ITP330 Refrigeration 2016 - phariyadi's blogphariyadi.staff.ipb.ac.id/files/2016/11/ITP330... ·...

ITP330/Fateta/IPB/Refrigeration11/25/2016

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Purwiyatno HariyadiEmail: [email protected]: phariyadi.staff.ipb.ac.id

ITP330

REFRIGERATIONLecture Note

Principles of Food Engineering

Prof. Purwiyatno HariyadiDr. Nur Wulandari

Dept of Food Science & Technology Faculty of Agricultural Engineering & Technology

Bogor Agricultural University

Learning Outcomes:

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

• To implement the basic concepts in the calculation of a refrigeration system

• To determine the performance of a refrigeration system


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OUTLINE

• The principle of refrigeration and freezing

• Selection of a refrigerant

• Components of a refrigerant system (evaporator, compressor, condenser, and expansion valve)

• Pressure-Enthalpy charts

• Mathematical expressions useful in analysis of vapor-compression refrigeration (cooling load, compressor, condenser, evaporator, coefficient of performance, and refrigerant flow rate)

REFRIGERATION

• Provides cool storage of foods

• T ......> 60°F (16°C) to 28°F (-2°C)

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

extended only for days or a few weeks

• Growth of nearly all pathogenic m.o. is prevented

• some spoilage microorganisms (psychrophiles) may thrive


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Purwiyatno Hariyadi/ITP330/Fateta/IPB

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

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

10°C decrease in temperature) UNDESIRABLE EFFECTS

a. Textural deterioration b. Chilling injury

EFFECTS OF REFRIGERATION ON FOODS

Removal of heat (Q) :

Q = mCpT

m = mass/weight of food

Cp = specific heat of food above freezing

T = temperature difference

ENERGY REMOVAL DURING REFRIGERATION


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1834 : Jacob Perkins patented refrigeration by vapourcompression

Use of Natural Refrigerants:

1880’s : NH3, SO2, CO2, HC’s

Toxic and flammable refrigerants led to fatal accidents

Use of Synthetic Refrigerants: (Stability, Non-toxicity and efficiency)

1930 : R11, R12

1936 : R22

1961 : R507

REFRIGERATION:Important Dates in Refrigeration History

A. REFRIGERATOR : Vapor Compression Refrigeration Systems


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

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

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

VAPOR COMPRESSION REFRIGERATION SYSTEMS

REFRIGERANT

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

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

• A good example of a common refrigerant is ammonia (NH3).


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• Ammonia boils at -33.3C, compared to 100oC for water at atmospheric pressure.

• Similar to water, ammonia needs latent heat of vaporization to change from liquid to vapor, and it discharges latent heat of condensation to change from vapor to liquid.

• The boiling point of a refrigerant can be varied by changing the pressure.

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

REFRIGERANT



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SELECTION OF A REFRIGERANT

The following characteristics are important in the selection of a refrigerant:

1. A high latent heat of vaporization is preferred.2. Excessively high condensing pressures should be

avoided3. The freezing temperature of the refrigerant should be

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

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

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

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

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

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



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Table. Properties of refrigerants used in warehouse refrigeration at -15oC evaporator temperature and 30oC condenser pressure

Refrigerant

Evaporator pressure, kPa

Condenser pressure, kPa

Latent heat of vaporization @ -15 C, kj/kg

Liquid refrigeration circulated per ton of refrigeration, kg/s

Stability (Toxic products)

Flammability

Odor

Evaporator temperature range

Ammonia

236.5

1166.5

1314.2

31 x 10-2

no

yes

acrid

-68 to -7

Freon 12

182.7

744.6

161.7

2.8 x 10-2

yes

no

ethereal

-73 to 10

1. Halocarbons

2. Azeotropic Refrigerants

3. Zeotropic Refrigerants

4. Inorganic Refrigerants

5. Hydrocarbon Refrigerants

SELECTION OF A REFRIGERANT:

Type of refrigerants


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Halocarbon refrigerant are all synthetically produced and were developed as the Freon family of refrigerants.

