investigation on effects of piping on heating …1 1 ziaili department of building science, tsinghua...

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Ziai LiDepartment of Building Science, Tsinghua University, Beijing

May 16, 2017

Investigation on effects of piping on heating performance of multi-split variable refrigerant

flow system

12th IEA Heat Pump Conference, 15th-18th May, 2017, Rotterdam

2

Contents

Background

Motivation and method

Results and discussion

Conclusions and outlook

3

Multi-split variable refrigerant flow (VRF) system

Types: Air-cooled heat pump type, heat recovery type, water-cooled type, gas engine heat pump type, etc.Configuration: ODU up to 70kW, IDU number up to 60, outdoor-to-indoor pipeline length up to 149m, height difference up to 46m

ü Space saving ü Fewer transportation loss ü Efficient part-load operationü Individual control of indoor unitsü Easy maintenance and management

Outdoor units

Indoor units

Refrigerant pipe

Office Hotel VillaHospital

• Widely applied in east Asian and European countries• Sales of VRF units shared larger than 40% of China

market of central air-conditioning products in 2016

4

0.00.51.01.52.02.53.03.54.0

0% 20% 40% 60% 80% 100%

Syst

em C

OP

Part-loadratio

Firstfloor

Fifthfloor

Seventhfloor

FourthfloorSeventh floor: average COP =3.32 Fifth floor: average COP=3.11First floor: average COP=2.92

Issue about piping of VRF system in application

Field cooling performance of multi-split VRF systems in one office building in Beijing, 2005 (Note: applying centralized control method)

Data resource: Department of building science , Tsinghua University. Reports on public building energy saving diagnosis in the summer of 2005.

H Lpipe=L1+L2

L1

L2

Refrigerant pipe

Performance degradation caused by piping (L and H) in application

Concern on piping effect on VRF system performance

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Issue about piping of VRF system in application

Performance degradation caused by piping (L and H) in application

• Cooling mode: Lpipe ↑ à Psuc ↓ and Tshsuc ↑ à ηvcp ↓, mrcp ↓, Qe ↓ and COP ↓• Heating mode: Lpipe ↑ à Tcond ↓ à Qc↓ and COP ↓• Cooling/Heating mode: H↑à Insufficiency of ΔP of indoor EEVs or overpressure at

the entry of indoor EEVs

1

1’

23

3’

4

ΔPLP

ΔPGP

R410A, cooling mode

ΔPEEV1~N

ΔPEEV0

1

2’

23

4

5

ΔPLP

ΔPGP

R410A, heating mode

4’

ΔPEEV1~N

ΔPEEV0

6

Research status of piping effect of VRF system Reference Approach Control method Condition Highlights

W. Shi, et al (2007)

Simulation, R22

Constant outlet superheat of evaporators

Rated cooling/heatingcondition (given fixed compressor speed)

The cooling and heating COP decrease 25% and 11% respectively at pipe length of 100m

K. Zheng, et al (2007)

Field test, R22 Not mentioned Field cooling/heating

condition

The cooling capacity degradation is mainly caused insufficiency of ΔP for indoor EEV caused by pressure drop along liquid pipe while the heating capacity degradation is mainly caused by the heat leakage to environment along gas pipe

Z. Guo(2008)

Experiment, R410A Not mentioned

Variable cooling conditions (given differentactive indoor units)

The effect of pipe length on performance under part-load cooling conditions is smaller than that under full load condition

X. Wang (2010)

Simulation, R410A

Constant suction pressure and continuous operation of indoor units

Rated cooling condition (given certain cooling capacity)

The cooling COP decreases as pipe length increases without bypass loop while it is almost not affected by pipe length when suction superheat is controlled by bypass loop

D. Zhou, et al (2011)

Simulation,R410A

Constant suction superheat and continuous operation of indoor units

Rated cooling condition (given certain cooling capacity)

The effect of pipe length on performance under part-load cooling conditions is less than that under full load condition

