Kuppan T. Heat Exchanger Design Handbook

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1004

Chapter

Rear

door

khqnkal

Crmt

door

qY='em

Roughing

Fore

valve

Figure

Vacuum type batch furnace. (From Ref.

69.)

In a

semicontinuous vacuum furnace,

the brazing chamber is divided into two or more

heating stations

that part movement from station to station brings about the braze. The

number of hot zones is determined by the parts to be brazed and the desired production rate.

For example, in a three-zone brazing chamber, the braze is completed in three stages, and three

carriers of parts, each at a different heating station, are contained in the brazing chamber

environment at all times. Cycle times for a three-zone chamber are approximately one-third

that for the batch process

[68].

semicontinuous or locked furnace is shown schematically in

Fig.

31.

It is developed to avoid contamination while loading or unloading work

[69].

The

entrance vestibule is generally used to preheat and outgas the parts prior to transfer into the

brazing zone. The exit vestibule is used for the initial phase of cooling and filler metal solidifi-

cation under an inert

dry air environment

[68].

For more details on batch type and continu-

ous type vacuum furnaces see Ref.

69.

Procedure.

Furnace brazing steps are listed in Table

Entry

Hot

zone

Hot

zone

Exit

Wstibu(c

vest

ibuk

Figure

Semicontinuous

locked furnace. (From Ref.

69.)

Heat Exchanger Fabrication

I005

Brazing Processes Without Corrosive

Flux.

Various processes have been developed for re-

ducing or eliminating corrosive

flux

in the brazing of heat exchangers. Three industrial pro-

cesses are followed to overcome the requirement of corrosive flux. They are

Controlled-atmosphere brazing

Use of noncorrosive flux under the trade name NOCOLOCK

Vacuum brazing

All three methods are commercially used for the manufacture of heat exchangers and high-

volume automotive heat exchangers, such as radiators, air conditioning evaporators, heater

cores, and turbo air coolers. In general, the fluxless processes require furnace atmosphere

having a dew point less than about

-60°C

(-75°F)

to produce better quality.

Controlled-Atmosphere Brazing.

In controlled-atmosphere brazing (excluding a vacuum

atmosphere), a continuous flow of a certain gas is maintained in the work zone to avoid

contamination from outgassing of the metal parts and dissociation of oxides. Controlled atmo-

spheres for furnace brazing include combusted fuel gas, dissociated ammonia, cryogenic or

purified N2

HZ, deoxygenated and dried hydrogen, purified inert gas, etc., as specified by

AWS designations. The controlled atmosphere such as pure dry hydrogen or inert gases inhibits

the oxidation and scaling of the surfaces. Inert gases, such as helium and argon, do not form

compounds with metals and inhibit vaporization of volatile elements. A dry-nitrogen atmo-

sphere is excellent for production volume brazing with a fluoride flux. With controlled-atmo-

sphere brazing, the need for a

flux

is eliminated or the flux requirement is considerably re-

duced.

The merits of controlled-atmosphere brazing include:

(1)

The need for a flux is eliminated

or the flux requirement is considerably reduced;

(2)

there is more rapid and uniform heating

than brazing under vacuum, due to convective heating of the component by the circulated gas;

and

(3)

it prevents oxide and scale formation on the parts being brazed and reduces evaporation

of volatile elements. Due to these reasons, the assembled components remain clean and bright

after brazing, postbraze cleaning is generally not necessary, and there are lower maintenance

requirements related to buildup of volatile alloying elements

[70].

However, controlled atmo-

spheres are not intended

perform the primary cleaning operation and surface oxide removal.

Use of Noncorrosive

Flux

Under the Trade Name NOCOLOCK.

Use of noncorrosive

flux under the Trade name NOCOLOCK pertains to aluminum brazing. Therefore, it is covered

in the next part, while discussing aluminum brazing.