Examples :◦ CFC’s : R11, R12, R113, R114, R115◦ HCFC’s : R22, R123◦ HFC’s : R134a, R404a, R407C, R410a


Halocarbon refrigerant


o Carbon Dioxide

o Water

o Ammonia

o Air

o Sulphur dioxide


Inorganic Refrigerants


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A stable mixture of two or several refrigerants whose vapour and liquid phases retain identical compositions over a wide range of temperatures.

Examples : R-500 : 73.8% R12 and 26.2% R152R-502 : 8.8% R22 and 51.2% R115R-503 : 40.1% R23 and 59.9% R13


Azeotropic Refrigerants


A zeotropic mixture is one whose composition in liquid phase differs to that in vapour phase. Zeotropic refrigerants therefore do not boil at constant temperatures unlike azeotropic refrigerants.

Examples : R404a : R125/143a/134a (44%,52%,4%)

R407c : R32/125/134a (23%, 25%, 52%)

R410a : R32/125 (50%, 50%)

R413a : R600a/218/134a (3%, 9%, 88%)


Zeotropic Refrigerants


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Many hydrocarbon gases have successfully been used as refrigerants in industrial, commercial and domestic applications.

Examples: R170, Ethane, C2H6

R290, Propane C3H3

R600, Butane, C4H10

R600a, Isobutane, C4H10

Blends of the above gases


Hydrocarbon Refrigerants


- Depletion of the ozone layer in the stratosphere

- Global warming :

Refrigerants directly contributing to global warming when released to the atmosphere

Indirect contribution based on the energy consumption of among others the compressors

( CO2 produced by power stations )


Current Issure: Environmental Effects of Refrigerants


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Global Warming Potential (GWP) is a simplified index that estimates the potential future influence on global warming associated with different gases when released to the atmosphere.


Current Issue: Environmental Effects of Refrigerants



Current Issue: Environmental Effects of Refrigerants

Refrigerants now used in the food industry include:

• R502 for transport refrigeration

• R502 and R22 for retail display cases and retail central storage

• R502 for cold storage

• R22 for refrigerated storage and refrigerated vending machines

• R717forlarge freezers, frozen storage warehouses, and large refrigerated warehouses


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Most common refrigeration cycle in use today:Vapor-Compression Refrigeration Cycle

There are four (4) principal components:

• Evaporator

• Compressor

• Condenser

• Expansion valveTwo-phase

liquid-vapor mixture

COMPONENT OF A REFRIGERATION SYSTEM

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

CONDENSOR

EVAPORATOR

COMPRESSOREXPANSION VALVE

a

b

c

d

e


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

MECHANISM: Component of a Refrigerator

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

CONDENSOR

EVAPORATOR


a

b

c

d

e

Function as heat pumps and contain four essential mechanical components



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B. Compressor(1) Where the T and P of the refrigerant vapor is increased (2) When this happens, the heat in the refrigerant is released

CONDENSOR

EVAPORATOR


a

bc

d

e



C. Condensor(1) Where the heat is transferred from the refrigerant to another

medium (air or water) (2) When this happens, the refrigerant decreases in T and

condenses

CONDENSOR

EVAPORATOR

COMPRESSOREXPANSION VALVE a

b

c

d

e




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D. Expansion valve (1) Where the flow of liquid refrigerant is controlled (2) When this happens, the evaporator receives a

constant supply of refrigerant

CONDENSOR

EVAPORATOR


a

b

c

d

e




MECHANISM


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P (

kPa)

Enthalpy (H; kJ/kg)

a

bcd

eP2

P1

H1 H2 H3

Diagram P-H

MECHANISM of REFRIGERATION

CONDENSOR

EVAPORATOR


a

bc

d

e

Diagram P-H


CONDENSOR

EVAPORATOR


a

bc

d

e


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P (

kPa)

Enthalpy (H; kJ/kg)

a

bcd

eP2

P1

H1 H2 H3

PRESSURE ENTHALPY (P-H) DIAGRAMS

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

• These diagrams are available for all type of refrigerants

Liquid & vapor

Saturated vapor lineLiquid

Saturated liquid line

Vapor

Constant entropy line

P (

kPa)

