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: $0.10 /kWh Natural gas: $10 /mmBtu Water: $6 /thousand gallons.
Near order of magnitude difference in costs!
Qh1 = 100
Qc1 = 100
If Qhx = 30,
Qh2 = 70
Qc2 = 30
HX reduces both heating and cooling loads!
Cost effective to transfer heat between processes, whenever the processes that need cooling are 10 F higher than the process that need heating
Add Heat Exchangers
T = 145 F
T = 120 F
For multiple heating and cooling opportunities, optimize heat exchanger network using Pinch Analysis.
Cross flow cooling of extruded plastic with 50 F chilled water from chiller
e = 0.78
e = 0.62
e = 0.50
NTU = 3 and Cmin/Cmax = 1
Cross Flow: e = 0.69
Counter Flow: e = 0.78
Cooling towers can deliver 79 F
water much of the year using 1/10
as much energy as chillers!
Separate hot and cold water tanks
Lower temperature, less pumping energy to process
Higher temperature, less fan energy to cooling tower
Cooling towers can deliver water at about outside air temperature
CoolSim reports number hours CT delivers target temperature.
E/Q (Air-cooled) = 1.0 kW/ton E/Q (Water-cooled) = 0.8 kW/ton
Multiple compressors, stages, evaporative condensers