3 heat exchangers

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Table of Contents

Heat Transfer

1.1 Heat Transfer Mechanisms

1.2 Factors Affecting Heat Transfer1.3 Basic Conduction / Convection Equations

Types of Heat Exchangers

2.1 Type of Service

2.2 Type of Equipment

Shell and Tube Exchangers

3.1 Floating Tube sheet Exchanger

3.2 Tube Arrangement

3.3 Fluid Placement

Double pipe Exchangers

4.1 Indirect Heaters

AirCooled Exchangers

5.1 Heat Transfer Calculations

5.2 Fans5.3 Combination Coolers

5.4 Choice of Heat Exchangers

Heat Exchanger Operations

6.1 Procedure to Take a Heat Exchanger Out of Service.

6.2 Procedure to Place heat Exchanger in Service.

6.3 Testing Heat Exchanger for Leaks.

Heat Exchanger Problems

7.1 Heat Exchanger Fouling and Corrosion.

7.2 Vibration in Heat Exchangers.

7.3 Cleaning of Heat Exchangers.


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Radiation is the process whereby a body emits heat waves that may be absorbed,

reflected or transmitted through a colder body. The sun heats the earth by means of

electromagnetic waves. A hot body emits a whole spectrum of wave lengths.

Radiation which affects the eye as light extends roughly from 0.00004-0.00008 cm in

wave length. To the right of this visual spectrum is the infraredregion; to the left is

the ultraviolet region. Heat is transferred throughout the full wave length range. Astemperature increases the predominant wave lengths become shorter. A detailed

understanding of radiation is provided by the quantum theoryof physics.

1.2 Factors Affecting Heat Transfer

(a) Temperature difference - the greater the temperature difference

between two materials the greater the driving force causing heat transfer.

(b) Thermal conductivity - every substance has a definite thermal

conductivity which affects the amount of heat transferred. Metals are

good conductors while wood and carbon are very poor conductors.

(c) Area- the cross-sectional area affects the heat transfer. The larger the

area, the more heat can be transferred.

(d) Velocityof the fluids in the tube affects the amount of heat transfer.The velocity also affects the fouling with higher velocities reducing the

possibility of scale or dirt deposits on the tubes. An increase in the

velocity of the fluids increases the heat transfer rate.

(e) Di rection of fl owof the liquids exchanging heat influences the rate of

heat transfer. It is seen from the diagrams that when using identicalequipment with equal rates of flow, the one with counter-current flow and

the other with parallel flow, the final temperature will be higher with

counter current flow.

Therefore, in the design of this equipment, countercurrent flow is usually

preferred to parallel flow due to the fact that the cooler medium can beraised to a higher temperature and that in general, a smaller area is needed

for the same amount of heat transfer. As can be seen from the diagrams,

countercurrent flow occurs when hot and cold fluids travel through the

exchanger in opposite directions; while in parallel flow, both hot and cold

fluids travel through the apparatus in the same direction.


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Values of the overall coefficient "U" may be predicted from Equations 3 or 4 or from

actual performance. Most heat exchanger quotations show the overall "U" used in

preparing them. These, plus plant operating data, are a valuable source of information

for future planning. Equations 3 and 4 differ only in the surface area used for

reference. They assume one fluid is flowing inside of the exchanger tubing and one is

flowing along the outside surface. "U" will vary with area so that U 1A1= U2A2. Inshell-and-tube exchangers the heat transfer area "A" is almost always based on

outside tube wall area.

1.3.1 Effective T

Equation 2 is the basic equation used for design. It contains the term tm. This is themean t because the t across the wall surface varies with location as shown below.

(A) Two fluids flowing countercurrent, no phase change.

(B) Two fluids flowing concurrent, no phase change.

(C) One fluid flowing and one boiling (or condensing).

(D) Superheated vapor being cooled to saturation (a) condensing (b) and being

subcooled as liquid. The other fluid is boiling or condensing.

(C) (D)

The only temperatures that we can measure conveniently are at the inlet and outletends of the exchanger. Thus, we can measure two ts. The larger we will call t1,the smaller t2. t2 is also called the approach. It designates how close thetemperatures of the two fluids approach each other in the exchanger.

In concurrent flow the fluids flow in the same direction. In countercurrent flow they

flow in opposite directions. Most exchangers use countercurrent flow, or as close to it

as possible, since it is more efficient.


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The basic equation for estimating tmis:

tm= (F))t/t(Ln

tt

21

21

(5)

Where tm= log mean temperature difference (LMTD)F = factor for heat exchanger

t1= largest At (at one end of the heat exchanger) t2= smallest At (at one end of the heat exchanger)

In= logarithm to the base e

The value of F depends on the geometry of the fluid flow in the exchanger and will be

discussed later for each type. F = 1.0 for a concentric pipe-in-pipe exchanger.

Equation 5 can be derived from the calculus for this situation.

1.3.2 Approach

The approach t2, is an economic choice. Its specification governs heat exchangercost. As t2gets smaller, LMTD becomes smaller and area required becomes larger.As LMTD approaches zero, area approaches infinity. Since the cost of the heat

exchanger is a direct function of area, specification of approach has a direct effect on

cost. In order to have the optimum cost installation, a series of exchangers in series

may be used.

The approach used often will be in the following range:

Aerial coolers, 10-25C [18-45F]Water cooling of hydrocarbon liquids and gases, 8-12C [14-22F]

Liquid-liquid heat exchange, 11-25C [20-45F]

Refrigeration chillers on gas-liquid streams, 4-6C [7-11F]

When specifying heat exchangers it often is desirable to specify a

maximum or minimum approach to the vendor. This does not fix the

actual approach. It merely establishes an upper or lower limit, below or

above which the actual approach must occur.


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Figure 1 Nomograph for LMTD

1.3.3 Vaporizing (Boiling) Liquids

There is a special concern when one of the heat exchanger fluids is vaporizing. Thisoccurs in refrigeration chillers and fractionation reboilers, as two examples.