Vacuum Brazing

Vacuum brazing is a form of furnace brazing process for brazing assemblies in a vacuum

environment without the use of flux. Vacuum brazing is being used commercially to produce

automotive and aircraft heat exchangers. Large prototype industrial heat exchangers and mas-

sive cryogenic heat exchangers have been successfully vacuum brazed. Commercial vacuum

brazing generally is done at pressure varying from

10-5

10-'

torr, depending on the materials

brazed and the filler metals being used.

Parent Metals Brazed.

Vacuum brazing

most suitable to brazing aluminum alloys, stainless

steels, superalloys, titanium alloys, zirconium, or other reactive elements with particularly sta-

ble oxides.

Merits

Vacuum Brazing.

Merits of vacuum brazing are:

The method eliminates the need for fluxes and postbraze cleaning.

It is nonpolluting and possesses high braze performance capabilities.

Vacuum prevents oxidation of metals by removing air from around the assembly and also

removes volatile impurities and gases from the metals.

I006

Chapter

The method imparts a high standard of cleanliness to the work that could not be achieved

by any other method.

The overall production time is minimized since there is no fluxing action and subsequent

postbraze cleaning.

5.7

Postbraze Cleaning

Brazing fluxes are chemically active, and their residues are highly corrosive, especially

the

presence of moisture. Therefore, after brazing, flux residuals must be removed to avoid both

corrosion and contamination problems. Use boiling water, which usually removes chloride type

flux

residue, or a strong acid such as nitric, followed by water rinsing. Oxidized areas adjacent

to the brazed joint can be cleaned by pickling, wire brushing, or blast cleaning.

Braze Stopoffs

When filler metal flow must be restricted to definite areas, “stopoffs” are employed to outline

the areas that are not to be brazed. Braze stopoff materials, if used, must be compatible with

base metal, filler metal, fluxes, and furnace atmosphere

[71].

Stopoff residue must be removed

from the joint after brazing. Wire brush, air blast, or water flush will remove the stopoff

residue.

FUNDAMENTALS OF BRAZING PROCESS CONTROL

It is generally accepted that the most important variables involved

the brazing process

include the brazing temperature, time at temperature, brazing atmosphere, and the rate and

mode of heating. Some of the these factors are discussed next.

6.1

Heating Rate

Geometry and the distribution of mass within the part, heating element configuration relative

to part location, and vacuum environment characteristics will determine the heating rates

needed to realize a good braze.

6.2

Brazing Temperature

Brazing temperature is mostly governed by the base metal characteristics and the type of filler

metal. In general, the filler metals with the higher melting points correlate with higher strength

joints. Various base metals with their melting and brazing temperatures are shown

Table

6.3

Brazing Time

Brazing time will depend somewhat on the mass/thickness of the parts and the amount

fixturing necessary to hold them. It has become industry practice to minimize the holding time

Table

Base

Metals

and

Their

Melting

Temperatures

and

Brazing

Temperatures

Base

metal

Melting

temperature

Brazing

temperature

Aluminum

and

aluminum

588-657°C 57 1-62

“C

alloys

(1090-1215°F)

(

1060-

150°F)

Stainless

steels

1370- 1532°C 6

18-

1232°C

(2500-2790°F)

145-2250°F)

Heat Exchanger Fabrication

1007

at brazing temperatures to minimize excessive filler metal flow andor erosion, evaporation,

and oxidation. Brazing times in the order of less than a minute are common.

6.4

Tem perat u re Uniformity

Producing consistent braze joint quality throughout the padfurnace loadtemperature unifor-

mity

all of the workpieces depends on the configuration of the work, its placement, how

interrelates with the hot zone, etc.

6.5

Control

Distortion During the Furnace Cycle

Control of distortion is vitally important for successful furnace brazing, particularly complex

parts. Distortion can result from fast heating, fast cooling, stresses that develop during heating,

residual stresses in a workpiece, phase transformation, and properties of dissimilar base materi-

als [72]. According to Tennenhouse [72], distortion can be minimized or eliminated by mea-

sures like uniform heating and cooling of the part, reduced and controlled heating rates, use of

dummy weights, heat shields and lightened fixtures, and adequate supports to the parts.

