refrigeration (kylteknik)

Microsoft PowerPoint - REF18-OH3Ron Zevenhoven Åbo Akademi University
Thermal and Flow Engineering Laboratory / Värme- och strömningsteknik tel. 3223 ; [email protected]
Refrigeration (Kylteknik) course # 424519.0 v. 2018
4 slides 57-60 added 6.11.2018
ÅA 424519 Refrigeration / Kylteknik
06.11.2018Åbo Akademi Univ - Thermal and Flow Engineering Piispankatu 8, 20500 Turku 2/60
3.1 Absorption refrigeration (AR) processes 1: NH3 / H2O
See also A11 7.1-7.9 & 7.11
Absorption refrigeration: general
Instead of compressing a vapour, it can be absorbed in a liquid; after compressing the liquid solution a high-pressure vapour can be obtained
06.11.2018Åbo Akademi Univ - Thermal and Flow Engineering Piispankatu 8, 20500 Turku 3
Picture: HTW08
Absorption refrigeration (AR)
Instead of using a net work input, a refrigeration cycle can be driven by heat (preferably 100-200°C)
The box replaces the compressor in a vapour- compression cycle
Sources for ”cheap heat” could be waste heat from power generation or steam plants, geothermal energy, solar energy, biogas fuel, etc.
Absorption refrigeration involves absorption of refrigerant by a transport medium
Most widely used is ammonia / water (refrigerant/transport medium), also used are water / LiBr and water / LiCl. (The use of water as refrigerant requires temperatures above 0 °C.)
Picture: ÇB98
Ammonia / water AR system /1 Hot, pressurised ammonia
(NH3, R-717) from the box in the Figure gives of heat to the surroundings in a condenser, is throttled to low pressure and takes up heat in the evaporator
In the box, the ammonia is absorbed and dissolved in water, giving off dissolution heat. The lower the absorber temperature, the higher the solubility of the NH3. Note that the absorber needs cooling! ...continues
Picture: ÇB98
Ammonia / water AR system /2 The rich NH3/H2O mix is
pumped via a heat exchanger to a regenerator, where heat is used to vaporise part of the solution
A rectifier separates ammonia vapour from the lean NH3/H2O mix
The ammonia vapour enters a new cycle
The lean NH3/H2O mix is returned to the absorber, on the way exchanging heat with the rich NH3/H2O mix, before throttling to the absorber pressure
ADVANTAGE compared to vapour - compression processes: a liquid is compressed instead of a gas; the pump power is very small, often negligible. → the process is heat-driven.
Picture: ÇB98
06.11.2018 Åbo Akademi Univ - Thermal and Flow Engineering Piispankatu 8, 20500 Turku
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Absorption refrigeration: COPR
With Ts = 120°C, TL = -10°C and T0 = 25°C → COPR,rev = 1.8 In real systems COPR will be < 1.
Picture: ÇB98
L
L
COP 100/165 = 0.60 100/30 = 3.33
Table: HTW08
A practical NH3/H2O AR system A basic scheme ↓ and a practical system ← with heat exchangers and some generator details
Pictures: D03
high p
low p
Log p , 1/T diagram (Dühring plot)
System calculations are not easy; T,s and p,h diagrams can’t be used.....
Log p, 1/T diagrams can be used the find p,T boundaries for an AR process
A second diagram, the h,w diagram can be used to find heat in-/outputs for the process
a = vapour pressure line for pure refrigerant w = 100% b = vapour pressure line for pure absorbent w = 0%
Evaporation: p2, T2
Condensation: p1, T1
Picture: S90
Ref. Abs.