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From Equation 2, you would expect a plot of Q/A versus t to yield a line like thestraight line at left. It does except for boiling liquids. The solid curve at right is what

really occurs with boiling liquids. At some value of t the curve changes directionand Q/A decreases rapidly to a minimum, after which it begins to rise again. Why?

As shown in the sketch above, a layer of gas bubbles can build up around a tube if

vaporization occurs at the tube wall faster than the vapor can disengage and rise

through the liquid. This layer of bubbles forms an extra resistance in series and is atype of fouling factor.

When t across the tube reaches a critical point, the bubble layer forms and Q/Adecreases. If t continues to increase, the layer resistance stabilizes and Q/A beginsto increase again.

The critical t depends on the liquid and the character of the tube surface. Thecritical t may occur as low as 20-35C. Special tube surfaces are marketed whichare designed to minimize bubble layer formation.

There are two basic mechanical factors which affect vapor disengagement - spacing

and arrangement of the exchanger, and the area available between the liquid and

vapor phases. As vapor forms it must get away from the surface quickly. There also

must be enough surface area so that the resistance at the vapor-liquid interface does

not limit vapor disengagement.


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In the figure below are shown two tube configurations among the many available.

This is known as triangular layout since the tubes in adjacent rows are not directly

above or below each other. To improve vapor disengagement between tubes, the tube

pitch is typically 1.5 to 2 rimes the tube diameter.

Another alternative is the square layout where tubes in adjacent rows are directly

above or below each other. Although not as common as triangular layout, square

layout has been used in corrosive service such as amine regeneration.


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Figure 2 Typical Exchanger Tube Layout Patterns


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Notice in the previous figure that there is room above the tubes for vapor. The

arrangement shown is typical for chillers and reboilers where the liquid covers all

tubes. The area of the liquid surface must be sufficient, which means that the shell

diameter must be larger than that needed to merely hold the tubes. The arrangement

at the right might be used for a condenser to allow good vapor distribution. We have

not shown baffles which might also be needed for good distribution.

As the tubes are farther apart there is more room for vapor to rise. But, the cost of the

exchanger increases. Be sure the low bid on your reboiler or chiller has enough vapor

space.

Also, be sure the vapor outlet flanges and piping have sufficient area.. If not, vapor

can back up, "choke" the exchanger, and limit capacity even though the tube area is

adequate.

1.3.4 Flow Path

Fluids flowing through a heat exchanger can take one or a combination of

these paths: parallel flow, counter flow, or cross-flow.

a- Parallel Flow. in parallel flow, fluid flowing inside the tubes flows in thesame direction as the fluid flowing outside the tubes. This flow pattern yields

the least amount of heat transfer because it does not maintain a high

temperature difference between the fluids.

Suppose the hotter fluid is flowing inside the tubes and the colder fluidoutside the tubes. At the inlets the temperature difference is the greatest,


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but at the outlets the colder fluid has absorbed enough heat from the

hotter fluid that the temperature difference is relatively small. Therefore,

heat transfer at the outlets drops off considerably.

b. Counter Flow: In counter flow, sometimes called reverse flow, fluid inside thetubes flows in one direction while the fluid outside the tubes flows in the other

direction. This flow pattern yields the most heat transfer because temperature

difference remains relatively high all the way through the heat exchanger.

Suppose the hotter fluid is flowing inside the tubes and the colder fluid

outside the tubes. Although the colder fluid picks up heat along its path, it

will exit the heat exchanger at the point where the hotter fluid is enteringat its highest temperature. At the point here the hotter fluid has been

cooled and is existing the heat exchanger, the colder fluid is entering at its

lowest temperature. Therefore, the temperature difference between thefluids remains higher throughout the heat exchanger.

Cross-Flow:In cross-flow, fluid outside the tubes flows at right angles to fluid inside

the tubes. This flow pattern creates more turbulence in the fluid outside the tubes

which increase the amount of heat transfer. Cross-flow is commonly used in

conjunction, with parallel flow and/or counter flow fluid paths.


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1.3.5 Fouling Factors

The fouling factor (Ff) shown in Equations 3 and 4 must be estimated from

experience. The figure at right shows a temperature gradient across a wall, including

the possible corrosion or depositional scales. If scale forms, how fast will it form?

This is a question that requires a detailed analysis.

Some erroneously use a fouling factor as an arbitrary safety factor. Use of a fouling

factor is all right so long as the number used is a realistic one compatible with

expected performance. If large a number is used it controls U and invalidates the

calculation.

In Equations 3 and 4 the units of Ff are the reciprocal of those for "U" or

"h." It is customary to talk about a fouling factor by quoting a whole

number. The number quoted must be inserted in these equations with two

zeros in front of it. For example, a fouling factor of 5 would be written as

0.005.

We hesitate to quote any fouling factor for fear it will be misused. The ones shown

below are ones that we often note. We offer them without comment.

Units of "h" FfBtu/hr-ft -F

W/(m2.C)

0.001-0.0015

0.006-0.009

These numbers are used often for nonscale forming liquids, free of suspended solids.


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2. Types of Heat Exchangers

The kinds of exchangers commonly found in oil or gas processing are now described

and references given for further study. Type of service is discussed first, followed by

a description of the various equipment types and their application.

2.1 Type of Service

The functions of heat exchangers are manifold. The following are typical

of gas processing.

2.1.1 Heating.At the wellhead natural gas is often passed through a choke to regulate

the flow. In some instances advantage can be taken of this to deliberately lower the

gas temperature, as in the LTS process. In other cases, the gas from the choke is

delivered directly into gathering lines. In this latter case, the gas must be kept abovethe hydrate temperature at all times. The temperature-lowering effect of the choke or

Joule-Thomson expansion valve may have to be offset by heating the gas upstream of

the choke.

Another instance is long gathering lines. Again the gas may be passed through heaters

at various points along the line to maintain the temperature above the hydrate point.

2.1.2 Gas-to-Gas Exchange.This important service is often found in NGL recovery.

Here the goal is to allow the cold residue gas to approach as closely as possible the

temperature of the inlet gas, thus either maximizing the savings in refrigeration or

allowing a lower processing temperature.

