08 - DYNSIM Modelling Heat Exchangers.pdf

Title KNPC (Kuwait) 1st  – 5th February February,, 2008

DYNSIM OTS Training Course 8.- DYN DYNSIM SIM Mod Modell elling ing:: Heat Exchangers

8.- DYNSIM Modelling: Heat Exchangers •

Heat Exchangers

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Utility Exchangers

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Multi Exchangers

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Heat Exchanger 

Heat Exchanger: Definition •

Two flow devices to model SINGLE pass shell and tube heat exchange

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Uses log mean temperature difference (LMTD)

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Fouling resistance

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Model film coefficients with

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Heat Exchanger Configuration •

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Two flow devices to model SINGLE pass shell and tube heat exchange Uses log mean temperature difference (LMTD) Specify  –

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 Area and either Film coefficients or overall U Tube/Shell fluid volumes (Vol) and Metal Mass (MM). Tube/Shell flow conductance (J)

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Heat Exchanger: Boundary Calculations •

Four combined fluid/metal nodes provide calculation stability during crossover transients SHELL

Bypass

Inlet stream

Inlet node

Exit stream

Exit node

 Ambient heat loss

 Ambient heat loss

Heat transfer (LMTD & Natural Convection)

 Ambient heat loss

Exit stream

 Ambient heat loss

Exit node

Inlet node Bypass

Inlet stream

TUBE

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Calculate natural convection duty and heat loss to ambient Inlet and Outlet boundary temperatures can be set to tune heat transfer coefficients  –

Tune heat exchanger by setting inlet node boundaries

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Tune downstream equipment by setting outlet node boundaries

The boundary conditions should be removed once model tuning is complete 6

Utility Exchanger 

Utility Exchanger: Definition •

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Models heat exchangers with only one process side (one inlet and one outlet stream) Simplified thermo system or a heat stream connection to vessel on utility side Can model the submerged tubes in a drum or separator using a Heat Stream Can model fans and louvers on an air cooler. To do that, better to change the heat exchanger image Process side of a heat exchanger is identical to a Heat Exchanger (two sided) with same data requirements

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Utility Exchanger Configuration •

Must specify  –

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Type

 Air 

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Water 

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Constant Cp fluid

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Constant temperature source

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Heat stream

Should specify  –

Process Side: •

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 Area, Film Coefficients or Overall U

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Fluid Volume & Metal mass

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Flow conductance

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Reference flow rate

Utility Side: •

Inlet temp

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Utility mass & flow rate, Metal mass

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Heat transfer coefficient 9

Multi Exchanger 

Multi Exchanger: Definition •

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Flexible heat exchanger model that can simulate:  –

Pipe with heat loss

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Shell and tube exchanger with multiple zones

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NGL plate/fin core exchanger 

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LNG spiral-wound Main Cryogenic Heat Exchanger (MCHE)

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Ethylene cold box

Uses multiple flow passes, multiple metal nodes, each with multiple elements Any wall may exchange heat with any fluid or the surroundings Fluid / Metal Heat transfer calculation and Metal temperature calculation is made for each element 11

Multi Exchanger Configuration (I) •

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Configure  –

Number of flow paths

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Number of metal walls

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Number of elements

Specify Wall Geometry  –

Total metal mass

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Total heat transfer area

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Heat loss coefficient

Specify Flow Path Geometry  –

Flow conductance

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Reverse flow factor 

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Total fluid volume

Multi Exchanger distributes total values evenly over each element.

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Multi Exchanger Configuration (II) •

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Specify Tube Pass Heat Transfer   –

Reference Heat transfer coefficient

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Reference mass flow

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Natural convection heat transfer coefficient

Specify the area fraction of each wall area to each flow path  –

0 = no contact

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1 = Full contact

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0
Flow Direction  –

 Ascending = In first element and out last

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Descending = In last element and out first

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Custom = Define custom flow order 

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Multi Exchanger: U-Tube •

Why ever use a custom flow order? Consider a U-tube shell and tube exchanger with a single shell side pass.  –

Flow1Wall1.Dir = Ascending

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Flow2.Wall1.Dir = Custom

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Flow2Wall1.Order = [3 4 2 1 5]

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Questions & Answers

Thank You 15