Comparative Analysis of Low Temperature Refrigeration Systems

Transcription

1 Comparative Analysis of Low Temperature Refrigeration Systems IRC Research and Technology Forum 2009 Pete Mumanachit Advisors Prof. Gregory F. Nellis Prof. Douglas T. Reindl 13 May 2009

2 NH 3 Compound System heat rejected HPR condenser High temperature circuit high stage compressor Work intercooler Low temperature circuit booster compressor Work evaporator refrigeration load 2

3 Why Using Ammonia? 3

4 10 2 CO 2 as an alternative to NH 3 Also a natural refrigerant Low CFM/Ton at low evaporating temperatures Very high operating pressure at high saturation temperatures CO 2 CFM/Ton 10 1 NH 3 Pressure [kpa] NH Pressure [psia] CO Evaporating temperature [F] Saturation temperature [F] 10 0 NH 3 CO 2 Taylor, 2003 ASHRAE Presentation 4

5 NH 3 /CO 2 Cascade System heat rejected condenser High temperature circuit high temp circuit compressor Work NH 3 Cascade heat exchanger CO 2 Low temperature circuit evaporator low temp circuit compressor Work refrigeration load 5

6 Operating Requirements Based on freeze-drying plant operated by Nestle UK NH 3 /CO 2 cascade system (replaced R-22) Constant freezer load of 680 Tons (2,390 kw) Evaporating temperature range below -40 o F (-40 o C) 6

7 Major System Components 7

8 System Component Models Utilize performance data from manufacturer Compressors Compressor manufacturer s selection programs Evaporative condenser and evaporator Rated capacity at design conditions Determine the conductance rate (UA) from effectiveness-ntu method Relate the change in conductance rate to the change in air-side properties at off-design conditions Cascade heat exchanger (CHE) Physics-based model is developed using principles of heat transfer 8

9 Cascade Heat Exchanger NH 3 evaporates (nucleate boiling) on the shell side CO 2 is cooled by flowing through tube bundle Two distinctive sections accommodate CO 2 cooling process De-superheating Condensing CO 2 4-pass shell-and-tube HX NH 3 9

10 Cascade Heat Exchanger Model Based on geometry of actual heat exchanger located in US Relate UA to thermal resistances in each HX section Condensing section pinch-point temperature difference De-superheating section effectiveness-ntu Translate required UA into a physical size Utilize the same size and geometry for off-design conditions Pinch-point temperature difference influences CHE size 10

11 System Performance Optimization Intermediate condition is varied to maximize COP Simplified optimization method Specify a range of typical head pressures ( psia) Min/Max function in EES Optimum intermediate condition is curve-fitted as a biquadratic function A function of evaporating temperature and head pressure Coefficient of performance (COP) Intermediate temperature [K] 11

12 System Performance Comparison Coefficient of performance (COP) cascade system compound system (simplified method) 135 psia 160 psia 190 psia Break-even temperature Evaporating temperature [F] Point of equal performance efficiency (COP) Cascade system is more efficient below this temperature Compound system is more efficient above this temperature Break-even temperature is dependent on condensing pressure 12

13 Integrated System Model baseline of 12-month Simulation Parameters Description System Input(s) 8670-hour TMY2 weather data Locations Miami, FL., Madison, WI., Los Angeles, CA., Houston, TX. Mode(s) of operation 8 hr/day (2,920 hr/yr) and 10 hr/day (3,650 hr/yr) Head pressure limit Variable based on weather but with a 120 psig (135 psia) minimum Cascade pinch-point 10 o F at design conditions Evaporator heat load 680 Tons (constant) Evaporating temperatures -40 o F to -65 o F (in steps of 5 o F) Locations are selected based on climate Combinations of temperature and humidity Head pressure is allowed to float with ambient weather 120 psig (135 psia) represents a typical set-point in industry Yearly energy usage results are used to calculate operating cost 13

