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Shell and Tube Heat Exchangers

- Goals
- By the end of todays lecture, you should be able

to - describe the common shell-and-tube HE designs
- draw temperature profiles for parallel and

counter-current flow in a - shell-and-tube HE
- calculate the true mean temperature difference

for a shell-and-tube - HE (use FG chart)
- make heat transfer calculations for

shell-and-tube HEs - describe how the inside and outside heat

transfer coefficients are - determined for shell-and-tube HEs
- use the Donohue equation to estimate ho in a

multiple pass heat - exchanger
- make heat transfer calculations for multiple

pass shell-and-tube - heat exchangers

Outline I. Review II. Shell-and-tube

equipment III. Rate equation and DTTM IV. Example

Problem - FG for multiple pass HE V. Heat

transfer coefficients VI. Example Problem -

Handout

I. Review Last time, we reviewed heat transfer

in double pipe (concentric pipe) heat

exchangers. We considered cases of parallel and

countercurrent flow of the hot and cold fluids

in the concentric pipe design. The basic

equations required in the design of a heat

exchanger are the enthalpy balances on both

fluid streams and a rate equation that defines

the heat transfer rate.

Enthalpy balances for fluids without phase

change (hot stream) and

(cold stream) If a phase change

occurs (the hot stream is condensed), then the

heat of condensation (vaporization) must be

accounted for In most applications, the heat

gained by the cold stream can be assumed to

equal the heat lost by the hot stream (i.e., qh

qc).

The rate of heat transfer for a concentric pipe

heat exchanger with parallel or countercurrent

flow can be written as

or where DTTM is the true mean temperature

difference. For concentric pipe heat exchangers,

the true mean temperature difference is equal to

the log mean temperature difference (DTLM).

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II. Shell-and-Tube Equipment Concentric Pipe

vs. Shell Tube Heat Exchangers The simple

double pipe heat exchanger is inadequate for flow

rates that cannot readily be handled in a few

tubes. Double pipe heat exchangers are not used

for required heat exchange areas in excess of

100-150 ft2. Several double pipe heat

exchangers can be used in parallel, but it

proves more economical to have a single shell

serve for multiple tubes.

Typical Shell-and-Tube Heat Exchanger

Shell-and-tube heat exchangers are described

based on the number of passes the shell-side and

tube-side fluids must undergo. Exchangers are

listed as 1-1, 1-2, 2-4, etc. in which the first

number signifies the number of passes for the

shell-side fluid and the second number refers to

the tube-side fluid.

TEMA Designations

TEMA AES Exchanger

Baffles How do baffles help? Where are

they installed and which fluid is directly

affected? Common practice is to cut away a

segment having a height equal to one- fourth the

inside diameter of the shell. Such baffles are

called 25 percent baffles.

Baffle Arrangement

The RODbaffle heat exchanger design (Phillips

Petroleum Co.)

Tube Bundles

Tube sizes

Tubes Standard tube lengths are 8, 12, 16 and 20

ft. Tubes are drawn to definite wall thickness

in terms of BWG and true outside diameter (OD),

and they are available in all common metals.

Tube Pitch

The spacing between the tubes (center to

center) is referred to as the tube pitch (PT).

Triangular or square pitch arrangements are

used. Unless the shell side tends to foul

badly, triangular pitch is Used. Dimensions of

standard tubes are given in the Handout and in

MSH Appendix 6.

Tube Pitch

III. Rate equation and DTTM The rate equation

for a shell-and-tube heat exchanger is the same

as for a concentric pipe exchanger However,

Ui and DTTM are evaluated somewhat differently

for shell-and-tube exchangers. We will first

discuss how to evaluate DTTM and then a little

later in the notes we will discuss how to

evaluate Ui for shell-and-tube exchangers. In a

shell-and-tube exchanger, the flow can be single

or multipass. As a result, the temperature

profiles for the two fluids in a shell-and-tube

heat exchanger are more complex, as shown

below.

Computation of DTTM For the concentric pipe

heat exchanger, we showed the following (parallel

and countercurrent flow) DTTM When a

fluid flows perpendicular to a heated or cooled

tube bank, and if both of the fluid temperatures

are varying, then the temperature conditions do

not correspond to either parallel or

countercurrent. Instead, this is called

crossflow.

DTlm

FG DTLM

The factor Z is the ratio of the fall in

temperature of the hot fluid to the rise in

temperature of the cold fluid. The factor hH

is the heating effectiveness, or the ratio of the

actual temperature rise of the cold fluid to the

maximum possible temperature rise obtainable (if

the warm-end approach were zero, based on

countercurrent flow).

From the given values of hH and Z, the factor FG

can be read from the text book figures

Therefore, as with the concentric pipe heat

exchanger, the true mean temperature difference

for the 1-1 exchanger is equal to the log mean

temperature difference (DTLM). For multiple

pass shell-and-tube designs, the flow is complex

and the DTLM is less than that for a pure

countercurrent design. We must account for the

smaller temperature driving force using a

correction factor, FG, which is less than 1 and

typically greater than 0.8. The rate of heat

transfer in multiple pass heat exchangers is

written as

where DTLM is the log mean temperature difference

for pure countercurrent flow

Textbook Figures 15.6 a, b

1-2 exchangers

2-4 exchangers

Ten Minute Problem -- FG for multiple pass

HE For a 2-4 heat exchanger with the cold fluid

inside the tubes and the following

temperatures Tca 85F Tha 200F Tcb

125F Thb 100F (a) What is the true mean

temperature difference? (answer D TTM

31.7F) (b) What exchanger area is required to

cool 50,000 lbm/hr of product (shell-side fluid)

if the overall heat transfer coefficient is 100

Btu/hr-ft2-F and Cp for the product is 0.45

Btu/lbm-F? (answer A 710 ft2)

V. Heat transfer coefficients In a

shell-and-tube exchanger, the shell-side and

tube-side heat transfer coefficients are of

comparable importance and both must be large if a

satisfactory overall coefficient is to be

attained.

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Gb

Gc

Gb

Exchanger Fouling

Electron microscope image showing fibers, dust,

and other deposited material on a residential air

conditioner coil and a fouled water line in a

water heater.

Exchanger Fouling

VI. Text Example Problem 15.3

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