Energy-Efficient Process Cooling

Process 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

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

Chillers 4

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

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

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

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

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

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 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: Qh1 = 100 Qc1 = 100 With HX: If Qhx = 30, Qh2 = 70 Qc2 = 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 Cross flow Parallel flow e = 0.78 e = 0.62 e = 0.50 NTU = 3 and Cmin/Cmax = 1

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 = e mcpmin (Tp – Tw1) = 14,352 Btu/min = 0.78 83.2 (300 – Tw1) Tw1 = 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 CoolSim reports number hours CT delivers target temperature. Model cooling tower performance

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

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