14 Annual Energy Usage Comparison Cascade system Compound system Annual energy usage [kwh] Miami Houston Los Angeles Madison Annual energy usage [kwh] Miami Houston Los Angeles Madison Evaporating temperature [F] Evaporating temperature [F] Energy usage trend follows Local climate (wet-bulb temperature) System COP Energy usage in Los Angeles and Madison is close despite climates Attributes to minimum head pressure limit 14

15 Operating Cost Comparison Energy usage for Houston, TX. Total operating cost for Houston, TX Energy usage [kwh] cascade system compound system (simplified method) Total operating cost [$] cascade system compound system (simplified method) Evaporating temperature [F] Evaporating temperature [F] Energy usage trend reflects system COP behavior Average electricity cost is 0.06$/kWh (EIA, 2008) 15

16 Operating Cost Comparison (cont.) Assuming equal life-cycle cost LCC cascade = LCC compound More efficient system will have: Operating cost savings at the end of the life cycle Allowable first cost advantage (Premium difference) Defined with respect to the cascade system (cascade - compound) Positive difference = advantage for compound system Negative difference = advantage for cascade system 16

17 Premium Difference by Location 8-hr day mode 10-hr day mode Premium difference [$] Miami Houston Los Angeles Madison Premium difference [$] Miami Houston Los Angeles Madison Evaporating temperature [F] Evaporating temperature [F] The premium difference is negative below the break-even temperature Cascade system has an operating cost advantage Premium difference increases with System COP Energy usage 17

18 Effect of Cascade Approach Temperature on Premium Difference Average head pressure of 160 psia (8-hr day mode) Premium difference [$] F 8 F 5 F Evaporating temperature [F] Cascade system becomes more efficient with smaller approach Premium difference increases Break-even temperature shifts to higher temperature Capital cost must be estimated to evaluate economic feasibility 18

19 Capital Cost Estimation Adjusted capital cost (ACC) Assume that common component costs are equal Condensers, evaporators, and etc. Compressor cost Based on aggregate CFM in $/CFM (EPD, 1996) Cascade heat exchanger cost Cost correlation from industrial survey (Lachner, 2004) Difference in system components Adjusted capital cost difference (ACD) Defined with respect to cascade system (cascade - compound) 19

20 Compressor Cost Comparison Total compressor cost [$] compound system cascade system Evaporating temperature [F] Compressor cost savings for cascade system Lower aggregated CFM to meet load Attributes to high vapor density of CO 2 20

21 Cascade Heat Exchanger Cost Cost [$] Total CHE cost Tube bundle cost Shell cost Pinch-point temperature [F] As cascade pinch-point temperature difference decreases Cascade heat exchanger cost increases dramatically Cost changes more rapidly over low pinch-point difference range 21

22 Adjusted Capital Cost Comparison 10 o F cascade approach temperature Adjusted capital cost (cascade - compound) [$] ACC compound ACC cascade Compressor cost (cascade system) CHE cost Adjusted capital difference (cascade - compound) [$] ACC compound ACC cascade ACD Evaporating temperature [F] Evaporating temperature [F] Cascade system has adjusted capital cost savings advantage Cascade heat exchanger cost is lower than compressor cost Adjusted capital cost difference (ACD) is negative with this cascade pinchpoint temperature difference 22

23 Life-Cycle Cost Savings Over the life cycle of the systems 23

24 Life-Cycle Cost Savings (cont.) Sum of operating cost savings and adjusted capital cost savings Life-cycle savings (cascade - compound) [$] ACD premium difference LCS break-even point Evaporating temperature [F] Break-even point shifts from -54 o F to -52 o F True break-even temperature Attributes to adjusted capital cost savings 24

25 Effects of Approach Temperature on Adjusted Capital Cost Difference 8 o F approach 5 o F approach Adjusted capital difference (cascade - compound) [$] ACC cascade ACC compound ACD Evaporating temperature [F] Adjusted capital difference (cascade - compound) [$] ACC compound ACD ACC cascade Evaporating temperature [F] Adjusted capital cost saving is positive throughout Cascade heat exchanger cost overwhelms compressor cost savings Cascade system no longer has capital cost savings advantage Economic disadvantage for reducing approach temperature 25

