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Energy-Efficient Process Cooling

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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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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

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Chillers

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

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

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

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

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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

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

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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

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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

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

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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

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

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

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

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End Use: Improve Heat Transfer NTU = 3 and Cmin/Cmax = 1 = 0.78 = 0.62 = 0.50 Counter flowCross flowParallel flow

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Cooling Product: Cross vs Counter Flow Cross Flow: = 0.69 Tw1 = 50 F Tp = 300 F Mcp min = 83.2 Btu/min-F Q = mcp min (Tp – Tw1) = 0.69 83.2 (300 – 50) Q = 14,352 Btu/min Counter Flow: = 0.78 Q = 14,352 Btu/min Tp = 300 F Mcp min = 83.2 Btu/min-F Q = mcp min (Tp – Tw1) = 14,352 Btu/min = 0.78 83.2 (300 – Tw1) Tw1 = 79 F

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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!

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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

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Primary Cooling: Match Cooling Source to End Use

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

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

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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

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Primary Cooling: Stage Multiple Constant Speed Chillers

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Primary Cooling: Use Variable-Speed Chiller

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Ammonia Refrigeration Systems Multiple compressors, stages, evaporative condensers

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Ammonia Refrigeration Savings Opportunities Reclaim heat Variable head-pressure control

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