2.1.3 Chilling.Recovery of NGL from natural gas can be increased by cooling the

gas in an exchanger with a refrigerant scream, such as liquid propane. The cold

propane removes heat from the gas, vaporizing in the process.

2.1.4 Reboiling. This service is very similar to chilling, except that the vaporizing

fluid is now the process stream and the energy source is the heating medium, which

can be hot gas, hot water, steam, hot oil or hot combustion gases.

Reboiling is required in such services as condensate stabilizers or fractionators and

amine and glycol solution regenerators. Boiling also occurs in the refrigerant side of a

chiller.


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2.1.5 Inter- and After-Cooling. When gas is compressed it is healed and must be

cooled prior to further compression to avoid excessive temperatures or to reduce

horsepower. Hot gas must be cooled also before injection into a pipeline to avoid

higher pressure drop and thus higher compression cost.

Cooling is accomplished by three media: cooling air, cooling-tower water, ortempered cooling water (Brown and Benkly, 1974). Air cooling is accomplished by

fintubed heat exchangers, discussed below. Cooling towers are beyond the present

scope; cooling towers cool recirculated water by evaporating a portion of it.

Tempered cooling water is usually an ethylene glycol solution in water. which, in turn

is cooled using seawater.

2.1.6 Desuperheating and Condensing.After a refrigerant is vaporized in the chiller,

it is compressed to a pressure and temperature at which it can be condensed by

rejecting heat to the surroundings. This condensation occurs in a cooling-water or air-

cooled exchanger.

2.1.7 Condensing. Conventional distillation columns require overhead product

condensation to provide the necessary reflux and to supply the distillate product in

convenient liquid form. A coolant (air, water) or refrigerant is used for this purpose.

Exchanger Materials.Like most process equipment, heat exchangers are fabricated

from carbon steel where possible. Exceptions are made for low temperatures and

corrosive materials.

Carbon steelbecomes brittle at approximately 20F. Charpy-impact-tested carbon

steel can be used to40F. Between -40 to - 150T, 3.5% Ni steel is used and below -

150F stainless steel is used. Stainless steel (300 series) may be more readily

available or more economical than low Ni steel. In plate-fin heat exchangers one

exception is aluminum,which can be used at any cryogenic processing temperature.

Gas containing hydrogen sulfide and carbon dioxide can cause severe corrosion.

Stainless steel is often required for this service. Carbon steel and a type of brass called

Admiralty metal are sometimes used for cooling water.

2.2 Type of Equipment

The exchangers used in gas processing are of several different basic geometricalconfigurations or types. The more important types and their appropriate services are

now reviewed.


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3. Shell-and-Tube Exchangers

The name describes the geometrya bundle of tubes mounted within a cylindrical

shell. The "tube-side" fluid flows inside the tubes and the "shell-side" fluid passes

inside the shell but outside the tubes. The two fluids exchange heat through the tube

walls. Shell-and-tube exchangers are by far the most common type in gas processing.

Shell-and-tube exchangers are named by the Tubular Exchanger Manufacturers

Association TEMA (1988) as shown in Figure 5. Letters are given to indicate the

style of head (or front) end. shell, and rear end. Selection guidelines may be

summarized as follows (McGlynn. 1989).

Head. Type B. the bonnet (integral cover) is often used for hazardous (H2 or HF)

gasses in refineries, for high pressure service in gas plants, and for clean fluids.

Removable covers. type A or N, are used when cleaning is required. Bonnets are

cheaper than removable covers and reduce leaks by eliminating one gasket.

Shell.The one-passshell, type E. is most common. A close temperature approach or

pinch or a temperature cross can require two or more shells in series to achieve an

acceptably high LMTD (or F factor). A two-pass shell, type F, has a much higher

LMTD F factor but has a much higher pressure drop. Fluid leakage past a longitudinal

baffle (unless welded to the shell) can reduce heat transfer dramatically. Less than

0.01 in. clearance between the baffle and the shell can reduce heat transfer by 30% or

more. Divided flow shells (type J) reduce shell-side pressure drop to about one-eighth

of a comparable E shell.Kettle(type K) shells are used for reboiling or vaporizing, as

in a chiller.

Rear.There are three types: first is thefixed-tube-sheet exchanger, shown in Figure 8.

This figure also shows the standard TEMA nomenclature. Fixed-tube sheets are

relatively hard to remove or replace; therefore, they are used for clean streams and

low temperature differences.

The second rear-end type is thefloating-headexchanger, depicted in Figures 3 & 4 &

6. Floating-head exchangers are used in a variety of services. Manufacture is more

expensive than for the fixed-tube sheet, but the channel head permits easier access for

maintenance. Also, the floating head allows large temperature difference between

ambient and operating conditions without excessive thermal stress on the equipment.


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The third type is the U-tube (Fig. 8A). Like the floating-head, the U-tube is a

removable bundle and has similar advantages. However, cleaning inside the tubes is

extremely difficult.

In gas processing three kinds of shell-and-tube exchangers predominatefixed

tubesheet (Fig. 7 A & B), floating head (Fig. 6), and kettle (Fig 8 A). Figure 7 Cshows exchanger type BEM, for bonnet front end, one-pass shell, with fixed tube

sheet. Similarly, the Figure 6 exchanger is an AES for the channel head, one-pass

shell, and floating head. The Figure 8 A exchanger is an AKT to designate the

channel head, kettle shell, and "pull-through" floating head (needed here for

maintenance purposes). Kettles are also built with U-tube rear heads (e.g.,AKU).

Exchanger size is indicated by two numbers, the inside diameter (ID) of the shell and

the tube length, both in inches. For example, a 29-in. ID shell with 16-ft long tubes is

referred to as size 29-192. A kettle with 23-in. ID front-end flange, a 37-in. ID kettle

shell, and 16-ft long tubes is 23/37-192.

Common tube diameters are 0.75 and 1.0 in. outside diameter (OD) with varying

thickness (usually 12 to 16 BWG). Standard lengths are 8. 10. 12. 16, and 20 ft.