Dühring plot NH3/water
Dühring plot NH3/water S
Saturated vapour
Saturated liquid
at various pressures
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: h tt
p: //m
ai l.a
ir ah
.o rg
.a u/
do w
nl oa
ds /2
00 2-
08 -0
2. pd
4)
15/60
Ammonia / water AR system COP
COPR of a single-stage NH3/H2O AR system as function of evaporation temperature and condenser temperature
Picture: D03
Condenser temperature
A CC-HP system with gas engines and water/ammonia absorption chiller in a dairy factory: (a) schematic diagram of a CCHP in Burgos, Spain; (b) ARP-M10 water/ammonia absorption chiller
Source: DWH11
[50] = Appl Therm Eng 2008 (28) 975-987. Source: DWH11
NH3 – water: basic vs generator- absorber exchanger (GAX) /2
Source: DWH11
19/60
3-fluid AR system (von Platen - Munters) /1
In this system the pump has been removed and replaced by third fluid: H2
H2 provides inert gas partial pressure* and acts as carrier gas for transport of NH3 from the evaporator to the absorber by natural convection circulation through a gas-gas heat exchanger
Gravity and density differences drive the system
Complicated and therefore considered expensive
Picture: D03
* It makes the total gas pressure in evaporator and absorber equal to that in generator and condenser
Electrolux
See also A11 7.11
20/60
3-fluid AR system (von Platen - Munters) /2
21/60
See also: http://www.cam.net.uk/~aaa314/electrolux.html (Oct. 2012) or: http://www.portablefridgesonline.com.au/fridge-faqs/fridge-faq-how-3-way-fridges-work.html (Nov. 2014)
Heat is supplied as steam (A) or electricity (B)
In siphon (C) NH3 vapour is separated from NH3/H2O
NH3/H2O returns to absorber via pipe (D); the NH3 proceeds to the evaporator via pipe (E)
Liq. NH3
3.2 Absorption refrigeration (AR) processes 2: LiBr (or LiCl) / H2O
See also A11 7.10
Single-effect LiBr AR system /1
An absorption chiller with H2O as refrigerant and LiBr as absorbent is widely used for water chilling to ~5°C Pressure is low: < 135 kPa. COPR ~ 0.7
1. Metering device 2. Evaporator ~ 5°C 3. Absorber ~ 25°C 4. Pump 5. Generator 6. Condenser ~ 30°C
Picture: after D03
high p low
Picture: A11
~100 °C
Subcooling / superheating heat exchanger
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Absorption sequence is 1→2→3 in p, T diagram
Evaporated water vapour is absorbed into the solution and is pumped up to the generator for heat-up and the generated water vapour comes into the condenser.
Major risk/problem: LiBr crystallisation, especially in the heat exchangers
S ou
rc e/
pi ct
ur es
: h ttp
(water 12 7 °C)
S ou
rc e:
ht tp
(water 12 7 °C)
S ou
rc e:
ht tp
Dühring plot LiBr/water
xLiBr ~ 0.61
xLiBr ~ 0.66
Pressure-temperature-concentration diagram for H2O-LiBr solution
Mass fraction of lithium bromide versus saturation temperature of pure water and vapour pressure. Also given are lines of constant solution temperature.
Picture: IIT08
Enthalpy LiBr/water solutions
Picture: HRK96
Single-effect LiBr AR
Example data for operating conditionsf = m(3)/m(7)
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Double-effect LiBr AR system
A significant improvement is made with a higher temperature heat source and a second generator in the cycle
COPR ~ 1.0 ~1.2
LiBr / H2O chiller
Double / single effect LiBr AR units
S ou
rc e/
pi ct
ur es
: h ttp
Source: DWH11
Constant temperature stages are more reversible than constant pressure stages
Fluids for absorption refrigeration Refrigerant
Similar as for vapour- compression systems: reasonable pressures, safe, cheap, stable
Here vaporisation heat per kg is important, not per m3
Air reduces pressure, and heat/mass transfer
Water is a problem (except when it is the absorbent !)