Enthalpy (H; kJ/kg)

a

bcd

eP2

P1

H1 H2 H3

PRESSURE ENTHALPY (P-H) DIAGRAMS

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

• These diagrams are available for all type of refrigerants

Liquid & vapor

Saturated vapor lineLiquid

Saturated liquid line

Vapor

Constant entropy line

Constant temperature line


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

Location b : - superheated compressed gas exits the compressor

MECHANISM of REFRIGERATION- description

CONDENSOR

EVAPORATOR


a

bc

d

e

Location c : - compressed gas enters the condenser- the condensing temperature must be higher than that of an

easily available heat sink, e.g., ambient air, water, etc.- the refrigerant gas discharges latent heat of condensation the

heat sink and changes phase to liquid


CONDENSOR

EVAPORATOR


a

bc

d

e


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Location d : - refrigerant in a saturated liquid state

- expansion valve separates high as refrigerant passes through the expansion valve the sudden decrease in pressure causes some of the refrigerant to change into gas


CONDENSOR

EVAPORATOR


a

bc

d

e

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


CONDENSOR

EVAPORATOR


a

bc

d

e


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COOLING LOAD:

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

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

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

MECHANISM of REFRIGERATION- description on mathematical

expressions useful in the analysis of vapor-compression

refrigeration

P (

kPa)

Enthalpy (H; kJ/kg)

a

bcd

eP2

P1

H1 H2H3

REFRIGERATION LOAD:

• Unsteady-state load: rate of heat removal necessary to reduce the temperature of the material being refrigerated to storage temperature within a specific period of time sensible heat of the product, heat of respiration of fresh products

• Steady-state load: the amount of heat removal necessary to maintain the storage temperature heat incursion through enclosures, cracks and crevices, open doors, heat from motors/blowers

P (

kPa)

Enthalpy (H; kJ/kg)

a

bcd

eP2

P1

H1 H2H3


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refrigeration

REFRIGERANT FLOW RATE

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

• Refrigerant flow rate = (Cooling Load) / (H2 - H1)

P (

kPa)

Enthalpy (H; kJ/kg)

a

bcd

eP2

P1

H1 H2H3



refrigeration

COMPRESSOR

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

• Rate of work done on the compressor = (refrigerant flow rate) (H3 - H2)

P (

kPa)

Enthalpy (H; kJ/kg)

a

bcd

eP2

P1

H1 H2H3


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refrigeration

CONDENSER

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

• Heat rejected in the condenser = (refrigerant flow rate) (H3 - H1)

P (

kPa)

Enthalpy (H; kJ/kg)

a

bcd

eP2

P1

H1 H2H3



refrigeration

EVAPORATOR

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

• Heat absorbed by the evaporator = (refrigerant flow rate) (H2 - H1)

P (

kPa)

Enthalpy (H; kJ/kg)

a

bcd

eP2

P1

H1 H2H3


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refrigeration

COEFFICIENT of PERFORMANCE (COP)

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

• COP = (H2 - H1) / (H3- H2)

P (

kPa)

Enthalpy (H; kJ/kg)

a

bcd

eP2

P1

H1 H2H3


MECHANISM


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Power requirement in horsepower/ton refrigerant

HP/(ton)r = 12,000 BTU 1 HP

(COP) h (ton) (2545 BTU/h)

4.715

(COP)=

MATHEMATICAL EXPRESSIONS USEFUL IN THE ANALYSIS OF VAPOR-COMPRESSION REFRIGERATION

Compression EfficiencyThe ratio between the theoretical

HP as calculated / actual HP expended


TONSRefrigeration capacity of cooling systems is sometimes given in “tons.” This rating is basedupon the cooling capacity of one ton of ice melted over a 24-hour period [(2000 lb × 144 BTU/lb)/24 hr = 12,000 BTU/hr = 3.5 kW].

While the use of “ton” to indicate cooling capacitydoes have a logical basis this is another example of using one unit (mass) to represent something completely different (rate of energy transfer, or power).


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B. REFRIGERATION PROCESSFOR “LIFE” AGRICULTURAL PRODUCTS

Lutz, J.M. and Hardenburg, R.E., Agricultural Handbook No. 66, USDA, Washington, D.C., 1968.

B. Good Refrigeration Practices- chilling Injury?


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Lutz, J.M. and Hardenburg, R.E., Agricultural Handbook No. 66, USDA, Washington, D.C., 1968.