26 Effects of Cascade Pinch-Point Temperature Difference on Life-Cycle Cost Savings 8 o F pinch point 5 o F pinch point Life-cycle savings (cascade - compound) [$] LCS ACD premium difference break-even point Life-cycle savings (cascade - compound) [$] ACD LCS premium difference Evaporating temperature [F] Evaporating temperature [F] Break-even temperature shifts to lower temperatures Adjusted capital cost increase outweighs operating cost savings Pinch-point temperature difference reduction negatively impacts life-cycle cost savings No break-even temperature above -65 o F for 5 o F pinch-point 26

27 Optimal Cascade Pinch-Point Temperature Difference -51 Break-even temperature [F] Optimal cascade pinch-point temperature difference is 10 o F Operating cost saving balances with adjusted capital cost Highest break-even temperature Cascade pinch-point temperature difference [F] Cascade system can operate below -52 o F while maintaining economic benefit 27

28 Conclusion NH 3 /CO 2 cascade system is compared an ammonia compound system Detailed component models are used to perform annual system simulations Intermediate condition is optimized for maximum system efficiency (COP); optimization function is curve-fitted and integrated into the system model 28

29 Conclusion (cont.) Cascade system is more efficient at low temperatures (below the break-even point), energy usage is lower and the premium difference is higher Reducing CHE approach temperature increases COP and premium difference, but greatly increases capital cost Optimal cascade pinch-point temperature difference of 10 o F provides maximized life-cycle savings; cascade system is selected to operate below -52 o F 29

30 Future Research Account for frost accumulation on evaporator coil surfaces and different defrosting techniques and penalty associated with each system configuration Examine pressure losses and parasitic loads in system components Integrate different penalizing effects on performance, such as oil cooling and de-superheating loss, into the each compressor model. Obtain more accurate equipment cost data or estimation as well as difference in material for each refrigerant. 30

31 References Ayub, Z.H, Industrial Refrigeration and Ammonia Enhanced Heat Transfer, Preceedings of the ASME-ZSIS International Thermal Science Seminar II (CD-ROM), Slovenia, June, 2004, pp Cavallini, A., Censi G., Del Col D., Doretti L., Longo G.A., Rossetto L., In-tube Condensation of Halogenated Refrigerants, ASHRAE Transactions, 2002, paper H Coolware, Frick Inc., Duffie, J.A. and Beckman, W.A. (2006) Solar Engineering of Thermal Processes. New Jersey: John Wiley & Sons, 3 rd edition. EPD, Design of Ammonia Refrigeration Systems, University of Wisconsin-Madison short course notes, directed by Professor Douglas Reindl, (1996) Homsy, P., Ammonia/CO 2 Cascade System in a Large Freeze-Drying Plant: Lessons Learned During Installation and Commissioning, Technical Paper #10, IIAR Ammonia Refrigeration Conference, Albuquerque, New Mexico, 2003, pp

32 References (cont.) Incropera, F. and DeWitt, D. (2002) Fundamentals of Heat and Mass Transfer. New Jersey: John Wiley & Sons, 5 th edition. Lachner, B. F., (2004) The Use of Water as a Refrigerant: Impact of Cycle Modifications on Commercial Feasibility. M.S. Thesis, Mechanical Engineering, Solar Energy Laboratory, University of Wisconsin-Madison. Lawrence, J. N. (2003) Refrigeration Fundamentals Throughout History: Methods Used to Obtain Colder Temperatures, and Principles Governing Them. Literature Seminar, Department of Chemistry, The University of Alabama. Manske, K.A., (1999) Performance Optimization of Industrial Refrigeration Systems. M.S. Thesis, Mechanical Engineering, Solar Energy Laboratory, University of Wisconsin-Madison. Product Features & Engineering Brochure, Evapco Inc., Single Screw Compressors Design & Operation, VSM Bulletin No (2006), Vilter Manufacturing LLC, Cudahy, WI. Winterton, R.H.S., Int. J. Heat Mass Transfer, 41, 809,