Y. Pan et al. （2012）

Simulation, R22


Rated cooling condition (given fixed compressor speed)

The highest COP can be obtained when indoor units are located equally from the outdoor unit

Z. Li, et al (2016)

Simulation, R410A


Rated cooling condition (given fixed compressor speed)

The cooling capacity reduces by 14% and the COP reduces by 15% as the main pipe length increases from 10 m to 190 m

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Research status of piping effect of VRF system

Brief summary

• The available simulation work focuses on pipe length effect on the cooling performance of VRF system given fixed compressor speed. There is lack of study on the piping effect on its heating performance given various heating load

• The control method of the simulated systems is either indefinite or unpopular currently

• The simulation assumed a constant subcooling degree of condenser exit instead of using refrigerant mass charge conservation under part-load conditions

• The experiment and field test study need more detailed information of control method of the tested system to analyse the piping effect under various operation conditions

This study works on the effect of pipe length and height difference on VRF system performance under part-load heating conditions with the current heating control method based on simulation

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Contents

Background




9

Current heating control method of VRF systems

Compressor

Gas-liquid separator

Four-way valve

Outdoor heat exchanger

EEV0 EEV1

EEV2

EEVN

Indoor heat exchanger #1

…

Indoor heat exchanger #2

Indoor heat exchanger #N

Outdoor unit Indoor unit

Main liquid pipe

Main gas pipe

Secondary liquid pipe

Secondary gas pipe

Actuator Control Target / Set value

Indoor unit

Indoor fan User control Rated air volume

EEV1~N Automatic Target air temperature Tai =20℃& indoor HEX exit subcooling degree ΔTsc ≥ 1℃

Outdoor unit

Compressorspeed Automatic Constant discharge pressure set value

Tsat (Pdis, cp set)=54℃Outdoor fan Automatic Rated air volume

EEV0 Automatic Compressor suction superheat set value ΔTsh, suc, set

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Methodology

Model simplification

• Regardless of the effect of lubricating oil• Inlet refrigerant of gas-liquid separator is superheated and no refrigerant is

accumulated• No frosting occurred out of the outdoor heat exchanger

Establishment of steady-state heating model of VRF system

Validation of refrigerant pipe model

Component design of a VRF system

Simulation on heating performance using constant discharge pressure control method (variable pipe length and height

difference)

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Model development

[1] R. Koury, L. Machado, K. Ismail, Numerical simulation of a variable speed refrigeration system, International Journal of Refrigeration 24 (2001) 192-200.[2] Shao S, Li X, Shi W, et al. A universal simulation model of air-cooled condenser consisting of plate-fin-tube[C]//ASME 2003 Heat Transfer Summer Conference. American Society of Mechanical Engineers, 2003: 729-734.[3] S. W. Churchill, 1977. Friction-factor equation spans all fluid-flow regimes. Chemical Engineering 84: 91-92.[4] Zhou Qiangtai. Two-phase flow and heat transfer [M] . Water conservancy and hydropower press, 1990.

Variable speed compressor: Efficiency model [1]

cp th v i,cp/V vm n h=

cp cp o,cp i,cp loss( ) /h h fW m -=

o,cp s i,cpo,cp i,cp

s

|h hh h

h-

+=

v 1 i,cp o,cp, )( Pf Ph = s 2 i,cp o,cp, )( Pf Ph =

Fin-and-tube evaporator, condenser: One-dimensional distributed-parameter model [2]

EEV: Correlation for the variable area expansion device

Pressure drop along refrigerant pipeline : Two-phase flow empirical model

Single phase[3]:

Two-phase [4]:

eev i,eev i,eev o,eev( ) 2 ( )Dm C A z P Pr= -

o,eevi,eevh h= i,eev o,eev0.02005 +6.34 /DC r r=

2f

i

( )2

dP f Jdl D r

- = ×

( )1/1212 1.58 8 /f Re K -é ù= +ë û

1616 0.9

i

37530 7( ) 2.457 ln(( ) 0.27 )KRe Re D

eé ù= - +ê ú

ë û2

2L0fL0

i L

( )2fdP J

dl Df

r- = × ×

L0 8 kf c Re-= ×2 2 (2 )/2 (2 )/2 (2 )L0 1 ( 1)[ (1 ) ]q q qY bx x xf - - -= + - - +