B. Good Refrigeration Practices- chilling Injury?

General Removal of heat (Q) :

Q = mCpT

m = mass/weight of food

Cp = specific heat of food above freezing

T = temperature difference

For Life (Respiring) Agr Product; you MUST consider Heat of Respiration (QR):

Q total = mCpT + m (QR)

B. Good Refrigeration Practices- Heat of Respiration?


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B. Good Refrigeration Practices- Control of Humidity?

- Too low of humidity cause water evaporation- Evaporation (i) need energy increase refrigeration load (ii) removal of water quality? Loss of weight?

B. Good Refrigeration Practices- Control of Humidity?


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More:

http://www.globalspec.com/reference/65355/203279/chapter-17-the-refrigeration-and-freezing-of-food


Example

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 refrigerant; 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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-30

12.3

(psia)

oCoF

133

100

Pax104

Co

nst

ant

tem

p li

ne


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e a

7432

12.3

133

94

100oFH3

e a

bd

7432

12.3

133

Evaporator

Condenser

Exp

ansi

on

va

lve

94

100oF


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e a

7432

12.3

133

e a

bd

7432

12.3

133

Evaporator

Condenser

Exp

ansi

on

va

lve

94


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See Figure 10.1, p. 400

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

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

See Figure 10.5, p. 406

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

H2 P= 133 psia 32 btu/lb (7.4 kj/kg)

H3 P 133 psia, TP2 94 btu/lb (21.8 kj/kg)

Refrigerant capacity (heat/kg refrigerant):

H1 – H2 = (17.2 – 7.4)x 104 = 98,000 J/kg = 42 BTU/lb

For Freon 12

P (

psi

a)

Enthalpy (H; btu/lb)

a

bcd

eP2

P1

32 74 94

12.3

133

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

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

= 2.1

HP per ton refrigerant:

See Figure 10.1, p 400:

Cp/cv F-12 = 1.14

HP/(ton)r = 4.715 / (COP)

= 4.715 /(1.14)(2.1) = 1.97

For Freon 12

P (

psi

a)

Enthalpy (H; btu/lb)

a

bcd

eP2

P1

32 74 94

12.3

133


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

Weight = Cooling capacity/ton refrigerant

Cooling capacity/unit weight of refrigerant

= 12,000 BTU/h

42 BTU/lb = 286 lb refrigerant/h

= 0.0359 kg refrigerant/s

Thank you…

See you in the cooler topic


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Quiz please……


Which of the following statements are true and which are false?

1. A disadvantage of ammonia as a refrigerant is its low value of latent heat of vaporization.

2. The higher the value of the latent heat of vaporization of a refrigerant, thelower the required refrigerant flow rate for a given refrigeration load.


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3. Hydrofluorocarbons (HFCs) are replacing chlorofluorocarbons (CFCs) as refrigerants.

4. Compressor, evaporator, condenser, and expansion valve are the maincomponents of a mechanical refrigeration system.



5. Expansion of the refrigerant in the expansion valve of a refrigeration cycle takes place at constant entropy.

6. Compression of the refrigerant in the compressor takes place at constantenthalpy.


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7. Evaporation of the refrigerant in the evaporator takes place at constantpressure.

8. The condenser is at the low pressure side of a mechanical refrigeration system.



9. Refrigerant vapors leaving the evaporator may be superheated.

10. Liquid refrigerant leaving the condenser may be sub cooled.


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11. The coefficient of performance (COP) is equal to the ratio of the refrigerationeffect to the net work input.

12. COP is always less than one.

13. The higher the temperature difference between condenser and evaporator, the higher the COP.


Contoh: Suatu sistem refrigerasi menggunakan R12 sebagai refrigeran. Tekanan refrigeran di dalam evaporator sebesar 20 lbf/in2, sedangkan tekanan refrigeran di dalam kondensor sebesar 160 lbf/in2. Gambarkanlah diagram Mollier-nya secara sederhana dan tentukanlah:

• Entalphi (H) refrigeran saat keluar dari evaporator

• Entalphi (H) refrigeran saat keluar dari kompressor

• Entalphi (H) refrigeran saat keluar dari kondensor

• COP

• HP/tonr yang dibutuhkan jika efisiensi kompresor90%

ITP330 Refrigeration 2016 - phariyadi's blogphariyadi.staff.ipb.ac.id/files/2016/11/ITP330... ·...

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