CONDENSATION AND FLOW BOILING HEAT TRANSFER OF HYDROCARBONS IN MINICHANNELS

Stefano Bortolin, Marco Azzolin,Arianna Berto, Davide Del Col

Department of Industrial EngineeringUniversity of Padova

Padova, Italy

Workshop on Heat Pumps for Low-GWP Refrigerants – December 7, 2020

Outline

2

▪ Introduction

▪ Description of the test facility

▪ Pressure drop measurements

▪ Condensation heat transfer coefficients

▪ Condensation performance and condenser charge evaluation

▪ Flow boiling heat transfer coefficients

▪ Applications: minichannels shell-and-tube condenser and evaporator

▪ Conclusions

Introduction

3

• In the last years, international organizations undertook some actions aimed at

HFCs phase-down:➢ F-gas Regulation (No 517/2014) of the European Union

➢ The Kigali Amendment to the Montreal Protocol (United Nations, 2016)

• The heat pump market continues to prosper in EU countries:

(19th EurObserv’ER Report):➢ At least 4 million HPs were sold in 2018

➢ Reversible air-air HPs still dominate EU market with 3.5 million systems sold in 2018

➢ The air-water HP market increased by 21.5% between 2017 and 2018

• Alternatives to the high GWP fluids currently employed in heat pumps (e.g.

R134a, R410A) must be investigated.

• The GWP100-years is lower than 4 for propane and propylene.

• The major issue related to a wide utilization of HCs is due to their flammability.

Introduction

• Hydrocarbons (HCs) present favorable thermodynamic and transport propertiesthat make them attractive for use in heat pumps.

• Refrigerant charge in heat pumps accumulates in the components (e.g. heatexchangers) where the liquid phase is present.

• Because of HCs flammability it is therefore essential to decrease the volume ofheat exchangers.

• Minichannel technology is a viable solution to reduce refrigerants charge.

• In the scientific literature, it is possible to find only few experimental data takenduring condensation/vaporization of HC refrigerants and the majority of them weretaken inside conventional channels (6-8 mm) or inside plate heat exchangers.

• Propane heat pumps can be used to produce hot water in residential buildings.

• Propane (R290) and propylene (R1270) can be also considered alternatives toR404A in commercial refrigeration and to R134a in R134a/CO2 cascaderefrigeration plants.

Experimental test rig

5

Pressure drop tests – Experimental technique

6

R290 - R1270

,

, ,

water water water PS

in MS in PSref

m c Th h

m

= −

, ( , )in PSh f p T=( ),, in MSx f p h=

ADIABATIC

enters as subcooled liquid

enters as superheated vapor

0.96 mm internal diameter

7

Adiabatic pressure drop

0

50

100

150

200

0 50 100 150 200

CA

LC

. P

RE

SS

UR

E G

RA

DIE

NT

[kP

a m

-1]

EXP. PRESSURE GRADIENT [kPa m-1]

Del Col et al. (2013a)

Friedel (1979)

Müller-Steinhagen and Heck (1986)

Zhang and Webb (2001)

-20%

+20%

Propane Propylene

Cavallini et al.

(2009)

40 °C saturation temperature

0.96 mm diameter channel

Del Col et al. IJR 47, 2014

Del Col et al. IJR 83, 2017

R290

R1270

Test section for HTC measurement

8

0.96 mm internal diameter

REFRIGERANT

Hole for wall

thermocouple

WATER

Heat transfer – Experimental technique

9

• Local heat transfer coefficient HTC

( )( )

in

r LG

q zx z x

m h= −

• Local vapor quality x(z)

( )( )

( ) ( )HTC

sat wall

q zz

d T z T z=

− ( )

( ),

d

d

w

w p w

T zq z m c

z= −

( )satT z by pressure measurement ( )wallT z directly measured

inx from hin,MS (p,T)

by heat balance

Condensation HTC

10

Propane Propylene

40 °C saturation temperature

0.96 mm diameter channel

Cavallini et al.

(2006)Cavallini et al.

(2006)

11

Fluid psat [bar] ρl [kg m-3] ρv [kg m-3] µl [µPa s] λl [W m-1 K-1] GWP100-yearsTemp.

Glide [K]

R1270 16.48 478.6 35.7 82.99 0.105 2 -

R290 13.69 467.5 30.2 82.84 0.087 3 -

R32 24.78 893.0 73.3 94.99 0.115 677 -

R134a 10.17 1146.7 50.1 161.45 0.075 1300 -

R452BR32/R1234yf/R125

67/26/7 %22.85 924.9 82.7 94.78 0.090 676 1.1

R32/R1234ze(E)

75/25%21.78 947.5 70.9 104.3 0.106 507 3.1

Fluid properties at 40°C (mean) saturation temperature

Performance in condensation

Performance in condensation

12

• Penalty Factor (PF) is employed to compare the

potential heat transfer performance of different

refrigerants.

• The parameter PF is a function of:➢ the driving temperature difference ΔTdr;

➢ the refrigerant temperature decrease due to

pressure drop ΔTsr.

• Considering PF=5 K2 at x=0.5, the mass velocity

that gives the same energy penalization is different

for each of the four fluids.

• R1270 displays HTC 26% higher than that of

propane and R134a.