GzL

L

( ) {1 (1 )}dP gdz

rr ar

- = × × - -

Potential pressure drop:

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Compressor

Gas-liquid separator

Condenser

EEV

Evaporator

Outdoor unit

Indoor unit

Liquid pipeGas pipeTP

TP

TP

TP

TP

T

P

Experimental data centre room air-conditioner for validation of refrigerant pipe model

• 5 HP variable-speed compressor• Indoor unit was located 30 m higher• 100-meter-long liquid pipe and a 100-meter-

long gas pipe• Wrapped with 18 mm insulation• Refrigerant mass flow rate is calculated based

the suction and discharge refrigerant state of the compressor by employing the compressor efficiency model

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Ambient temperature (oC) 45 35 25

Compressor frequency (Hz) 84 70 50 84 70 50 84 70 50

Measurement

Suction temperature (oC) 34.5 36.4 40.3 32.6 34.8 38.4 27.1 30.8 35.2

Suction pressure (MPa) 1.136 1.221 1.174 1.102 1.181 1.182 1.096 1.181 1.322

Discharge pressure (MPa) 3.612 3.478 3.205 2.953 2.818 2.567 2.631 2.547 2.418

Gas pipe pressure drop (MPa) 0.160 0.120 0.037 0.217 0.169 0.075 0.286 0.241 0.125

Liquid pipe pressure drop (MPa) 0.304 0.293 0.267 0.311 0.301 0.275 0.318 0.306 0.288

Simulation

Refrigerant mass flow rate (kg/h) 446.2 399.2 259.1 432.6 380.1 256.2 443.3 387.9 292.4

Gas pipe pressure drop (MPa) 0.155 0.121 0.043 0.191 0.148 0.074 0.227 0.173 0.092

Liquid pipe pressure drop (MPa) 0.295 0.297 0.290 0.334 0.333 0.315 0.360 0.347 0.330

Deviation of simulation

Gas pipe pressure drop -3.1% 0.8% 16.2% -12.0% -21.4% -1.3% -20.6% -19.2% -26.4%

Liquid pipe pressure drop -3.0% 1.4% 8.6% 7.4% 10.6% 14.6% 13.2% 13.4% 14.6%

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Solving procedure for VRF system heating simulation

[5] Li Z, Wang B, Li X, et al., Simulation on effects of subcooler on cooling performance of multi-split variable refrigerant flow systems with different lengths of refrigerant pipeline, Energy and Buildings, 126 (2016) 301-309.

Assuming fcp, Pi,cp, Po,MLP

Calculating mcp, Wcp, To,cp by compressor model

eh<eh' ?

End

Adjusting Pi,cp

Calculating Pi,e, hi,e, xeev0 by evaporator model and EEV model

Yes

Start

eQ<eQ' ?

Assuming mc, j, ho,SLP, j

Calculating Po,c, j, ho,c, j, xeevj by condenser model and EEV model

j=N？

emass<emass' ?

j=j+1

Adjusting Po,MLP

Inputting system configuration, operating condition, Mass_charge, ΔTsho,e,set, Po,cp,set, Qc,1~Qc,N

Calculating Pi,MLP, hi,MLP, Po,MGP, ho,MGP by liquid refrigerant pipe and gas

refrigerant pipe models, j=1

Adjusting mc, j

YesNo

No

YesNo

Yes

No

No

Calculating Pi,SLP,j, hi,SLP,j, Po,SGP,j, ho,SGP,j by liquid refrigerant pipe and gas

refrigerant pipe models

em<em' ?Adjusting fcp

Yes

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Contents

Background




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Configuration of VRF system for simulation

