5Water-Cooled Chiller E/Q = 0.8 kW/ton = 67 kWh/mmBtu Unit cost of cooling = $6.70 /mmBtu
6Air-Cooled Chiller E/Q = 1.0 kW/ton = 83 kWh/mmBtu Unit cost of cooling = $8.30 /mmBtu
7Absorption Chiller E/Q = 1 Btu-heat / Btu-cooling Eff-boiler = 80% Unit cost of cooling = $12.50 /mmBtu
8Open-Loop Water Cooling DT = 10 F V = 12,000 gallons / 1 mmBtuUnit cost of cooling = $72 /mmBtu
9Compressed Air Cooling 150 scfm at 100 psig to produce 10,200 Btu/hr cooling4.5 scfm per hpUnit cost of cooling = $272 /mmBtu
10Relative Process Cooling Costs Near order of magnitude difference in costs!
11Cooling Energy Saving Opportunities Reducing end use cooling loads and temperaturesAdd insulationAdd heat exchangersImprove heat transferImproving efficiency of distribution systemReducing friction using large smooth pipesAvoiding mixingEmploying variable-speed pumpingImproving efficiency of primary cooling unitsUse cooling tower when possibleUse water-cooled rather than air-cooled chillerUse variable speed chillers
12End Use: Add Insulation Reduces heat transfer into cooled tanks & pipingDecreases exterior condensationEven at small temperature differences insulating cold surfaces is generally cost effective
13End Use: Continuous Process with Sequential Heating and Cooling Current:Qh1 = 100Qc1 = 100With HX:If Qhx = 30,Qh2 = 70Qc2 = 30HX reduces both heating and cooling loads!
14End 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
15End Use: Batch Processes with Discrete Heating and Cooling Add Heat ExchangersT = 145 FRequires CoolingT = 120 FRequires Heating
16End Use: Optimize Heat Exchanger Network (Pinch Analysis) For multiple heating and cooling opportunities, optimize heat exchanger network using Pinch Analysis.
17End Use: Improve Heat Transfer Cross flow cooling of extruded plastic with 50 F chilled water from chiller