5 Water-Cooled Chiller E/Q = 0.8 kW/ton = 67 kWh/mmBtu Unit cost of cooling = $6.70 /mmBtu
6 Air-Cooled Chiller E/Q = 1.0 kW/ton = 83 kWh/mmBtu Unit cost of cooling = $8.30 /mmBtu
7 Absorption Chiller E/Q = 1 Btu-heat / Btu-cooling Eff-boiler = 80% Unit cost of cooling = $12.50 /mmBtu
8 Open-Loop Water Cooling DT = 10 F V = 12,000 gallons / 1 mmBtuUnit cost of cooling = $72 /mmBtu
9 Compressed Air Cooling 150 scfm at 100 psig to produce 10,200 Btu/hr cooling4.5 scfm per hpUnit cost of cooling = $272 /mmBtu
10 Relative Process Cooling Costs Near order of magnitude difference in costs!
11 Cooling 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
12 End Use: Add Insulation Reduces heat transfer into cooled tanks & pipingDecreases exterior condensationEven at small temperature differences insulating cold surfaces is generally cost effective
13 End 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!
14 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
15 End Use: Batch Processes with Discrete Heating and Cooling Add Heat ExchangersT = 145 FRequires CoolingT = 120 FRequires Heating
16 End Use: Optimize Heat Exchanger Network (Pinch Analysis) For multiple heating and cooling opportunities, optimize heat exchanger network using Pinch Analysis.
17 End Use: Improve Heat Transfer Cross flow cooling of extruded plastic with 50 F chilled water from chiller
18 End Use: Improve Heat Transfer Counter flowCross flowParallel flowe = 0.78e = 0.62e = 0.50NTU = 3 and Cmin/Cmax = 1
19 Cooling Product: Cross vs Counter Flow Cross Flow: e = 0.69Tw1 = 50 FTp = 300 FMcpmin = 83.2 Btu/min-FQ = e mcpmin (Tp – Tw1) = (300 – 50)Q = 14,352 Btu/minCounter Flow: e = 0.78Q = e mcpmin (Tp – Tw1) = 14,352 Btu/min = (300 – Tw1)Tw1 = 79 F
20 Cooling Product: Cross vs Counter Flow Cooling towers can deliver 79 Fwater much of the year using 1/10as much energy as chillers!
21 Distribution System: Avoid Mixing Separate hot and cold water tanksLower temperature, less pumping energy to processHigher temperature, less fan energy to cooling tower
22 Primary Cooling: Match Cooling Source to End Use
23 Primary Cooling: Use Cooling Tower When Possible Cooling towers can deliver water at about outside air temperature
24 Primary Cooling: Use Cooling Tower When Possible CoolSim reports number hours CT delivers target temperature.Model coolingtower performance
25 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