33 Thank you 33

34 34

35 Industrial Refrigeration Overview 35

36 Today in The Food Industry 36

37 Alternative Industrial Refrigerants 37

38 NH 3 /CO 2 Cascade System as an Alternative to Ammonia Compound System Utilize the best characteristics of each refrigerant NH 3 at high temperature High operating efficiency Moderate operating pressures CO 2 at low temperature High density of vapor (low CFM/Ton) Operating pressure (limit) Maintain below critical point Reasonable design pressures 38

39 Thesis Goal Research Question: Determine the most efficient and economically viable system between NH 3 compound and NH 3 /CO 2 cascade system Research Objective: Perform comparative analysis by simulating operation using detailed system model Operating efficiency (COP) Life-cycle cost savings 39

40 Compressor Model Utilize compressor selection program Coolware by Frick Inc. Curve-fit performance data from compressor maps Capacity = f(sst,sdt) Power = f(sst,sdt) Capacity [Tons] SDT = 105 o F SDT = 95 o F SDT = 85 o F SDT = 75 o F VSM single-screw compressor, Vilter Manufacturing LLC Saturated suction temperature [F] 40

41 Condensing Section Thermal resistance network UA Correlation for condensing CO 2 Cavallini et al. (2002) Correlation for nucleate boiling ammonia Ayub (2004) hc sat, cas CO 2, sat CO 2, sat CO 2, L 0.55 NH, sat γ qcascade, sat " 3 = 1 R + R + R = & CO, sat Tube, sat NH, sat Nus = 0.05 Re Pr T sur,sat q" cascade,sat T wall,sat T ev ap,sat,htc T cascade γ = T T T x10 T C C C C D i D o R NH3,sat R tube,sat R CO2,sat 41

42 De-superheating Section Thermal resistance network UA cascade, sh = 1 R + R + R CO, sh tube, sh NH, sh 2 3 Correlation for de-superheating CO 2 Dittus-Boelter Equation (Winterton, 1998) CO, sh = CO, sh CO, sh Nus Re Pr T ev ap,sat,htc T sur,sh q" cascade,sh R NH3,sh R tube,sh R CO2,sh T wall,sh TCO,2,sh D i D o 42

43 Influence of Cascade Pinch-Point Temperature Difference on CHE Size Cascade heat exchanger size varies exponentially with pinch-point Pinch-point temperature difference dictates the UA of condensing section Surface area depends on the UA Heat flux is dependent of the surface area Convective heat transfer coefficient of ammonia depends on the heat flux Number of tubes Pinch-point temperature difference [F] 43

44 Evaporative Condenser Evaporative cooling through sensible and latent heat rejection Drawn-in ambient air stream removes evaporated water vapor Effectiveness depends on ambient air temperature and humidity Saturated air at outlet cooling water spray superheated vapor saturated liquid ambient air inlet ambient air inlet recirculation water pump 44

45 Evaporative Condenser Model Modeled after ATC-486B by Evapco Inc. Capacity is provided with heat rejection factor (HRF) Over a range of condensing and wet-bulb temperatures Determine conductance rate at design operating conditions Effectiveness is based on enthalpy change Enthalpy-based effectiveness analysis Effectiveness-NTU solution Compare predicted capacity and rated capacity over the given range of operation Curve-fit UA as a function of condensing and wet-bulb temperatures Performance can be predicted over a typical range of normal operation 45

46 Evaporator Model Modeled after a 130-kW unit by King Corporation Provides cooling and de-humidification of air inside refrigerated space Absorbs sensible and latent energy Enthalpy-based effectiveness analysis Utilize effectiveness-ntu solution to determine conductance rate at the nominal (design) condition Relate the change in conductance rate to the change in airside properties at off-design conditions Thermal conductivity and Reynolds number 46

