alberto cavallini - università di padova - applicazioni a minicanali per sistemi di refrigerazione...

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 PadovaDepartment 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 EffectEnergy consumption

Direct EffectLeakage 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 1.0VAPOUR QUALITY [/]

HT

C [

W m

-2 K

-1]

0

5

10

15

20

25

30

35

40

45

50

Ts

at-

Tw

all

[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 1.0VAPOUR QUALITY [/]

HT

C [W

m-2

K-1

]

G790 SQUARE

G800 CIRCULAR

0

2000

4000

6000

8000

10000

0.0 0.2 0.4 0.6 0.8 1.0VAPOUR QUALITY [/]

HT

C [W

m-2

K-1

]

G400 SQUARE xin=0.92

G400 SQUARE xin=0.68

G400 CIRCULAR

G = 800 kg/(m2s)G = 400 kg/(m2s)

HEA

T TR

ANSF

ER C

OEF

FICI

ENT

[W/(

m2 K

)]



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 = 0.6G = 100 kg m-2s-1

x = 0.8G = 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

R2

90

Ch

arg

e R

ed

uct

ion

[kg

]

R 290

LOCKHARTMARTINELLI

CONDENSER CHARGE REDUCTIONMINICHANNEL 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

Char

ge [k

g]

Cise

Zivi

Baroczy

00,20,40,60,8

11,2

Char

ge [k

g]

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)

CONDENSERComparison of refrigerant charge

EVAPORATOR



Specific charge for systems usingHCs 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 !

[email protected]

Università degli Studi di PadovaDipartimento 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 …, 10 th IIR G-L Conf. on Natural Refrigerants, Delft.)



Heat exchangers data (KTH)

Propane heat pump developed at KTH of StockholmReferences: 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...

Business

square minichannel

yesmeasured charge

kgtarget charge

calculated charge reduction

8g100 normal gravity

pa s v pa s

6g100 normal gravity

liquid receiver