special slides - heat exchangers and airfin coolers

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Department of Chemical Engineering

College of Engineering

University of the Philippines Diliman

Second Semester, AY 2014-2015

ChE 142: Chemical

Engineering Plant Design

Detailed Design of

Static Equipment (HEx and AFC)

Introduction Heat Exchanger Types Design Calculations P&ID Representation Cost Estimation

Outline of Lecture

Devore et al recommend the

following heat exchanger types:

Spiral heat exchanger if area is less than 2 m2.

Double-pipe heat exchanger if area is between 2 and 50 m2.

Shell-and-tube heat exchanger if area is greater than 50 m2.

Heat Exchanger Types Heat Exchanger TypesSpiral Heat Exchanger

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Heat Exchanger TypesDouble Pipe Heat Exchanger

Heat Exchanger TypesShell and Tube Heat Exchanger

Shell and tube heat exchangers are

broadly classified into two: removable

and non-removable tube bundle.

Of the two classifications, the cheapest

options are the U-tube heat exchanger

and the fixed tubesheet heat exchanger

respectively.



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Cheaper for the same heat transfer

area, but cant clean shell side.

Costlier for the same heat transfer

area, but can clean shell side.

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Design Calculations

Design and draw the P&ID representation of

a shell-and-tube heat exchanger with the

following information:

Design CalculationsProblem Statement

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Fluid Placement

Place the fluid on the tube side if it is:

(arranged in order of priority)

1. Corrosive

2. Cooling water

3. More fouling

4. Less viscous

5. More pressurized

6. Hotter

Design CalculationsInitial Specifications

Fluid Placement

Fouling factors (refer to Table 3.3)

Crude oil = 0.004-0.005 hr-ft2-F/Btu

Kerosene = 0.001-0.003 hr-ft2-F/Btu

Place the crude oil and kerosene in tube

side and shell side respectively.


Shell and Head Type

What shell and tube heat exchanger type

is suitable for this service, BEU or BEM?


Tube and Tubing Layout

The following guidelines are

observed in the selection of tube

dimensions and layout:

14 BWG tubes with 1 triangular pitch for straight tubes

1 14 BWG tubes with 1 square pitch for U-tubes


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What are the advantages and disadvantages

of triangular pitch over square pitch?



The preferred straight tube lengths are 16 ft

and 20 ft. For the same heat transfer area,

which is more economical?


Longer but thinner

heat exchanger?

Shorter but fatter

heat exchanger?

Baffle Dimensions

The baffle spacing is recommended to

be between 20% and 100% of the

shell diameter. The default is 20%.


Baffle Dimensions

The baffle cut is recommended to be

between 15% and 45%. The default is 20%.


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Allowable pressure drop for shell and tube

exchangers and air coolers in pumped liquid

service may be considered as follows:

Design CalculationsMaximum Pressure Drop

Viscosity (cP)Allowable Pressure Drop (psi)

Shell Side Tube Side

Less than 1.0 5.0 10

1.0 to 5.0 7.5 10

5.0 to 15.0 10 15

15.0 to 25.0 15 20

25.0 to 50.0 15 25

Allowable pressure drop for shell and tube

exchangers and air coolers in condensing

service may be considered as follows:

Pressure (psig) Allowable Pressure Drop (psi)

Up to 50 2.5 per shell

50 and above 5.0 per shell

Design CalculationsMaximum Pressure Drop

The initial specifications are as follows: Kerosene at shell, crude oil at tube

o Kerosene = 0.003 hr-ft2-F/Btu

o Crude oil = 0.005 hr-ft2-F/Btu

Heat exchanger type BEU 1 14 BWG tubes with 1 square pitch

o Outer diameter (Do) = 1.000 in

o Inner diameter (Di) = 0.834 in

o Tube pitch (PT) = 1.250 in

o Clearance (C) = 0.250 in

Tube length of 20 ft Baffle spacing of 20% of shell diameter Baffle cut of 20% Maximum shell-side P = 5.0 psi Maximum tube-side P = 10.0 psi


The initial specifications are as follows: Kerosene at shell, crude oil at tube

o Kerosene = 5.248 x 10-4 m2-K/W

o Crude oil = 8.806 x 10-4 m2-K/W

Heat exchanger type BEU 1 14 BWG tubes with 1 square pitch

o Outer diameter (Do) = 0.02540 m

o Inner diameter (Di) = 0.02118 m

o Tube pitch (PT) = 0.03175 m

o Clearance (C) = 0.00635 m

Tube length of 6.096 m Baffle spacing of 20% of shell diameter Baffle cut of 20% Maximum shell-side P = 34.46 kPa Maximum tube-side P = 68.93 kPa


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Assume that only one shell pass will suffice.

