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Refrigiration Cycle

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Submitted By bloodlust
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Refrigeration Cycles
Content:
The ideal vapor-compression refrigeration cycle vaporActual Vapor-Compression Refrigeration Cycle VaporHeat Pump System for Heating and Cooling Innovative Vapor-Compression Refrigeration Cycle VaporGas Refrigeration Cycles Absorption Refrigeration Systems Refrigerants Selection

Refrigeration affects many areas of your life
The obvious:
Refrigerator/Freezers allow food preservation Air conditioning

Refrigerated fishing boats allow preservation of catch Refrigerated trucks are used to ship fruits/meats Refrigeration makes possible medical procedures that call for lowering body temperatures

Refrigerator and Heat Pump
The objective of a refrigerator is to remove heat (QL) from the cold medium; the objective of a heat pump is to supply heat (QH) to a warm medium.

Schematic diagram of the ideal vaporcompression refrigeration cycle

Compressor

Condenser Evaporator

Simple view of household refrigerator

T-s Diagram of the Ideal Refrigeration Cycle

Process 1-2 Isentropic Compression Process Process 2-3 P = const. Heat Rejection Process const. Process 3-4 Expansion under Throttling Process, h = const Process 4-1 P = const. Heat Addition Process const.

P-h Diagram of the Ideal Refrigeration Cycle

Process 1-2 Isentropic Compression Process Process 2-3 P = const. Heat Rejection Process const. Process 3-4 Expansion under Throttling Process, h = const Process 4-1 P = const. Heat Addition Process const.

Coefficient of Performance in Refrigeration Cycles
For Refrigerators and Air Conditioners:
COPR = Desired output Cooling effect = Required input Work input q h −h COPR = L = 1 4 wnet,in h2 − h1

For Heat Pumps:
COPHP =
COPHP =

Desired output Heating effect = Required input Work input qH h −h = 2 3 wnet,in h2 − h1

Example 10-1 A refrigerator uses refrigerant-134a as the working fluid and operates on an ideal vapor-compression refrigeration cycle between 0.14 and 0.8 MPa. The mass flow rate of the refrigerant is 0.05 kg/sec. Show the cycle on a T-s diagram with respect to saturation lines. Determine (a) the rate of heat removal from the refrigerated space and the power input to the compressor, (b) the rate of heat rejection to the environment and (c) the coefficient of performance.

Property of R-134a : Table A-11 – A-13
State 1 Sat. vapor @ P1 = 0.14 MPa h1 = hg@0.14MPa = 236.04 kJ/kg, s1 = sg@0.14MPa = 0.9322 kJ/kg-K State 2 Superheated@ P2 = 0.8 MPa and s2 = s1 = 0.9322 kJ/kg-K, h2 = 272.05 kJ/kg State 3 Sat.liquid @ P3 = P2= 0.8 MPa, h3 = hf@0.8MPa = 93.42 kJ/kg State 4 Throttling, h4 = h3 = 93.42 kJ/kg

(a) Heat remove from refrigerated space and power input : Evaporator 4 - 1, 1st law SSSF and w = 0 & & q = h − h , Q = mq = 0.05(236.04 − 93.42)
L 1 4 L L

& Q L = 7.13 kW

Compressor 1 - 2, 1st law SSSF and q = 0 & & w = h − h , W = mw = 0.05(272.05 − 236.04) in 1 2 in in

(c) The coefficient of performance : & Q 7.13kW COP = L = = 3.96 & W 1.80kW in & Win = 1.80 kW

(b) The rate of heat reject : Condenser 2 - 3, 1st law SSSF and w = 0 & & q = h − h , Q = mq = 0.05( 272 .05 − 93.42)
H 3 2 H H

& Q H = −8.93 kW

Actual Vapor-compression Refrigeration Cycle

Example 10-2 Refrigerant-134a enter the compressor of a refrigerator as superheated vapor at 0.14 MPa and -10oC at rate of 0.05 kg/sec and leave at 0.8 MPa and 50oC. The refrigerant is cooled in the condenser to 26oC and 0.72MPa and is throttled to 0.15MPa. Determine (a) the rate of heat removal from the refrigerated space and the power input to the compressor, (b) the isentropic efficiency of the compressor and (c) the coefficient of performance.

