Cooling Water Networks
G.T.Polley, M.Picon Nunez & E.Tamakloe
Dept. of Chemical Engineering, University of Guanajuato, Mexico
Simulations can be set up quickly
Analysis quite simple
Significant Benefits - power reduction, capital cost saving,
Topics: Design Principles
? Cooling Towers:
Individual Plants ? Plant Regions ? Centralized System
Packaged Units ? Specialist Suppliers
Selected with future expansion in mind
Thermal Performance well modeled using E-Ntu Method
Minimum wetting required. So, system may have flow set by cooling tower rather than process plant
Maximum water return temperatures apply (function of water chemistry)
Dominates Capital Cost
Set with allowance for plant expansion
Equations for economic pipe size
Capital Cost : diameter to power 3
Installation : again diameter to power 3
Function of Plant Layout ? not use of temperature driving force
Heat Exchanger Sizing
Must be undertaken in ?system context?
Cannot arbitrarily set pressure drop or cooling water temperature rise
Pressure drop : function of square of volumetric throughput
Components of pressure drop:
? Gravitational head
? Frictional losses
? Momentum changes
? MOMENTUM EFFECTS INDEPENDENT OF FLOW PATH
? GRAVITATIONAL EFFECTS INDEPENDENT OF FLOW PATH (Provide flow initiated)
? Initiation of flow may require provision of back pressure ? restriction at cooling tower return
? Vacuum can be pulled in heat exchangers ? checks for vapor blocking are required
? Great advantage in using equation based on Volumetric Flow Rate rather than velocity
? Velocity can vary along specific flow path ? but volume flow remains constant
Frictional Components are additive:
Pipe + Valve + Heat Exchanger + Valve
Kleg = 2Kvalve + Kexchanger + Kpipe
Pressure Drop Across Branches are Equal
Revamping Cooling Water Networks
Occasions arise when it is necessary to reduce load on cooling tower.
Example: installing new plant into existing factory and do not want expense of new cooling tower.
Best Option: Improve integration of plants in order to reduce demand for cooling water.
Problem: Load on existing exchangers is reduced, hot stream inlet temperature fall, outlet temperatures also fall : streams are now ?over-cooled?
Identifying ALL Options:
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