PDF MEP 460 Heat Exchanger design - Kau

MEP 460 Heat Exchanger design

King Abdulaziz University

Mechanical Engineering Department

April 2020

Introduction to EES (Engineering Equation Solver)

Introduction to EES professional version

1-Professional and commercial versions 2-EES user interface

3-Basic idea of EES 4-Built in functions

5-Thermodyncamics and physical properties of fluids & solids

6-Parametric tables 7-Plotting

8-External libraries

❖ Heat exchangers (LMTD, -NTU)

❖ Fins

❖ Compact heat exchangers 9-User Functions and procedures 10-Notes

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Commercial version

Limited number of equations

Professional version

Some additional capabilities larger systems of equations

ability to create special purpose

executable EES programs that can be freely distributed to others

1-Professional and commercial versions

External routines for heat exchangers, fins, boiling and condensation, compact heat exchangers, … etc

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Some features of EES

1-Thermodynamic properties are built in the program

2-Easy programing by just writing the equations in any order

3-Parametric table, plotting and array windows 4-lookup tables

2-EES user interface

4

Variable info

Units Check the

program for any errors

Run or solve

New Parametric table

2-EES user interface

plot Select window: solution, parametric table, plot, formatted equation …etc

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Windows

1-Equation window

2-Formatted equation window 2-Solution window

3-Diagram window 4-Array

5-Parametric window 6-Plot window

2-EES user interface

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Direct way of solving simultaneous equations

2𝑥 + 4𝑦 + 6𝑧 = 60 2𝑥 + 3𝑦 + 7𝑧 = 20.5 6𝑥 − 2𝑦 + 8𝑧 = 19

3-Basic idea of EES

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3-Basic idea of EES

Check your equations.

Number of equations must equal to number of

unknowns

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3-Basic idea of EES

Solution window

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2-Basic idea of EES

Variable information

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Formatted equation window

3-Basic idea of EES

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Math and string functions Fluid properties

Solid/liquid properties Heat transfer

Mechanical Design EES Library

External routines

4-EES Built in functions

Function info.

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4-EES Built in functions

Fluid properties

Example:

T1=50 [C]

P1=300 [kPa]

h=enthalpy(‘water ’ ,T=T1,p=P1)

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Solid properties functions

4-EES Built in functions

Function info.

Paste into equation window

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Solid properties functions

4-EES Built in functions

Example:

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4-EES Built in functions

EES library

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4-EES Built in functions

External routines

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String built in functions

A string variable must end with $ such as

Fluid1$=‘Air’

Fluid2$=‘water’

4-EES Built in functions

T1$=time$

D$=Date$

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5-Thermodyncamics and physical properties of fluids & solids

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5-Thermodyncamics and physical properties of fluids & solids

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6-Parametric tables

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Use parametric table when require to solve the problem several time by changing one or more variables

6-Parametric tables

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Parametric table to calculate the friction factor for different values of Re (

Reynold's number)

7- Plotting

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The results can be plotted using the plot window

7- Plotting

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plotting the variation of the friction factor with Re.

You can select which parameter to be in x or y axis.

6-External libraries

(Related to Heat Exchangers)

❖ Heat exchangers (LMTD, e-NTU) methods

❖ Fins

❖ Compact heat exchangers

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6-External libraries

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6-External libraries

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6-External libraries

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Usage: The general form is:

F=LMTD_CF(TypeHX$,P,R)

Ti and To, and ti and to are the

temperatures at the inlet and outlet of the two streams respectively.

TypeHX$ - specifies the heat exchanger flow configuration;

applicable heat exchanger geometries are listed below

𝑃 = 𝑡_𝑜 − 𝑡_𝑖

𝑇_𝑖 − 𝑡_𝑖 𝑅 = 𝑇_𝑖 − 𝑇_𝑜 𝑡_𝑜 − 𝑡_𝑖

Flow Configurations: The heat exchanger flow configuration, TypeHX$, must be set to one of the following (case-insensitive)

strings:

'parallelflow'

'crossflow_both_unmixed' 'crossflow_one_unmixed'

'shell&tube_N' {where N is an integer between 1 and 9, specifying the number of shell pass. The number of tube passes is then, 2N, 4N, 6N, .... . }

LMTD=Log Mean Temperature Difference

6-External libraries- LMTD correction factor F

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6-External libraries-Heat exchangers LMTD Correction factor F

Example

T_hi=100 [C]

T_ho=60 [C]

T_ci=30 [C]