1 1 dPF

4 d

satdr sr

fL V

G d T pT T

z

= = −

0

2

4

6

8

10

12

14

16

18

20

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

HE

AT

TR

AN

SF

ER

CO

EF

FIC

IEN

T [

kW

m-2

K-1

]

VAPOR QUALITY [/]

R32 G797

R1270 G375

R290 G300

R134a G420

R452B G822

R32/R1234ze(E)75%/25% G664

Condenser charge evaluation

13

• Minichannels condenser with 1 mm internal diameter.

• 40°C saturation temperature and condensation from

x=1 to x=0.

• The mass velocity G is calculated with PF = 5 K2 at

x=0.5.

• A fixed wall-to-saturation temperature difference ΔTdr

has been assumed.

• Considering the condition for minimum Total

Temperature Penalization (TTP)

minimum TTP 0.5 2srdr sr

dr opt

TT T

T

= + =

• For each element, the HTC has been evaluated

using the Cavallini et al. (2006) correlation.

0.5PF

2drT

=

14

Flow boiling HTC

Sun and Mishima

(2009)

Sun and Mishima

(2009)

30 °C saturation temperature

0.96 mm diameter channel

15

Performance in flow boiling

R1270 and R290 HTC calculated using the model by

Sun and Mishima (2009) at G = 400 kg m-2 s-1 and two

different heat fluxes.

Saturation temperature drop for R1270 and R290

calculated with the Del Col et al. (2013) model.

16

Propane heat pump

• 100 kW water-to-water heat pump

• Two commercial PHEs can be used as an

evaporator and a condenser.

• Two prototypes heat exchangers using

minichannels can be used as a condenser and

as an internal heat exchanger.

• A semi-hermetic reciprocating compressor is

installed in the heat pump.

• The internal volume is 2.9 L in the case of the

minichannel prototype and 8.4 L in the case of

the PHE; 65% volume reduction has been

obtained.Cavallini et al. IJR 33, 2010

17

Minichannels condenser

• When using the minichannel condenser instead of the plate

condenser, a very small COP reduction (around 2%) is observed.

• By using the minichannel condenser instead of the plate

condenser, 0.8 kg refrigerant charge reduction can be obtained,

corresponding to 25% of the total mass.

• Segmentally baffled shell-and-tube

heat exchanger using 2 mm i.d. copper

minichannels.

• Propane flows inside the tubes and

water flows on the shell side.

18

Minichannels evaporator

• Segmentally baffled shell-and-tube evaporator using 2 mm i.d. copper

minichannels.

• Refrigerant flows inside the tubes and water flows on the shell side.

• A single shell pass has been adopted with two tube passes by using a U-tube

bundle.

• The design capacity of the prototype is 80 kW when operating with propane

• A perforated plate having 624 holes with 1 mm diameter is installed inside the

header.

• In the design of the evaporator, particular attention has been paid to minimize the

internal volume of the cylinder head.

Del Col et al. STBE 21, 2015

19

0

2

4

6

8

10

12

70 80 90 100 110 120

Ou

tle

t E

va

po

rati

ng

Te

mp

era

ture

[°C

]

Heat Flow Rate [kW]

BPHE (0.33 inlet quality, 7 K superheat)BPHE (0.25 inlet quality, 5 K superheat)PROTOTYPE (0.33 inlet quality, 7 K superheat)PROTOTYPE (0.25 inlet quality, 5 K superheat)

R290

WATER TEMPERATURE 12-7 C

0

0.05

0.1

0.15

0.2

0.25

0.3

Pro

pa

ne

Ch

arg

e In

ve

nto

ry [kg

]

ROUHANI NIÑO LOCKHART

MARTINELLI

CISE ZIVI BAROCZY HOMOGENEOUS

Minichannels evaporator

Estimated total charge in the microchannel evaporator

when using propane (80 kW, 12 °C – 7 °C water

temperature, 0.33 inlet quality, 7 K superheat).

• Calculated outlet evaporating temperature vs heat flow

rate when using propane in the microchannel evaporator

and in a commercial BPHE.

• The refrigerant-side internal volume is 5.8 L in the case of

the prototype and 8.4 L in the case of the BPHE.

Conclusions

20

• Condensation HTCs and adiabatic two-phase pressure drop have been measured with

propane (R290) and propylene (R1270) inside a 0.96 mm diameter channel at 40°C

saturation temperature.

• The Del Col et al. (2013) model for frictional pressure gradient and the Cavallini et al.

(2006) model for condensation well predict experimental data.

• A comparative analysis has been conducted using the parameter Penalty Factor as

performance evaluation criterion: propylene displays higher heat transfer performance

as compared to propane and R134a.

• An evaluation of the refrigerant charge inside the condenser has been done: when using

HCs in minichannels, the specific charge is found to less than half the charge of R134a.

• HTCs have been measured during flow boiling at 30 °C saturation temperature.

Considering also pressure drop, propylene shows better performance with respect to

R290.

• A shell-and-tube minichannel condenser and a shell-and-tube minichannel evaporator

operating with propane lead to a reduction of the refrigerant charge compared with a

commercial BPHXs without loss of performance.

Thank you for your

attention!

http://stet.dii.unipd.it/

Sustainable Thermal Energy Technologies LAB

Department of Industrial Engineering

University of Padova

Stefano Bortolin

stefano.bortolin@unipd.it