30079_52.pdf

CHAPTER 52

HEAT EXCHANGERS,

VAPORIZERS, CONDENSERS

Joseph W. Palen

Heat Transfer Research, Inc.

College Station, Texas

52.1 HEAT EXCHANGER TYPES

AND CONSTRUCTION

52.3.4 Air-Cooled Heat

Exchangers

1607

1625

52.1.1 Shell and Tube Heat

Exchangers

52.3.5 Other Exchangers

1627

1607

52.1.2 Plate-Type Heat

Exchangers

52.4 COMMON OPERATIONAL

PROBLEMS

1610

1627

52.4.1 Fouling

1627

52.1.3 Spiral Plate Heat

Exchangers

1610

52.4.2 Vibration

1628

52. 1 .4 Air-Cooled Heat

Exchangers 1611

52.1.5 Compact Heat Exchangers 1611

52.1.6 Boiler Feedwater Heaters

52.4.3 Flow Maldistribution

1629

52.4.4 Temperature Pinch

1629

52.4.5 Critical Heat Flux in

Vaporizers

1613

1630

52.1.7 Recuperators and

Regenerators

52.4.6 Instability

1630

1613

52.4.7 Inadequate Venting,

Drainage, or Blowdown

1630

52.2 ESTIMATION OF SIZE AND

COST

1613

52.5 USE OF COMPUTERS IN

THERMAL DESIGN OF

PROCESS HEAT

EXCHANGERS 1631

52.5.1 Introduction 1631

52.5.2 Incrementation 1631

52.5.3 Main Convergence Loops 1631

52.5.4 Rating, Design, or

Simulation

52.2.1 Basic Equations for

Required Surface

1614

52.2.2 Mean Temperature

Difference

1615

52.2.3 Overall Heat-Transfer

Coefficient

1615

52.2.4 Pressure Drop

1616

1632

52.3 RATINGMETHODS

1616

52.5.5 Program Quality and

Selection

52.3.1 Shell and Tube

Single-Phase Exchangers 1616

52.3.2 Shell and Tube Condensers 1619

52.3.3 Shell and Tube Reboilers

and Vaporizers

1633

52.5.6 Determining and

Organizing Input Data

1633

1622

52.1 HEAT EXCHANGER TYPES AND CONSTRUCTION

Heat exchangers permit exchange of energy from one fluid to another, usually without permitting

physical contact between the fluids. The following configurations are commonly used in the power

and process industries.

52.1.1 Shell and Tube Heat Exchangers

Shell and tube heat exchangers normally consist of a bundle of tubes fastened into holes, drilled in

metal plates called tubesheets. The tubes may be rolled into grooves in the tubesheet, welded to the

tubesheet, or both to ensure against leakage. When possible, U-tubes are used, requiring only one

Mechanical Engineers' Handbook, 2nd ed., Edited by Myer Kutz.

Fig. 52.1 Schematic illustration of shell and tube heat exchanger construction.

tubesheet. The tube bundle is placed inside a large pipe called a shell, see Fig. 52.1. Heat is exchanged

between a fluid flowing inside the tubes and a fluid flowing outside the tubes in the shell.

When the tubeside heat-transfer coefficient is as high as three times the shellside heat-transfer

coefficient, it may be advantageous to use low integral finned tubes. These tubes can have outside

heat-transfer coefficients as high as plain tubes, or even higher, but increase the outside heat-transfer

area by a factor of about 2.5-4. For design methods using finned tubes, see Ref. 11 for single-phase

heat exchangers and Ref. 14 for condensers. Details of construction practices are described by

Saunders. 5 8

The Tubular Exchanger Manufacturers Association (TEMA) provides a manual of standards for

construction of shell and tube heat exchangers, 1 which contains designations for various types of

shell and tube heat exchanger configurations. The most common types are summarized below.

E-Type

The E-type shell and tube heat exchanger, illustrated in Figs. 52.2a and 52.2Z?, is the workhorse of

the process industries, providing economical rugged construction and a wide range of capabilities.

Baffles support the tubes and increase shellside velocity to improve heat transfer. More than one

pass is usually provided for tubeside flow to increase the velocity, Fig. 52. 2a. However, for some

cases, notably vertical thermosiphon vaporizers, a single tubepass is used, as shown in Fig. 52.2/?.

Fig. 52.2 TEMA E-type shell: (a) horizontal multitubepass; (b) vertical single tubepass.

Fig. 52.3 TEMA F-type shell.

The E-type shell is usually the first choice of shell types because of lowest cost, but sometimes

requires more than the allowable pressure drop, or produces a temperature "pinch" (see Section

52.4.4), so other, more complicated types are used.

F-Type Shell

If the exit temperature of the cold fluid is greater than the exit temperature of the hot fluid, a

temperature cross is said to exist. A slight temperature cross can be tolerated in a multitubepass E-

type shell (see below), but if the cross is appreciable, either units in series or complete countercurrent

flow is required. A solution sometimes used is the F-type or two-pass shell, as shown in Fig. 52.3.

The F-type shell has a number of potential disadvantages, such as thermal and fluid leakage

around the longitudinal baffle and high pressure drop, but it can be effective in some cases if well

designed.

J-Type

When an E-type shell cannot be used because of high pressure drop, a J-type or divided flow ex-

changer, shown in Fig. 52.4, is considered. Since the flow is divided and the flow length is also cut

in half, the shellside pressure drop is only about one-eighth to one-fifth that of an E-type shell of

the same dimensions.

X-Type

When a J-type shell would still produce too high a pressure drop, an X-type shell, shown in Fig.