BRAZING OF ALUMINUM

Brazed aluminum assemblies are all aluminum with excellent corrosion resistance when prop-

erly cleaned of any residual corrosive flux. Brazed aluminum assemblies conduct heat uni-

formly. Therefore, brazed aluminum heat exchangers are long-lasting and highly efficient.

Techniques for brazing aluminum are similar to those used for brazing other metals. Com-

mercially the same equipment is often used. But there are some important differences between

aluminum and other brazeable metals that affect the standard brazing techniques. These differ-

ences stem from the refractory nature of the surface oxide film on aluminum, from the oxide

that forms on aluminum having a high melting point (3722"F), and from the low melting point

of aluminum [62]. Other characteristic features of aluminum relevant for brazing include: It

conducts heat quickly, surface oxides form rapidly, thermal expansion is greater than many

other common metals, and it doesn't change color as its temperature changes

[50].

7.1

Need for Closer Temperature Control

Aluminum alloys are brazed with aluminum filler metals that are similar to the base metals.

Filler metals have liquidus temperatures closer to the solidus temperature of the parent metals,

and hence close temperature control is very important [51]. This can be easily understood from

the Table 3. The brazing atmosphere should be approximately 70°F (39°C) below the solidus

temperature of the base metal, but if temperature is accurately controlled and the brazing cycle

is short, the difference can be as close as 10°F

(5.5"C)

[59].

7.2

Aluminum Alloys That Can Be Brazed

A majority of the non-heat-treatable aluminum alloys like 1100, 3003, 3004, and

5005

and

many of the heat-treatable alloys like 6053, 6061, 6063, 6951, 7005, and 7072 can be brazed.

Aluminum alloys containing more than 2.5% magnesium are difficult to braze. Common braze-

able wrought alloys, their composition, and approximate melting range are shown

Table 4.

Approximate brazing temperatures for common aluminum alloys are shown in Fig. 32. In

addition to these aluminum can be brazed to many dissimilar metals and alloys.

partial list

includes the ferrous alloys, nickel, titanium, Monel, and Inconel [50].

I008

Chapter

Table

Common Brazeable Wrought Alloys With Their Compositions and

Approximate Melting Ranges

[50]

Nominal composition

(%),

remainder AI and impurities

Melting range

Alloy Cu Si Mn Mg

Ni Cr

1100

99.00%

minimum aluminum

643-657

1190-1215

3003 0.12

0.6 1.2

643-654

1190-1210

3004

1.2 1.0

629-652

1165-1205

3005

1.2 0.4

638-657

1180-1215

5005

0.8

627-652

1160-1205

5050

1.2

627-652

160- 1205

- -

2.5

5052

0.25

593-649

1100-1200

6061 0.25

0.6

1.0

0.25

616-652

1140-1205

6063

0.4

0.7

6 16-652

140- 1205

6151

1.0

0.6

0.25

588-649

1090-

200

6951 0.25

0.35

0.6

6 16-654

1140-1210

7004

- -

0.45

1.5

4.2

0.13

604-646

1120-1 195

7005

0.45 1.4 4.5

0.13

607-646

1125-1 195

7072

- - -

1.0

646-657

1195-1215

7.3

Elements

Aluminum Brazing

Joint Clearance

The joint clearances established for aluminum brazing are furnished in Ref.

50.

Precleaning

Brazing aluminum requires that the metal be free from surface contaminants and its oxide layer

thin enough to be displaced by the brazing flux. Surface contaminants such as greases, oil,

fatty acids, and marking crayons can be removed by vapor degreasing using inhibited trichloro-

ethylene or ultrasonic degreasing.

Surface Oxide Removal

highly reactive material, aluminum and its alloys are covered with an oxide. This oxide

can be an important hinderant in brazing process, where the oxide film acts as a barrier to the

wetting and

flow

filler metal

[68].

