Thermal Unit Operation (ChEg3113) -...

Thermal Unit Operation

(ChEg3113)

Instructor: Mr. Tedla Yeshitila (M.Sc.)

Lecture 9- Shell and Tube Heat Exchanger Design

Today…

Review

Shell and tube heat exchanger

Steps in Shell and tube heat exchanger

Review

Double pipe heat exchanger design

– Thermal design , and

– Hydraulic design

Example

Chapter 5

Design of Shell and Tube Heat Exchanger

Many industrial service requires use of large number of double

pipe hairpins, but it consumes considerable ground area and also

entail a large number of points at which leakage may occur. It

also result high pressure drop.

So shell and tube heat exchanger are best choice when large heat

transfer surface is required.

Shell and tube equipment involves expanding a tube into tube

sheet and forming a seal which does not leak under reasonable

operating condition.

Chapter 5

Design of Shell and Tube Heat Exchanger Stationary Tube Sheet Exchanger

The simplest type of shell and tube heat exchanger is fixed or

stationary tube-shell exchanger.

The essential parts are a shell (1) equipped with two nozzles and

having tube sheets (2) at both ends, which also serve as flanges

for the attachment of the two channels (3) and their respective

channel covers (4).

The tubes are expanded into both tube sheets and equipped with

transverse baffles (5) on the shell side.

The baffle are held securely by means of baffle spacer (6).

Chapter 5


Main components of shell and tube heat

exchangers are:

– Shell: is the case for the heat exchange

– Tube bundles

– Tube sheet (to support the tube bundles)

– Head

– Baffles

Chapter 5

Design of Shell and Tube Heat Exchanger The area between inside the shell and outside the tube is called

shell side of heat exchanger. It has its inlet and outlet.

Each end of tube opens to the head, so one head into the tube

and the other out of the tube.

The area inside the tube and head called tube side of heat

exchanger.

The tube side supported inside by partition called baffle. It also

direct flow to the shell side of the HX which increases the

efficiency of the unit.

Chapter 5


A tube hole is drilled in a tube sheet with slightly greater

diameter than the outside diameter of the tube, and two or more

grooves are cut in the wall of the hole.

The tubes are placed inside the tube hole,

and a tube roller is inserted into the end of the tube.

The roller is rotating mandrill having a slighter taper (thickness

reduce at the end). It is capable of exceeding the elastic limit of

the tube metal and transforming in to a semi plastic condition so

that it flows into the grooves and form extremely tight seal.

Tube rolling need great care to prevent tube damage.

Chapter 5

Design of Shell and Tube Heat Exchanger In some industry, the tubes are actually packed in the tube sheet by

means of ferrules (ring or cup) using a soft metal packing ring, so

that the tubes can be easily removed easily.

Chapter 5

Design of Shell and Tube Heat Exchanger Heat exchanger tubes (condenser tubes):

– They are different from steel pipes or other types of pipes which are

extruded to iron pipe size.

– The outside diameter is the actual outside diameter in inches with a

very strict tolerance.

– Can be made of steel, copper, aluminum, aluminum bronze, 70-30

copper-nickel, and stainless steel.

– They are obtained in a number of different wall thickness defined by

the Birmingham wire gage (BWG) or gage of the tubes.

– The sizes of tubes are listed in Table 10 of Appendix of which the ¾

outside diameter and 1in. OD are the most common in heat exchanger

design.

Chapter 5


Tube pitches:

– Tube holes can not be dilled very close together, since too small a

width of metal between adjacent tube structurally weakens the

tubes walls.

– The shortest distance between two adjacent tube holes called

clearance (ligament).

– Tube pitch (PT) is the shortest center to center distance

between two adjacent tubes.

– Tubes are laid out either in square or triangular pattern.

– The advantage of square pitch is that the tube accessible for

external cleaning and cause a lower pressure drop when a fluid

flow in the direction indicated.

Chapter 5


Square pitch can be rotated 450 or if the tubes are spread wide

enough, triangular pitch can include mechanically cleanable

modification.

The common pitches for square layouts are:

• ¾ in. OD on 1 in. square pitch

• 1 in. on 1 ¼ in. square pitch

The common pitches for triangular layouts:

• ¾ in. OD on 15

16in. triangular pitch

• ¾ in. OD on 1 in. triangular pitch

• 1 in. on 1 ¼ in. triangular pitch

Chapter 5

Design of Shell and Tube Heat ExchangerShell:

– They are fabricated from steel pipe with nominal IPS diameter

up to 12in. as given in the Table 11.

– Above 12in. and including 24 in. the actual outside diameter

and the nominal pipe diameter are the same.

– The standard wall thickness for shells with inside diameters

from 12 to 24 in. inclusive is 3/8 in. which is satisfactory for

shell-side operating pressure up to 300 psi.