Temperatures may be higher than with vapour- compression systems
Absorbent Liquid (at least when mixed
with refrigerant) Low vapour pressure Low specific heat Safe, cheap, stable
Refrigerant + absorbent Deviation from ideal behaviour
(Raoult’s Law) is beneficial Low mixing heat Low viscosity, high density Chemical and thermal stability Low corrosivity No crystallisation
P ic
tu re
: h ttp
3.3 Gas expansion refrigeration processes
See also A11 13.6 (& 13.7)
Gas refrigeration cycle /1
A gas power cycle can also be run in reverse → reversed Rankine cycle or gas refrigeration cycle
Used with air as refrigerant in aircraft, and for liquefaction of gases
Picture: ÇB98
! Process efficiency (COPR) drops rapidly for compressor efficiency and/or turbine efficiency << 100%
isentropic compression isobaric heat rejection isentropic expansion isobaric heat take-up
Gas refrigeration cycle /2 The heat transfer per kg
refrigerant gas depends on the product of specific heat and temperature change, which is small compared to vaporisation/ condensation heat → large mass streams of refrigerant are needed → elevated pressures (and closed loops) make equipment compact
The gas cycle involves non-isothermal heat transfer This deviation from a reversed Carnot cycle results in lower
efficiencies, i.e. lower COPR values
Picture: ÇB98
COP vs pressure ratio for air, γ = cp/cv = 1.4
Source: A11
Example: air refrigeration cycle (ideal)
Consider an gas refrigeration cycle using air:
Enters compressor at T1 = -20°C, exits at T2 = 40°C, 8 bar
Enters evaporator at T4 = -80°C. With γ = cp/cv = 1.4 and cp = 1.0035
kJ/(kgK) : calculate the specific work (kJ/kg) for compressor and expander, and COPR
Calculate, using polytropic relations, T1, T2, T3 and T4
p2/p1 = (T2/T1)γ /(γ -1) → p1 = 3.8 bar; T3/T4 = (p3/p4)(γ -1)/ γ → T3 = -34°C wcompr = cp(T2-T1) = 60.2 kJ/kg; wexpan = cp(T3-T4) = 45.9 kJ/kg; qH = cp(T2-T3) = 74.5 kJ/kg; qL = cp(T1-T4) = 60.2 kJ/kg COPR = qL/ wnet,in = qL/ (wcompr - wexpan) = 4.21
Picture: ÇB98
44/60
Gas refrigeration systems In aircraft, a gas
refrigeration cycle can be run as an open cycle, routing the cool air into to the cabin space
Regenerative cooling is achieved by inserting a heat exchanger → this lowers turbine inlet temperature and exit temperature → improves COPR
Pictures: ÇB98
Pics: A11
Sub-atmospheric pressure cycles open at warm end (left) and at both ends (right)
Aircraft refrigeration system (simple air conditioning without evaporative cooling)
Pics: A11
Gas expansion: Joule-Thomson effect /1 Throttling (= isenthalpic pressure
reduction of gases) can have a temperature effect as a result of deviations from ideal gas behaviour:
For the states (for example in a T,s diagram) where (∂T/∂p)h > 0, reducing pressure will give a lower temperature: the Joule-Thomson effect Picture: S90
0

0 Liquid-vapour dome
48/60
At the inversion temperature of a gas, µJT = 0
Application: cooling and liquefaction of gases (Linde)
Some tabelised data:
Picture & table: A83
3.4 Thermo-electric refrigeration processes
See also A11 14.5
50/60
Thermo-electric effects
Pictures: ÇB98
Heating one of the junctions (sv: kontaktställe) of two different metals gives an electric current I (A) Seebeck effect
Similarly, breaking the circuit gives an electric potential difference V (V)
This can be used for thermo-electric power generation → power out Wnet
If irreversibilities (such as I²R heating) are small, efficiency ≈ Carnot efficiency
51/60
Thermo-electric power generator The low efficiency of a T-E
generator can be improved by using semi- conductors instead of simple metal pairs → higher voltages
Advantages are small weight and compactness, reliability, no sound, little service needs
Used in space applications (Voyager)
Picture: ÇB98
P ic
tu re
: h ttp
)
52/60
a T-E circuit creates a temperature difference: Peltier effect
This can be used for thermo- electric refrigeration
Effficiency (COPR) not very good at values ~ 0.10 (like T-E power units)
Small weight and compact, reliable, no sound, little service needs
I²R heating may increase temperatures → convective cooling may be necessary
Pictures: ÇB98
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Pictures: D03
P ic
tu re
: h ttp
Example: thermo-electric cooler A thermo-electric device is
used to cool an object to Tc = 5°C at an ambient temperature Ta = 25°C, which acts as a forced convection heat sink with thermal resistance Rt = (ΔT/Q) = 0.15°C/W. The hot-side temperature is Th = 35°C, electric current and voltage are 3.6 A and 10 V.