Figures 6 show the basic characteristics of shell-and-tube exchangers. The major

manufacturers of such equipment have a trade association (TEMA) which has a set of

standards. They are not a code but are used commonly in bid specifications. Class R

exchangers are used most commonly in the petroleum industry.

The choice of configuration depends on a number of considerations - fluids involved,

corrosion potential, problems of cleaning, pressure drop, heat transfer efficiency. Heat

exchanger selection is not routine.

Do you need removable or nonremovable tube bundles? The latter are relatively

inexpensive and provide maximum protection against shell-side leakage but they are

not accessible for mechanical shell-side cleaning. A type of expansion joint is

sometimes needed to relieve differential thermal expansion stresses.

Removable tube bundles consist of U tubes (hairpin type) or straight tubes with a

floating head. The former is the least expensive, can be used with very high pressures

on the tube-side and no shell-side impingement plates are necessary but, mechanical

cleaning is difficult and it is very difficult to replace tubes.


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The floating head exchanger is the most versatile and most expensive. Obtaining a

positive seal between tube-side and shell-side fluids is critical in many cases. The

pressure differential between shell- and tube-sides is limited by the seal.

Although there are exceptions, most tubes used are 1.5-2.5 cm [5/8-1.0 in.] diameter,

1.9 cm [3/4 in.] is the most common. The larger size normally is used when fouling isanticipated, to facilitate mechanical cleaning. The tube length may be as large as 12 m

[40 ft] but tubes about half this length are more commonly employed.

The tube bundle can be arrayed in a triangular, square or rotated-square layout.

Triangular usually gives better shell-side "h" values and more heat transfer area for a

given shell diameter. However, the other arrangements are easier to clean and have a

lower pressure drop.

3.1 Floating Tube Sheet Exchangers

This type of exchanger is the most common type used in refinery operation. Note theflow arrows through the different tube passes and how the floating head has room tomove inside the shell cover.

The shell side flow is single pass but the liquid path is controlled by the baffles.

Without baffles, the tubes would sag and the flow would be

parallel to the tubes and give poor heat transfer. Each exchanger isdesigned for a specific heat duty with baffles at set distances.

Details of a Floating Head

At one time, the floating head was bolted directly to the floating tubesheet and could

be left on the tubesheet when it was removed from the shell this assembly however

left a large dead space between the outside row of tubes and the shell.

The split backing ring makes full use of the shell but has to be removed before the

bundle can be pulled. Note the gaskets and how internal gasket failure can cause

contamination of the liquids between shell and tube.


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Figure3C

ross

SectionalVie

w

of

FloatingT

ubesheetExchanger


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Figure4Shelland

TubeHeatExchanger

Detailofa

FloatingHead


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3.2 Tube Arrangement

Heat exchangers are built so fluids will have one pass, two passes, or multiple passes

through the exchanger, depending on the arrangement of the tubes. They may also

have a combination of flow paths.

The tubes in heat exchangers have either straight tube arrangements or U-tube

arrangements. In straight tube heat exchangers, fluid enters one end of the tubes,

flows straight through and exits the other end. This is called a single pass heat

exchanger.

In U-tube heat exchangers, fluid enters one end of the tubes and flows to

the other end. However, instead of exiting, the tubes bend back in the

shape of a U. Fluid flows around the bend and back to the first end, then

exits the heat exchanger. This is called a double pass heat exchanger

Heat exchangers can also be designed for multiple passes. Tubes are

built to change the direction of flow through the heat exchanger severaltimes before the fluid exits. The more passes between the fluids, the moreheat can be transferred.


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Figure 5 Basic Mechanical TEMA Characteristics


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Figure 6 1-Pass Shell, 2-Pass Tube Exchanger


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A- 1- Pass Shell and Tube with Expansion Joint on Shell Side

B- 1- Pass Shell and 2-Pass Tube

C- Hairpin

Figure 7 Three Other Examples of Tubular


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Exchangers


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A- Kettle Reboiler

B- Thermosiphone Reboiler

Figure 8 Two Common Types of Reboilers

Figures 10l5 provide a means to estimate the factor F shown on the left ordinate.

Values of P and R on these figures are found by the equations

P =11

12

tT

tt

, R =

12

21

tt

TT

For a given value of P and R, find the corresponding value of F. If the values of P and

R do not intersect within the grid, simply record F as less than 0.5.


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As shown on each figure, T1 and T2 represent the shell-side fluid and t1 and t2 the

tube-side fluid.

This correlation is a modified version that has been in use for a long time. An

alternate calculation to that shown has been developed. The net results are essentially

the same.

3.3 Fluid Placement

This obviously affects the value of F in the LMTD calculation. But, the major

consideration may be the character of the fluid itself. The following general

guidelines are useful.

A. Shell-Side

1. Viscous fluid to increase (generally) the value of "U"

2. Fluid having the lowest flow rate

3. Condensing or boiling fluid

B. Tube-Side

1. Toxic and lethal fluids to minimize leakage

2. Corrosive fluids

3. Fouling fluids; increased velocity minimizes fouling but enhances erosion

4. High temperature fluids requiring alloy materials

5. High pressure fluids to minimize cost

6. Fluid on which pressure drop is most critical

These are not mutually exclusive considerations. Some priorities must be established;

some compromises are necessary. For example, condensing may be done on the tube

side when special metallurgy is required. In this case, vertical tubes normally are a

better choice than horizontal tubes.

In some cases a series of exchangers (train) is required. One then must divide the total

heat transfer duty to optimize the number and size of each unit

Estimation of Mechanical Design

As part of the early planning function, it may be desirable to estimate the physical size

of the exchanger being considered. Figure 16 provides an easy method to accomplish

this. The equation for use with this figure is

A=AoF1F2F3 (6)

Where: A = area on left-hand ordinate of figure

A0= area calculated from heat transfer equation

F1,F2,F3= correction factors


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F1,F2and F3 are equal to unity for 3/4-in. tubes on a 15/16-in. triangular pitch, one

tube pass and a fixed tube sheet exchanger, respectively.