47 System Head Pressure Control System head pressure is an equilibrium Condenser capacity matches with system heat rejection Float with ambient condition (wet-bulb temperature) Head pressure dictates energy usage of the system Condenser fan motor speed control Variable Frequency Drive (VFD) Normally operates at full speed De-rates when condenser has excess capacity Minimum head pressure limit 120 psig (135 psia) 47

48 Frequency of occurrence study Wet-bulb temperature bins Madison, WI. Head pressure bins Hours of occurrence entire year 8-hr day mode 10-hr day mode Hours of occurrence entire year 8-hr day mode 10-hr day mode Wet-bulb temperature [F] Head pressure [psia] High frequency of occurrence in low wet-bulb temperature High frequency in lowest head pressure bin Frequency-weighted wet-bulb temperature and head pressure are close to their yearly average value 48

49 Frequency of occurrence study (cont.) Wet-bulb temperature [F] Wet-bulb temperature [F] Head pressure [psia] entire year (arithmetic mean) 8-hr day mode (w eighted average) 10-hr day mode (w eighted average) Madison Houston Los Angeles Miami Energy usage [kwh] Madison Los Angeles Houston Miami without head pressure limit with head pressure limit Wet-bulb temperature [K] Wet-bulb temperature [K] High frequency of occurrence in low wet-bulb temperature Average head pressure does not correlate well with average wet-bulb temperature Minimum head pressure limit (135 psia) bounds head pressure 49

50 Economic Parameters in P 1, P 2 Method Affected multiplier Economic parameter Value P 1 Fuel inflation rate 5.5 % P 2 General interest rate 2.5 % Down payment fraction 20 % Mortgage interest rate 7.5 % Term of loan 20 years Depreciation lifetime 20 years Property tax 3.5 % Salvage value fraction 20 % Maintenance cost fraction 5 % P 1 and P 2 Period of analysis 20 years Discount rate 5.25 % Effective tax rate 40% 50

51 Effects of head pressure 8-hr day mode 10-hr day mode Premium difference [$] psia 180 psia 170 psia 160 psia 150 psia 140 psia 135 psia Premium difference [$] psia 180 psia 170 psia 160 psia 150 psia 140 psia 135 psia Evaporating temperature [F] Evaporating temperature [F] Premium difference varies slightly with head pressure change Results can be used to represent a location based on average head pressure 51

52 Effects of economic parameters Economic parameter Value ± 20% δp 1 δp 2 δ FC Fuel inflation rate ± % 0% 14.82% Down payment fraction 0.2 ± % 0% 0% General interest rate ± % 4.71% 4.15% Mortgage interest rate ± % 2.48% 2.18% Term of loan 20 ± 4 0% 2.54% 2.24% Depreciation lifetime 20 ± 4 0% 0.79% 0.7% Property tax ± % 3.97% 3.5% Salvage value fraction 0.2 ± % 0.05% 0.04% Maintenance cost fraction 0.05 ± % 8.10% 7.14% Discount rate ± % 12.48% 0.57% Period of analysis 20 ± % 35.72% 5.21% Effective tax rate 0.4 ± % 29.15% 0.01% Fuel (electricity) cost 0.06 ± % 0% 59.44% 52

53 Compressor Cost Prediction Reciprocating compressor Screw compressor Cost/CFM [$] $/CFM = CFM Cost/CFM [$] $/CFM = CFM Installed CFM [ft 3 /min] Installed CFM [ft 3 /min] Cost correlation based on aggregate CFM (EPD, 1996) Reciprocating compressor is a relatively cheaper technology Lower $/CFM 53

54 CHE Cost Prediction Correlation based on industrial survey (Lachner, 2004) CHE cost = shell cost + tube bundle cost $ cost = A L ft shell, cas 3 shell shell $ cost = 0.91 N L ft tube, cas tube, cas tube, pass Tube bundle cost depends on pinch-point temperature difference 54