Environmental Thermal
Engineering
Lecture Note #6
Absorption
Refrigeration
System
Backgrounds Absorption Refrigeration System
❑ Vapor compression refrigeration system
(1) It uses electrical energy for compression work.
(2) Electrical energy is an advanced energy form.
(3) It demands a large amount of energy in vapor compression due to the significant change in specific volume.
❑ To increase coolant pressure while minimizing work
▪ Vapor Compression → Liquid Compression
❑ Absorption refrigeration system
▪ Uses natural coolant such as water or ammonia
▪ Operates with gases, wasted heated or solar heat
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Comparison Absorption Refrigeration System
Type Compression
(mechanical, electric drive) Absorption
(Chemical, heat-driven)
Diagram
Energy
Source Electricity Gas combustion heat,
Steam, Hot water Operating
Pressure Condenser : higher than atmosphere
Evaporator : Lower than atmosphere Vacuum
Indoor
Cooling
Condenser
Evaporator Expansion
Valve Compressor
Work
Refrigerant Heat
Heat
Condenser
Evaporator Nozzle
Refrigerant
Generator
Absorber
Weak Solution Pump
Strong Solution
Work Heat
Heat
Heat Heat
Indoor
Cooling
Refrigerant / Absorbent of
Absorption system
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Refrigerant / Absorbent Refrigerant / Absorbent of Absorption system
• A binary system based on H2O/LiBr, and a advanced ternary and quaternary system.
• Water has large evaporative latent heat, and high COP and easily acquire.
• Due to large boiling point, it is hard to air cooling.
• It cannot acquire cold heat source below 0℃.
• Due to strong corrosiveness, it is hard to manage solution.
• Recently, water receives attention as coolant/absorber of absorption refrigerating system for solar heat.
❑ Water Refrigerant System (H
2O/LiBr)
• Due to high operating pressure, it needs pressure vessel.
• Ammonia has adequate characteristic for coolant.
• Toxicity, flammable and explosiveness is critical defects.
• Applications : large absorption refrigerating system for industry, small absorption refrigerant system for air conditioning.
❑ Ammonia Refrigerant System (NH
3/H
2O)
• The latent heat of evaporation is large and the freezing point is lower than that of water.
• Flammability problem.
❑ Alcohol Refrigerant System ❑ Halocarbon refrigerant System
• Use dimethylformide (DMF) as absorbent.
• Refrigerant includes R21, R22, R124, R31, R123, R133a, etc
Comparison Refrigerant / Absorbent of Absorption system
Refrigerant/
Absorbent
NH
3/H
2O H
2O/LiBr
Refrigerant NH3 H2O
Absorbent H2O LiBr
COP 0.3~0.7 (Single effect) 0.7~1.2 (Single effect)
High Pressure 4~5 bar 0.01bar
Low Pressure 10~15 bar 0.07~0.1 bar
Boiling Point -33 ℃ at 1 atm 100 ℃
Freezing point -77℃ 0 ℃
Crystallization N Y
Piping material Steal (C.S, S.S) Steal, Copper tube
O.D.P N N
G.W.P N N
Toxicity Y N
Flammability Some (Not Explosive) N
Cycles in
Absorption system
Components in Absorption cycle Cycles in Absorption System
Solution Heat Exchanger
Generator
Absorber Condenser
Evaporator
Solution Expansion Valve Pump
Refrigerant Expansion Valve
It absolves evaporated refrigerant steam in evaporator and sends regenerator.
It absolves coolant and heats solution from absorber, so it separates coolant and absorbent. Evaporated coolant is sanded to condenser and absorbent is sanded to absorber
❑ Absorber
❑ Generator
It condenses refrigerant steam by heat exchanging with cooling water.
❑ Condenser
It sprays liquid coolant from condenser on upper part of cooling pipe. Depriving the heat, this liquid is evaporated and sanded to absorber.
❑ Evaporator
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Components in Absorption cycle Cycles in Absorption System
Ref. Modern Refrigeration and Air Conditioning
Principle of Absorption System Cycles in Absorption System
FIGURE Schematics of single effect absorption cycle
Desorber
Absorber Condenser
SHX
Qc Qd
Qe Qa
P
T
1 2
3 4
7
5
6 8
9
10 W
Evaporator
• Absorber: Water vapor from an evaporator is absorbed into a lithium bromide solution (Strong Solution)
• 1 → 2: Increase the pressure of the solution by pump
• 2 → 3: The temperature rises through heat exchange with a dilute solution in the
solution heat exchanger.
