The Design and Revamp of Cooling Water Networks

Cooling Water Networks

G.T.Polley, M.Picon Nunez & E.Tamakloe

Dept. of Chemical Engineering, University of Guanajuato, Mexico

In Reality:

Simulations can be set up quickly

Analysis quite simple

Significant Benefits - power reduction, capital cost saving,

fouling mitigation

Topics: Design Principles

Flow Simulation

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)

       Piping

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

FLOW SIMULATION

Components of pressure drop:

?      Gravitational head

?      Frictional losses

?      Momentum changes

Implications:

?      MOMENTUM EFFECTS INDEPENDENT OF FLOW PATH

?      GRAVITATIONAL EFFECTS INDEPENDENT OF FLOW PATH (Provide flow initiated)

Gravitation

?      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

Friction

?      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: