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
Cooling Tower
Process
Load 1
Process
Load 2
Chilled Water Tank
Cooling Tower Pump
Process Pump
• 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
Bypass
Valve
Chillers
Water-Cooled Chiller
Cooling Tower
Process
Load 1
Chiller
Cooling Tower Pump
Process Pump
• E/Q = 0.8 kW/ton = 67 kWh/mmBtu
• Unit cost of cooling = $6.70 /mmBtu
Process
Load 2
Bypass
Valve
Air-Cooled Chiller
Process
Load 1
Chiller
Air
Process Pump
• E/Q = 1.0 kW/ton = 83 kWh/mmBtu
• Unit cost of cooling = $8.30 /mmBtu
Process
Load 2
Bypass
Valve
Absorption Chiller
Process
Load 1
Process
Load 2
Bypass
Valve
Boiler
Steam
Absorption
Chiller
Process Pump
• E/Q = 1 Btu-heat / Btu-cooling Eff-boiler = 80%
• Unit cost of cooling = $12.50 /mmBtu
Open-Loop Water Cooling
From City Water Supply
Process
Load 1
Process
Load 2
To Sewer
 DT = 10 F
V = 12,000 gallons / 1 mmBtu
 Unit cost of cooling = $72 /mmBtu
Compressed Air Cooling
Compressed Air In
Cold Air Out
Warm Air Out
• 150 scfm at 100 psig to produce 10,200 Btu/hr cooling
• 4.5 scfm per hp
• Unit cost of cooling = $272 /mmBtu
Relative Process Cooling Costs
300
$/mmBtu cooling
250
200
150
100
50
0
Compressed air
Open loop
cooling
Chillers
Cooling towers
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
End Use: Continuous Process
with Sequential Heating and Cooling
Qh1
T1
Qc1
T2
T3
A.
T2B
Qc2
Qh2
T1
T2A
T2
T3
Current:
Qh1 = 100
Qc1 = 100
With HX:
If Qhx = 30,
Qh2 = 70
Qc2 = 30
B.
New HX
HX reduces both
heating and
cooling loads!
End Use: Batch Processes
with Discrete Heating and Cooling
Tanks That Need Cooling
Name Tmax (F) Q (mmBtu/hr)
5.6
115
1
5.1
130
1
Work
145
3 x 15
V (gpm)
400
400
3 x 1,720
Cost effective to transfer
heat between processes,
whenever the processes
that need cooling are 10 F
higher than the process
that need heating
Tanks That Need Heating
Name Tmin (F) Q (mmBtu/hr)
5.1
165
5.2
5.2
165
5.2
5.3
165
2.2
5.4
160
2.4
5.5
165
2.4
5.8
125
2.0
5.9
130
2.1
5.11
130
2.1
5.13
130
2.1
5.15
145
2.2
5.16
120
2.0
5.17
120
2.0
5.18
145
2.2
V (gpm)
1,060
1,060
530
530
530
530
530
530
530
530
530
530
530
End Use: Batch Processes
with Discrete Heating and Cooling
Add Heat Exchangers
T = 145 F
Requires Cooling
T = 120 F
Requires Heating
End Use: Optimize Heat Exchanger Network
(Pinch Analysis)
T (F)
Shifted Composite Curves
180
170
160
150
140
130
120
110
100
Tc
Th
0
10
20
30
40
Q (mmBtu/hr)
For multiple heating and cooling opportunities, optimize
heat exchanger network using Pinch Analysis.
End Use: Improve Heat Transfer
Cross flow cooling of extruded plastic with 50 F chilled water from chiller
End Use: Improve Heat Transfer
Counter flow
e = 0.78
Cross flow
e = 0.62
NTU = 3 and Cmin/Cmax = 1
Parallel flow
e = 0.50
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
Cooling Tower
Process
Load 1
Process
Load 2
Bypass
Valve
Tp2
Chilled Water Tank
Tc1
Cooling Tower Pump
Tp1
Tc2
Process Pump
Separate hot and cold water tanks
Lower temperature, less pumping energy to process
Higher temperature, less fan energy to cooling tower
Primary Cooling:
Match Cooling Source to End Use
300
$/mmBtu cooling
250
200
150
100
50
0
Compressed air
Open loop
cooling
Chillers
Cooling towers
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