For a given value of A, various diameter/length combinations are suitable. These

would need to be checked for fluid velocities. L/D ratios less than 3:1 may suffer from

poor fluid distribution. Ratios in range 6:1-10:1 generally are a good compromise.These L/D ratios are shown as dashed lines in Figure 16.

4. Double-Pipe Exchangers

Figure 9 shows a double-pipe,or hairpin,exchanger. One fluid passes through the

inside of the inner pipe and the second fluid flows through the annulus between

the outside of the inner pipe and the inside of the outer pipe. The flow is

countercurrent, so that the F factor is 1.0 for this type of exchanger.

Double-pipe exchangers have limited area and are used for services with small

heat duties (UA < 100,000 Btu/hr F). If one of the fluids shows a very low heattransfer coefficient, that fluid can be placed in the annulus and longitudinal fins

used on the outside of the inside pipe. The extended surface of the fins provides

better heat transfer for the fluid with higher resistance. These exchangers are used

primarily for heating and cooling of gas streams. McDonough (1987) reviews

design procedures and areas of application. Manufacturers, e.g.,

F igure 9 Double Pipe Heat Exchanger (GPSA, 1987)

Brown Fintube (1989), present detailed information on fin efficiency,heat resistances, and pressure drops.


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4.1 Indirect Heaters

The indirect heater is similar to a shell and tube exchanger, being composed of a large

cylindrical shell with a tube bank mounted in the upper portion for heating the process

fluid (Fig. 10). A U-shaped fire tube is located in the bottom, and the shell is filled

with a heat transfer medium that transmits heat from the fire tube to the tube bank.

Heat is provided by burning gas or oil in the burner. The naming combustion gas

passes through the first section of the fire tube, giving up mainly radiant heat with a

lesser amount of convective heat. The hot gases then turn and pass through the return

section, in which the heat transfer is mainly by convection. The gases then flow up the

stack to the atmosphere. Indirect heaters are akin to fire-tube boilers.

Water has an unequaled ability to transfer heat and so is almost always used as the

heat transfer fluid for applications between 35 and 190F. Adding ethylene glycol

extends the range from 50 to 250F. Special heating oils are used for higher

temperature service, say up to 650F; these oils have a low vapor pressure and high

specific heat. Molten salt is used for high-temperature applications from 500 to

900F. Molten salt will not decompose, as will oils, and has good heat transfer

properties. Ballard and Manning (1989) discuss the design and operation of heat-

transfer-fluid systems and also discuss in detail the evaluation of heat-transfer fluids.

F igure 10 Water Bath I ndirect Heater (GPSA, 1987)


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The conventional glycol regenerator is essentially an indirect heater converted to

direct-heat service by placing the glycol outside the fire tubes. The regenerator has a

small packed column mounted atop it to furnish a bit of reflux for the outgoing

vapors, and thus prevent glycol vapor loss with the discharged water vapor.

In gas processing, indirect heaters are used primarily for hearing gas streams,including regeneration gas in small solid-desiccant dehydration units.

5. Air-Cooled Exchangers

Air-cooled exchangers have the process fluid inside the tubes and ambient air on the

outside, either moving by natural convection or blown by a fan. Because of the low

heat-transfer coefficient for atmospheric air, fins are used on the outside of the pipes

(Fig. 11).

Both inducedandforced draftfans are used (Fig. 12). The latter are specified in mostapplications. When recircularion of cooling air is a problem, induced draft fans are

used to provide positive outflow of the air.

F igure 11 Details of F inned Tubes and Exchanger Bundle (Cook,

1964, Vol. 95 No. 10,22-26, 1988)


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Special precautions are needed for cold climates (API, 1988; Shipes, 1974; Brown

and Benkly, 1974; Franklin and Munn, 1974). Wind skirts or housing may be

necessary, as well as air recirculation.

The flow pattern in air-cooled exchangers is cross flow, with either an odd or even

number of tube passes.

In air-cooled exchanger design, a difficulty arises with the exit air temperature, which

is needed to estimate the LMTD. Air enters the bottom of an air-cooled exchanger at.

essentially constant temperature but is heated differently in each location across the

exchanger. Rigorous estimation of the average outlet air temperature would be very

complex. GPSA (1987Section 10) details a method of estimating the outlet air

temperature and designing air-cooled exchangers. Brown (1978), Ganapathy (1978),

and Glass (1978) provide detailed design information.

Air-cooled exchangers are used for inter- and after-cooling of compressed gases,

desuperheating and condensing refrigerant streams, and fractionator condensers.

F igure 12 Typical Side Elevations of Air Coolers (GPSA, 1987)


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Figure13FinFanCoolers


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Advantages of Induced Draft Design

1. Easier to shop assemble, ship and install.

2. The hoods offer protection from weather.

3. Easier to clean underside when covered with lint, bugs, debris.4. More efficient air distribution over the bundle.

5. Less likely to be affected by hot air recireulation.

Disadvantages of Induced Draft Design

1. More difficult to remove bundles for maintenance.

2. High temperature service limited due to effect of hot air on the fans.

3. More difficult to work on fan assembly, i.e. adjust blades due to heat from

bundle, and their location.

Advantages of Forced Draft Design

1. Easy to remove and replace bundles.

2. Easier to mount motors or other drivers with short shafts.

3. Lubrication, maintenance, etc. more accessible.

4. With reinforced straight side panels to form a rectangular box type plenum,

shipping and mounting is greatly simplified, permitting complete

preassembled shop-tested units. Best adapted for cold climate operation

with warm air recirculation.

Disadvantages of the forced draftdesign are the list of "advantages of the induced

draft design."

5.1 Heat Transfer Calculations

The basic calculation approach is the same as other exchangers. Table 1 shows a

group of overall heat transfer coefficients based on bare tube area. These are useful as

a first step in planning before choosing a particular fin type on the outside of the tube.

