Control of FGB (Flash Gas Bypass) in Start-Up and Transients Mr. Nemanja Dzinic, Pega Hrnjak CTS, USA & MPG-KGH d.o.o., Serbia
Background Microchannel evaporators widely used in MAC systems and increasingly in stationary AC systems One of the issues: two phase refrigerant distribution Possible solutions: y Geometry remedies: baffles, flow constrains, etc. y Generate mist droplets flow at the evaporator inlet y Flash Gas Bypass Typical microchannel evaporator used in MAC system Two Phase Refrigerant Paper -39
Flash Gas Bypass (FGB) Concept used in industrial systems for many years: Separate & bypass flash gas Feed liquid only to MCHX Hrnjak proposed modification for MC systems: Beaver et al. 999 Elbel and Hrnjak, 3 Milosevic and Hrnjak, Tuo and Hrnjak, 3, Primary benefits Improve refrigerant distribution Lower refrigerant-side DP Effect on heat transfer FGB: Flash Gas Bypass DX: Direct Expansion
Example: Good performance demonstrated in MAC system: Original system (Toyota Camry 7) 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 modification Denso Evaporator Test Evaporator (two slabs, multi-pass) (single slab, single pass) Unit Tube Length Overall width No. of Tubes Depth Fin height Fin pitch Fin thickness Slabs Air side area mm mm mm mm mm mm m Denso evap. 7 39 3.. 3. (single slab, single pass) Test conditions Evaporator Specifications Items Test Evaporator with transparent headers Test evap. 33 3...9 Indoor Chamber Air inlet temperature 3 ºC Air flow rate.3 m3s- Dry condition Outdoor Chamber Air inlet temperature 3 ºC Air flow rate.3 m3s-
COP and Q improvements at 9 rpm FGB DX (b) DX mode
COP improvements at the same Q FGB DX Started at 9 rpm DX baseline FGB ran at only rpm Increased compressor speed for both systems at the same Q
Fixed compressor speed (9 rpm) Compressor inlet superheat ºC Compressor inlet pressure ~ 3kPa Tevap ~3. ºC DPevap DPIHX DX COP Q [kw].. Wcomp [kw]. Pevap [kpa]. PIHX [kpa] eisen 7..
Matched capacity (Q.9 kw) Compressor inlet superheat ºC Condensation Pressure ~ kpa Tevap ~7. ºC DPevap DPIHX Compressor inlet pressure ~ kpa DX COP Q [kw].3.3 Wcomp [kw].3 Pevap [kpa] 7 PIHX [kpa] eisen 79.7
Evaporator effectiveness is increased Q ε = Qmax Qmax = Ca (Teai Teri ) Teai: evaporator air inlet Teri: evaporator refrigerant inlet
Refrigerant distribution improved Compressor speed 9 rpm M Havg Hi N Average temperature Distribution factor* *(Bowers, Wujek, Hrnjak, )
Interim conclusions 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 But: Smaller evaporator amplified positive effects of FGB: Qualitative illustration Distributor could have improved performance in DX mode So, when we have better performance questions are: Can this system work in heat pump mode (reversible system)? How to control it?
FGB can easily work in reversible mode For conventional reversible systems -way valves are used For FGB a way valve should be modified FGB can use IHX and only one EV while conventional need two
Prototype FGB Reversible system AC mode HP mode Switching from AC to HP mode so far is controlled manually with eight ball valves new valve in development In the future one -way valve will replace manually controlled ball valves
The facility for exploration of reversing operation and transients developed FGB separator Denso compressor 9 cm3 TYI 3-pass condenser 33 tubes
We are exploring visually: Separation in the vessel Charge migration in transients vapor to suction line Bypass valve xxo Mr EEV valve Mr FGB separator liquid to evaporator (-xxo) Mr
Visualisation : Webcams Two webcams were installed on the system to record the system performance from the sightglass of the separator and evaporator outlet. HD p Webcam C. Logitech Record: Separator. Logitech HD p Webcam C Record: Evaporator Outlet
Control strategy: EV controls subcooling, BV superheat At this point, subcooling and superheat at compressor suction constant @ K Later, COP maximizing subcooling and superheat PID parameters P I D Subcooling controller.3 EEV. Superheat controller. for CDS-. for CDS-
Effects of Bypass Valve Size CDS is stepper motor operated conditions CDS- smaller Test vs. CDS- larger pressure controller 3 3 Face Velocity Temp ( C) Mass Flow (L/s) Setup Value Setup Value for for Subcooling Superheat fans on, V 7 fans on, 7 opening EEVV and bypass valve 7 3 Time (min) Superheat of compressor inlet and evaporator outlet Restriction of CDS- is too high even when fully open causing too low superheat Superheat ( C) Temp ( C) Evaporator 9 9 9 Condenser CDS- CDS- EEV/Bypass valve opening (%) Compressor Test Speed Name (rpm) 9 7 3 Time (min)
EEV is well selected Subcooling is maintained at set value, but opening of BV (CDS-) is %, so we should go for higher capacity (CDS) EEV and bypass valve opening Time (min) Subcooling of condenser outlet Subcooling ( C) 9 7 3 EEV/Bypass valve opening (%) 9 7 3 Time (min)
System performance under different test conditions So bypass valve is changed from CDS- to CDS- size The CDS- at % stroke ( steps open) is close to the same flow of a CDS- at % stroke ( steps open). Since the CDS- s opening would be bigger than CDS- s for each step, for the CDS- case, the Kp valve of superheat PID controller should be smaller, theoretically. Test conditions from test matrix Test Name Condenser Compressor Speed (rpm) Temp ( C) Face Velocity Evaporator I3a 9 3 fans on, V Temp ( C) I3a 9 3 fans on, V Mass Flow (L/s) Subcooling Superheat Charge (g)
I3a vs. I3a test condition 3 Time (min) 9 7 3 Air temperature of Evaporator 9 Time (min) 7 3 Air temperature of evaporator ( C) 3 Air temperature of condenser ( C) Air temperature of Condenser
EEV and bypass valve opening Time (min) 9 7 3 Subcooling of condenser outlet 9 7 3 Subcooling ( C) EEV/Bypass valve opening (%) Subooling and superheat control Time (min) superheat ( C) Superheat of compressor inlet and evaporator outlet - 9 7 3 Time (min)
High liquid level reduce separation I3a separator I3a separator
3 Qevap/Wcp (W) Cooling Capacity and Compressor Work 9 7 3 Time (min) COP 3 COP Cooling capacity, compressor work and COP 9 7 3 Time (min)
Conclusions We demonstrated earlier that FGB improves performance (capacity and efficiency) by reducing pressure drop and improving distribution Transients (start-ups, shut-downs, cycling, ) are manageable and start-ups are presented with selected characteristics of PID controller Analyses of start-ups indicated good sizes of by-pass and EXV valve We are working on performance maximizing control strategies Hope that next year we will have new results to present