Property of R-134a : Table A-11 – A-13
State 1 Superheated@P1 = 0.14 MPa and T1=-10oC State 2 Superheated@ P2 = 0.8 MPa and T2=50oC State 3 Sat.liquid @ = 0.72 MPa and T3=26oC State 4 Throttling, h4 = h3 = 85.75 kJ/kg h1 = 243.40 kJ/kg, h2 = 284.39 kJ/kg

h3 = hf@26C = 85.75 kJ/kg

(a) Heat remove from refrigerated space and power input : Evaporator 4 - 1, 1st law SSSF and w = 0 & & q = h − h , Q = mq = 0.05(243.40 − 85.75)
L 1 4 L L

ηc ≅

& Q L = 7.88 kW

281.05 − 243.40 = 0.919 284.39 − 243.40 = 91.9%

Compressor 1 - 2, 1st law SSSF and q = 0 & & w = h − h , W = mw = 0.05(284.39 − 243.40) in 1 2 in in

& Win = 2.05 kW

(c) The coefficient of performance : & Q 7.88 kW COP = L = = 3.84 & W 2.05 kW in (b) The isentropic efficiency of compressor : h −h ηc ≅ 2s 1 h2 − h1
State 2s Superheated@ P2 = 0.8 MPa and s2s=s1 =0.9606kJ/kgK h2s = 281.05 kJ/kg

Heat Pump System for Heating and Cooling

Cascade Refrigeration Systems

Improve COP of the system
Two cycles connected through HX.

Application: • Moderately low temperature (TL) • Large pressure range in cycle

Multistage Compression Refrigeration Systems

Better heat transfer than HX.

Application: • Moderately low temperature (TL) • Large pressure range in cycle

Multipurpose Refrigeration Systems with a Single Compressor

Application: • Refrigeration at more than one temperature

Liquefaction of Gases

Desire product

Application: • Operate at cryogenic temperatures (below -100oC)

Gas Refrigeration Cycles

Process 1-2 Isentropic Compression Process Process 2-3 P = const. Heat Rejection Process const. Process 3-4 Isentropic Expansion, s = const Expansion, Process 4-1 P = const. Heat Addition Process const.

Application: • Simple and lighter components • Refrigerate in the aircraft

An Open-Cycle Aircraft Cooling System

Absorption Refrigeration Systems

Heat-driven system

Definitions of Refrigerants
A refrigerant is a fluid used for heat transfer in a refrigeration system. Most refrigerants absorb heat during evaporation at low temperature and low pressure and reject heat during condensation at a higher temperature and higher pressure.

Classification of Refrigerants
Refrigerants most commonly used refrigeration systems can be classified into four groups:

halocarbons, azeotropes, hydrocarbons, and inorganic compounds

Hydrocarbons Refrigerants belonging to the hydrocarbon group are ethane, propane, butane and isobutane. isobutane. They are produced from petroleum in an oil refinery. This group of refrigerants is used in the refrigeration systems in oil refineries and the petrochemical industry due to their low cost and ready availablility. availablility. Hydrocarbons are flammable and so safety precautions are of utmost importance in the petrochemical industry. Inorganic compounds Inorganic compounds were refrigerants used in refrigerant this group group are ammonia NH3

Halocarbons Refrigerants belonging to the halocarbon group are derivatives of the hydrocarbons obtained by substituting chlorine or fluorine fluorine for the hydrogen atoms in methane and ethane. As chlorine and fluorine are both halogens, this group of refrigerants is called the halogenated hydrocarbons or halocarbons. They are sometimes referred to freons which are colourless, non-inflammable, non-corrodent to most metals and generally nonnonnon-toxic nonCommon refrigerants in this group are R-11, R-12, R-13 and RRRR-22.

Azeotropes An azeotrope is a mixture of two substances which cannot be separated into its components by distillation. It evaporates and condenses as a single substance and Its properties are completely different from its constituents. For example, azeotrope R-500 is a mixture composed of 73.8 per cent R-12 and 26.2 per cent R-152. RR-

Terminology of Refrigerants In the early days of refrigeration, refrigerants were called by their chemical names. However, a numbering system has been developed. Each kind of refrigerant is represented by a number, and in front front of this number is a prefix `R’ which represent 'Refrigerant'. `R’ Before 'R' is used as the prefix, 'Freon' & 'Genetron', the trade 'Genetron', names of refrigerants produced by two companies. For example, the R-12 they produced is called Freon-12 and RFreonGenetron-12. GenetronThe digits related to number of fluorine, hydrogen and carbon atoms in the molecule.

Selection of a suitable refrigerant It depends on many factors, such as the evaporating temperature required during operation, the coefficient of performance COP, safety safety requirements, and the size and location of the refrigeration plant. plant. In order to select a suitable refrigerant for a refrigeration system of system known size and evaporating temperature, the following factors must be must considered: The volume flow rate required per kW of refrigeration capacity The coefficient of performance COP Safety requirements Physical properties Operating properties Cost.

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