Compressor

Displacement, Vth 85 cm3/revRefrigerant R410A

Quantity 1Rotation speed 20~100 rps

Rated nominal cooling

performance (pipe

length=10m)

Rated rotation speed 60 rpsRefrigerant charge (circulating) 3.0 kg

Rated cooling capacity 28 kW

Rated suction & discharge pressure Tsat,suc=5.2 ℃, Tsat,dis=47.6℃

Indoor HEXRated cooling capacity 7.0 kW

Quantity 4

Outdoor HEXRated heat exchange capacity 35.4 kW

Quantity 1EEV Nominal diameter of EEV0/EEV1~N 6.0 mm/ 2.4 mm

Main pipe External diameter Do of main liquid/ gas pipe 12.7mm(liquid), 25.4mm(gas)

Addition of refrigerant charge 0.11 kg/m

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0 30 60 90 120 150 18030

40

50

60

70

80Compressor frequency

Qc=26.0kW Qc=23.6kW Qc=21.2kW Qc=18.8kW

Com

pres

sor f

requ

ency

(Hz)

Length of main gas pipe and main liquid pipe (m)

3.20

3.25

3.30

3.35

3.40

3.45

Inlet pressure of condensers Qc=26.0kW Qc=23.6kW Qc=21.2kW Qc=18.8kW

Pres

sure

(MPa

)0 30 60 90 120 150 180

3.0

3.2

3.4

3.6

3.8

4.0COP


CO

P (W

/W)

Length of main gas pipe and main liquid pipe (m)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Suction saturation temperature of compressor


Tem

pera

ture

(o C)

Effect of main pipe length (Tai=20℃, Tao=7℃, RHao=50%, Tsat (Pdis, cp set)=54℃)


As LMGP and LMLP increased from 5 m to 165 m, • Compressor frequency increases by 3~5 Hz to output the same heating capacity• Te reduces by about 0.5 oC• Pin of condensers reduces by 0.068 MPa (about 1oC of Tc drop) at Qc=26kW• COPcomp decreases from 3.28 to 3.11 at Qc=26 kW

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Effect of outdoor-to-indoor height difference (Tai=20℃, Tao=7℃, RHao=50%, Tsat(Pdis, cp set)=54℃)

-60 -40 -20 0 20 40 6030

40

50

60

70

80

90

100Compressor frequency


Com

pres

sor f

requ

ency

(Hz)

Height difference between the outdoor unit and indoor units (m)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5Pressure difference of EEV0


Pres

sure

diff

eren

ce(M

Pa)

-60 -40 -20 0 20 40 601.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

COP Qc=26.0kW Qc=23.6kW Qc=21.2kW Qc=18.8kW

CO

P (W

/W)

Height difference between the outdoor unit and indoor units (m)

0

5

10

15

20

25

30

35

40

Outlet subcooling degree of condensers Qc=26.0kW Qc=23.6kW Qc=21.2kW Qc=18.8kW

Subc

oolin

g de

gree

(o C)


• Compressor frequency and COPcomp show little change (tiny decrease of COPcompcaused by length increase) while ΔTsc of condensers increase slightly

• Available ΔP for EEV0 decreased markedly (decreased from 2.25 MPa to 1.55 MPa at Qc=26 kW), affecting adjustability and system stability

19

Contents

Background




20


Conclusions

• A model has been established for heating operation of multi-split VRF system applicable to analysis of piping effect and control method• According to simulation of a designed VRF system under the constant discharge pressure control method:

(1) Lengthening of horizontal pipe will reduce COP apparently in heating mode even under part-load conditions

(2) Height difference in range from -50 m to 50 m had low effect on COP but it would affect the available pressure drop of EEVs

Outlook

• Application of the developed model to study the effect of piping under more operation conditions and different control methods, aiming at a comprehensive evaluation on the application of multi-split VRF systems in buildings

2121

THANKS FOR YOUR ATTENTION !

Email: [email protected]

investigation on effects of piping on heating …1 1 ziaili department of building science, tsinghua...

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