The optimum air temperature rise across the tubes may be estimated by' the equation

(t2t1) = (o.005)(U)

1

12

2tTT (7)

Where: t2= outlet air temperature

t1= inlet air temperature

T2= temperature of process fluid out

T1= temperature of process fluid in

U = value from Table 1


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The optimum air temperature rise is also a function of the range (T 1 T2) of the

process fluid. The value of (t2 t1) calculated from Equation 7 should be corrected

using the equation

CF = 0.89 + A (T1T2) (8)

For a specified cooling load and conditions, the outlet air temperature can

be estimated. From this an LMTD can be found to calculate bare tube

area.

Where: CF = correction factor

A = constant


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Table 1

Typical Overall Heat Transfer Coefficients

Tube size can vary from 15.9-38.1 m [5/8-1 in.), but the standard size is 25.4 mm

[1 in.). Tube layout is triangular. Tube pitch is the minimum which avoids fin

contact or overlap.

The types of fins vary with the service. They are either tension wrapped, solder

bonded or extruded.

The latter are the most expensive. Fin height varies from -5/8 in. and normally

there are 8-11 fins per inch.


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The air-side film coefficient for a typical fin tube (based on extended area) can be

estimated from the equation

ha= 3.0

6.0

g )(vA

d (9)

This "h" would be used with other data in Equations 3 or 4 to find an overall "U" for

comparison with values in Table 1.

5.2 Fans

The fan power requirements can be estimated from the equation

kW =))((

))((

EfficiencyA

QP aa (10)

Efficiency varies from 0.4-0.75; 0.7 is a useful planning number

Pressure drop varies with air rate, tube diameter, pitch and number of tube rows. For

planning purposes a pressure drop of 25 Pa [0.10 in H2O] per tube row can be used.

For most gas processing applications the number of tube rows varies from 3-6.

Where: ha= air-side film coefficient

Vg= air velocity by tubes

d = outside diameter of bare tubes

A = constant

Where: Qa= air flow rate

Pa = air pressure drop in cooler

A = constant


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37

The following figure is a rough estimate of horsepower based on the value of "U"

from Table 1 for bare tubes. The number of fans and heat transfer bays will vary with

the installation. Two fans/unit are generally preferred because of the additional

flexibility in controlling air flow.

Noise control is a serious concern. A common specification is that fan and motornoise shall not exceed 85-90 dBA at a distance of three feet from the fan ring. One

can estimate the sound pressure level by the equation

dBA = 65+30 (log V) + 10 log (hp) + 20 (log d) (11)

Where: dBA = relative sound level in decibel

log = logarithm to base 10

V = fan tip speed. (0.001)(ft/min).

hp = fan horsepower

d = fan diameter, ft.

Overall Heat-Transfer Coefficient, But/(h)(ft2)(F)


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As noted previously, various methods of fan control are used. The primary criteria are

temperature control of the process fluid and power consumption. It is feasible to drive

variable speedfans with standard induction motors using some type of a.c. adjustable

frequency drives (AFD) - a variable voltage inverter, a pulse width modulator or

current source types.

One alternative is to use a variable pitch fan. It offers rather precise temperature

control, provides energy savings and is convenient for cold weather operations. It

also tends to cost more and may involve more routine maintenance. The choice

between variable speed and variable pitch depends on local circumstances and the

biases of the purchaser. If power costs are large and temperature control is critical,

one or the other normally will be chosen.

The other control alternatives are fluid by-pass, on-off operation (with possibly

several fans per cooling bay) and the use of louvers or shutters. By-pass and louvers

may be effective in some cases but they are energy inefficient. On-off fan control is

simple and may be used if there are a lot of fans in the same service. Winterprotection is required in cold climates. In this case, the use of louvers plus some form

of variable air rate control is desirable. This is one case where a variable pitch fan

plus louvers may be the best system to control internal air circulation.

Outlet temperature is controlled primarily, by air rate. Louvers, variable pitch fan

blades, and variable speed motors are all used to control temperature. Louvers may be

manually adjustable for seasonal or night-day air temperature changes, or controlled

automatically. We have found automatic louver control less than satisfactory in those

cases where a close tolerance is required on outlet fluid temperatures and the louvers

are operating almost closed (where a small change in position causes a large change in

air flow rate). In those cases where large air temperature changes are encountered, a

variable pitch fan may prove efficient. Some report trouble with the pitch control, but

this has not been a problem in my experience. Pitch and speed controls are expensive

but in this era of high energy costs they can prove profitable.

Fan power is an important operating cost consideration. A ten percent change in air

flow rate will cause about a 35 percent change in power used, assuming efficiency

stays constant. Actual power consumption required for a given heat transfer depends

on many factors. One is the clearance between the fan and the fan ring. Close

clearances are more expensive to fabricate. Consider this in comparing capital cost

from different vendors.


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5.4 Choice of Heat Exchangers

It should be apparent from preceding discussions that choice of heat exchangers

involves many factors. It is relatively easy to choose one that will work. An intelligent

choice however involves choosing equipment that optimizes cost of the total systemwithout compromising operating reliability.

Heat exchangers normally cost less per unit of energy transferred than any other type

of energy equipment. If you "chisel" on exchanger size you must pay dearly for this in

the cost of companion equipment in many instances. Since heat loads vary with flow

rates, some flexibility must be provided. If done wisely, a little extra heat exchange

capacity is the cheapest "insurance" one can purchase.

There are some "rules" one should follow.

1. Do not specify a HEX without consideration of its effect on the totalprocess.

2. Do not make the capital cost of the HEX alone a sole criterion for purchase.

3. Acquaint the vendor with details of service and point out that choice will bemade on both initial and operating cost, not initial capital cost alone.

4. Use realistic pressure drop specifications since this affects size and cost.Allow as much pres sure loss as economics dictate for the actual system and

not merely reproduce a standard spec that might not apply.


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6. Heat Exchanger Operation

6.1 Procedure to Take a Heat Exchanger out of Service

1. The hot fluid must be shut off before the cold fluid, This should be done slowly toallow the exchanger to cool down. The cold fluid must not be shut off first.

