ITP330/Fateta/IPB/Refrigeration 11/25/2016 1 Purwiyatno Hariyadi Email: [email protected]Web: phariyadi.staff.ipb.ac.id ITP330 REFRIGERATION Lecture Note Principles of Food Engineering Prof. Purwiyatno Hariyadi Dr. 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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• 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)
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
Purwiyatno Hariyadi/ITP330/Fateta/IPB
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.
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
Purwiyatno Hariyadi/ITP330/Fateta/IPB
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
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
• 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
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)
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.
MATHEMATICAL EXPRESSIONS USEFUL IN THE ANALYSIS OF VAPOR-COMPRESSION REFRIGERATION
Compression EfficiencyThe ratio between the theoretical
HP as calculated / actual HP expended
Purwiyatno Hariyadi/ITP330/Fateta/IPB
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).
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
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.
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%