• Generator : Water vapor generation in a high concentration solution by receiving heat (Weak Solution)
• 7 → 8: Heat exchange with cooling water and steam condensation in Condenser.
• 8 → 9: Pressure drop through the expansion valve.
• 9 → 10: Evaporator absorbs external heat and evaporates.
• 10 → 1: Steam enters Absorber and is absorbed into the lithium bromide solution.
• 4 → 5: The temperature of the diluted solution decreases through heat exchange with the high concentration solution in the solution heat exchanger.
• 5 → 6: Pressure drops through the expansion device.
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Absorption System (Double Effect, H Cycles in Absorption System 2 O, LiBr)
FIGURE Schematics of double effect absorption cycle (Series Flow)
High temp.
Desorber
Absorber Evaporator
Condenser
High Temp.
SHX P
Low temp.
Desorber
Low Temp.
SHX P
T P
Condenser High temp.
Desorber
Absorber Evaporator
Condenser
High Temp.
SHX P
Low temp.
Desorber
Low Temp.
SHX P
T P
Condenser
FIGURE Schematics of double effect absorption cycle (Parallel Flow)
Absorption System (Triple Effect, H Cycles in Absorption System 2 O, LiBr)
FIGURE Schematics of triple effect absorption cycle
Absorber Evaporator
T P
Condenser 3 Desorber 3
Condenser 1
Desorber 2
Desorber 1
P
P
SHX
SHX
P
SHX
Condenser 2
Q
Q
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Basic Process of Thermodynamics Cycles in Absorption System
FIGURE T-x diagram of isotropic 2-fluids mixture Superheat
region
Subcooled liquid region
Temperature
x Condensing
line
Boiling line
0
❑ Isotropic 2-fluid mixture
• Uniform composition
• It does not separated by mechanical ways
• It needs pressure, temperature and concentration
• Several saturated temperature in each pressure
1
h(i)-x diagram Cycles in Absorption System
FIGURE h(i)-x diagram of isotropic 2-fluids mixture in liquid-gas region
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LiBr/H 2 O h(i)-x diagram
Cycles in Absorption System
Adiabatic mixing – Cycles in Absorption System Two Streams
1
2
3
ሶ
𝑚
2, 𝑥
2, ℎ
2ሶ
𝑚
1, 𝑥
1, ℎ
1ሶ
𝑚
1ሶ
𝑚
2= ℎ
2− ℎ
3ℎ
3− ℎ
1= 𝑥
2− 𝑥
3𝑥
3− 𝑥
1𝑥
3= 𝑥
1+ 𝑚 ሶ
2ሶ
𝑚
3𝑥
2− 𝑥
1ℎ
3= ℎ
1+ 𝑚 ሶ
2ሶ
𝑚
3ℎ
2− ℎ
11
2 3
Mixing Chamber
ℎ2
ℎ3 ℎ1
𝑥1 𝑥3 𝑥2
0 1
ℎ
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Example of Adiabatic mixing Cycles in Absorption System
➢ A graphical method is used to obtain temperature and the composition of state 3
❑ Adiabatic mixing
▪ Stream 1 :
➢ 𝑥 = 0.7
➢ 4.54 kg/min
➢ 15.5℃
➢ 689 kPa
▪ Stream 2 :
➢ Saturated Liquid
➢ 2.27 kg/min
➢ 93.3℃
➢ 689 kPa
2 3Sat. Vapor 689 kPa
0.26 0.553 0.7
T=43.3℃
(689 kPa) Equilibrium
construction line 689 kPa
f
g
T=93.3℃
1.0 0
𝑥 ℎ
Sat. Liquid 689 kPa
T=43.3℃
T=15.5℃
1
Heating and Cooling Process Cycles in Absorption System
❑ To increase the purity of the water-ammonia system
Concentration
Enthalpy
1 2
3
4
5
6
7
FIGURE Simple heating and cooling processes for steady-flow conditions
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Adiabatic Throttling- Binary Liquid Mixture Cycles in Absorption System
1 2
f
P1
g
P2 t1
t2
1: High pressure, Saturated solution 2: Low Pressure, Saturated condition
➢ Temperature of the final state is obtained using a graphical
approach
𝑥1 = 𝑥2
0 1.0
ℎ
ℎ1 = ℎ2
T2=Tf=Tg Sat. vapor line (P2)
1 Throttle 2
𝑥
Absorption Refrigeration System COP Cycles in Absorption System
water vapor
LiBr-water solution Condenser
Evaporator
Generator
Absorber 40℃
①
②
③
④
⑤
ሶ 𝑄
𝐺ሶ 𝑄
𝐶ሶ 𝑄
𝐸ሶ 𝑄
𝐴ሶ 𝑊
𝑃ሶ 𝑄 𝐺 = ሶ 𝑚 3 ℎ 3 + ሶ 𝑚 2 ℎ 2 − ሶ 𝑚 1 ℎ 1
ሶ 𝑄 𝐸 = ሶ 𝑚 5 ℎ 5 − ሶ 𝑚 4 ℎ 4 𝐶𝑂𝑃 𝑎𝑏𝑠 = ሶ 𝑄 𝐸
ሶ 𝑄 𝐺
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Calculation of heat in Absorption Ref. system Cycles in Absorption System
Condenser
Evaporator
Generator
Absorber
Refrig.
