Alberto Cavallini - Università di Padova - APPLICAZIONI A MINICANALI PER SISTEMI DI REFRIGERAZIONE A CARICA RIDOTTA

XV EUROPEAN CONFERENCE – ITALY
THE LATEST TECHNOLOGY IN AIR CONDITIONING AND
REFRIGERATION INDUSTRY - Politecnico di Milano
Davide Del Col, Marco Azzolin, Stefano Bortolin, Alberto Cavallini
University of Padova
Department of Industrial Engineering
MINICHANNEL DEVICES FOR
LOW CHARGE
REFRIGERATING SYSTEMS
XV EUROPEAN CONFERENCE

Outline
Environmental impact and introduction to
minichannel technology
Heat transfer in minichannels
Effect of channel shape
Refrigerant properties and specific charge
Heat exchangers with minimum refrigerant charge
Conclusions

Environmental impact
The worldwide alert about global warming has led to an
increasing interest in new HVAC (heating, ventilation and
air conditioning) technologies with low environmental
impact
Effects to be considered:
Indirect Effect
Energy consumption
Direct Effect
Leakage of refrigerant

Why minichannel technology ?
- The refrigerant charge minimization can be considered
one of the most important targets for HVAC&R applications
to cope with the new environmental challenges.
- Minichannel technology appears to be a very good
opportunity to minimize the charge without energy
performance loss.
- Instead, minichannel technology can help reducing
greenhouse gas emissions by improving component and
system energy efficiencies.

SQUARE
CIRCULAR
Resarch on minichannels

Enlarged picture taken with a
micro-endoscope
Square cross section minichannel
(1.18 mm side length)

Condensation HTC: square vs. circular
G = 200 kg/(m2s)
HEAT TRANSFER COEFFICIENT [W/(m2K)]
0
1000
2000
3000
4000
5000
6000
7000
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
VAPOUR QUALITY [/]
HTC[Wm
-2
K
-1
]
0
5
10
15
20
25
30
35
40
45
50
Tsat-Twall[K]
G200
G200
G200 t
G200 t
T SAT - T WALL [K]

Condensation HTC: square vs. circular
0
2000
4000
6000
8000
10000
12000
14000
16000
0.0 0.2 0.4 0.6 0.8 1.0
VAPOUR QUALITY[/]
G790 SQUARE
G800 CIRCULAR
0
2000
4000
6000
8000
10000
0.0 0.2 0.4 0.6 0.8 1.0
VAPOUR QUALITY[/]
HTC[Wm
-2
K
-1
]
G400 SQUARE xin=0.92
G400 SQUARE xin=0.68
G400 CIRCULAR
G = 800 kg/(m2s)
G = 400 kg/(m2s)

Condensation: effect of channel shape
G=400 kg m-2 s-1
Modelled liquid-vapour interface in circular and square minichannel with R134a

G100 zero-gravity x=0.8
G100 normal gravity x=0.8
G100 zero-gravity x=0.6
G100 normal gravity x=0.6
Liquid-vapour interface during condensation
x = 0.6
G = 100 kg m-2s-1
x = 0.8
G = 100 kg m-2s-1

University of Padova
Tube sheet of minichannel HX

Condenser internal volume
BPHE
80 plates
8.4 L
MINICHANNEL S&T
2 mm i.d. tubes
2.9 L
Volume reduction = -65%
Charge reduction = -0.8 kg

Propane charge
Evaporator PHE PHE PHE PHE
Condenser PHE MC PHE MC
IHX no no yes yes
Measured charge ~3.9 kg ~ 3.1 kg ~ 5.1kg ~ 4.3 kg
Target charge ~ 3.1 kg ~ 2.3 kg ~ 3.5kg ~ 2.7 kg
Piping length not minimized (around 1 kg of R290)
~0.5 kg of R290 in the liquid receiver
~0.4 kg of R290 in the dehydrating filter
TARGET CHARGE: 0.5m liquid line, no receiver, with filter

Heat exchanger data
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
ROUHANI NIÑO CISE ZIVI BAROCZY HOMOGEN. EXPERIMENTAL
R290ChargeReduction[kg]
R 290
LOCKHART
MARTINELLI
CONDENSER CHARGE REDUCTION
MINICHANNEL vs PHE
Comparison between experimental and calculated charge reduction for minichannel
condenser vs. PHE
EXP

