energy-efficient process cooling. process cooling systems cooling systems –cooling tower...
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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
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
ProcessLoad 1
ProcessLoad 2
Chilled Water Tank
Cooling Tower
BypassValve
Process PumpCooling Tower Pump
Chillers
Water-Cooled Chiller
• E/Q = 0.8 kW/ton = 67 kWh/mmBtu• Unit cost of cooling = $6.70 /mmBtu
Process Pump
ProcessLoad 1
ProcessLoad 2
Chiller
Cooling Tower
Cooling Tower Pump
BypassValve
Air-Cooled Chiller
• E/Q = 1.0 kW/ton = 83 kWh/mmBtu• Unit cost of cooling = $8.30 /mmBtu
Process Pump
ProcessLoad 1
ProcessLoad 2
Chiller
BypassValve
Air
Absorption Chiller
• E/Q = 1 Btu-heat / Btu-cooling Eff-boiler = 80%• Unit cost of cooling = $12.50 /mmBtu
Process Pump
ProcessLoad 1
ProcessLoad 2
AbsorptionChiller
BypassValve
Boiler
Steam
Open-Loop Water Cooling
DT = 10 F V = 12,000 gallons / 1 mmBtu Unit cost of cooling = $72 /mmBtu
ProcessLoad 1
ProcessLoad 2
From City Water Supply
To Sewer
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
Compressed Air In
Cold Air Out Warm Air Out
Relative Process Cooling Costs
0
50
100
150
200
250
300
Compressed air Open loopcooling
Chillers Cooling towers
$/m
mB
tu c
oo
lin
g
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
• Insulation:–Reduces heat transfer into cooled tanks & piping–Decreases exterior condensation
• Even at small temperature differences insulating cold surfaces is generally cost effective
Current: Qh1 = 100 Qc1 = 100
With HX: If Qhx = 30,Qh2 = 70 Qc2 = 30
HX reduces both heating and cooling loads!
T1 T2 T3
Qh1 Qc1
T1 T2 T3
Qh2 Qc2
New HX
T2B
T2A
A.
B.
End Use: Continuous Process with Sequential Heating and Cooling
End Use: Batch Processes with Discrete Heating and Cooling
Tanks That Need Cooling Name Tmax (F) Q (mmBtu/hr) V (gpm)
5.6 115 1 400 5.1 130 1 400
Work 145 3 x 15 3 x 1,720
Tanks That Need Heating Name Tmin (F) Q (mmBtu/hr) V (gpm)
5.1 165 5.2 1,060 5.2 165 5.2 1,060 5.3 165 2.2 530 5.4 160 2.4 530 5.5 165 2.4 530 5.8 125 2.0 530 5.9 130 2.1 530 5.11 130 2.1 530 5.13 130 2.1 530 5.15 145 2.2 530 5.16 120 2.0 530 5.17 120 2.0 530 5.18 145 2.2 530
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 FRequires Cooling
T = 120 FRequires Heating
End Use: Optimize Heat Exchanger Network (Pinch Analysis)
For multiple heating and cooling opportunities, optimize heat exchanger network using Pinch Analysis.
Shifted Composite Curves
100110120130140150160170180
0 10 20 30 40
Q (mmBtu/hr)
T (
F) Tc
Th
End Use: Improve Heat Transfer
Cross flow cooling of extruded plastic with 50 F chilled water from chiller
End Use: Improve Heat Transfer
NTU = 3 and Cmin/Cmax = 1
e = 0.78 e = 0.62 e = 0.50
Counter flow Cross flow Parallel flow
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 = 14,352 Btu/min • Tp = 300 F• Mcpmin = 83.2 Btu/min-F• 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 tanksLower temperature, less pumping energy to processHigher temperature, less fan energy to cooling tower
ProcessLoad 1
ProcessLoad 2
Chilled Water Tank
Cooling Tower
BypassValve
Tp2
Tc2 Process Pump
Tp1
Cooling Tower Pump
Tc1
Primary Cooling: Match Cooling Source to End Use
0
50
100
150
200
250
300
Compressed air Open loopcooling
Chillers Cooling towers
$/m
mB
tu c
oo
lin
g
Primary Cooling: Use Cooling Tower When Possible
Cooling towers can deliver water at about outside air temperature
Primary Cooling: Use Cooling Tower When Possible
Model cooling tower performance
CoolSim reports number hours CT delivers target temperature.
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