vapor
Exp. Valve A
Exp. Valve B
Weak solution
Strong solution Refrigerant
only Refrigerant + Absorbent
Pump
H X
ሶ 𝑄
𝐸ሶ 𝑄
𝐴ሶ 𝑊
𝑃ሶ 𝑄
𝐶ሶ 𝑄
𝐺Ideal System COP Cycles in Absorption System
ሶ 𝑄 𝐴 + ሶ 𝑄 𝐶 = ሶ 𝑄 𝐸 + ሶ 𝑄 𝐺 + ሶ 𝑊 𝑃
𝐶𝑂𝑃 = 𝑄 ሶ 𝐸
ሶ 𝑄 𝐺 ≤ 𝑇 𝐸 𝑇 𝐺 − 𝑇 𝑂
𝑇 𝐺 𝑇 𝑂 − 𝑇 𝐸 𝐶𝑂𝑃 max = 𝑇 𝐸 𝑇 𝐺 − 𝑇 𝑂 𝑇 𝐺 𝑇 𝑂 − 𝑇 𝐸 ሶ 𝑄 𝑂 = ሶ 𝑄 𝐴 + ሶ 𝑄 𝐶
∆𝑆 𝐺 = − 𝑄 ሶ 𝐺
𝑇 𝐺 ∆𝑆 𝐸 = − 𝑄 ሶ 𝐸
𝑇 𝐸 ∆𝑆 𝑂 = 𝑄 ሶ 𝑂 𝑇 𝑂
∆𝑆 𝑡𝑜𝑡𝑎𝑙 = ∆𝑆 𝐺 + ∆𝑆 𝐸 + ∆𝑆 𝑂 = − 𝑄 ሶ 𝐺
𝑇 𝐺 − 𝑄 ሶ 𝐸
𝑇 𝐸 + 𝑄 ሶ 𝑂
𝑇 𝑂 ≥ 0 ሶ 𝑄 𝐺 𝑇 𝐺 − 𝑇 𝑂
𝑇 𝐺 ≥ ሶ 𝑄 𝐸 𝑇 𝑂 − 𝑇 𝐸
𝑇 𝐸 − ሶ 𝑊 𝑃
Classification of
Various Absorption Refrigeration
Systems
Absorption Chiller Classification of Various Absorption Refrigeration Systems
Ref. World energy catalog
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Absorption Chiller Classification of Various Absorption Refrigeration Systems
Ref. World energy catalog
Absorption Heat Pump – Classification of Various Absorption Refrigeration Systems Type 1
Ref. World energy catalog
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Absorption Heat Pump – Classification of Various Absorption Refrigeration Systems Type 2
Ref. World energy catalog
Water-Ammonia system for Industry Classification of Various Absorption Refrigeration Systems
FIGURE Water-Ammonia Absorption refrigerating system for industry Condenser
Evaporator Absorber
Generator
Phase Separator
Pump
Q
EQ
CQ
GW
PQ
AQ
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Solar-Powered NH Cycles in Absorption System 3 /H 2 O Absorption System
FIGURE Solar-Powered ammonia absorption refrigeration cycle
Ref. Cengel, Boles, Thermodynamics