– Pressure drop at a given thickness is to be used.

– Greater wall thickness may be obtained for grater pressure.

– Shells above 24 in in diameter are fabricated by rolling steel

plate.

Chapter 5

Design of Shell and Tube Heat Exchanger The calculation of the effective heat-transfer surface is

frequently based on the distance between the tube sheets instead

of the overall tube length.

Baffles

Higher heat transfer coefficient results when a liquid is

maintained in a state of turbulence.

To induce turbulence outside the tube, baffles are used which

cause liquid to flow through the shell at right angles to axes of

the tubes.

Baffle spacer

The center to center distance between baffles called baffle

pitch or baffle spacing.

Chapter 5


Chapter 5


Since the baffle may be spaced close together or far apart, the

mass velocity is not entirely dependent upon the diameter of the

shell. That means, it depends also on the baffle distance.

The baffle spacing is usually not greater than a distance equal to

the inside diameter of the shell or closer than one-fifth the inside

diameter of the shell.

Common types of baffles are:

Segmental baffle Disc and doughnut baffle Orifice baffle

Chapter 5


Segmental baffles are dilled plates with heights which are

generally 75 percent of the inside diameter of the shell. These

are known as 25 percent cut baffle.

Even though there are different percent cut baffle, we use 25

percent cut baffle in the next lectures.

The baffle pitch, not the 25 percent cut of the baffles determines

the effective velocity of the shell fluid .

Figure. Temperature relationship in 1-2 exchanger

Chapter 5


1-2 exchanger means

one pass in the shell

and two pass in the

tubes.

The assumptions in 1-2 exchanger:

1. The shell fluid temperature is an average isothermal

temperature at any cross section.

2. There is an equal amount of heating surface in each pass.

3. The overall coefficient of heat transfer is constant.

4. The rate of flow of each fluid is constant.

5. The specific heat of each fluid is constant.

6. There are no phase changes of evaporation or condensation

in a part of the exchanger.

7. Heat losses are negligible.

Chapter 5


Chapter 5


Steps in Shell and tube heat exchanger design (1-2 exchanger)

Process conditions required:

– Hot fluid: T1, T2, W, C, s or 𝛒, μ, k, 𝜟P, Rd

– Cold fluid: t1,t2, w, c, s or 𝛒, μ, k, 𝜟P, Rd

For the exchanger the following data must be given:

Tube side

Number and length

OD, BWG, and pitch

Passes

Shell side

ID

Baffle space

Passes

Chapter 5

Design of Shell and Tube Heat Exchanger1. Heat balance

Q=WC(T1-T2) = wc(t2-t1)

2. True temperature difference

For a heat exchanger with countercurrent flow, the mean

temperature difference is known as the log mean temperature

difference, ΔTLM or LMTD.

𝐿𝑀𝑇𝐷 =𝜟𝑡2−𝜟𝑡1

2.3 log (𝜟𝑡2/𝜟𝑡1)

The log mean temperature difference is the maximum mean

temperature difference that can be achieved in any geometry of

heat exchanger for any given set of inlet and outlet temperatures.

Chapter 5

Design of Shell and Tube Heat Exchanger For any other type of heat exchanger, the mean temperature

difference can be expressed as

Δt = 𝐿𝑀𝑇𝐷* FT (FT from figure 18)

Where FT is always less than or equal to 1.

Estimating the mean temperature difference in a heat exchanger

by calculating the log mean temperature difference and

estimating FT is known as the F factor method.

FT varies with geometry and thermal conditions.

The thermal conditions are defined by parameters such as the

overall heat transfer coefficient, U, the area available for heat

transfer, A, the mass flow rates of the two steams and , the

specific heat capacities of the two streams c1 and c2, and the

temperature change in each stream (T1 - T2) and (t2 – t1).

Chapter 5


For any given geometry, FT is often presented as a function of

two non dimensional parameters:

– R, the ratio of the thermal capacities of the two streams, and

– P (sometimes known as effectiveness, E), the ratio of the

achieved heat transfer rate to the maximum possible heat

transfer rate.

R= wcWC

=T1−T2

t2−t1, S=

achived heat transfer ratemaximum heat trassfer rate

=t2−t1

T1 −t1

Chapter 5


Figure. Typical relationship between F and P for various values of S (Based on

Single pass shell with any even number of tube passes).

S

Chapter 5


3. Caloric temperature (TC and t C):

TC=T2+FC (T1-T2)

t C=t1+FC (t2-t1)

At the end of this class:

You will have more know how about shell and tube exchanger

design

You will have the concept of true temperature difference and

caloric temperature calculation

You will be able to do the first 3 steps in shell and tube

exchanger design

End of lecture -9

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