Calculate: temperature difference ΔT (°C) across the thermo-electric device and the extracted cold heat stream Qc (W)
Answer: ΔT = 35 – 5 = 30°C Qh = (Th – Ta) / Rt = 66.7 W Qh = Qc + Pelec
Pelec = 3.6·10 = 36 W Qc = 30.7 W
Tc = 5°C
Th = 35°C
Rt = 0.15°C/W
Sources #3 /1 of 2
A83: P.W. Atkins ”Physical chemistry”, 2nd ed., Oxford Univ. Press (1983) A11: R. C. Arora ”Refrigeration and air conditioning”, 2nd. Ed. PHI Learning Private
Limited , New Delhi (2011) Chapter 7, 13.6-13.7, 14.5 CB98: Y.A. Çengel, M.A. Boles “Thermodynamics. An Engineering Approach”, McGraw-Hill
(1998) D03: . Dinçer “Refrigeration systems and applications” Wiley (2003)
DWH11: J Deng, R.Z. Wang, G.Y. Han “A review of thermally activated cooling technologies for combined cooling, heating and power systems.” Progr. Energy and Combust. Sci. 37 (2011) 172 - 203
HTW08: G.F. Hundy, A.R. Trott, T.C. Welsh “Refrigeration and air conditioning 4th ed. Butterworth-Heinemann (2008)
HRK96: K.E. Herold, R. Radermacher, S.A. Klein ”Absorption Chillers and Heat Pumps”, CRC Press (1996)
IIT08: Refrigeration and air conditioning, IIT Kharagpur (2008) Lessons 14, 15, 16, 17 see: http://nptel.iitm.ac.in/courses/Webcoursecontents/IIT%20Kharagpur/Ref%20and%20Air%20Cond/New_index1.h tml
KJ05: D. Kaminski, M. Jensen ”Introduction to Thermal and Fluids Engineering”, Wiley (2005) SEHB06: P.S. Schmidt, O. Ezekoye, J. R Howell, D. Baker
“Thermodynamics: An Integrated Learning System” (Text + Web) Wiley (2006)
Sources #3 /2 of 2
S90: A.L. Stolk ”Koudetechniek A1”, Delft University of Technology (1990) T06: S.R. Turns ”Thermal – Fluid Sciences”, Cambridge Univ. Press (2006) WL99: Wang, S.K., Lavan, Z. “Air-Conditioning and Refrigeration” §19.14
in: Mechanical Engineering Handbook, Ed. Frank Kreith CRC Press (1999) Ö96: G. Öhman ”Kylteknik”, Åbo Akademi University (1996) See also: Martinez, I. ”Lectures on Thermodynamics” – lecture 18 (English or Spanish)
http://webserver.dmt.upm.es/~isidoro/bk3/index.html updated and based on “Termodinámica básica y aplicada", Ed. Dossat, Madrid (1992) ISBN 84-237-0810-1
P ic
tu re
: w w
w .je
m im
ar .fi
Source: Arora, Refrigeration and air conditioning, 3rd ed. Chapter 12
Assume an ammonia absorption refrigeration system with an analyser but no dephlegmator
”Analyser”
”Dephlegmator”
ÅA 424519 Refrigeration / Kylteknik
See table previous page
7(ξ7 = ξ5) + 2ξa = 1aξr
f = 1a/7 & 2 = 1a - 7 2/7 = 1-f (ξ7 = ξ5) + (f-1)ξa = fξr
”Analyser”
”Dephlegmator”
”Analyser”
”Dephlegmator”
”Analyser”
”Dephlegmator”

refrigeration (kylteknik)

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