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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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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.
SELECTION OF A REFRIGERANT
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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
SELECTION OF A REFRIGERANT:
Halocarbon refrigerant
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o Carbon Dioxide
o Water
o Ammonia
o Air
o Sulphur dioxide
SELECTION OF A REFRIGERANT:
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
SELECTION OF A REFRIGERANT:
Azeotropic Refrigerants
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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%)
SELECTION OF A REFRIGERANT:
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
SELECTION OF A REFRIGERANT:
Hydrocarbon Refrigerants
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- 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 )
SELECTION OF A REFRIGERANT
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.
SELECTION OF A REFRIGERANT
Current Issue: Environmental Effects of Refrigerants
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SELECTION OF A REFRIGERANT
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
COMPRESSOREXPANSION VALVE
a
b
c
d
e
Function as heat pumps and contain four essential mechanical components
MECHANISM: Component of a Refrigerator
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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
COMPRESSOREXPANSION VALVE
a
bc
d
e
Function as heat pumps and contain four essential mechanical components
MECHANISM: Component of a Refrigerator
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
Function as heat pumps and contain four essential mechanical components
MECHANISM: Component of a Refrigerator
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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
COMPRESSOREXPANSION VALVE
a
b
c
d
e
Function as heat pumps and contain four essential mechanical components
MECHANISM: Component of a Refrigerator
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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
COMPRESSOREXPANSION VALVE
a
bc
d
e
Diagram P-H
MECHANISM of REFRIGERATION
CONDENSOR
EVAPORATOR
COMPRESSOREXPANSION VALVE
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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MECHANISM of REFRIGERATION
MECHANISM of REFRIGERATION
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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
COMPRESSOREXPANSION VALVE
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
MECHANISM of REFRIGERATION- description
CONDENSOR
EVAPORATOR
COMPRESSOREXPANSION VALVE
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
MECHANISM of REFRIGERATION- description
CONDENSOR
EVAPORATOR
COMPRESSOREXPANSION VALVE
a
bc
d
e
Location e : - the refrigerant absorbs heat, equivalent to its latent heat of vaporization, and completely converts into gas
MECHANISM of REFRIGERATION- description
CONDENSOR
EVAPORATOR
COMPRESSOREXPANSION VALVE
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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MECHANISM of REFRIGERATION- description on mathematical
expressions useful in the analysis of vapor-compression
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
MECHANISM of REFRIGERATION- description on mathematical
expressions useful in the analysis of vapor-compression
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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MECHANISM of REFRIGERATION- description on mathematical
expressions useful in the analysis of vapor-compression
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
MECHANISM of REFRIGERATION- description on mathematical
expressions useful in the analysis of vapor-compression
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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MECHANISM of REFRIGERATION- description on mathematical
expressions useful in the analysis of vapor-compression
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
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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
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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
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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……
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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.
ITP330/Fateta/IPB/Refrigeration11/25/2016
37
Purwiyatno HariyadiEmail: [email protected]: phariyadi.staff.ipb.ac.id
Purwiyatno Hariyadi/ITP330/Fateta/IPB
Which of the following statements are true and which are false?
3. Hydrofluorocarbons (HFCs) are replacing chlorofluorocarbons (CFCs) as refrigerants.
4. Compressor, evaporator, condenser, and expansion valve are the maincomponents of a mechanical refrigeration system.
Purwiyatno Hariyadi/ITP330/Fateta/IPB
Which of the following statements are true and which are false?
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.
ITP330/Fateta/IPB/Refrigeration11/25/2016
38
Purwiyatno HariyadiEmail: [email protected]: phariyadi.staff.ipb.ac.id
Purwiyatno Hariyadi/ITP330/Fateta/IPB
Which of the following statements are true and which are false?
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.
Purwiyatno Hariyadi/ITP330/Fateta/IPB
Which of the following statements are true and which are false?
9. Refrigerant vapors leaving the evaporator may be superheated.
10. Liquid refrigerant leaving the condenser may be sub cooled.
ITP330/Fateta/IPB/Refrigeration11/25/2016
39
Purwiyatno HariyadiEmail: [email protected]: phariyadi.staff.ipb.ac.id
Purwiyatno Hariyadi/ITP330/Fateta/IPB
Which of the following statements are true and which are false?
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.
Purwiyatno Hariyadi/ITP330/Fateta/IPB
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%