Application of FGB Concept in Reversible Cooling & AC Systems

45th KGH Congress, Belgrade 3-5 Dec 2014

Nemanja DzinicUniversity of Illinois/University of BelgradeMPG-KGH Belgrade , CTS USA

Pega HrnjakCo-Director ACRC, Res. Professor, University of Illinois, Urbana-Champaign, USAPresident, CTS

Background• Microchannel evaporators widely used in MAC system• One of the issues: two phase refrigerant distribution• Possible solutions:

yGeometry remedies: baffles, flow constrains, etc.

yGenerate mist droplets flow at the evaporator inlet

yFlash Gas Bypass

Paper 01-0139

Two Phase Refrigerant

Typical microchannel evaporator used in MAC

system

Flash Gas Bypass (FGB)

FGB: Flash Gas BypassDX: Direct Expansion

Concept used in industrial systems for many years: • Separate & bypass flash gas• Feed liquid only to MCHX

Hrnjak proposed for MC systems:• Beaver et al. 1999• Elbel and Hrnjak, 2003• Milosevic and Hrnjak, 2011• Tuo and Hrnjak, 2013, 2014

• Primary benefits • Improve refrigerant distribution• Lower refrigerant-side DP• Effect on heat transfer

Example: Good performance demonstrated in MAC system:Original system (Toyota Camry 2007) had to be modified• Same compressor and condenser• Evaporator had to be modified: flash gas bypass requires

single pass design to preserve initial distribution

• We had difficulties to get single pass evaporator in the same size as original Denso evaporator

Evaporator SpecificationsItems Unit Denso

evap.Test evap.

Tube Length mm 255 255Overall width mm 267 233No. of Tubes - 39 23Depth mm 32 20Fin height mm 5 8Fin pitch mm 1.25 1.56Fin thickness mm 0.05 0.1Slabs - 2 1Air side area m2 3.25 1.49

Denso Evaporator(two slabs, multi-pass)

Test Evaporator(single slab, single pass)

Evaporator modification

Indoor Chamber

Air inlet temperature 35 ºC

Air flow rate 0.203 m3s-1

Dry condition

Outdoor Chamber

Air inlet temperature 35 ºC

Air flow rate 0.342 m3s-1

Compressor speed 900 rpm (baseline)

Test conditions

Test Evaporator with transparent headers

(single slab, single pass)

COP and Q improvements at 900 rpm

FGB

(b) DX modeDX

COP improvements at the same Q

FGB

DX

§ Started at 900 rpm DX baseline

• FGB ran at only 500 rpm• Increased compressor

speed for both systems at the same Q

Fixed compressor speed (900 rpm)

Compressor inlet pressure ↑~ 31kPaTevap ↑~3.5 ºC

  COP Q [kW] Wcomp [kW]

∆Pevap [kPa]

∆PIHX [kPa] eisen

DX 2.11 2.64 1.00 11.5 27.4 0.62FGB 2.35 3.07 1.08 7.9 48.2 0.64

DPevap

DPIHX

Compressor inlet superheat 25ºC

Matched capacity (Q≈2.9 kW)

Compressor inlet pressure↑~ 140kPa

  COP Q [kW] Wcomp [kW]

∆Pevap [kPa]

∆PIHX [kPa] eisen

DX 1.53 2.83 1.83 17 79 0.57FGB 2.80 2.88 1.00 6 33 0.62

Tevap ↑~7.8 ºC

DPevap

DPIHX

Condensation Pressure

↓ ~140 kPa

Compressor inlet superheat 22ºC

Evaporator effectiveness is increased

Teai: evaporator air inlet Teri: evaporator refrigerant inlet

max

QQ

ε = max ( )a eai eriQ C T T= −

Refrigerant distribution improved

 

HiHavg

N

M

Distribution rating parameter*

*(Bowers, Wujek, Hrnjak, 2010)

Compressor speed 900 rpm

Interim conclusions

• So, when we have better performance questions are:• Can this system work in heat pump mode?• How to control it?

But: • Smaller evaporator amplified positive effects

of FGB: • Qualitative illustration

• Distributor could have improved performance in DX mode

• System COP and capacity significantly improved by FGB due to:• More uniform distribution• Lower refrigerant-side DP (matched Q)

• Evaporator has better performance in FGB than in DX:• Higher effectiveness• Lower pressure drop

The facility for exploration of reversing operation and transients developed

TYI 3-pass condenser 33 tubes

Denso compressor 90 cm3

FGB separator

Operation diagram

Reversible FGB system

• It is possible to reverse the cycle while using only one EXV and IHX

4way valve

Hrnjak, 1999

We are exploring visually:

Bypass valve

EEV valve

vapor to suction line

FGB separator

liquid to evaporator

(1-xxo)∙ Mr

Mr

xxo∙Mr

• Separation in the vessel• Charge migration in transients

Is bypass valve really needed – first results• Same conditions - different set up of bypass

valve ( 900 rpm , 25 C ambient, AFRe 150 l/s, AFRc 3 m/s)

Conclusions

• We have demonstrated that FGB improves performance (capacity and efficiency) by reducing pressure drop and improving distribution

• The reversible operation (a/c to heat pump) is possible

• Transients (start-ups, shut-downs, cycling, …) are manageable

• We are working on performance maximizing control strategies

• Hope that next year we will have new results to present