Hence, oxide barrier dispersal

a prerequisite for suc-

Figure

Clad aluminum brazing sheet. (From Ref.

74.)

Heat Exchanger Fabrication

1009

cessful brazing. For non-heat-treatable aluminum alloys, vapor degreasing or ultrasonic de-

greasing is adequate

[65].

Overly thick oxide layers on heat-treatable alloys can be reduced by

either mechanical means or chemical means

[50].

Chemical means will consist of a degreasing

either in trichloroethylene or a chemical cleaner followed by an acid or alkaline etch. The parts

will normally need desmuting in nitric acid, after which they are washed in clean water and

dried

[65].

Chemical cleaning is not recommended for fluxless vacuum brazing

[59].

Aluminum Filler Metals

In the case

aluminum, the major elements in the filler metal are the same

the base

metal except for some additional melting-point depressants. Complex parts can be formed and

assembled using clad brazing sheet to eliminate the need for separate filler wire or shim.

Various flux brazing filler alloys, vacuum brazing filler alloys, and clad aluminum

flux

brazing

sheet are dealt with in Ref.

50.

Aluminurn Brazing Sheet.

Aluminum brazing sheet consists of a base metal core (typically

3003

6951

alloy) and a filler metal, typically either A1-7.5Si

(4343)

or AI-1OSi

(4045),

clad roll bonded to either to one or both sides as illustrated in Fig.

32.

The core provides

structural integrity, while the clad plays the role of filler metal. An oxide surface layer is

depicted to emphasize the importance

this barrier to the wetting and flow of the filler metal

and the requirement that the barrier be dispersed for a successful braze

[74].

Fluxing

Except for vacuum brazing or controlled-atmosphere (without oxygen) brazing, or salt-bath

brazing processes where the salt bath itself consists of flux, all surfaces must be fluxed to

break down the oxide film on the surface of the work and to promote wetting, and the filler

itself must be fluxed. Fluxes for aluminurn brazing contain fluorides and chlorides of the alkali

metals. To the base are added “activators” such as fluorides, lithium compounds, and some-

times zinc chloride

[62].

7.4

Brazing

Methods

The main processes used commercially to braze aluminum heat exchangers are

Torch brazing

Furnace brazing: controlled-gas atmosphere brazing and vacuum brazing

Flux dip brazing

Torch brazing and dip brazing employ a flux to break down the oxide film on the surface

of the work and promote wetting. Furnace brazing with controlled-gas atmosphere requires

less flux

no flux, whereas vacuum brazing does not require a flux and the mechanism of

oxide breakdown is quite different.

Aluminum Dip Brazing

Aluminum dip brazing has been practiced commercially for many years. The molten salt bath

contains chemically active salts that promote brazing. These help to displace the adherent oxide

film on aluminum surfaces

that the molten filler metal wets the surfaces and forms the

brazed joints. Because of the stability of surface oxide film, and because of low brazing temper-

atures that have to be used, a highly active flux is required for brazing aluminum. Important

parameters pertaining to dip brazing aluminum are given in Table

Manufacturing procedure

for dip brazing of PFHE is shown in Figure

33.

1010

Chapter

Table

Aluminurn Dip Brazing Parameters [54]

Bath Preheating Brazing

Bath composition melting point temperature temperature

~ ~~ ~

AIF,

NaCl, KCI,

900-1000"F

100°F

1030-

190°F

LiCl

(482-5 38°C) (538°C)

(555-643 "C)

Furnace Brazing

Furnace brazing is the second most popular method of brazing aluminum in use today. Furnace

brazing's popularity derives from the comparatively low cost of equipment, from the ease with

which the existing furnaces can be adapted to aluminum brazing, and from the minimal fixtur-

ing required

[50].

Inert-Gas or Controlled-Atmosphere Brazing

Aluminum.