Calculate the LMTD correction factor (F).

Design CalculationsNumber of Shell Passes

Assume that only one shell pass will suffice.

Calculate the LMTD correction factor (F).


If calculated F is less than 0.80, set the

number of shell passes to two.


Calculate the area using the equation

Q = UAFTlm

Estimate the overall heat transfer

coefficient using the individual heat

transfer coefficients (should have been

done during ChE 141).

Design CalculationsEstimated Area

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The heat exchanger area is equal to the

surface area of each tube multiplied by

the number of tubes:

A = nt x (DoL)

Design CalculationsMinimum Number of Tubes

The tube-side fluid loses pressure as it

expands at the inlet nozzle, flows inside the

tubes, and contracts at the outlet nozzle.

Assume that nozzle losses are negligible.

How can the maximum number of tube

passes be calculated using the equation

above?

Design CalculationsMaximum Number of Tube Passes

Given the heat exchanger type and the tube-

side details, a shell diameter can be selected:

Design CalculationsShell Diameter

Given the selected tube length and

the tube count on the selected shell

diameter, calculate the area available

for heat exchange:

A = nt x (DoL)

Calculate the required overall heat

transfer coefficient:

Ureq = Q/AFTlm

Design CalculationsRequired Overall Heat Transfer Coefficient

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The tube-side heat transfer coefficient is

calculated using the Seider-Tate and Hausen

equations. Viscosity correction is neglected:

Design CalculationsInside Heat Transfer Coefficient

The shell-side heat transfer coefficient is

calculated using the following correlation.

Viscosity correction is neglected:

Design CalculationsOutside Heat Transfer Coefficient

Calculate the overall heat transfer coefficient

using the equation below. Metal resistance is

neglected:

The magnitude of oversize is based on the

required overall heat transfer coefficient.

Design CalculationsOverall Heat Transfer Coefficient

The tube-side fluid loses pressure as it

expands at the inlet nozzle, flows inside the



Design CalculationsTube-Side Pressure Drop

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Design CalculationsShell-Side Pressure Drop

The shell-side fluid loses pressure as it

expands at the inlet nozzle, flows outside the



What adjustments need to be done?

Design CalculationsDesign Assessment

Case

Is overall HTC

greater than

required?

Is tube-side

P less than

maximum?

Is shell-side

P less than

maximum?

1 YES YES YES

2 YES YES NO

3 YES NO YES

4 NO YES YES

P&ID Representation P&ID Representation

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P&ID Representation P&ID Representation

P&ID Representation Cost Estimation

Towler and Sinnott (2008) expressed the

January 2006 purchased cost of heat

exchangers as a function of area.

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Air Fin Coolers

Because of low heat

transfer coefficient on

the air side, the tubes

are finned to increase

the area available for

heat transfer.

Air Fin Coolers

Air fin coolers are second only to shell-

and-tube heat exchangers in frequency

of occurrence in chemical and

petroleum processing operations.

Assuming no process restrictions,

when is air cooling economically

advantageous over water cooling?

Air Fin Coolers Air Fin CoolersForced Draft Air Fin Coolers

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Air Fin CoolersInduced Draft Air Fin Coolers

Air Fin CoolersForced vs. Induced Draft Air Fin Coolers

Differentiate the two configurations in

terms of:

1. Accessibility of tubes and fan parts

2. Fan power consumption for the same

mass flowrate of air

3. Area for the same air fin cooler duty



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For inlet process fluids above 350F,

use forced draft configuration.

Air Fin CoolersTube Dimensions

Tube lengths are typically from 6 ft to 50 ft,

with 40 ft commonly used. Tubes are

typically stacked from three to eight rows,

with six rows commonly used.

Air Fin CoolersTube Dimensions

Air Fin CoolersBay Dimensions

Bay widths are typically from 4 ft to 30 ft, with

14 ft commonly used. Axial-flow fans with four

or six blades and diameters of 6 ft to 18 ft are

typically employed.

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Department of Chemical Engineering

College of Engineering

University of the Philippines Diliman

Second Semester, AY 2014-2015

ChE 142: Chemical

Engineering Plant Design

Detailed Design of

Static Equipment (HEx and AFC)

special slides - heat exchangers and airfin coolers

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