow rate) (H= (refrigerant flow rate) (H33 -- HH22))


CONDENSERCONDENSER


CONDENSERCONDENSER

•• The heat rejected to the environment in the The heat rejected to the environment in the condenser depends upon the refrigerant flow condenser depends upon the refrigerant flow rate and the latent heat of condensation of the rate and the latent heat of condensation of the refrigerantrefrigerantrefrigerantrefrigerant

heat rejected in the condenser heat rejected in the condenser = (refrigerant flow rate) (H= (refrigerant flow rate) (H33 -- HH11))



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•• The heat absorbed by the evaporator depends The heat absorbed by the evaporator depends upon the refrigerant flow rate and the latent upon the refrigerant flow rate and the latent heat of evaporation of the refrigerant.heat of evaporation of the refrigerant.

heat absorbed by the evaporator heat absorbed by the evaporator = (refrigerant flow rate) (H= (refrigerant flow rate) (H22 -- HH11))


COEFFICIENT PERFORMANCECOEFFICIENT PERFORMANCE


COEFFICIENT PERFORMANCECOEFFICIENT PERFORMANCE

•• The coefficient performance is a ratio The coefficient performance is a ratio between the heat absorbed by the between the heat absorbed by the refrigerant as it flows through the refrigerant as it flows through the evaporator to the heat equivalent of theevaporator to the heat equivalent of theevaporator to the heat equivalent of the evaporator to the heat equivalent of the energy supplied to the compressor.energy supplied to the compressor.

COP = (HCOP = (H22 -- HH11) / (H) / (H33-- HH22))



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Power requirement in horsepower/ton refrigerant (for F12)

HP/(ton)r = 12,000 BTU 1 HPγ(COP) h (ton) (2545 BTU/h)

4.715γ(COP)

=γ(COP)

Example

• A refrigeration system is to be operated at an• A refrigeration system is to be operated at an evaporator coil temperature of -30oF (-34oC) and a condenser temperature of 100oF (37.8oC) for the liquid refrigerant. For Freon 12, determine: (a) the high-side pressure; (b) the low-side pressure; (c) the refrigeration capacity per unit weight of g p y p grefrigerant; (d) COP; (e) HP of compressor per ton of refrigerant; (f) quantity of refrigerant circulated through the system per ton of refrigeration


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PRESSURE ENTHALPY (PPRESSURE ENTHALPY (P--H) DIAGRAMSH) DIAGRAMS

•• PP--H diagrams are useful in designing and analyzing H diagrams are useful in designing and analyzing vapor compression refrigeration systemsvapor compression refrigeration systems

•• These diagrams are available for all type of refrigerantsThese diagrams are available for all type of refrigerantsSaturated liquid line

P (k

Pa)

P (k

Pa)

aa

bbccdd

eePP22

PP11 Liquid & vapor

Saturated vapor lineLiquid

Saturated liquid line

Vapor

Constant temperature line

Constant entropy line

Enthalpy (H; kJ/kg)Enthalpy (H; kJ/kg)

HH11 HH22 HH33


(psia) Pax10

133

-30

12.3oCoF100


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nt te

mp

line

Con

stan

133

e a12.3

7432 94100oF

H3


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bd133

Condenser

n

e a12.3

Evaporator

Expa

nsio

nva

lve

7432 94100oF

Lihat Gambar 10.1, p. 400

T = -30oF (-34oC) P= 12.3 psia (85 kPa)

T = 100oF (37.8oC) P=133 psia (910 kPa)

For Freon 12

P (p

sia)

P (p

sia) bbccddPP11

133

Lihat Gambar 10.5, p. 406

H1 P= 12.3 psia 74 btu/lb (17.2 kj/kg)

H2 P= 85 psia 32 btu/lb (7.4 kj/kg)

H3 pada P 910, TP2 94 btu/lb (21.8 kj/kg)

Enthalpy (H; btu/lb)Enthalpy (H; btu/lb)

aaeePP22

3232 7474 9494

12.3

Refrigerant capacity (heat/kg refrigerant):

H1 – H2 = (17.2 – 7.4)x 104 = 98,000 J/kg


8/24/2011


COP = (H2-H1)/(H3-H2)

= (17.2-7.4)/(21.8-17.2)

= 2.1

For Freon 12P

(psi

a)P

(psi

a) bbccddPP11133

HP per ton refrigerant:

Lihat Gambar 10.1, p 400:

Cp/cv F-12 = 1.14

HP/(ton) = 4.715/γ(COP)Enthalpy (H; btu/lb)Enthalpy (H; btu/lb)

aaeePP22

3232 7474 9494

12.3

HP/(ton) 4.715/γ(COP)

= 4.715/(1.14)(2.1) = 1.97

One ton refrigerant for F12 = 12,000 BTU/h or 3517 W

W i ht Cooling capacity/ton refrigerantWeight = Cooling capacity/ton refrigerantCooling capacity/unit weight of refrigerant

= 12,000 BTU/h42 BTU/lb = 286 lb refrigerant/h

= 0.0359 kg refrigerant/s


8/24/2011


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