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: