páginas desdeteach yourself the basics of aspen plus by ralph schefflan 2011

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CHAPTER NINE

HEAT EXCHANGERS

The implementation of heat exchanger models is different from that of most Aspen

Plus models in that some are capable of detail design using very high quality industrial

programs which are integrated into Aspen Plus These are the Hetran and Tasc shell-

and-tube heat exchanger and the Aerotran air-cooled exchanger programs They are

documented in Aspen Plus Help EDR (Exchanger Design and Rating) Most can be

used for design as well as being used to rate existing exchangers that is they can be

used as sequential modular models

To simulate a heat exchanger one must solve the primary equations

q minusmhotchotp (T hot

in minus T hotout ) = 0 (91)

q minus mcoldccoldp (T cold

out minus T coldin ) = 0 (92)

q minus UAF T LM = 0 (93)

T LM

=

T 1 minusT 2

ln(T 1T 2)

(94)

Here q is the exchanger duty m a flow rate cp the heat capacity T the temperature

T the temperature difference at an end of the exchanger U an overall heat transfer

coefficient which depends on temperature transport properties and exchanger geom-

etry and F a correction factor for multiple tube-side andor shell-side passes The

factor F derived through the work of Nagle (1933) and Underwood (1934) can be

calculated as

Teach Yourself the Basics of Aspen Plus991522 By Ralph SchefflanCopyright copy 2011 John Wiley amp Sons Inc

111



112 HEAT EXCHANGERS

F =radic

R2 + 1 ln[(1minus S)(1 minus RS )]

(R minus 1) ln 2minusS(R+1minus

radic R2+1)

2minusS(R+1+radic

R2+1)

(95)

where

R = T hotin minus T hot

out

T coldout minus T cold

in

(96)

S = T coldout minus T cold

in

T hotin minus T cold

in

(97)

When used in simulation mode the state of the exchanger feeds must be specified

Depending on the complexity of the model chosen the heat transfer area A is either

specified or calculated from the heat exchanger physical layout Heat transfer coeffi-

cients are calculated from correlations such as the Hewitt (1992) correlation preparedby Gnielinski which involves the Nusselt N NU = hDk Reynolds N RE = DGmicro and

Prandtl N PR = cpmicrok numbers and the Darcy friction factor given by

N NU =hi Di

k= (f D 8)(N RE minus 1000)N PR

1+ 127radic

F D8(N 23PR minus 1)

1+ D

L

23

(98)

f D = (182 log10 N RE minus 164)minus2 (99)

Here hi is the inside pipe heat transfer coefficient Di the inside pipe diameter k

the thermal conductivity G the mass flow rate micro the viscosity cp the heat capacity

and f D the Darcy friction factor Complete documentation of all correlations used inAspen Plus can be found in Help Heatx Reference and Model Reference Depending

on which model is chosen U is either specified or calculated iteratively during the

convergence process The four heat-exchange-related models can be found in the model

library under the tab Heat Exchangers

91 HEATER BLOCK

An example of the primary input form of the Heater block which shows the possible

specifications is shown in Figure 91 The Heater block offers a variety of ways to

specify the output stream state all of which result in calculation of the energy requiredto heat (or cool) a stream Alternatively one may specify the energy added to or

removed from a heater which is used by the block to establish the state of the output

stream

An important capability is the use of two heaters to model a heat exchanger

bypassing the use of equations (93) and (94) as shown in Figure 92 Note the use

of a heat stream to connect the two heaters The heat stream should be aligned in the

correct direction which depends on which heater will receive the heat either positive

or negative Care must be taken with the sign of the heat transferred (heat added is

positive) In this example the outlet temperature of heater H2 is specified and the heat

stream 5 flows to heater H1 This example may be found at Chapter NineExamplesHeaters



112 HEAT EXCHANGERS

F =radic

R2 + 1 ln[(1minus S)(1 minus RS )]

(R minus 1) ln 2minusS(R+1minus

radic R2+1)

2minusS(R+1+radic

R2+1)

(95)

where

R = T hotin minus T hot

out

T coldout minus T cold

in

(96)

S = T coldout minus T cold

in

T hotin minus T cold

in

(97)

When used in simulation mode the state of the exchanger feeds must be specified

Depending on the complexity of the model chosen the heat transfer area A is either

specified or calculated from the heat exchanger physical layout Heat transfer coeffi-

cients are calculated from correlations such as the Hewitt (1992) correlation preparedby Gnielinski which involves the Nusselt N NU = hDk Reynolds N RE = DGmicro and

Prandtl N PR = cpmicrok numbers and the Darcy friction factor given by

N NU =hi Di

k= (f D 8)(N RE minus 1000)N PR

1+ 127radic

F D8(N 23PR minus 1)

1+ D

L

23

(98)

f D = (182 log10 N RE minus 164)minus2 (99)

Here hi is the inside pipe heat transfer coefficient Di the inside pipe diameter k

the thermal conductivity G the mass flow rate micro the viscosity cp the heat capacity

and f D the Darcy friction factor Complete documentation of all correlations used inAspen Plus can be found in Help Heatx Reference and Model Reference Depending

on which model is chosen U is either specified or calculated iteratively during the

convergence process The four heat-exchange-related models can be found in the model

library under the tab Heat Exchangers

91 HEATER BLOCK

An example of the primary input form of the Heater block which shows the possible

specifications is shown in Figure 91 The Heater block offers a variety of ways to

specify the output stream state all of which result in calculation of the energy requiredto heat (or cool) a stream Alternatively one may specify the energy added to or

removed from a heater which is used by the block to establish the state of the output

stream

An important capability is the use of two heaters to model a heat exchanger

bypassing the use of equations (93) and (94) as shown in Figure 92 Note the use

of a heat stream to connect the two heaters The heat stream should be aligned in the

correct direction which depends on which heater will receive the heat either positive

or negative Care must be taken with the sign of the heat transferred (heat added is

positive) In this example the outlet temperature of heater H2 is specified and the heat

stream 5 flows to heater H1 This example may be found at Chapter NineExamplesHeaters

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