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Energy-Efficient Process Cooling

Energy-Efficient Process Cooling. Process Cooling Systems . Cooling systems Cooling tower Water-cooled chiller Air-cooled chiller Absorption chiller Compressed air cooling Cooling costs assume: Electricity: $0.10 /kWh Natural gas: $10 /mmBtu Water: $6 /thousand gallons.

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Energy-Efficient Process Cooling

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  1. Energy-Efficient Process Cooling

  2. Process Cooling Systems • Cooling systems • Cooling tower • Water-cooled chiller • Air-cooled chiller • Absorption chiller • Compressed air cooling • Cooling costs assume: • Electricity: $0.10 /kWh • Natural gas: $10 /mmBtu • Water: $6 /thousand gallons

  3. Cooling Tower • 500-ton tower delivers 7.5 mmBtu/hr • Ppump = 18 kW Pfan = 20 kW Water = 120 gal/mmBtu • Unit cost of cooling = $1.22 /mmBtu

  4. Chillers

  5. Water-Cooled Chiller • E/Q = 0.8 kW/ton = 67 kWh/mmBtu • Unit cost of cooling = $6.70 /mmBtu

  6. Air-Cooled Chiller • E/Q = 1.0 kW/ton = 83 kWh/mmBtu • Unit cost of cooling = $8.30 /mmBtu

  7. Absorption Chiller • E/Q = 1 Btu-heat / Btu-cooling Eff-boiler = 80% • Unit cost of cooling = $12.50 /mmBtu

  8. Open-Loop Water Cooling • DT = 10 F V = 12,000 gallons / 1 mmBtu • Unit cost of cooling = $72 /mmBtu

  9. Compressed Air Cooling • 150 scfm at 100 psig to produce 10,200 Btu/hr cooling • 4.5 scfm per hp • Unit cost of cooling = $272 /mmBtu

  10. Relative Process Cooling Costs Near order of magnitude difference in costs!

  11. Cooling Energy Saving Opportunities • Reducing end use cooling loads and temperatures • Add insulation • Add heat exchangers • Improve heat transfer • Improving efficiency of distribution system • Reducing friction using large smooth pipes • Avoiding mixing • Employing variable-speed pumping • Improving efficiency of primary cooling units • Use cooling tower when possible • Use water-cooled rather than air-cooled chiller • Use variable speed chillers

  12. End Use: Add Insulation • Insulation: • Reduces heat transfer into cooled tanks & piping • Decreases exterior condensation • Even at small temperature differences insulating cold surfaces is generally cost effective

  13. End Use: Continuous Process with Sequential Heating and Cooling Current: Qh1 = 100 Qc1 = 100 With HX: If Qhx = 30, Qh2 = 70 Qc2 = 30 HX reduces both heating and cooling loads!

  14. End Use: Batch Processes with Discrete Heating and Cooling Cost effective to transfer heat between processes, whenever the processes that need cooling are 10 F higher than the process that need heating

  15. End Use: Batch Processes with Discrete Heating and Cooling Add Heat Exchangers T = 145 F Requires Cooling T = 120 F Requires Heating

  16. End Use: Optimize Heat Exchanger Network (Pinch Analysis) For multiple heating and cooling opportunities, optimize heat exchanger network using Pinch Analysis.

  17. End Use: Improve Heat Transfer Cross flow cooling of extruded plastic with 50 F chilled water from chiller

  18. End Use: Improve Heat Transfer Counter flow Cross flow Parallel flow e = 0.78 e = 0.62 e = 0.50 NTU = 3 and Cmin/Cmax = 1

  19. Cooling Product: Cross vs Counter Flow Cross Flow: e = 0.69 • Tw1 = 50 F • Tp = 300 F • Mcpmin = 83.2 Btu/min-F • Q = e mcpmin (Tp – Tw1) = 0.69 83.2 (300 – 50) • Q = 14,352 Btu/min Counter Flow: e = 0.78 • Q = 14,352 Btu/min • Tp = 300 F • Mcpmin = 83.2 Btu/min-F • Q = e mcpmin (Tp – Tw1) = 14,352 Btu/min = 0.78 83.2 (300 – Tw1) • Tw1 = 79 F

  20. Cooling Product: Cross vs Counter Flow Cooling towers can deliver 79 F water much of the year using 1/10 as much energy as chillers!

  21. Distribution System: Avoid Mixing Separate hot and cold water tanks Lower temperature, less pumping energy to process Higher temperature, less fan energy to cooling tower

  22. Primary Cooling: Match Cooling Source to End Use

  23. Primary Cooling: Use Cooling Tower When Possible Cooling towers can deliver water at about outside air temperature

  24. Primary Cooling: Use Cooling Tower When Possible CoolSim reports number hours CT delivers target temperature. Model cooling tower performance

  25. Primary Cooling: Use Water Cooled Chillers for Year Round Loads E/Q (Air-cooled) = 1.0 kW/ton E/Q (Water-cooled) = 0.8 kW/ton

  26. Primary Cooling:Stage Multiple Constant Speed Chillers

  27. Primary Cooling: Use Variable-Speed Chiller

  28. Ammonia Refrigeration Systems Multiple compressors, stages, evaporative condensers

  29. Ammonia Refrigeration Savings Opportunities • Reclaim heat • Variable head-pressure control

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