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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