Otherwise, the heat from the hot side will cause the cold fluid to increase in

temperature and as there is no place for the expansion, the pressure would build up

and cause exchanger ruptures.

2. After the hot fluid has been shut off, both on inlet and outlet of the exchangers and

the temperature has cooled to that of the cold fluid, then the cold fluid can be shut

off on both inlet and outlet valves.

3. Both shell and tube side should now be pumped out to slop or drained

down.

4. Both inlet and outlet lines should be blanked off for safety.

5. If the exchanger is in sour oil service or any iron sulfide scale is expected, the

exchanger should be water washed before opening to the atmosphere.

6.2 Procedure to Place Heat Exchanger in Service

1. Cheek the exchanger carefully to ensure that all plugs have been replaced and that

all pipe work is ready for the exchanger to be placed in service -(no loose bolts,gaskets in flanges).

2. All valves should be in the shut position.

3. Purging and testing.

4. Line up the system.

5. Open hot and cold fluid vent valves.

6. Crack open cold fluid inlet valve vent all air when liquid full. Close cold fluid ventvalve.

7. Crack open hot fluid outlet valve and vent all the air, then close hot fluid vent

valve. At this stage, the exchanger is liquid full of both hot and cold flowing

fluids - open coldfluidinlet and hotfluidoutlet valves fully.


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8. The cold fluid valve outlet may be opened slowly until the cold fluid is passing

through the exchanger.

9. Start opening the hot fluid inlet valve slowly.

10. Both valves, the cold fluid outlet valve and the hot fluid inlet should be openedslowly until fully open.

All operations should be performed slowly and care must be taken not to cause

sudden temperature changes.

6.3 Testing Heat Exchangers for Leaks

In some cases, before the equipment has been operated, it is

hydrostatically tested to check for leaks, although all tubular equipment is

normally tested at its place of manufacture. Sometimes, during operation,the products become contaminated and this could be due to a leaking heat

exchanger tube. The basic method for testing is as follows. In a fixed tubesheet exchanger, after the end covers have been removed, a hydrostatic

test pressure is applied to the shell and leaking tubes will be detected by

water running out of the tube. The tube is sealed by driving in a tapered

plug of suitable metal at each end of the tube and the test repeated until

all the leaks have been cured. In a floating head exchanger, the testprocedure is a little different. After the end covers are removed, a special

test ring sized to fit the exchanger is fitted so as to seal the tubes andshell. The procedure then becomes the same as for a fixed tube sheet

exchanger. Always use a cold liquid for testing, because a hot liquid

affects the expansion of tube and shell and can cause damage. Hydrostatic

test pressures at ambient temperature, normally are 1.5 times the designpressure corrected for temperature, except for cast iron parts where other

codes govern. It should be noted, however, that when testing, the

maximum specified D P between tube and shell sides should not be

exceeded.


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7. Heat Exchangers Problems

7.1 Heat Exchanger Fouling and Corrosion

Fouling of heat transfer surfaces-introduces perhaps the major uncertainty

into the design and operation of heat exchange equipment. Fouling in

equipment involving boiling and evaporation is often more severe than in

single phase heat exchangers and moreover, in aqueous systems, is

frequently associated with corrosion. Finally the modification of heat

transfer and pressure drop characteristics by fouling layers is briefly

reviewed.

7.1.1 Introduction

Although fouling is by no means confined to heat transfer equipment, it is

in this particular. field that its unwanted presence is perhaps most acutelyfelt. As research work on the various aspects of single-phase and two-

phase heat transfer have progressed so the uncertainties in heat transfer

rates from clean surfaces have been markedly reduced. However, in

practice industrial heat exchangers rarely operate with non-fouling fluids.

Low temperature cryogenic heat exchangers are perhaps the onlyexception. The probability that fouling will occur in a heat exchanger is

therefore normally taken into account at the design stage by the use of anassumed fouling resistance or fouling factor. However, few systematic

investigations of fouling have been carried out and the uncertainty in the

fouling factor now greatly exceeds the uncertainty in the other terms ofthe overall heat transfer equation

7.1.2 Types of Fouling

Epstein has delineated six classes or types of fouling depending upon the

immediate cause of the fouling.

(a) Scaling involve the crystallization of inverse solubility salts (such as

CACO , CASO , Na SO in water) onto a superheated heat transfer

surface. This process can occur under both evaporating or non-

evaporating conditions.

(b) Particulate Fouling involves the deposition of particles suspended in the

fluid stream onto the heat transfer surface. This process includes

sedimentation, i.e. settling under gravitational forces as well as other

deposition. mechanisms.


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(c) Chemical Reaction Fouling involves deposits caused by some form of

chemical reaction within the fluid stream itself (but not with the heat

transfer surface). Polymerization, cracking and coking of hydrocarbon

liquids at high temperature are prime examples.

(d) Corrosion Fouling involves a chemical reaction between the heattransfer surface and the fluid stream to produce corrosion products

which, in turn, foul the surface. Examples of this would be the on-load

aqueous corrosion process often experienced within nuclear and waste

heat boilers.

(e) Biofouling involves the accumulation of biological organisms at the heat

transfer surface.

(f) Freezing Fouling occurs as a result of the crystallization of a pure liquid

or one component from a liquid phase on to a subcooled heat transfer

surface.

Not all these mechanisms are mutually exclusive; often more than one mechanism

will be occurring simultaneously.

7.2 Vibration in Heat Exchangers

7.2.1 Introduction

To improve thermal efficiency heat exchangers are commonly equipped with baffles.

These devices produce a low f around the tube bundles which is favorable for theheat transport, which also may induce vibrations. If the amplitudes of the vibrationsbecome too high, corrosion and erosion of the tubes at the position of-the baffles may

occur.

Since more than 30 years research activities are underway to study the

vibration phenomena in cross flow bundles of heat exchangers. Mostly it

is assumed that oscillations are excited by vortices departing from the

tubes, then the strongest vibrations should be observed if the departure

frequency of the vortices and the resonance frequency of the tubes are

identical. A safe layout of the tube banks -would then be not too difficult,one just has to avoid the coincidence of these both frequencies. In

literature it is clearly stated that there is a linear connection between the

vortex frequency and the flow velocity, which means that the Strouhal-

number is constant.


