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**Energy-Efficient Process Cooling**

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**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

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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 4

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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**

DT = 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**

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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**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!

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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**

Counter flow Cross flow Parallel flow e = 0.78 e = 0.62 e = 0.50 NTU = 3 and Cmin/Cmax = 1

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**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) = (300 – 50) Q = 14,352 Btu/min Counter Flow: e = 0.78 Q = e mcpmin (Tp – Tw1) = 14,352 Btu/min = (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**

CoolSim reports number hours CT delivers target temperature. Model cooling tower performance

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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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