T_co=50 [C]

hx_type$='crossflow_both_unmixed '

P=(T_co-T_ci)/(T_hi-T_ci) R=(T_hi-T_ho)/(T_co-T_ci)

F=LMTD_CF(hx_type$,P,R)

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LMTD Correction factor F

Flow Configurations: The heat exchanger flow configuration, TypeHX$, must be set to one of the following (case-insensitive)

strings:

'parallelflow'

'crossflow_both_unmixed' 'crossflow_one_unmixed'

'shell&tube_N' {where N is an integer between 1 and 9, specifying the number of shell pass. The number of tube passes is then, 2N, 4N, 6N, .... . }

The general form is either:

epsilon=HX(TypeHX$, Ntu, C_1, C_2 , Return$) or

Ntu=HX(TypeHX$, epsilon, C_1, C_2 , Return$)

If epsilon is known, then it is more efficient to set Return$='Ntu'. If Ntu is known, then it is more efficient to set

Return$='epsilon'.

There is always a

solution for epsilon given any valid Ntu but there may not be a solution for Ntu given any valid

epsilon.

Capacitance rates, C_1 and C_2 :

6-External libraries-Heat exchangers effectiveness

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// UnitSystem SI C J Pa

"knowns:"

T_h_i=100 [C]

T_h_o=60 [C]

T_c_i=30 [C]

m_dot_h=0.1 [kg/s]

m_dot_c=0.1 [kg/s]

U=60 [W/m^2-K]

D=0.025 [m]

c_p_oil=1900 [J/kg-K]

c_p_water=4200 [J/kg-K]

"a) What is the heat transfer and the water outlet temperature?"

C_dot_h=m_dot_h*c_p_oil C_dot_c=m_dot_c*c_p_water q=C_dot_h * (T_h_i-T_h_o) q=C_dot_c * (T_c_o-T_c_i)

"b) What is the HX length?"

C_dot_min=min(C_dot_h, C_dot_c) TypeHX$='counterflow'

q_max=C_dot_min * (T_h_i-T_c_i) epsilon=q/q_max

Ntu=HX(TypeHX$, epsilon, C_dot_h, C_dot_c, 'Ntu') Ntu=(U*A)/C_dot_min

A=pi*D*L

6-External libraries-Heat exchangers effectiveness

Example

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6-External libraries-Heat exchangers

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6-External libraries-Fins

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eta_fin_straight_rect(th, L, h, k)

Example:

$UnitSystem SI K Pa J L=0.06 [m]

th=0.003 [m]

h=60 [W/m^2-K]

k=200 [W/m-K]

eta=eta_fin_straight_rect(th, L, h, k) {Solution:eta=0.8063}

Function

Non-dimensional:

eta_fin_straight_rect_ND(mL)

Function eta_fin_straight_rect_ND

returns the fin efficiency of a straight fin with a rectangular base given the

product of the fin parameter, m, and fin length, L, where m is defined as:

𝑚 = 2ℎ 𝑘 𝑡ℎ

L_c =L+th/2 mL=m*L_c

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eta_fin_spine_rect(D, L, h, k)

Example:

$UnitSystem SI K Pa J L=0.075 [m]

D=0.005 [m]

h=60 [W/m^2-K]

k=200 [W/m-K]

eta=eta_fin_spine_rect(D, L, h, k) {Solution: eta=0.7008}

Non-dimensional:

eta_fin_spine_rect_ND(mL) Function

eta_fin_spine_rect_ND

returns the fin efficiency of a rectangular fin with a circular base given the product of the fin parameter, m, and fin

length, L, where m is defined as

Function

𝑚 = 4ℎ

𝑘𝐷 mL=m*L

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Example:

$UnitSystem SI K Pa J r_in=0.01[m]

r_out=0.03 [m]

th=0.004 [m]

h=60 [W/m^2-K]

k=200 [W/m-K]

eta=eta_fin_annular_rect(th, r_in, r_out, h, k)

Function

Non-dimensional:

eta_fin_annular_rect_ND(mr_out, r_in\r_out)

Function eta_fin_annular_rect_ND returns the fin efficiency of an annular fin given the product of the fin parameter, m, and out radius, r_out and the ratio of the inner to outer radii. The first parameter is defined as:

𝑚𝑟_𝑜𝑢𝑡 = 𝑟_𝑜𝑢𝑡 2ℎ 𝑘 𝑡ℎ eta=eta_fin_annular_rect(th, r_in, r_out, h, k)

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Example on rectangular fin:

