Lecture4-Example_Shellandtubedesign.pdf

8/11/2019 Lecture4-Example_Shellandtubedesign.pdf

1/4

Shell and tube heat exchanger

design

Chee 331/332/333 1

Design of Shell and Tube Heat

Exchangers

Design strategy


design

Chee 331/332/333 2

Design of a shell and tube heat exchanger is an iterative process. The

suggested steps are outlined below.

Obtain an initial configuration, using the preliminary design techniquesthat you have learned.

Follow established design guidelines (see for example Serth, 5.7) and

tables to specify an initial configuration.

Rate the design, as you have learned in the rating example to

determine if the design is thermally suitable.

Use principles of hydraulics/fluid mechanics to assess if the design is

hydraulically suitable (see section 5.3 Serth and Heaslip notes for hydrauliccalculations for S&T, and section 4.3 Serth for double-pipe HEs).

Modify the design if necessary

Iterate, until an acceptable design has been obtained.

Solved example (5.1 From Serth)

Design of a shell and tube heat exchanger

45,000 lb/h of kerosene are to be cooled from 390F to 250F by heat

exchange with 150,000 lb/h of crude oil, which is at 100F. A maximum

pressure drop of 15 psi has been specified for each stream. Crude oil

exhibits significant tendency for fouling, with a fouling factor of 0.003

h.ft2.F/Btu. Design a shell and tube heat exchanger for this service.

Fluid properties are as follows:

Kerosene cp=0.59 Btu/lbm.F, =0.97 lbm/ft.h, k=0.079 Btu/h.ft.F, specific

gravity 0.785, Pr=7.24

Crude Oil cp=0.49 Btu/lbm.F, =8.7 lbm/ft.h, k=0.077 Btu/h.ft.F, specific

gravity 0.85, Pr=55.36


design

Chee 331/332/333 3

Initial specifications

Need to consider:

i. Fluid placement (see section 5.7.1 and Table 3.4)

ii. Shell and head types (i.e. floating head vs. fixed tubesheet), Type E

shell (single pass) vs. Type F shell (two-pass) see section 5.7.5


design

Chee 331/332/333 4

Tube bank can be removed from the shell for cleaning


2/4

Tubular Exchanger

Manufacturers Association

(TEMA)

Three columns designate design for:

(1) Front end

(2) Shell types

(3) Rear end

e.g. Shell type E is com monly us ed

Shell type relates to the

shell side fluid pattern

Front and rear end relate to

the tube side flow pattern

Design s tandards:

Serth, pg. 88Shell and tube heat exchanger

design

Chee 331/332/333 5


iii. Tubing (see section 5.7.2).

Outer diameters Do= and 1 in. are most commonly used. For water service

in., 16 BWG tubes are recommended. For oils, in. 14 BWT for non-fouling, or

1 in. 14 BWG tubes for fouling fluids. Tube length from 8-30 ft. A good startingpoint is 16 or 20 ft.

iv. Tube layout

Most commonly triangular or square, with a pitch of 1 in. (for in. tubes) or 1.25

in. (for 1-in.tubes).


design

Chee 331/332/333 6

Square and rotated square patterns permit mechanical cleaning of the outside of the

tubes

Tube dimensions


design

Chee 331/332/333 7


v. Baffles (see section 5.7.6)

Segmental baffles with a 20% cut are a good starting point. This corresponds to a

baffle spacing of 0.3 shell diameters (i.e. B/ds=0.3)

vi. Other details such as sealing strips and construction materials.

Plain steel for tubes and shell if neither fluid is corrosive, one pair of sealing stripsper 10 tube rows.


design

Chee 331/332/333 8

Sealing strips are thin strips of metal that reduce the effect of the bypass flow that flows

around the tube bundle. Mainly used in floating head heat exchangers.


3/4

Estimation of heat transfer area

Follow procedure from Example 1 (3.4 Serth) on estimation of heat transfer area

(a) Use energy balances to find the heat duty and any unknown parameters(temperatures or mass flow rate).

(b) Calculate LMTD and correction factor assuming 1-2 heat exchanger:


design

Chee 331/332/333 9

F=0.97>0.8 therefore one shell pass is suitable.

Estimation of heat transfer area

(d) Estimate the overall heat transfer coefficient, UD, from tables (for example

Table 3.5 Serth).

(e) Assuming UD=25 Btu/h.ft2.F, use the heat exchanger design equation to

calculate the required area and number of tubes.


design

Chee 331/332/333 10

This is your starting point!

Initial design

(a) Find the number of tube passes, keeping in mind that turbulent flow

inside the tubes is desired (ideally Re>10,000)

The allowable range for the corresponding velocity is from 3-8 ft/s

(b) Determine the actual tube count and corresponding shell inner

diameter, ds (Table C.5 Serth)

This concludes the initial design. Summary:

Tube-side fluid: crude oil

Shell-side fluid: kerosene

Shell: type AES ds=21.25- in.

Tube bundle: 156 tubes, 1-in. OD, 0.834 ID, 14 BWG, 20 ft long on 1.25-in.square pitch, arranged for 6 passes.

Baffle spacing 0.3 ds


design

Chee 331/332/333 11 Shell and tube heat exchanger

design

Chee 331/332/333 12


4/4

Documents

Lecture4-Example_Shellandtubedesign.pdf