For inert-gas brazing of alumi-

num, the process variables that affect moisture and oxygen levels in the brazing chamber must

be controlled. This requires that the workpiece along with the fixture and the carrier be ade-

quately outgassed to remove adsorbed moisture and oxygen prior to entering the inert-gas

chamber. This is achieved by either repeated heating and evacuations, or a vacuum heating of

aluminum with magnesium brazing filler metal clad to getter the contaminants.

Controlled

Dry

Air Brazing.

The amount of moisture in the brazing atmosphere influences

the amount of flux required for successful brazing. When the moisture is not controlled, thick

flux

slunies, about

to 75% by weight, are required. Only a small amount of

flux

serves the

intended purpose, and much of it becomes ineffective because it is hydrolyzed by reaction with

moisture present in the air, before the brazing alloy melts [75]. In a dry-air atmosphere of

-40°F

(-40°C)

dewpoint or lower, thin slurries containing

20-25%

flux

will serve the purpose.

This brazing process also eases the postbrazing cleaning requirements.

Furnace Brazing

Aluminum with a NonCorrosive Flux-NOCOLOCK Flux (NOCOLOK

Owned

Alcan Aluminum Ltd.).

It was discovered that a brazing method using a nonhydro-

scopic, noncorrosive, potassium fluo-aluminate

flux

material offered the benefits of flux braz-

ing without a requisite postbraze cleaning. When used in an inert-gas atmosphere,

has a

significant tolerance to impurities in the furnace atmosphere [76]. The

flux

is active only at or

near the brazing temperature, and is completely inactive at lower temperatures, which allows

the flux residue to be left on the work without any danger of subsequent corrosion. The trade

name for the noncorrosive

flux

is known as

NOCOLOCK.

Heat exchangers such as automotive

radiators, heater cores, condensers, evaporators, oil coolers, and charge air coolers are being

mass produced using the

NOCOLOK

flux

brazing process [77]. The

flux

a fine white powder

mixture of potassium fluo-aluminates. It is mixed with water to form a dilute slurry, applied

by spray or other methods to cleaned aluminum assemblies,

and

dried. Parts are normally

brazed

a nitrogen atmosphere tunnel furnace. Brazing temperatures of 590"-620°C

(

1095-

1150°F)

are suitable.

Postbraze Treatments.

Generally speaking, no postbraze treatments are required. The flux

residue is noncorrosive, strongly adherent to the aluminum surface, and provides good corro-

sion protection. However, in recent years, there continues to be a demand for air-oxidized or

blackened-surface radiators and condensers [77]. These are based on the controlled introduction

dry air for surface oxidation or carbon dioxide for surface blackening into an inert atmo-

sphere brazing furnace soon after the completion of the braze. These after-treatments are

re-

quired to improve corrosion resistance of aluminum heat exchangers.

Heat Exchanger Fabrication

I01

separator

plote

fins

side-bars

row material row material raw material

meosuring,cuttinq

stamping,meosurinq,cutting

meosuring,cutting

washing

bGd

stocking

headers-sections nozzles

1111

cooling brazing pre-heating

raw moterial

washing of blocks drying

flux brazing

measuring,cutting

ossembly

packing

Figure

Manufacturing procedure of dip brazing with

flux.

(From

Ref.

58.)

1012

Chapter

Vacuum Brazing of Aluminum

In vacuum brazing, aluminum assemblies are heated in a vacuum environment by radiation.

The principal differences between vacuum brazing and furnace brazing are that

(1)

the brazing

done

a vacuum of the order of

104

torr,

(2)

flux

is used, (3) clad brazing sheet

and other forms of filler materials usually contain

0.1

2.0%

magnesium as braze promoter,

and (4) heating is by radiation rather than convection.

Typical Procedures

for

Vacuum Brazing Aluminurn Heat Exchanger.

These include:

Precleaning of components.

Assembly and fixturing.

Heathraze

vacuum.

Back-fill the furnace: Dry nitrogen is often used, although dry air can be used under some

circumstances.

Remove the brazed assemblies, which should be air quenched with a fan or allowed to

stand until cool.

