(61a) 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: