EnergyEfficient Process Cooling Process Cooling Systems Cooling systems

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: – Natural gas: – Water: $0. 10 /k. Wh $10 /mm. Btu $6 /thousand gallons

Cooling Tower • 500 -ton tower delivers 7. 5 mm. Btu/hr • Ppump = 18 k. W Pfan = 20 k. W Water = 120 gal/mm. Btu • Unit cost of cooling = $1. 22 /mm. Btu

Chillers

Water-Cooled Chiller • E/Q = 0. 8 k. W/ton = 67 k. Wh/mm. Btu • Unit cost of cooling = $6. 70 /mm. Btu

Air-Cooled Chiller • E/Q = 1. 0 k. W/ton = 83 k. Wh/mm. Btu • Unit cost of cooling = $8. 30 /mm. Btu

Absorption Chiller • E/Q = 1 Btu-heat / Btu-cooling Eff-boiler = 80% • Unit cost of cooling = $12. 50 /mm. Btu

Open-Loop Water Cooling § DT = 10 F V = 12, 000 gallons / 1 mm. Btu § Unit cost of cooling = $72 /mm. Btu

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 /mm. Btu

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

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

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

End Use: Continuous Process with Sequential Heating and Cooling Current: Qh 1 = 100 Qc 1 = 100 With HX: If Qhx = 30, Qh 2 = 70 Qc 2 = 30 HX reduces both heating and cooling loads!

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

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

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

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

End Use: Improve Heat Transfer Counter flow e = 0. 78 Cross flow e = 0. 62 NTU = 3 and Cmin/Cmax = 1 Parallel flow e = 0. 50

Cooling Product: Cross vs Counter Flow Cross Flow: e = 0. 69 • Tw 1 = 50 F • Tp = 300 F • Mcpmin = 83. 2 Btu/min-F • • Q = e mcpmin (Tp – Tw 1) = 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 – Tw 1) = 14, 352 Btu/min = 0. 78 83. 2 (300 – Tw 1) Tw 1 = 79 F

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!

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

Primary Cooling: Match Cooling Source to End Use

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

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

Primary Cooling: Use Water Cooled Chillers for Year Round Loads E/Q (Air-cooled) = 1. 0 k. W/ton E/Q (Water-cooled) = 0. 8

Primary Cooling: Stage Multiple Constant Speed Chillers

Primary Cooling: Use Variable-Speed Chiller

Ammonia Refrigeration Systems Multiple compressors, stages, evaporative condensers

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