1. Energy-Efficient Process Cooling

2. Process 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

3. 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

4. Chillers 4

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 mmBtu
Unit cost of cooling = $72 /mmBtu

9. 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

10. Relative Process Cooling Costs
Near order of magnitude difference in costs!

11. 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

12. End Use: Add Insulation
Reduces heat transfer into cooled tanks & piping
Decreases exterior condensation
Even at small temperature differences insulating cold surfaces is generally cost effective

13. End Use: Continuous Process with Sequential Heating and Cooling
Current:
Qh1 = 100
Qc1 = 100
With HX:
If Qhx = 30,
Qh2 = 70
Qc2 = 30
HX 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 Exchangers
T = 145 F
Requires Cooling
T = 120 F
Requires 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 flow
Cross flow
Parallel flow
e = 0.78
e = 0.62
e = 0.50
NTU = 3 and Cmin/Cmax = 1

19. 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) = (300 – 50)
Q = 14,352 Btu/min
Counter Flow: e = 0.78
Q = 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 F
water much of the year using 1/10
as much energy as chillers!

21. Distribution System: Avoid Mixing
Separate hot and cold water tanks
Lower temperature, less pumping energy to process
Higher 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 cooling
tower 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

26. Primary Cooling: Stage Multiple Constant Speed Chillers

27. Primary Cooling: Use Variable-Speed Chiller

28. Ammonia Refrigeration Systems
Multiple compressors, stages, evaporative condensers

29. Ammonia Refrigeration Savings Opportunities
Reclaim heat
Variable head-pressure control