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Heat Exchangers

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After a careful literature survey Chen found that the value of the

Strouhal-number should be between 0, 17 and 0, 21, for Reynolds-

numbers from 300 up to more than 2x10

Owen studied the turbulence behind the tubes and found that this may cause vibration.

He called this aeroelastic exciting phenomenon buffeting. These turbulent andstochastic velocity fluctuations are mainly due to the perturbation of the boundary

layer on the rear side of the tube. These fluctuations have a very wide frequency

spectrum.

A special case is the resonant buffeting, which also has a statistical

energy distribution, however, in addition a periodical velocity fluctuationis superimposed. If the frequency of this periodical fluid dynamic

exciting force coincides with the resonance frequency of the tubes,

vibrations of large amplitudes.

7.2.2 Galloping - Wake Galloping

In civil engineering another vibration exciting phenomenon is well

known, which is called galloping - for a single obstacle - or wake

galloping - for a group of obstacles like cylinders -. Galloping wasobserved with chimneys or with the cables of high voltage transportation

lines. As shown in Figure 15 for the example of a tube or rod bundle with

3 rows galloping can produce a lifting force due to the partial deflection

of the flow. This phenomenon mainly in the second and in the third row

may perform a vibration rectangular to the direction of the inlet flow.

Figure 15 Flow Path in Rod Bundle


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7.2.3 Aeroelastic Coupling

Another mode exciting vibrations in cylindrical tube banks may be the

aeroelastic coupling, first mentioned by Livesey and later by Connors. In

contrast to the vortex or galloping induced vibration, the tubes not onlymove rectangular to the f low but also in the flow direction.

The aeroelastic coupling is a consequence of the movement of the rods. If the rod

leaves its original or stationary position, the fluidynamic forces around the rod, which

are influenced by the relative position to the neighboring rods change. So a new

exciting force for vibration may be created. The frequency of the vibration, however,

is then not only depending on the flow velocity, but also on the resonant frequency of

the surrounding rods. The main influencing factor with aeroelastic coupling is the

movement of the neighboring tubes, which means that a small vibration of a few rods

in the tube bank may excite other rods by fluid dynamic forces and therefore this

phenomenon is called aeroelastic coupling. Contrary to the exciting modes discussedbefore - like buffeting or galloping - with aeroelastic coupling no favored vibration

direction can be observed. The vibration movement of each rod is depending on and

influenced by the movement of its neighboring rods.

It is found that the vortices cannot be the only and main reason for inducing vibrations

in tube banks. Most of the experiments in the literature studying vibrations in heat

exchangers used tube banks where. only one tube could freely move and all others

were fixed.

7.2.4 Conclusion and Measures to Reduce Vibration

There seem to be two effects mainly influencing the vibration of tube or rod bundles,namely the wake galloping and the aeroelastic coupling. From this information the

conclusion can be drawn that two measures could be taken in account to reduce the

sensibility of a bundle against vibrations namely

1. Increasing of the inlet turbulence

2. Putting out of tune the resonance frequencies of neighboring rods.

The inlet turbulence can be easily increased by placing a grid upstream ofthe first row of the bundle. Already Vickery found a reduction of the

oscillating pressure onto a prismatic rod in the order of 100% byincreasing the inlet turbulence. Using a punched plate with wholes of 10

mm diameter and placed 20 cm upstream of the first row a remarkable

improvement of the stability against exciting vibrations could be

observed. By this measure the grade of turbulence which in the testsbefore was in the order of 0, 7% could be raised up to 50%.


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The improvement was much more pronounced for the staggered

arrangement than for the inline one and the onset of vibrations could be

shifted to velocities which were almost twice of that under low turbulence

conditions. One can see that the grade of turbulence mainly influences

the vibration behavior of the critical rows, which are usually the threeinlet rows. Further downstream there is no effect of the turbulence

promotor which can be easily explained by the fact that then the grade of

turbulence is anyhow high enough due to the perturbation of the flow in

the first rows. It can be assumed that the increased inlet turbulence

affects the drag coefficient rectangular to the flow direction and reduces

by this the onset of aeroelastic coupling.

Whilst the increasing of the inlet turbulence is certainly measure of

practical use, the mistuning of the resonance frequency of neighbored

rods seems to be more of academic interest. Never the less it should be

briefly pointed out here that this can reduce the vibration amplitudesremarkably. It, however, does not change the critical velocity, for the

most sensitive s/d ratio of 1,3 and a staggered-arrangement.

In a staggered arrangement the vibration is mainly induced by aeroelastic coupling -

ass we can conclude from the experimental results discussed before. This aeroelastic

coupling is introduced by the beginning of oscillations in the critical row.

7.3 Cleaning of Heat ExchangersFive possible cleaning techniques are recognized for condenser tubes,

based on field testing.

1. Hydroblast. Small sections of the sample tube were sent to a localhydroblast company. The results showed fairly clean tubes with

some pitting; however, a significant roughness remained to impede

fluid flow.

2. Acid Cleaning. A section of tube was sent to a local firm for acid cleaningutilizing a 12% foaming hydrochloric acid solution. This technique also

produced a clean tube with pitting continuing, and surface roughness again

was evident.

3. Chemical Additive. A sample tube was tested utilizing five differentchemical agents. These chemicals were ineffective in removing the tenacious

mineral deposits.


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4. Brushes and Rubber Plugs. Several sample tubes were shot with nylonbrushes and rubber plugs. Both techniques were ineffective in removing

internal tube deposits.

5. Tube Scrapers. A medium-pressure (150 to 250 psi) water gun was used to

propel spring-loaded metal scrapers. This technique was used in several tubesprior to removal from the condenser for inspection. The scrapers cleaned

down to bare metal and polished the tube surface for minimum flow

restrictions.

3 heat exchangers

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