10 replies to this topic

[gecos]

how can I specify a certain liquid (or gas) fraction as inlet condition when simulating a heat exchanger in Excel with Prode ?

I have an exchanger with saturated steam on hot side  and a multicomponent mixture on cold side,

the manual gives examples about how to solve the exchanger with a series of flash operation at specified pressure and enthalpy (I need to create a table with properties for different zones) but I do not know how to define the initial condition for saturated steam.

[serra]

to initialize at desired condition you may solve a flash at specified pressure and liquid fraction,

in Excel put

 

=LFPF(1,p,lf)

 

which returns the desired equilibrium temperature and initializes the stream at desired condition

 

N.B. works for both pure components and mixtures

[PaoloPemi]

as said there are several ways to initialize a stream to a specific condition in Prode Properties,

specifically, in Microsoft Excel, there are different alternatives

 

utilize the Properties Editor and solve a operation with specified pressure (or temperature) and phase fraction

 

put a MACRO in Excel cell, you can choose between two alternatives

 

methods =LfPF() and =LfTF() are based on a liquid fraction specification, they returns the first point (along the specified liquid fraction line) at the specified pressure (or temperature)

 

methods =PfPF() and =PfTF() can accept a gas, liquid, solid fraction as specification, they can calculate up to 5 points (at specified pressure or temperature) along the line with specified phase fraction

 

note that PfPF() and PfTF() are mainly of interest with mixtures as they allow to calculate several equilibrium points for example in retrograde region...

[gecos]

thanks serra & PaoloPemi,

I am able to initialize the stream (in cold and hot sides) with the Excel macro

 

=LFPF(1,p,lf)

 

then I solve a series of flash operations at specified P & H with the Excel macro

 

=HPF(1,p,h,0)

 

to simulate the different areas of heat exchanger

 

on each area I calculate properties as density, viscosity etc. with Excel macros

 

; liquid density

=StrLD(1)

; liquid viscosity

=StrLV(1)

; gas density

=StrGD(1)

; gas viscosity

=StrGV(1)

 

then I apply some general purpose correlation for calculating pressure drop and k

 

is this the correct way to proceed ?

(I do not wish to obtain a rigorous design, just some preliminary numbers) 

Edited by gecos, 05 June 2013 - 03:28 AM.

[PaoloPemi]

it's correct,

with PRODE PROPERTIES working with Excel or Matlab

it's easy simulate the different areas in heat exchangers

then calculate the transport properties

with these values you can feed a code as HTRI

or include in Excel or Matlab some general purpose

correlation which allow to estimate pressure drop and

heat transfer coefficient,

note that for best results this should be a iterative solution

i.e. you define a initial dp (or some other parameter) then solve

and iterate to find the final solution.

 

Also note that Excel macro

 

=LFPF(1,p,lf)

 

alows to define a initial vapor/liquid fraction,

in general you have a stream at specified

pressure and temperature so you need to solve a

isothermal flash operation,

for that PRODE PROPERTIES exports the Excel macro

 

= SetOp(1,t,p)

 

hoping this helps,

Paolo

[marchem]

Gecos,
as probably you are doing,
I have created a Excel page for preliminary design of heat exchangers
with PRODE and simplified formulations based on
"Heat Exchangers selection, rating and thermal design" book,

Of course results are not very accurate
(I expect errors up to 20-30% in dP and areas)
neverless it is very useful for preliminary
design before to run a tool such as HTRI

Edited by marchem, 30 August 2013 - 02:05 AM.

[gecos]

thanks for the comments,

actually I have created a EXCEL VBA code based on
the procedures described on volume 3 of
"Heat Exchangers Design Handbook" (Hemisphere)

it applies Bell-Delaware method to design
shell and tube heat exchangers with single phase flow

next steps will be evaporators and condensers

I have a few questions concerning the application
of PRODE PROPERTIES for thermodynamic and
transport properties of pure fluids and mixtures.

SPECIFIC HEATS (vapor and liquid)
PRODE PROPERTIES has the methods

=StrGCp(stream) cp for vapor
=StrGCv(stream) cv for vapor
=StrLCp(stream) cp for liquid
=StrLCv(stream) cv for liquid

my first question is about solving the different zones
in heat exchamger, should I adopt constant cp
or use instead difference in enthalpies ?
I presume that enthalpies should be more accurate
since cp (and cv) are not constant.


LATENT HEATS
with PRODE PROPERTIES I can calculate latent heats
in different ways
actually I calculate latent heats as difference
in vapor and liquid enthapies,
is that the correct way?

Edited by gecos, 25 September 2013 - 02:05 AM.

[PaoloPemi]

I simulate the different zones in heat exchangers

with the HPF() method available in PRODE PROPERTIES,

the procedure is very simple and I use EXCEL for that,

 

as first step I calculate the difference in enthalpy

(exchanger heat duty) from outlet and inlet conditions,

I divide heat duty by the number of required zones,

and I calculate temperature and properties

on each zone solving a series of HPF()

flash operations,

 

for solvinga H-P flash operations

there is the macro

 

=HPF(stream,p,h,0.0)

 

where p is the pressure,

and h the required enthalpy

 

I think it is preferable to calculate

enthalpies instead of assuming

constant properties as cp,

also solving a HPF() operation

accounts for phase equilibria

and you can model a

liquid,

vapor-liquid

or vapor zone

without additional effort.

 

The same for latent heats.

[gecos]

thanks Paolo, have you an idea of the accuracy in calculated cp ? I did some tests with Extended Peng Robinson and BWR and results (compared with tables) are within 5-10% , is that normal ?

[PaoloPemi]

of course that depends from the capability of selected thermodynamics

to calculate the properties of the different fluids in the mixture,

I have found the Extended versions of Peng Robinson and Soave Redlich Kwong

available in PRODE quite accurate for hydrocarbons,

for non-ideal , polar fluids such as water things are a bit

more delicate but you may get reasonable values

far from critical area.

[gecos]

thanks Paolo,
I have some other questions about to simulate
a heat exchanger when there are two liquid phases
and I need the properties of each phase,
I'll open a specific thread.

Edited by gecos, 13 November 2013 - 02:58 AM.