$UnitSystem SI K Pa J L=0.06 [m]

th=0.003 [m]

h=60 [W/m^2-K]

k=200 [W/m-K]

eta=eta_fin_straight_rect(th, L, h, k) 6-External libraries-Fins

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6-External libraries-Fins

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6-External libraries- Compact heat exchangers

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6-External libraries- Compact heat exchangers

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No Kay & London designation

EES designation 1 surface CF-8.8-1.0J

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Surface designation in Kays and London book and in EES

6-External libraries- Compact heat exchangers

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Tube fin compact heat exchangers

Finned circular tube Finned flat tube

8 surfaces 5 surfaces

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Plate fin gas-gas compact heat exchangers

Louvered fin Pin fin Strip fin Plain fin Wavy fin

10 6 21 18 3

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10 surfaces

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6 surfaces

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21 surfaces

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18 surfaces

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3 surfaces

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Wavy fins

Call procedure for geometric information

Call CHX_geom_finned_tube(TypeHX$: D_o, fin_pitch, D_h, fin_thk, sigma, alpha, A_fin\A)

For finned tube

For plate fin

Applies to finned tube type heat exchangers including finned circular tubes and finned flat tubes.

Call CHX_geom_plate_fin(TypeHX$, a, b_2: b_1, D_o, fin_pitch, D_h, fin_thk, sigma, alpha, A_fin\A)

input

TypeHX$ - a representative string variable corresponding to the heat exchanger type and surface

a - plate thickness (applies only to plate-fin heat exchangers) [m]

b_2 - thickness of passages through which the second fluid passes (applicable to plate-fin heat exchangers only) [m]

6-External libraries- Compact heat exchangers

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D_o - the outside diameter of the tube (applicable to finned circular tubes and pin-fin plate fin) [m]

fin_pitch - the number of fins per meter [1/m] or [1/ft]

D_h - the hydraulic diameter [m] of [ft]

fin_thk - thickness of fins (not applicable to pin-fin) [m] of [ft]

sigma - minimum free flow area/frontal area

alpha - heat transfer area/total volume [m^2/m^3] or [ft^2/ft^3]

A_fin\A - fin area/total area

b_1 - thickness of passages through which the first (considered) fluid passes [m] or [ft]

output

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Call CHX_geom_plate_fin(TypeHX$, a, b_2: b_1, D_o, fin_pitch, D_h, fin_thk, sigma, alpha, A_fin\A)

For plate fin

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Tube-fin surface data

8-User defined functions and procedures

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a) functions b) Procedures

functions are used to find the resulted single value as a function of several variables

Procedure can have more than one variable in the input and in the output

Uses defined Functions

must be at the top of the equation window Start with function fname(arg1,ag2,arg3…) fname=…………..

- - - - - - - - - - - - End

Then you call the function using R=fname(x1,x2,x3,…)

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Example of user defined function

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User defined Procedures

Procedure pname(arg1, arg2, arg2,…: output1, output2,…) Output1=…….

Output2=……

End

Call pname(inp1, inp2, inp3…:out1,ou2,ou3..)

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9- Additional Notes

a) Remarks statement b) Variable information c) Units

d) Useful notation and abbreviations

e) Highlighting variables in solution windows

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a) Remarks

Use “ “ for remarks statements (blue) Use { } for remarks (blue)

Use // for one line comment statement (blue)

Use {! } to make the comment statement in red

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Comment statements

The comments are displayed in blue (by default) in the Equations Window to indicate that they are not part of the

equation set that will be solved. Comments that begin with the ! character (referred to as comment type 2) are displayed in red by default. The default colors for comments can be changed in the Preferences dialog (Options menu).

One line comment statement using //

Several line comments using either { }, or “ “ Comments default color is blue

Red comments statement with “! “

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You can convert several lines in equation window into comment statement or vis versa using mouse tight click

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Comment statements

b) Variables information

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c) Units

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d) Some useful notation and abbreviations

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Highlighted and key variables

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You can not use if statement in the main program. But it can be used in the

functions or procedures

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Lookup table

If you have a table of properties such as a table for the density, specific heat, thermal conductivity, Prandtl number as a function of temperature, and you require to interpolate the table for any given temperature, then lookuptable is very useful.

Example: Engine oil properties as a function of temperature

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Create a look table using Table and then new look up table with number of column and row as needed

Lookup table

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Lookup table

Save the table as oil_prop.lkt

save the file in EE32 directory

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Lookup table

Use of

lookup table