52.5, may be used. This type is especially applicable for vacuum condensers, and can be equipped

with integral finned tubes to counteract the effect of low shellside velocity on heat transfer. It is

usually necessary to provide a flow distribution device under the inlet nozzle.

G-Type

This shell type, shown in Fig. 52.6, is sometimes used for horizontal thermosiphon shellside vapor-

izers. The horizontal baffle is used especially for boiling range mixtures and provides better flow

distribution than would be the case with the X-type shell. The G-type shell also permits a larger

temperature cross than the E-type shell with about the same pressure drop.

H-Type

If a G-type is being considered but pressure drop would be too high, an H-type may be used. This

configuration is essentially just two G-types in parallel, as shown in Fig. 52.7.

Fig. 52.4 TEMA J-type shell.

Fig. 52.5 TEMA X-type shell.

K-Type

This type is used exclusively for kettle reboilers and vaporizers, and is characterized by the oversized

shell intended to separate vapor and liquid phases, Fig. 52.8. Shell-sizing relationships are given in

Ref. 25. Usually, the shell diameter is about 1.6-2.0 times the bundle diameter. Design should

consider amount of acceptable entrainment, height required for flow over the weir, and minimum

clearance in case of foaming.

Baffle Types

Baffles are used to increase velocity of the fluid flowing outside the tubes ("shellside" fluid) and to

support the tubes. Higher velocities have the advantage of increasing heat transfer and decreasing

fouling (material deposit on the tubes), but have the disadvantage of increasing pressure drop (more

energy consumption per unit of fluid flow). The amount of pressure drop on the shellside is a function

of baffle spacing, baffle cut, and baffle type.

Baffle types commonly used are shown in Fig. 52.9, with pressure drop decreasing from Fig.

52.9a to Fig. 52.9c.

Baffle spacing is increased when it is necessary to decrease pressure drop. A limit must be

imposed to prevent tube sagging or flow-induced tube vibration. Recommendations for maximum

baffle spacing are given in Ref. 1. Tube vibration is discussed in more detail in Section 52.4.2. When

the maximum spacing still produces too much pressure drop, a baffle type is considered that produces

less cross flow and more longitudinal flow, for example, double segmental instead of segmental.

Minimum pressure drop is obtained if baffles are replaced by rod-type tube supports. 5 2

52.1.2 Plate-Type Heat Exchangers

Composed of a series of corrugated or embossed plates clamped between a stationary and a movable

support plate, these exchangers were originally used in the food-processing industry. They have the

advantages of low fouling rates, easy cleaning, and generally high heat-transfer coefficients, and are

becoming more frequently used in the chemical process and power industries. They have the disad-

vantage that available gaskets for the plates are not compatible with all combinations of pressure,

temperature, and chemical composition. Suitability for specific applications must be checked. The

maximum operating pressure is usually considered to be about 1.5 MPa (220 psia). 3 However, welded

plate versions are now available for much higher pressures. A typical plate heat exchanger is shown

in Fig. 52.10.

52.1.3 Spiral Plate Heat Exchangers

These exchangers are also becoming more widely used, despite limitations on maximum size and

maximum operating pressure. They are made by wrapping two parallel metal plates, separated by

Fig. 52.6 TEMA G-type shell.

Fig. 52.7 TEMA H-type shell.

spacers, into a spiral to form two concentric spiral passages. A schematic example is shown in Fig.

52.11.

Spiral plate heat exchangers can provide completely countercurrent flow, permitting temperature

crosses and close approaches, while maintaining high velocity and high heat-transfer coefficients.

Since all flow for each fluid is in a single channel, the channel tends to be flushed of particles by

the flow, and the exchanger can handle sludges and slurries more effectively than can shell and tube

heat exchangers. The most common uses are for difficult-to-handle fluids with no phase change.

However, the low-pressure-drop characteristics are beginning to promote some use in two-phase flow

as condensers and reboilers. For this purpose the two-phase fluid normally flows axially in a single

pass rather than spirally.

52.1.4 Air-Cooled Heat Exchangers

It is sometimes economical to condense or cool hot streams inside tubes by blowing air across the

tubes rather than using water or other cooling liquid. They usually consist of a horizontal bank of

finned tubes with a fan at the bottom (forced draft) or top (induced draft) of the bank, as illustrated

schematically in Fig. 52.12.

Tubes in air-cooled heat exchangers (Fig. 52.12) are often 1 in. (25.4 mm) in outside diameter

with 5 Xs in. (15.9 mm) high annular fins, 0.4-0.5 mm thick. The fins are usually aluminum and may

be attached in a number of ways, ranging from tension wrapped to integrally extruded (requiring a

steel or alloy insert), depending on the severity of service. Tension wrapped fins have an upper

temperature limit (~300°F) above which the fin may no longer be in good contact with the tube,

greatly decreasing the heat-transfer effectiveness. Various types of fins and attachments are illustrated

in Fig. 52.13.

A more detailed description of air-cooled heat exchanger geometries is given Refs. 2 and 3.

52.1.5 Compact Heat Exchangers

The term compact heat exchanger normally refers to one of the many types of plate fin exchangers

used extensively in the aerospace and cryogenics industries. The fluids flow alternately between

parallel plates separated by corrugated metal strips that act as fins and that may be perforated or

interrupted to increase turbulence. Although relatively expensive to construct, these units pack a very

large amount of heat-transfer surface into a small volume, and are therefore used when exchanger

volume or weight must be minimized. A detailed description with design methods is given in

Ref. 4.

Fig. 52.8 TEMA K-type shell.

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