Typical manufacturing procedure for vacuum brazing

PFHE is shown

Fig. 34.

Important Parameters

for

Vacuum Brazing Aluminum.

Important process variables that con-

trol brazing and braze quality are

(1)

vacuum environment,

(2)

brazing assembly temperature

and temperature uniformity,

(3)

oxide dispersion and wetting,

(4)

special vacuum brazing filler

and brazing sheet alloy, and

(5)

vacuum brazing furnace parameters control. Some

all of

these parameters are discussed in Refs.

68,

69,

and

74.

Vacuum Environment. The most important and difficult to control parameter is the vac-

uum itself. Pressure is generally monitored for most brazing processes, with the total pressure

being the sum of the partial pressure of all the gases present. It is particularly important to

monitor and control the partial pressure of contaminants such as oxygen and water vapor

[69].

Typical commercial furnaces designed for vacuum brazing operate in the

104

10-'

torr

pressure range. Mechanical roughing and oil diffusion pumps are used to achieve the vacuum.

Temperature Control.

The brazing assembly temperature and temperature uniformity are

important processing factors in brazing under vacuum.

designing the temperature program-

ming for the brazing process, several objectives must be borne

mind

[68]:

minimization of

(

)

the time for reaction with vapor-phase contaminants,

(2)

the temperature gradients within

the brazed assembly, and

(3)

the time at brazing temperature, to reduce erosion and other

unwanted core and cladding interactions

the brazing sheet. Braze performance

degraded

when the interactions causes a reduction

the available filler metal andor erosion of the core.

Vaporization losses of volatile solute from the molten clad can cause similar problems

[74].

The most important transport processes are indicated schematically in Fig.

35.

The adverse

effects

silicon diffusion from core alloy to the braze alloy and vaporization of zinc from

base metal on corrosion performance of braze are discussed later.

Oxide Dispersion and Wetting.

is well known that oxygen and water vapor are the

important process contaminants. In fluxless vacuum brazing, the presence of a promoter, either a

metal or reactive gas, is needed to getter oxygen and water vapor from the furnace and to suppress

reoxidation of the aluminum at the joints, once the oxide film is removed. The fluxless processes

use either elemental magnesium, magnesium containing filler metal, or magnesium bearing base

metals to getter oxygen [71]. Alternatively, brazing also can

achieved in a high vacuum by

prolonged exposure at elevated temperatures without using metal activators

[75].

Magnesium as a Braze Promoter.

Many metals can fulfil the function of a braze pro-

moter, but magnesium is the best, because, magnesium has the highest vapor pressure of the

metals that promote vacuum brazing

[75],

ease of alloying with aluminum, and low cost

1691.

1013

Heat Exchanger Fabrication

seporotor plate fins

side-bors

row moteriol

row rnoteriol

meosurinq,cutting

stornping,meosuring,cuttinq

meosurinq.cutting

woshing

,+,

headers nozzles

OWQOlU

rneasuring,cutting

testina occeotonce

x-roying helium flow pressure

leokoqe

test

test test

Figure

Manufacturing procedure of flux-free brazing in a vacuum. (From Ref.

58.)

Vacuum Brazing Filler Metals. Generally, vacuum brazing filler metals and brazing

sheets are aluminum alloys containing

7.5

to 12% silicon and up to 1.5 to

2.5%

magnesium

[60]. Since gravity adversely affects vertical capillary filler metal flow in vacuum brazing,

preplace filler metal in vertical joints or use a brazing sheet. Alloys of the lxxx, ~XXX, Sxxx,

~XXX,and 7xxx series can

vacuum brazed using

No.

13, and 14 brazing sheets, whose

compositions are shown in Table 6.

Vacuum Brazing Furnaces. Vacuum furnaces capable of reaching at least 1200

5°F

(650

3°C)

temperature uniformity in the normal brazing range of 1080-1 140°F (582-616°F)

are suitable for brazing aluminum [69]. Systems for vacuum brazing of aluminum are discussed