Refrigerants properties
Fluid psat [bar] pRED [ - ] ρl [kg m-3
] ρv [kg m-3
] µl [µPa s] µv [µPa s] λl [W m-1
K-1
] GWP
R32 24.783 0.429 893.0 73.3 94.99 13.83 0.115 550
R134a 10.166 0.250 1146.7 50.1 161.45 12.37 0.075 1430
R410A 24.256 0.495 975.33 101.71 95.861 14.882 0.081 2100
R22 15.336 0.307 1128.5 66.193 138.67 13.359 0.077 1810
R1234ze 7.666 0.211 1111.3 40.7 167.00 12.93 0.069 6
R290 13.694 0.322 467.46 30.165 82.844 8.8918 0.087 3
R717 15.554 0.137 579.44 12.034 114.04 10.325 0.443 <1
Properties at TSAT = 40°C

0
0,2
0,4
0,6
0,8
Charge[kg]
Cise
Zivi
Baroczy
0
0,2
0,4
0,6
0,8
1
1,2
Charge[kg]
Cise
Zivi
Baroczy
Calculated refrigerant
charge in minichannels
HEXs for different
refrigerants (80 kW heat
flux at the
evaporator, 100 kW at
the condenser, 5K
subcooling and 5K
superheating)
CONDENSER
Comparison
of refrigerant
charge
EVAPORATOR

Specific charge for systems using
HCs and ammonia
Capacity [kW] Refrigerant Specific charge [g/kW]
Cavallini et al. (2010) 100 Propane 25-30
Fernando et al. (2004) 5 Propane 37
Hrnjak and Litch (2008) 13 Ammonia 20
Corberan (2008) 14 Propane 30-40
Park and Jung (2009)
3.8 (H)
3.5 (C)
R170/R290
157 (H)
142 (C)
(H): Heating
(C): Cooling

Conclusions
- The refrigerant charge reduction in refrigerating systems is a goal
to reduce the refrigerants contribution to the greenhouse effect
and to reduce atmospheric emissions.
- Minichannels are used to reduce the refrigerant charge in the HEs
without decreasing performance.
- The understanding of dominant mechanisms during boiling and
condensation in minichannels is the fundamental basis for the
development of accurate design methods.
- To evaluate the refrigerant charge in the heat exchangers void
fraction correlations must be used.
- Systems using minichannels heat exchangers that could work with
a specific charge of 30 g/kW with propane have already been
developed.

Thank you
for your attention !
davide.delcol@unipd.it
Università degli Studi di Padova
Dipartimento di Ingegneria Industriale

Specific charge: potential charge reduction
Specific charge in the
condenser [g/kW]
Specific charge in the
evaporator [g/kW]
Ammonia (R717) 13.4 5.38
Propane (R290) 34.4 4.24
Carbon dioxide (R744) 29.8 2.49
R32 44.9 5.34
R134a 124.2 23.16
R410A 65.6 6.51
(Padilla Fuentes Y., Hrnjak P., 2012, Charge reduction potentials of several refrigerants …, Int. Refr. Conf. at Purdue, July 16-19;
Padilla Fuentes Y., Hrnjak P., 2012, Experimentally validated microchannel heat exchanger performance …, 10th IIR G-L Conf. on
Natural Refrigerants, Delft.)

Heat exchangers data (KTH)
Propane heat pump developed at KTH of Stockholm
References: Fernando et al. (2004)
Heating capacity 4-7.23 kW
Hydraulic diameter 1.42 mm
Number of channels 6
Tube length 651 mm
Baffle plates 31
Refrigerant R290
Evaporator Condenser
Number of tube 30 36
Outer area 0.822 m2 0.985 m2
37 g/kW
of R290
Fernando P., Palm B., Lundqvist P., Granryd E., 2004, Propane heat pump with low refrigerant charge: design and laboratory tests, Int. J.
of Refrig., 27, 761-773

Alberto Cavallini - Università di Padova - APPLICAZIONI A MINICANALI PER SISTEMI DI REFRIGERAZIONE A CARICA RIDOTTA

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Alberto Cavallini - Università di Padova - APPLICAZIONI A MINICANALI PER SISTEMI DI REFRIGERAZIONE A CARICA RIDOTTA